Chapter 1

INTRODUCTION

1.1 Introduction

A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. In a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder. In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder wall.

1.2 Piston Engines1.2.1 Internal combustion engines

Fig-1.2.1 Internal combustion engine piston, sectioned to show the gudgeon pin

The piston of an internal combustion engine is acted upon by the pressure of the expanding combustion gases in the combustion chamber space at the top of the cylinder. This force then acts downwards through the connecting rod and onto the crankshaft. The connecting rod is attached to the piston by a swiveling gudgeon pin. This pin is mounted within the piston: unlike the steam engine, there is no piston rod or crosshead. The pin itself is of hardened steel and is fixed in the piston, but free to move in the connecting rod. A few designs use a fully floating design that is loose in both components. All pins must be prevented from moving sideways and the ends of the pin digging into the cylinder wall, usually by circlips.

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Gas sealing is achieved by the use of piston rings. These are a number of narrow iron rings, fitted loosely into grooves in the piston, just below the crown. The rings are split at a point in the rim, allowing them to press against the cylinder with a light spring pressure. Two types of ring are used: the upper rings have solid faces and provide gas sealing; lower rings have narrow edges and a U-shaped profile, to act as oil scrapers. There are many proprietary and detail design features associated with piston rings.

Pistons are cast from aluminum alloys. For better strength and fatigue life, some racing pistons may be forged instead. Early pistons were of cast iron, but there were obvious benefits for engine balancing if a lighter alloy could be used. To produce pistons that could survive engine combustion temperatures, it was necessary to develop new alloys such as Y alloy and Hiduminium, specifically for use as pistons.

A few early gas engines had double-acting cylinders, but otherwise effectively all internal combustion engine pistons are single-acting. During World War II, the US submarine Pompano as fitted with a prototype of the infamously unreliable H.O.R. double-acting two-stroke diesel engine. Although compact, for use in a cramped submarine, this design of engine was not repeated.

1.2.2 Trunk pistons

Fig-1.2.2 Trunk piston for a modern diesel engine

Trunk pistons are long, relative to their diameter. They act as both piston and also as a cylindrical crosshead. As the connecting rod is angled for part of its rotation, there is also a side force that reacts along the side of the piston against the cylinder wall. A longer piston helps to support this.

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Trunk pistons have been a common design of piston since the early days of the reciprocating internal combustion engine. They were used for both petrol and diesel engines, although high speed engines have now adopted the lighter weight slipper piston.A characteristic of most trunk pistons, particularly for diesel engines, is that they have a groove for an oil ring below the gudgeon pin, not just the rings between the gudgeon pin and crown.

The name trunk piston derives from the trunk engine, an early design of marine steam engine. To make these more compact, they avoided the steam engine's usual piston rod and separate crosshead and were instead the first engine design to place the gudgeon pin directly within the piston. Otherwise these trunk engine pistons bore little resemblance to the trunk piston: they were of extremely large diameter and were double-acting. Their trunk was a narrow cylinder placed mounted in the centre of this piston

1.2.3 Crosshead pistons

Large slow-speed Diesel engines may require additional support for the side forces on the piston. These engines typically use crosshead pistons. The main piston has a large piston rod extending downwards from the piston to what is effectively a second smaller-diameter piston. The main piston is responsible for gas sealing and carries the piston rings. The smaller piston is purely a mechanical guide. It runs within a small cylinder as a trunk guide and also carries the gudgeon pin.

1.2.4 Slipper pistons

Fig-1.2.4 Slipper piston

A slipper piston is a piston for a petrol engine that has been reduced in size and weight as much as possible. In the extreme case, they are reduced to the piston crown, support for the piston rings, and just enough of the piston skirt remaining to leave two lands so as to stop the piston rocking in the bore. The sides of the piston skirt around the gudgeon pin are reduced away from the cylinder wall. The purpose is mostly to reduce the reciprocating mass, thus making it easier to balance the engine and so permit high speeds.

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A secondary benefit may be some reduction in friction with the cylinder wall, however as most of this is due to the parts of the piston that are left behind, the benefit is minor.

1.2.5 Deflector pistons

Fig-1.2.5 Two-stroke deflector piston

Deflector pistons are used in two-stroke engines with crankcase compression, where the gas flow within the cylinder must be carefully directed in order to provide efficient scavenging. With cross scavenging, the transfer inlet to the cylinder and exhaust ports are on directly facing sides of the cylinder wall. To prevent the incoming mixture passing straight across from one port to the other, the piston has a raised rib on its crown. This is intended to deflect the incoming mixture upwards, around the combustion chamber. Much effort, and many different designs of piston crown, went into developing improved scavenging. The crowns developed from a simple rib to a large asymmetric bulge, usually with a steep face on the inlet side and a gentle curve on the exhaust. Despite this, cross scavenging was never as effective as hoped. Most engines today use Schnuerle porting instead. This places a pair of transfer ports in the sides of the cylinder and encourages gas flow to rotate around a vertical axis, rather than a horizontal axis.

1.2.6 Steam engines

Fig-1.2.6.1 Cast-iron steam engine piston, with a metal piston ring spring-loaded against the cylinder wall.

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Steam engines are usually double-acting i.e. steam pressure acts alternately on each side of the piston and the admission and release of steam is controlled by slide valves, piston valves or poppet valves. Consequently, steam engine pistons are nearly always comparatively thin discs: their diameter is several times their thickness. One exception is the trunk engine piston, shaped more like those in a modern internal-combustion engine.

Fig-1.2.6.2 Early piston for a beam engine. The piston seal is made by turns of wrapped rope.

1.3 Drawbacks

Since the piston is the main reciprocating part of an engine, its movement creates an imbalance. This imbalance generally manifests itself as a vibration, which causes the engine to be perceivably harsh. The friction between the walls of the cylinder and the piston rings eventually results in wear, reducing the effective life of the mechanism.

The sound generated by a reciprocating engine can be intolerable and as a result, many reciprocating engines rely on heavy noise suppression equipment to diminish droning and loudness. To transmit the energy of the piston to the crank, the piston is connected to a connecting rod which is in turn connected to the crank. Because the linear movement of the piston must be converted to a rotational movement of the crank, mechanical loss is experienced as a consequence. Overall, this leads to a decrease in the overall efficiency of the combustion process. The motion of the crank shaft is not smooth, since energy supplied by the piston is not continuous and it is impulsive in nature. To address this, manufacturers fit heavy flywheels which supply constant inertia to the crank. Balance shafts are also fitted to some engines, and diminish the instability generated by the piston's movement.

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Chapter 2

LITERATURE SURVEY

2.1 Piston Functions

In an internal combustion engine, pistons convert the thermal into mechanical energy. The functions of the pistons are

To transmit the gas forces via the connecting rod to the crank shaft, To seal - in conjunction with the piston rings - the combustion chamber against gas

leakage to the crankcase and to prevent the infiltration of oil from the crankcase into the combustion chamber,

To dissipate the absorbed combustion heat to the cylinder liner and the cooling oil.

Aluminum alloys are the preferred material for pistons both in gasoline and diesel engines due to their specific characteristics: low density, high thermal conductivity, simple net-shape fabrication techniques casting and forging, easy machinability, high reliability and very good recycling characteristics. Proper control of the chemical composition, the processing conditions and the final heat treatment results in a microstructure which ensures the required mechanical and thermal performance, in particular the high thermal fatigue resistance.

The continuing development of modern gasoline and diesel engines leads to specific objectives for further piston development: reduction of piston weight, increase of mechanical and thermal load capacity, lower friction and thus improved scuffing resistance, etc. In addition, the basic requirements for durability, low noise level and minimum oil consumption have to be taken into account. These goals are achieved by a targeted combination of high performance aluminum piston materials, novel piston designs and the application of innovative coating technologies.

For future development, new aluminum materials using e.g. powder-metallurgical production methods or aluminum-based metal matrix composites produced by various methods as well as other lightweight materials such as magnesium alloys, carbon, etc., are being investigated. However, the ongoing improvements achieved with cast and forged aluminum alloys reveal that aluminum piston materials still offer great optimization potential and will continue to play a dominant role as piston material in the future.

2.2 Operating Conditions

Pistons are subjected to high mechanical and thermal loads. The mechanical loads on the piston result from extreme pressure cycles with peak

pressures up to 200 bar in the combustion chamber and

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Huge forces of inertia caused the by extremely high acceleration during the reciprocating motion of pistons.

These mechanical loads are superimposed by thermal stresses which are primarily generated by the high temperature gradients prevalent on the piston top. Ever rising demands regarding power density as well as the need for reduced emissions, low noise and more efficient fuel and oil consumption are the main engineering challenges for engines. For the pistons, these challenges translate into maximum strength requirements in the relevant temperature range combined with minimum weight.

In gasoline engines, the thermal loads have risen significantly during the last years as a result of higher power demands. Also the stresses at average ignition pressure have increased as a consequence of the introduction of knock control, direct fuel injection and turbocharging. Moreover, high speed concepts have led to an increase in inertia load. The requirements for pistons for diesel engines are even more demanding. Modern diesel engines for passenger cars equipped either with direct injection or super-charging with charge cooling operate with injection pressures up to 2,000 bar, mean effective pressures over 20 bar, peak pressures of 170 to 200 bar, and achieve specific powers of up to 80 kW per liter. But also the demand for ever lower exhaust gas emissions asks for significantly improved piston material characteristics.

The different elements of the piston system are indicated in the following schematic drawing:

Fig-2.2.1 Important piston terms

The thermal loads on the piston result from the combustion process with peak gas temperatures in the combustion chamber between 1800 and 2600°C depending on type of engine, fuel, gas exchange, compression, and fuel/gas ratio. Exhaust gases have

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temperatures between 500 and 800 °C. Combustion heat is transferred to the chamber walls and piston top primarily by convection. The heat is then dissipated by the water cooling of the chamber walls and by the oil cooling of the piston. A large share of the heat absorbed by the piston top is transferred by the piston ring belt area. The remainder is essentially removed by the oil lubricant impinging on the underside of the piston. The resulting temperature profile within the piston is schematically outlined in the following figure:

Fig-2.2.2 Operating temperatures in automotive engines under full load

2.3 Piston Materials

Pistons are produced from cast or forged, high-temperature resistant aluminum silicon alloys. There are three basic types of aluminum piston alloys. The standard piston alloy is a eutectic Al-12%Si alloy containing in addition approx. 1% each of Cu, Ni and Mg. Special eutectic alloys have been developed for improved strength at high temperatures. Hypereutectic alloys with 18 and 24% Si provide lower thermal expansion and wear, but have lower strength. In practice, the supplier of aluminum pistons use a wide range of further optimized alloy compositions, but generally based on these basic alloy types.

The majority of pistons are produced by gravity die casting. Optimized alloy compositions and a properly controlled solidification conditions allow the production of pistons with low weight and high structural strength. Forged pistons from eutectic and hypereutectic alloys exhibit higher strength and are used in high performance engines where the pistons are subject to even high stresses. Forged pistons have a finer microstructure than cast pistons with the same alloy composition. The production process

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results in greater strength in the lower temperature range. A further advantage is the possibility to produce lower wall thicknesses - and hence reducing the piston weight.

Also aluminum metal matrix composite materials are used in special cases. Pistons with Al2O3 fiber reinforced bottoms are produced by squeeze casting and used mainly in direct injection diesel engines. The main advantage, apart from a general improvement of the mechanical properties, is an improvement of the thermal fatigue behavior.

2.4 Design Considerations for Automotive Pistons

In engines for passenger cars, the diameter of the aluminum pistons for both gasoline and diesel engines ranges typically between 65 and 110 mm. There are two basic types:

mono-metal aluminum pistons Aluminum pistons with cast-in elements.

Steel or ceramic cast-in elements are used as local reinforcements to improve the high temperature mechanical properties and/or to control thermal expansion i.e. reduce the effects of different thermal expansion coefficients in contact areas with other materials.

Mono-metal pistons can be used in combination with cast iron engine blocks, but only in low-performance engines due to the larger clearance needed on account of the difference in thermal expansion between cast iron and aluminum. In engines with an aluminum engine block, this effect causes no problem, but special care must be taken to properly control friction and wear in the tribological system cylinder-piston-piston ring.

Fig-2.4 Mono metal piston

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2.4.1Pistons with cast-in control elements

When used in cast iron engine blocks, the thermal expansion of aluminum pistons is usually controlled by cast-in steel struts in the pin boss area. During engine operation, undesired thermal expansions are thereby avoided and the advantages of small clearances can be fully utilized.

Fig-2.4.1. Cast-in steel control strut

Diesel engines with pre-chamber, swirl chamber or direct injection operate under higher gas pressures and temperatures compared to gasoline engines. This increases the loads on the first ring groove, which is consequently strengthened by a cast-in stainless steel ring carrier in standard piston designs.

The even higher thermal loads in supercharged diesel engines are reduced by efficient cooling through a cooling gallery, a hollow annular cooling channel filled with oil through a nozzle installed in the crankcase. The cooling channel is usually produced using a salt core technology, but other methods are also possible, in particular for squeeze cast pistons where the cooling gallery is used in combination with ceramic fiber reinforcements.

Fig-2.4.2 other cast-in features10

For improved running properties, the piston skirt is generally protected by a wear resistant coating to reduce friction and hence to increase the scuffing resistance. Different coating methods are used as chromium plating, chemical nickel deposition, etc.

During the last two decades, different measures allowed a reduction of the oscillating masses by 20 - 25 % in the system piston – connecting rod. The suitable choice of piston material proved to be just as crucial as an optimum production process and an appropriate design to achieve the ideal combination of low weight and high stability/reliability. A critical factor is also the application of the proper piston rings. Piston rings are produced from cast iron and steel and optimized in their performance with electroplated, thermal-sprayed or vapour-deposited coatings whether for reducing the flank wear, longer service intervals, better conformity to the cylinder, reducing oil consumption, or reducing friction

2.5 Current Examples of Aluminium Pistons

Modern cast aluminium pistons for gasoline engines such as the ECOFORM piston concept developed by MAHLE are designed for minimum weight while increasing the load-bearing capacity. The inclination of the box walls enables relatively large skirt widths in the lower region and improves the stress distribution in the support area. For the next generation of lightweight pistons - the EVOTEC piston - additional changes relating to the skirt connection, enlarged recesses behind the ring belt on the pin axis, an asymmetrical skirt width as well as supporting ribs on the pin axis result in further weight reductions of up to 10%.

Fig-2.5.1 Cast piston

The piston skirt for gasoline engines with cast iron or steel cylinder surfaces is usually coated with GRAFAL. GRAFAL helps to reduce friction and hence increases the scuffing resistance. For the application in aluminum cylinder surfaces, MAHLE uses the iron particle reinforced synthetic resin coating FERROPRINT. MAHLE's new Ferro Tec galvanic iron layer is another ongoing development available on the market. These

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coatings are necessary to enable the combination of aluminum pistons with pure aluminum engine blocks and hence represent an essential contribution to an overall reduction in engine weight.

Another piston system optimized for fuel economy and CO2 emissions is offered by KS Kolbenschmidt.

The totally harmonized piston-cylinder system consists of the LITEKS2 “advanced” piston generation, the NANOFRIKS coating, the high duty alloy KS 309TM, a low friction ring pack, a lightweight bushless sinter-forged conrod, and a DLC-coated piston pin. As a result, the system friction could be reduced by 32% and the system weight by 10%. At the same time, the fatigue strength was improved and an excellent balance between noise excitation and scuff resistance was achieved.

Fig-2.5.2 Forged piston

Forged pistons are common in motor racing, but they are increasingly used also in series-produced engines subject to high stresses. Forged pistons have a finer microstructure than cast pistons with the same alloys. The production process results in greater strength in the lower temperature range. A further advantage is the opportunity for producing lower wall thicknesses-and hence reducing the weight.

Aluminium pistons for diesel engines require improved material properties with respect to the high temperature loads, especially a greater fatigue resistance over a wide temperature range. Standard features include ring carriers made from high-strength, austenitic cast iron for increasing the wear resistance of the first ring groove, salt core cooling channels or cooled ring carriers. For engines with especially high loads, bushings are used in the piston pin bores.

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Fig-2.5.3 Aluminium piston for diesel engine

In piston production for diesel engines, MAHLE utilizes the extremely heat-resistant aluminium alloy "M174+". In addition, MAHLE improved the casting process with its newly developed ADC Advanced Diesel Casting method. With ADC, a fine microstructure can be achieved in the high-stress zone of the bowl rim, which improves fatigue resistance and the resistance to temperature fluctuations. To improve the piston properties at critical points the piston structure or inserted subsequently.

In addition to the measures described earlier, such as casting-in a ring carrier and inserting bushings, fibers made from aluminum oxides are infiltrated for strengthening the combustion bowl subject to high thermal stresses. The fibre reinforcement enables an increased fatigue resistance, improved rigidity as well as increased thermal shock resistance.

With its cooled ring carrier, MAHLE has developed a solution for high volume production which achieves a significant improvement in piston cooling in the critical areas of the bowl rim and first ring groove. The cooled ring carrier consists of a Niresist ring carrier onto which a thin austenitic steel sheet is welded with inlet and outlet openings. Cast-in in the piston, the combined insert brings the cooling oil even closer to the combustion chamber and the first ring groove.

A critical area of high-loaded state-of-the-art diesel pistons is the combustion chamber bowl. Specific engine performance outputs of 70 kW/l and more result in bowl edge temperature exceeding 400 °C. The combination of thermal-mechanical fatigue and high frequency fatigue resulting from the gas forces may lead to cracking at the bowl edge or other areas of the bowl. In diesel pistons from KS Kolbenschmidt, the required improvement of the material characteristics is achieved by the newly developed alloy V4 and a process-controlled microstructure adapted to the specific thermal and mechanical piston loads.

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For especially high thermal and mechanical loads at the bowl edge, KS Kolbenschmidt has developed a laser re-melting technology where the zone subjected to high loads is re-melted under controlled conditions to produce an optimized, fine and homogeneous microstructure. This process improves the thermal fatigue properties of the critical zone by up to 60%.

Fig-2.5.4 Critical zone

2.6 Outlook

Modern engines with variable valve train or different direct injection concepts require pistons with complex crown shapes which would often lead to a higher piston weight. Therefore in every new piston development, the piston geometry is optimized in particular in the ring belt/piston skirt area. Intensive application of numerical simulation methods enables significant weight reductions while increasing at the same time the load-bearing capacity. Newly developed alloys with better cast ability, but also higher fatigue resistance in the critical temperature and stress region, allow the realization of thinner wall structures. Improved casting methods enable large recesses for the ring belt and hence a considerable reduction in the piston weight. Boring or milling the internal areas of the pistons also helps reduce the weight. Improved piston cooling and the reduction of piston friction are other features which have to be considered. Local reinforcements with cast-in metallic or ceramic inserts offer further development potential. Thus the aluminum piston has not yet reached its limits.

But also the use of steel pistons in diesel engines for passenger cars is discussed again and again. The advantages of steel pistons such as reduced installation clearances, low fuel consumption figures and long service life would have to be evaluated against customer demands such as low emission levels, lightweight, efficient cooling and a competitive price. But up to now, there are no definite indications that steel pistons would be a viable concept for mass production.

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Chapter 3

DESIGN OF PISTON

3.1 Function of Piston

The piston transforms the energy of the expanding gasses into mechanical energy. The piston rides in the cylinder liner or sleeve. Pistons are commonly made of aluminum or cast iron alloys. To prevent the combustion gasses from bypassing the piston and to keep friction to a minimum, each piston has several metal rings around it.

These rings function as the seal between the piston and the cylinder wall and also act to reduce friction by minimizing the contact area between the piston and the cylinder wall. The rings are usually made of cast iron and coated with chrome or molybdenum. Most diesel engine pistons have several rings, usually 2 to 5, with each ring performing a distinct function. The top rings acts primarily as the pressure seal. The intermediate rings acts as a wiper ring to remove and control the amount of oil film on the cylinder walls. The bottom rings is an oiler ring and ensures that a supply of lubricating oil is evenly deposited on the cylinder walls.

3.2 Piston Assembly

Engine pistons serve several purposes.   They transmit the force of combustion to the crankshaft through the connecting rod. They act as a guide for the upper end of the connecting rod. And they also serve as a   carrier   for   the   piston   rings   used   to   seal the compression in the cylinder.

The  piston  must  come  to  a  complete  stop  at  the end of each stroke before reversing its  course  in  the cylinder.  To withstand this rugged treatment and wear, it must be made of tough material, yet be lighting weight.  To overcome inertia and momentum at high   speed,   it   must   be   carefully   balanced   and weighed. All the pistons used in any one engine must be of similar weight to avoid excessive vibration. Ribs are used on the underside of the piston to reinforce the hand.

The ribs also help to conduct heat from the head of the piston to the piston rings and out through the cylinder walls.

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Fig-3.2 Piston and piston rod

The structural components of the piston are the head,   skirt,   ring   grooves,   and land. However, all pistons do not look like the typical one. Some have differently shaped heads. Diesel engine pistons usually have more ring grooves and rings than gasoline engine pistons. Some of these rings may be installed below as well as above the wrist or piston pin.

Fitting   pistons   properly   is   important.   Because metal   expands   when   heated and space   must   be provided  for  lubricants  between  the  pistons  and  the cylinder walls, the  pistons  are  fitted  to  the  engine with  a  specified  clearance.  This  clearance depends upon  the  size  or  diameter  of  the  piston  and  the material  form  which  it  is made.  Cast iron does not expand as fast or as much as aluminum.  Aluminum pistons require more clearance to prevent binding or seizing when the engine gets hot. The skirt of bottom part  of  the  piston  runs  much  cooler  than  the  top; therefore,  it  does  not require  as  much  clearance  as the head.

The piston is kept in alignment by the skirt, which is usually cam ground elliptical in cross section. This elliptical shape permits the piston to fit the cylinder, regardless of whether the  piston  is  cold  or  at  operating temperature. The narrowest diameter of the piston is at the piston pin bosses, where the piston skirt is thickest. At the widest diameter of the piston, the piston skirt is thinnest. The piston is fitted to close limit sat  its  widest diameter  so  that  the  piston  noise  is prevented during the engine warm-up. As the piston is expanded by the heat generated during operation, it becomes round because the expansion is proportional to the temperature of the metal. The walls of the skirt are cut away as much as possible to reduce weight and to prevent excessive expansion during engine operation. Many aluminum pistons are made with split skirts so that when the pistons expand, the skirt diameter will not increase.

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The two types of piston skirts found in most engines are the full trunk and the slipper. The full-trunk-type skirt, more widely used, has a full cylindrical shape with bearing surfaces parallel to those of the cylinder, giving more strength and better control of the oil film. The slipper-type  cutaway skirt  has  considerable  relief  on the sides of the skirt, leaving less area for possible contact  with  the  cylinder  walls  and  thereby  reducing friction.

3.2.1 Piston Pin

The piston is attached to the connecting rod by the piston pin or wrist pin. The pin passes through the piston pin bosses and through the upper end of the connecting rod, which rides within the piston on the middle of the pin. Piston pins are made of alloy steel with a precision finish and are case hardened and sometimes chromium plated to increase their wearing qualities. Their tubular construction gives them maximum strength with minimum weight. They are lubricated by splash from the crankcase or by pressure through passages bored in the connecting rods.

Three methods are commonly used for fastening a piston pin to the  piston  and  the connecting  rod:  fixed pin, semi floating pin, and full-floating pin. The anchored, or fixed, pin attaches to the piston by as crew running through one of the bosses; the connecting rod oscillates on the pin.  The semi floating pin is anchored to the connecting rod and turns in the piston pin bosses. The full-floating pin is free to rotate in the connecting  rod  and  in the  bosses,  while  plugs  or snap-ring locks prevent it from working out against the sides of  the  cylinder.

3.2.2 Piston Rings

Piston rings are used  on pistons to maintain gastight seals between the pistons and cylinders, to aid in cooling the piston, and to control cylinder-wall  lubrication.  About one-third of the heat absorbed by the piston passes through the rings to the cylinder wall. Piston rings are often complicated in design, are heat treated in various ways, and are plated with other metals.  Piston rings are of two distinct classifications: compression rings and oil control rings.

The principal function  of  a  compression  ring  is  to prevent gases from leaking by the piston during the compression and power strokes. All piston rings are split to permit assembly  to  the  piston  and  to  allow  for expansion. When the ring is in place, the ends of the split joint do not form a perfect seal; therefore, more than one ring must be used, and the joints must be staggered around the piston. If cylinders are worn, expanders are sometimes used to ensure a perfect seal. The bottom ring, usually located just above the piston pin, is an oil-regulating ring. This ring scrapes the excess oil from the cylinder

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walls and returns some of it, through slots, to the piston ring grooves. The ring groove under an oil ring has openings through which the oil flows back into the crankcase. In some engines, additional oil rings are used in the piston skirt below the piston pin.

In every engine, piston plays an important role in working and producing results. Piston forms a guide and bearing for the small end of connecting rod and also transmits the force of explosion in the cylinder, to the crank shaft through connecting rod.

The piston is the single, most active and very critical component of the automotive engine. The Piston is one of the most crucial, but very much behind-the-stage parts of the engine which does the critical work of passing on the energy derived from the combustion within the combustion chamber to the crankshaft. Simply said, it carries the force of explosion of the combustion process to the crankshaft. Apart from the critical job that it does above, there are certain other functions that a piston invariably does-It forms a sort of a seal between the combustion chambers formed within the cylinders and the crankcase. The pistons do not let the high pressure mixture from the combustion chambers over to the crankcase.

3.3 Construction of Piston

Its top known by many names such as crown, head or ceiling and thicker than bottom portion. Bottom portion is known as skirt. There are grooves made to accommodate the compression rings and oil rings. The groove, made for oil ring, is wider and deeper than the grooves made for compression ring. The oil ring scraps the excess oil which flows into the piston interior through the oil return holes and thus avoiding reaching the combustion chamber but helps to lubricate the gudgeon pin to some extent. In some designs the oil ring is provided below the gudgeon pin boss .The space between the grooves are called as land.

The diameter of piston always kept smaller than that of cylinder because the piston reaches a temperature higher than cylinder wall and expands during engine operation. The space between the cylinder wall and piston is known as piston clearance. The diameter of the piston at crown is slightly less than at the skirt due to variation in the operating temperatures. Again the skirt itself is also slightly tapered to allow for unequal expansion due to temperature difference as we move vertically along the skirt the working temperature is not uniform but slightly decrease.

3.4 Design of Piston

A piston does the dirty work of actually taking the brunt of the force of explosion arising of the combustion of the fuel and passes it onto the crankshaft the big, heavy part

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of an engine that rotates due to the movement of the piston. It takes a tremendous amount of pressure about 1000 Psi notwithstanding the severe heat that it has to take.

Fig-3.5. Components of a typical, four stroke cycle, DOHC piston engine. (E) Exhaust camshaft, (I) Intake camshaft, (S) Spark plug, (V) Valves, (P) Piston, (R) Connecting rod, (C) Crankshaft, (W) Water jacket for coolant flow.

Now, when designing pistons, the weight is a serious determining factor. Imagine the scenario on one hand you would need the pistons to be able to pick up all that heat and pressure, but on the other hand, you still want it light. Material sciences come to the rescue again with aluminum leading the pack for the choice with its favorable strength-to-weight ratio; the fact that it is easily machinable, has a great thermal conductivity can transfer heat quickly and most importantly, it is light weight, aluminum is the choice material for making pistons today.

However, the big brother cast iron is also used for the construction of pistons for the above mentioned reasons, except that it is heavy and hence is used for limited applications like slow-speed engines and the like.

The crown head of the piston takes heat and hence expands more than the other parts of the piston. So this area, the upper part of the piston, is machined to a diameter slightly lesser than the rest of the piston the skirt, mainly. Yet another way of controlling the piston’s expansion is cut a slot into the skirt the main body of the piston. So when the piston heats, up the skirt simply closes itself due to the metal expansion and prevents the piston to expand outwards and touch the cylinder.

In order to reduce wear and increase the life of piston grooves in high speed engines, a ferrous metal rings are inserted into the grooves.

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The piston rings, which are also called as compression rings are fit closely in the grooves provided in the piston. These rings are worn out before the wearing of the piston and cylinder wall. Hence by replacing the same, we can avoid replacement of piston or cylinder.

The leakage of the high temperature gases produced during power stroke in the combustion chamber is prevented by piston rings. The piston rings form an effective seal and at the same time transmit heat from crown to the cylinder walls and hence keep the temperature within the workable limit. There should be at least two piston rings in each piston of internal combustion engine. For the higher capacity engines, there are four or even six piston rings have been used. The number of rings is depending upon the capacity and size of the I.C.Engine.

In order to achieve the effective seal against lubricating oil and high pressure gases leakage, a great pressure must be exerted, by each ring on the cylinder walls. To produce this effect, the rings are made slightly larger in the diameter than that of cylinder bore and cutting small gap which is partly narrowed when the ring is fitted. The end gap in the piston ring provides flexibility to the ring and the same time allowing for thermal expansion. There are another rings used in piston grooves, called as, Oil Scraper Rings. The function of these rings are, only as much quantity of the oil as it just sufficient to maintain proper lubrication is allowed to reach the skit. The excess oil which would have leaked in the combustion chamber without serving any useful purpose and rather leading to carbonization is scraped off by the oil scraper ring.

While mounting the piston rings over the piston, a great care should be taken to ensure that the gaps of various rings should not fall in the same vertical line. The piston rings of internal combustion engines are made in various sections such as, standard, tapered, grooved, wedge and L shape. Whereas oil scraper rings are made as, narrow, wide,tapered and six segment cord.

The cast iron along with 2.5% silicon will provide a good wear resistance to piston ring. In case of passenger cars, the piston rings are usually plated with Chromium Tin or Cadmium. The plating reduces the rate of cylinder wear and hence increases the life of internal combustion engine.

The piston engine was first proposed by R.P. Pescara and the original application was a single piston air compressor. The engine concept was a topic of much interest in the period 1930-1960. These first generation piston engines were without exception opposed piston engines, in which the two pistons were mechanically linked to ensure symmetric motion. Piston engines provided some advantages over conventional technology, including compactness and a vibration-free design. The first successful application of the

20

piston engine concept was as air compressors. In these engines, air compressor cylinders were coupled to the moving pistons, often in a multi-stage configuration. Some of these engines utilized the air remaining in the compressor cylinders to return the piston, thereby eliminating the need for a rebound device. Piston air compressors were in use because it has advantages of high efficiency, compactness and low noise and vibration after the success of the piston air compressor. A number of piston gas generators were developed, and such units were in widespread use in large-scale applications such as stationary and marine power plants.

High operational flexibility, and excellent part load performance has been reported for such engines.

3.5 Piston Description

Pistons move up and down in the cylinders which exerts a force on a fluid inside the cylinder. Pistons have rings which serve to keep the oil out of the combustion chamber and the fuel and air out of the oil. Most pistons fitted in a cylinder have piston rings. Usually there are two spring-compression rings that act as a seal between the piston and the cylinder wall, and one or more oil control ring s below the compression rings. The head of the piston can be flat, bulged or otherwise shaped. Pistons can be forged or cast. The shape of the piston is normally rounded but can be different. A special type of cast piston is the hypereutectic piston. The piston is an important component of a piston engine and of hydraulic pneumatic systems. Piston heads form one wall of an expansion chamber inside the cylinder. The opposite wall, called the cylinder head, contains inlet and exhaust valves for gases. As the piston moves inside the cylinder, it transforms the energy from the expansion of a burning gas usually a mixture of petrol or diesel and air into mechanical power in the form of a reciprocating linear motion. From there the power is conveyed through a connecting rod to a crankshaft, which transforms it into a rotary motion, which usually

3.5.1 Piston head or crown

The piston head or crown may be that convex or concave depending upon the design of combustion chamber.

It with stands the pressure of gas in the cylinder. The selection of piston crown primarily depends upon the requirement of values

for the combustion chamber.

3.5.2. Piston ringsThese are used to seal the cylinder in order to prevent leakage of the gas past the

piston.

21

To act as passage of heat flow from piston crown to the wall of the cylinder. To act as a lubricating oil controller on the cylinder wall so as to minimize wear. To absorb some part of the piston due to side thrust. The material for piston rings is usually cast iron & alloy cast iron due to their good

wearing qualities & also they retain the spring characteristics ever at high temperatures.

Piston Rings are of Two Types

Compression rings - Sealing of the combustion gas. Heat transfer from piston crown to the cylinder wall.

Oil control rings - To prevent excessive oil from passing through the end gap of rings and between the cylinder wall & the ring face.

3.6 Different Types of Pistons

Various types of pistons are employed on different engines. This is because each type fulfils some specific requirements on a particular engine. Some pistons have complex head formation, some have specially formed skirts, and other have geometrical peculiarities. Based on various considerations, the pistons may be categorized as follows

On the basis of head formation: Deflector head piston combustion chamber type piston Domed and depression headed piston.

On the basis of skirt profile : Slipper piston Cutway piston

On the basis of skirt piston: solid skirt piston split skirt piston

On the basis of other specialties: Cam ground piston Taper piston Oval piston

3.7 Materials for Manufacturing Pistons

Aluminium alloys give light pistons and for better heat dissipation, aluminium alloys are the ideal materials due to their very high thermal conductivity. Aluminium is 3 times lighter than cast iron. Its strength is good at low temperatures but is looses about 50% of its strength at temperatures above about 320οc .Its expansion is about 2 ½ times that of

22

cast iron and the resistance to abrasion is low at height temperatures. However these disadvantageous properties of aluminium have now been ever come by alloying it with other materials and by developing advanced designs of pistons. The split skirt, T-slotted as well as cam ground, oval sectioned pistons made from aluminium alloys are mostly used which can be tightly fitted into the cylinder born to eliminate piston slap. A coating of aluminium oxide or tin on aluminium alloys pistons has been found to be protective against scuffing or partial seizure during running in after overhaul.

For a cast iron piston the temperature at the centre of the piston head Tc is about 425οc to 450οc under full load conditions and the temperatures at the edges of the piston head Tb is about 200οc to 225οc.

For aluminium alloy piston, Temp is about 260οc to 290οc and Te is about 185οc to 215οc.

Since the aluminium alloys are about three times lighter than cast iron, Therefore its mechanical strength is good at low temperatures, but they lose their strength about 50% at temperatures above 325οc.

3.7.1Cast Iron

It is obtained by re-melting pig iron with coke and furnaces by the definition cast iron is an alloy and iron and carbon containing more 2% of carbon. It contains

carbon -3.0-4.0% Silver -1.0 Manganese -0.5-1.0% Sulphur -upto0.1% Phosphors -upto0.1% Iron -remainder.

3.7.2 Properties of Cast Iron

It is brittle material. Good casting High compressive strength. High wheel resistance. Poor machine ability Tensile strength -100 to 200mpa Compressive strength-400 to 1000mpa Shear strength -120mpa

Chapter 4

23

INTRODUCTION TO ALUMINIUM ALLOYS

4.1 History

Aluminum is the world’s most abundant metal and is the third most common element, comprising 8% of the earth’s crust. The versatility of aluminum makes it the most widely used metal after steel. Although aluminum compounds have been used for thousands of years, aluminum metal was first produced around 170 years ago.

In the 100 years since the first industrial quantities of aluminum were produced, worldwide demand for aluminium has grown to around 29 million tons per year. About 22 million tons is new aluminium and 7 million tons is recycled aluminium scrap. The use of recycled aluminium is economically and environmentally compelling. It takes 14,000 kWh to produce 1 tonne of new aluminium. Conversely it takes only 5% of this to remelt and recycle one tonne of aluminium. There is no difference in quality between virgin and recycled aluminium alloys.

Pure aluminium is soft, ductile, corrosion resistant and has a high electrical conductivity. It is widely used for foil and conductor cables, but alloying with other elements is necessary to provide the higher strengths needed for other applications. Aluminium is one of the lightest engineering metals, having a strength to weight ratio superior to steel.

By utilising various combinations of its advantageous properties such as strength, lightness, corrosion resistance, recyclability and formability, aluminium is being employed in an ever-increasing number of applications. This array of products ranges from structural materials through to thin packaging foils.

4.2 Properties of Aluminium

The major advantages of using aluminium are tied directly to its’ remarkable properties. Some of these properties are outlined in the following sections.

4.2.1 Strength to Weight Ratio

Aluminium has a density around one third that of steel and is used advantageously in applications where high strength and low weight are required. This includes vehicles where low mass results in greater load capacity and reduced fuel consumption.4.2.2 Corrosion Resistance

24

When the surface of aluminum metal is exposed to air,a protective oxide coating forms almost instantaneously. This oxide layer is corrosion resistant and can be further enhanced with surface treatments such as anodizing.

 4.2.3 Electrical and Thermal Conductivity

Aluminum is an excellent conductor of both heat and electricity. The great advantage of aluminum is that by weight, the conductivity of aluminum is around twice that of copper. This means that aluminum is now the most commonly used material in large power transmission lines. The best alternatives to copper are aluminum alloys in the 1000 or 6000 series. These can be used for all electrical conduction applications including domestic wiring. Weight considerations mean that a large proportion of overhead, high voltage power lines now use aluminum rather than copper. They do however, have a low strength and need to be reinforced with a galvanized or aluminum coated high tensile steel wire in each strand.

4.2.4 Light and Heat Reflectivity Aluminum is a good reflector of both visible light and heat making it an ideal material for light fittings, thermal rescue blankets and architectural insulation.

4.2.5 Toxicity

Aluminium is not only non-toxic but also does not release any odours or taint products with which it is in contact. This makes aluminium suitable for use in packaging for sensitive products such as food or pharmaceuticals where aluminium foil is used.

4.2.6 Recycling

The recyclability of aluminium is unparalleled. When recycled there is no degradation in properties when recycled aluminium is compared to virgin aluminium. Furthermore, recycling of aluminium only requires around 5 percent of the input energy required to produce virgin aluminium metal. 

The combination of two remarkable properties of aluminium makes the need to recycle the metal obvious. These first of these factors is that there is no difference between virgin and recycled aluminium. The second factor is that recycled aluminium only uses 5% of the energy required to produce virgin material. Currently around 60% of aluminium metal is recycled at the end of its lifecycle but this percentage can still be vastly improved.

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4.3 Aluminum Production

Aluminium is extracted from the principal ore, bauxite. Significant bauxite deposits are found throughout Australia, the Caribbean, Africa, China and South America. Open cut techniques are commonly used to mine the bauxite.

The bauxite is purified using the Bayer process. This process involves dissolving aluminumtrihydrate to leave alumina plus iron and titanium oxides. The iron and titanium oxides are by-products of the process and are often referred to as red mud. Red mud must be disposed of with strong consideration given to environmental concerns.

Approximately two tones of bauxite are required to yield one tonne of alumina. The extraction of aluminum from alumina is achieved using an electrolytic process. A cell or pot is used that consists of a carbon lined steel shell. This shell forms a cathode. A consumable carbon anode is suspended in liquid cryolite sodium aluminium fluoride held within the pot at 950°C. Alumina is dissolved in the cryolite by passing low voltages at high amperages through the pot. This results in pure aluminium being deposited at the cathode.

4.4 Environmental Considerations

The aluminium industry is very conscious of the environmental impact of its activities. The mining and smelting of aluminium, plus the disposal of red mud can have a major environmental impact if not done properly. The industry is proud of its efforts and achievements in rehabilitating open cut mine sites and the restoring flora and fauna to these sites. Such efforts have been rewarded with awards from the United Nations Environment Programme and red mud disposal areas are now being successfully revegetated. Environmental requirements are met on pot line emissions through the use of specialist scrubbing system.

4.5 Applications

The properties of the various aluminium alloys has resulted in aluminium being used in industries as diverse as transport, food preparation, energy generation, packaging, architecture, and electrical transmission applications.

Depending upon the application, aluminium can be used to replace other materials like copper, steel, zinc, tin plate, stainless steel, titanium, wood, paper, concrete and

26

composites. Some examples of the areas where aluminium is used are given in the following sections.

4.5.1 Packaging

Corrosion resistance and protection against UV light combined with moisture and odour containment plus the fact that aluminium is non-toxic and will not leach or taint the products has resulted in the widespread use of aluminium foils and sheet in food packaging and protection. The most common use of aluminium for packaging has been in aluminium beverage cans. Aluminium cans now account for around 15% of the global consumption of aluminium.

4.5.2 Transport

After the very earliest days of manned flight, the excellent strength to weight ratio of aluminium have made it the prime material for the construction of aircraft. These same properties of aluminium mean various alloys are now also used in passenger and freight rail cars, commercial vehicles, military vehicles, ships & boats, buses & coaches, bicycles and increasingly in motor cars. The sustainable nature of aluminium with regards to corrosion resistance and recyclability has helped drive the recent increases in demand for aluminium vehicle components.

4.5.3 Marine Applications

Aluminium plate and extrusions are used extensively for the superstructures of ships. The use of these materials allows designers to increase the above waterline size of the vessel without creating stability problems. The weight advantage of aluminium has allowed marine architects to gain better performance from the available power by using aluminium in the hulls of hovercraft, fast multi-hulled catamarans and surface planning vessels. Lower weight and longer lifecycles have seen aluminium become the established material for helidecks and helideck support structures on offshore oil and gas rigs. The same reasons have resulted in the widespread use of aluminium in oil rig stair towers and telescopic personnel bridges.

4.5.4 Building and Architecture

27

Aluminium use in buildings covers a wide range of applications. The applications include roofing, foil insulation, windows, cladding, doors, shop fronts, balustrading, architectural hardware and guttering. Aluminium is also commonly used as the in the form of tread plate and industrial flooring.

4.5.5 Foils

Aluminum is produced in commercial foils as thin as 0.0065 mm or 6.5 µm. Material thicker than 0.2mm is called sheet or strip. Aluminum foil is impervious to light, gases, oils and fats, volatile compounds and water vapour. These properties combined with high formability, heat and cold resistance, non toxicity, strength and reflectivity to heat and light mean aluminum foil is used in many applications. These applications include:

Pharmaceutical packaging Food protection and packaging Insulation Electrical shielding Laminates

4.5.6 Other Applications

The above applications account for approximately 85% of the aluminium consumed annually. The remaining 15% is used in a wide variety of applications including:

 Ladders  High pressure gas cylinders  Sporting goods  Machined components Road barriers and signs  Furniture  Lithographic printing plates

This Data is indicative only and must not be seen as a substitute for the full specification from which it is drawn. In particular, the mechanical property requirements vary widely with temper, product and product dimensions. The information is based on our present knowledge and is given in good faith. However, no liability will be accepted by the Company is respect of any action taken by any third party in reliance thereon. As the products detailed may be used for a wide variety of purposes and as the Company has no control over their use; the Company specifically excludes all conditions

28

or warranties expressed or implied by statute or otherwise as to dimensions, properties and/or fitness for any particular purpose.

4.6 Aluminum Alloy LM25

This alloy conforms to British Standard 1490 LM25. Castings are standardized in the following conditions – as Cast, LM25-M precipitation treated LM25 - TE; solution treated and stabilized LM25 - TB7; and fully heat - treated LM25 - TF.

Element %Copper 0.1 max.

Magnesium 0.20-0.60Silicon 6.5-7.5

Iron 05 maxManganese 03 max

Nickel 0.1maxZinc 01maxLead 01maxTin 0.05max

Titanium* 02maxAluminium Remainder

*0.05% min if Titanium alone used for grain refining.

Fig-4.6.1 Chemical Composition of LM 25

4.6.1 Strength at Elevated Temperatures

The tensile properties of LM25 alloy at elevated temperatures are influenced by the condition heat -treatment of the castings and the duration at the elevated temperature. For short term testing e.g. 30 minutes at temperature, the properties fall only slowly and uniformly up to about 200oC at which temperature for example, the strength of LM25-TF is reduced by approximately 20%, Very prolonged heating 10,000 hours results in Sharp, loss of strength at about135oC and at 200oC the strength on LM25-TS is less than half of that at room temperature. For prolonged service at elevated temperatures above 130oC there is, therefore, no practical advantage to be gained by heat treatment.

4.6.2 Machinability

The heat treated alloy has fairly good machining properties, but tools should preferably be of high speed steel and must be kept sharp. A moderately high rate of tool wear may be expected. Liberal cutting lubricant should be employed.4.6.3 Corrosion Resistance

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Resistance to corrosive attack by sea water and marine atmospheres is high.

4.6.4 Anodizing

A protective anodic film can be obtained by either the sulphuric or chromic acid process but the grey opaque character of coatings of normal thickness precludes their colouring in light shades for decorative purposes. Achievement of the specified minimum tensile properties is dependent on maintaining the optimum magnesium content. Care must therefore be taken during melting and degassing to avoid loss of magnesium by oxidation resulting from overheating or by excessive chlorination.

4.6.5 Heat Treatment

LM25-TE Precipitation treated - Heat for 8-12 hours at 155- 175οc and allow to cool in airLM25-TB7 Solution treated and stabilized heat for 4-12 hours at 525-545ο

C and quench in hot water, followed by a stabilizing treatment at 250οC for 2-4 hours.LM25-TF fully heat treated- heat for 4-12 hours at 525- 545οC and quench in hot water, followed by a precipitation treatment of 8-12 hours at 155-175OC.

4.6.6 Application

LM25 alloy is mainly used where good mechanical properties are required in castings of a shape or dimensions requiring an alloy of excellent castability in order to achieve the desired standard of soundness.

The alloy is also used where resistance to corrosion is an important consideration particularly where high strength is also required.

Consequently LM25 finds application in the food, chemical, marine, electrical and many other industries and above all in road transport vehicles where it is used for cylinder blocks and heads, and other engine and body castings. Its potential uses are increased by its availability in four conditions of heat treatment in both sanded chill castings. It is, in practice, the general purpose high strength casting alloy.

4.7 Aluminum Alloy 7475-T761

4.7.1 General Characteristics of Aluminum 7475

Aluminum 7475 offers strength and fracture toughness while resisting fatigue crack propagation. It’s an ideal aircraft alloy appropriate for fuselage skins and bulkheads, and wing parts for commercial, fighter and transport airplanes.

4.7.2 Composition Notes:

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Aluminum content reported is calculated as remainder.Composition information provided by the Aluminum Association and is not for design.

Key Words: UNS A97475; ISO AlZn5.5MgCu; Aluminum 7475-T761; AA7475-T761

Component Wt. % Component

Wt. % Component Wt. %

Al 88.5-91.5 Mg 1.9-2.6 Si Max 0.1

Cr 0.18-0.25 Mn Max 0.06 Ti Max 0.06

Cu 1.2-1.9 Other,each Max 0.05 Zn 5.2-6.2

Fe Max 0.12 Other,total Max 0.15

Table 4.7.2 Composition of Aluminium Alloy 7475-T761

4.7.3 Material Properties

Physical PropertiesMetric English Comments

Density 2.81 g/cc 0.102 lb/in³  AA; Typical

Mechanical PropertiesHardness, Brinell 140 140  500 kg load

with 10 mm ball. Calculated value.

Hardness, Knoop 177 177  Converted from Brinell

Hardness ValueHardness, Rockwell A

51.6 51.6  Converted from Brinell

Hardness ValueHardness, Rockwell B

84 84  Converted from Brinell

Hardness Value

Hardness, Vickers

162 162  Converted from Brinell

Hardness ValueUltimate Tensile Strength

517 MPa 75000 psi  AA; Typical

Tensile Yield Strength

448MPa 65000 psi  AA; Typical

Elongation at 12% 12 %  AA; Typical;

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Break 1/16 in. (1.6 mm) Thickness

Modulus of Elasticity

70.3 GPa 10200 ksi  AA; Typical; Average of tension

and compression. Compression

modulus is about 2% greater than tensile

modulus.Poisson's Ratio 0.33 0.33  

Shear Modulus 27GPa 3920 ksi  

Allowable Strength

310 MPa 45000 psi  Calculated value.

Electrical PropertiesElectrical Resistivity

4.32e-006 ohm-cm

4.32e-006 ohm-cm

 AA; Typical at 68°F

Thermal PropertiesCTE, linear 68°F 23.2 µm/m-°C 12.9 µin/in-

°F AA; Typical;

Average over 68-212°F range.

CTE, linear 250°C

25.2 µm/m-°C 14 µin/in-°F  Average over the range 20-

300ºC

Specific Heat Capacity

0.88 J/g-°C 0.21 BTU/lb-°F

 Estimated from trends in similar

Al alloys.

Thermal Conductivity

147 W/m-K 1020 BTU-in/hr-ft²-°F

 AA; Typical at 77°F

Melting Point 477 - 635 °C 890 - 1175 °F  AA; Typical range based on

typical composition for wrought products 1/4 inch thickness or

greater

Solidus 477 °C 890 °F  AA; Typical

Liquidus 635 °C 1175 °F  AA; Typical

32

Processing PropertiesAnnealing Temperature

413 °C 775 °F  

Solution Temperature

516 °C 960 °F  must be preceded by soak at

870 to 890°F

Aging Temperature

121 - 177 °C 250 - 350 °F

Table 4.7.3 Material Properties of Aluminium Alloy 7475-T761

4.7.4 Heat Treatment of Aluminum 7475

Strength, fracture toughness optimization Solution treating Aging

4.7.5 Corrosion Resistance of Aluminum 7475

Acceptable exfoliation resistance in -T61 and -T651 tempers Comparable or better than other high-strength alloys, including 7075, 7050, and

2024

4.7.6 Workability of Aluminum 7475

Improved forming vs. 7075 7475 sheet offers super plastic forming qualities in some instances Heat treat to standard -T62 or -T762 temper for required corrosion resistance and

toughness

4.7.7 Applications of Aluminum 7475

Fuselage parts Skin Bulkheads Wing components Upper and lower skins Spars

4.8 Aluminum Alloy 6061

33

4.8.1 Background

Aluminum alloy 6061 is one of the most extensively used of the 6000 series aluminum alloys. It is a versatile heat treatable extruded alloy with medium to high strength capabilities.

4.8.2 Composition

Typical composition of aluminium alloy 6061Component Amount (wt.%)

Aluminium Balance

Magnesium 0.8-1.2

Silicon 0.4 – 0.8

Iron Max. 0.7

Copper 0.15-0.40

Zinc Max. 0.25

Titanium Max. 0.15

Manganese Max. 0.15

Chromium 0.04-0.35

Others 0.05

Table 4.8.2 Composition of Aluminium Alloy 60614.8.3 Key Properties

Typical properties of aluminium alloy 6061 include:

Medium to high strength

Good toughness

Good surface finish

Excellent corrosion resistance to atmospheric conditions

Good corrosion resistance to sea water

Can be anodized

Good weldability and brazability

Good workability

34

Widely available

4.8.4 Physical Properties

Density: 2.7 g/cm3

Melting Point: Approx 580°C

Modulus of Elasticity: 70-80 GPa

Poisson’s Ratio: 0.33

4.8.5 Mechanical Properties

Temper Ultimate

Tensile

Strength

(MPa)

0.2% Proof

Stress (MPa)

Brinell

Hardness

(500kg load,

10mm ball)

Elongation

50mm dia

(%)

0 110-152 65-110 30-33 14-16

T1 180 95-96   16

T4 179 min 110 min    

T6 260-310 240-276 95-97 9-13Table 4.8.5 Mechanical Properties of Alluminium Alloy 6061

4.8.6 Thermal Properties

Co-Efficient of Thermal Expansion (20-100°C): 23.5x10-6 m/m.°C

Thermal Conductivity: 173 W/m.K

4.8.7 Electrical PropertiesElectrical Resistivity: 3.7 – 4.0 x10-6 Ω.cm

4.8.8 Typical Heat Treatment/Temper StatesTreatment Definition

F As fabricated

0 Annealed to obtain lower strength temper

T1 Cooled from an elevated shaping process and

naturally agedT4, T4511 Solution heat treated and naturally aged

T51 Cooled from an elevated shaping process

35

and

T6, T6511 Solution heat treated and artificially aged

Table 4.8.8 Heat Treatment

This designation applies to products which are not cold worked after cooling from an elevated temperature shaping process, or in which the effect of cold work in flattening or straightening has no effect on mechanical properties

This designation applies to products which are not cold worked after solution heat-treated, or in which the effect of cold work in flattening or straightening has no effect on mechanical properties

This designation applies to products which are not cold worked after solution heat-treatment, or in which the effect of cold work in flattening or straightening does not effect mechanical properties.

4.8.9 Material Properties

Physical Properties Metric

Density  2.70 g/cc

Mechanical Properties Metric

Hardness, Brinell  30

Tensile Strength, Ultimate  124 MPa

Tensile Strength, Yield  55.2 MPa

Elongation at Break  25.0 % @Thickness 1.59 mm

  30.0 % @Diameter 12.7 mm

Modulus of Elasticity  68.9 GPa

Ultimate Bearing Strength  228 MPa

Bearing Yield Strength  103 MPa

Poissons Ratio  0.330

Fatigue Strength  62.1 MPa

@# of Cycles 5.00e+8

Machinability  30 %

Shear Modulus  26.0 GPa

Allowable Strength 82.7 MPa 

Electrical Properties Metric

36

Electrical Resistivity  0.00000366 ohm-cm

@Temperature 20.0 °C

Thermal Properties Metric

CTE, linear  23.6 µm/m-°C

@Temperature 20.0 - 100 °C

  25.2 µm/m-°C

@Temperature 20.0 - 300 °C

Specific Heat Capacity  0.896 J/g-°C

Thermal Conductivity  180 W/m-K

Melting Point  582 - 651.7 °C

Solidus  582 °C

Liquidus  651.7 °C

 

Processing Properties Metric

Solution Temperature  529 °C

Aging Temperature  160 °C

 

Component Elements Properties Metric

Aluminum, Al  95.8 - 98.6 %

Chromium, Cr  0.040 - 0.35 %

Copper, Cu  0.15 - 0.40 %

Iron, Fe  <= 0.70 %

Magnesium, Mg  0.80 - 1.20 %

Manganese, Mn  <= 0.15 %

Other, each  <= 0.050 %

Other, total  <= 0.15 %

Silicon, Si  0.40 - 0.80 %

Titanium, Ti  <= 0.15 %

37

Zinc, Zn  <= 0.25 %

Table 4.8.10 Material Properties of Aluminum Alloy 6061

4.8.10 Applications

Typical applications for aluminium alloy 6061 include:

Aircraft and aerospace components

Marine fittings

Transport

Bicycle frames

Camera lenses

Driveshafts

Electrical fittings and connectors

Brake components

Valves

Couplings

38

Chapter 5

DESIGN CALCULATIONS OF PISTON

5.1 Pressure Calculations

Suzuki GS 150 R specificationsEngine type : air cooled 4-stroke SOHCBore× stroke (mm )=57 ×58.6

Displacement =149.5CCMaximum power = 13.8bhp @8500rpmMaximum torque = 13.4Nm @ 6000 rpmCompression ratio = 9.35/1

Density of petrol C8 H 18=737.22kg

m3at 60 F

= 0.00073722 kg/cm3

= 0.00000073722 kg/mm3

T = 60F =288.855K =15.550CMass = density × volumem = 0.00000073722×149500m = 0.11kgmolecular wt. for petrol 144.2285 g/mole R = Gas constantPV = mRTwhere m = mass/molecular wt.R = Gas constant

P = mRT

V=0.11× 8.3143 ×288.555

0.11422×0.0001495= 263.9

0.00001707P = 15454538.533 j/m3 = N/m2

P =15.454 N/mm2

5.2 Design calculation for Material - Aluminum Alloy 6061

Temperature at the center of piston head TC = 2600c to 2900cTemperature at the edge of piston head TE = 1850c to 2150cMaximum gas pressure p = 15.454N/mm2

Bore or outside diameter of piston = 57mm

39

5.2.1 Thickness of piston head

th =√ 3 p D2

16σ t

where σt = allowable stress = σ t= 82.7Mpa

th = √ 3 × 15.454 ×572

16 ×82.7

th = √29.6983 = 10.669mm orConsidering heat transfer

th =(H

12.56 k (TC – TE))

Thermal Conductivity= 180w/mkTC – TE = The temperature differenceTC – TE = 750c

H = Heat flowing the piston head = C HCV m B.P(in KW)C = constant representing the portion of the heat supplied to the engine which is absorbed by the piston = 0.05

HCV =Higher Calorific Value of the fuel = 47 103KJ/kg for petrolm = mass of fuel used in kg per brake power per secondBP = brake power of the engine per cylinderH = CxHCVx(m/BP)xBPH = 0.05x47x103x0.11H = 258.5KJ/sth= 258.5/(12.56x180x75) = 0.001524 mth = 1.524 mmth = 10.699mm

5.2.2Piston rings

Radial thickness t1 = D

t1 = 57

= pressure of the gas on the cylinder wall = 0.042N/mm2

= allowable bending(tensile stress) for cast iron rings

40

= 110Mpa

t1 = 57t1 = 1.93mm

axial thickness t2 = D/10nr = 57/10 3 = 1.9mmnr = no of rings = 3width of the top land b1= 1.2b1 = 1.2(10.669) =12.8028 mmwith of other land i.e distance between ring groovesb2 = t2 = 1.9mmthe gap between the free ends of the ring = 3.5t to 4t = 7.72mm

5.2.3 Piston barrel

t3 = 0.03D + b +4.5b = radial depth of piston ringb = t1 +0.4 = 2.33mm

t3 = 0.03 57+2.33+4.5t3 = 8.54mmThe piston wall thickness towards the open endt4 = 0.35t3 = 2.989mm

5.2.4 Piston skirt

Maximum gas load on the piston

P = p πD2/4 = (15.454 572)/4P = 30414.88611NMaximum side thrust on the cylinderR = p/10 = 3941.488611R = bearing pressure x bearing area of the piston skirt

R = pb D ll = length of the piston skirt in mml =45.6N/mm2

Bearing pressure pb = 1.5N/mm2

Total length of the pistonL = length of the skirt+length of ring section + top land

Length of ring section = 5 b2 or t2 = 9.5mmL = 45.6 + 9.5 + 6.54 = 61.64mm

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5.2.5Piston pin - material heat treated alloy steel

Center of piston pin should be 0.02D to 0.04D aboveThe center of skirt = 0.04D = 2.28mm above center of skirtTensile strength = 710 to 910MpaLength of the pin in the connecting rod bushingl1 = 0.45D = 25.65mmload on the piston due to gas pressure = 39414.88611Np =bearing pressure×bearing areap = pb1×d0×l1

l1 = 25.65mmpb1 = 50 – 100Mpa for bronze pb1 = 100Mpa

d0 = p/pb1 l1 = 15.36mmInner diameter of piston pin di = 0.6d0 = 9.21mmMaximum bending moment at the center of pinM = P.D/8 = (39414.88611x57)/8M = 280831.06

Z = /32[(d0)4 – (dc)4/d0]

Z=

=

= Z = 2478.48Allowable bending stress σb = M/Z = 113.3This is less then the allowable value 140mpa for heat treated alloy steelThe mean diameter of the piston losses = 1.5d0

= 23.04mm

5.3 Design Calculation for Material - Aluminum Alloy 7475-T761

Material: Aluminum alloy 7475-T761Temperature at the center of piston head Tc = 2600c to 2900cTemperature at the edge of piston head Te = 1850c to 2150cMaximum gas pressure p = 15.454N/mm2

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Bore or outside diameter of piston = 57mm

5.3.1 Thickness of piston head

th =√ 3 p D2

16 σ t

¿¿

σ t = 310Mpa

th = √ 3 × 15.454 ×572

16 ×310¿

th = √29.6983 = 5.5108mm orConsidering heat transfer

th =(H

12.56 k (T C –T E))

Thermalconductivity = 147w/mkTC – TE= 750c

H = C HCV m B.P(in KW)C = constant = 0.05

HCV = 47 103KJ/kg for petrolm = mass of fuel for brake power per secondBP = brake power

H = C HCV

H = 0.05 47 103 0.11H = 258.5

th =(H

12.56 k (t c−t e))

th= 258.5/(12.56 147 75) = 0.001866 mth = 1.866 mmth = 5.5108mm

5.3.2 Piston rings

Radial thickness t1 = D

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t1 = 57

= pressure of the gas on the cylinder wall = 0.042N/mm2

= allowable bending(tensile stress) for cast iron rings = 110Mpa

t1 = 57t1 = 1.93mm

axial thickness t2 = D/10nr = 57/10 3 = 1.9mmnr = no of rings = 3width of the top land b1= 1.2th

b1 = 1.2(5.5108) =6.61296 mmWith of other land (i.e) distance between ring grooves

b2 = t2 = 1.9mmThe gap between the free ends of the ring = 3.5t to 4t = 7.72mm

5.3.3 Piston barrel

t3 = 0.03D + b +4.5b = radial depth of piston ringb = t1 +0.4 = 2.33mm

t3 = 0.03 57+2.33+4.5t3 = 8.54mmThe piston wall thickness towards the open endt4 = 0.35t3 = 2.989mm

5.3.4 Piston skirt

Maximum gas load on the piston

P = p πD2/4 = (15.454 572)/4P = 30414.88611NMaximum side thrust on the cylinderR = p/10 = 3941.488611R = bearing pressure x bearing area of the piston skirt

R = pb D ll = length of the piston skirt in mml =45.6N/mm2

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Bearing pressure pb = 1.5N/mm2

Total length of the pistonL = length of the skirt+length of ring section + top land

Length of ring section = 5 b2 or t2 = 9.5mmL = 45.6 + 9.5 + 6.54 = 61.64mm

5.3.5 Piston pin - material heat treated alloy steel

Center of piston pin should be 0.02D to 0.04D aboveThe center of skirt = 0.04D = 2.28mm above center of skirtTensile strength = 710 to 910MpaLength of the pin in the connecting rod bushingl1 = 0.45D = 25.65mmload on the piston due to gas pressure = 39414.88611Np =bearing pressure×bearing area

p = pb1xd0xl1

l1 = 25.65mmpb1 = 50 – 100Mpa for bronze pb1 = 100Mpa

d0 = p/pb1 l1 = 15.36mmInner diameter of piston pin di = 0.6d0 = 9.21mmMaximum bending moment at the center of pinM = P.D/8 = (39414.88611x57)/8M = 280831.06

Z = /32[(d0)4 – (dc)4/d0]

=

=

=Z = 2478.48Allowable bending stress σb = M/Z = 113.3This is less then the allowable value 140mpa for heat treated alloy steelThe mean diameter of the piston losses = 1.5d0

= 23.04mm

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Chapter 6

INTRODUCTION TO CAD

6.1- CAD

Computer Aided Design is a technique in which man and machine are blended in to problem solving team, intimately coupling the best characteristics of each. The result of this combination works better than either man or machine would work alone , and by using a multi discipline approach, it offers the advantages of integrated team work.The advances in Computer Science and Technology resulted in the emergence of very powerful hardware and software tool. It offers scope for use in the entire design process resulting in improvement in the quality of design. The emergency of CAD as a field of specialization will help the engineer to acquire the knowledge and skills needed in the use of these tools in an efficient and effective way on the design process.

Computer Aided Design is an interactive process, where the exchange of information between the designer and the computer is made as simple and effective as possible. Computer aided design encompasses a wide variety of computer based methodologies and tools for a spectrum of engineering activities planning, analysis, detailing, drafting, construction, manufacturing, monitoring, management, process control and maintenance. CAD is more concerned with the use of computer-based tools to support the entire life cycle of engineering system.

6.2 Introduction to pro/engineer

Pro,ENGINEER is the industry’s de facto standard 3D mechanical design suit. It is the world’s leading CAD/CAM /CAE software, gives a broad range of integrated solutions to cover all aspects of product design and manufacturing. Much of its success can be attributed to its technology which spurs its customer’s to more quickly and consistently

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innovate a new robust, parametric, feature based model. Because that PRO/ENGINEER is unmatched in this field, in all processes, in all countries, in all kind of companies along the supply chains. Pro/Engineer is also the perfect solution for the manufacturing enterprise, with associative applications, robust responsiveness and web connectivity that make it the ideal flexible engineering solution to accelerate innovations. Pro/Engineer provides easy to use solution tailored to the needs of small medium sized enterprises as well as large industrial corporations in all industries, consumer goods, fabrications and assembly. Electrical and electronics goods, automotive, aerospace, shipbuilding and plant design. It is user friendly solid and surface modeling can be done easily.

6.3 Advantages of Pro/Engineer

Ability to changes in late design process is possible.

It provides a very accurate representation of model specifying all other dimensions hidden geometry etc.

It is user friendly it is much faster and more accurate.

Once a design is completed. 2d and 3d views are readily obtainable.

The both solid and surface modeling can be done.

It provides a greater flexibility for change. For example if we like to change the dimensions of our model, all the related dimensions in design assembly, manufacturing etc. Will automatically change.

It provides clear 3d models, which are easy to visualize and understand. Pro/engineer provides easy assembly of the individual parts or models created it

also decreases the time required for the assembly to a large extent.

Pro/ENGINEER is a feature-based, parametric solid modeling system with many extended designed manufacturing applications. As a comprehensive CAD/CAE/CAM system, covering many aspects of mechanical design, analysis and manufacturing, Pro/ENGINEER represents threading edge of CAD/CAE/CAM technology.

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Chapter 7

3D MODELS OF PISTON

7.1 Aluminum Alloy 7475-T7617.1.1 2 D model of piston

Fig-7.1.1 2 D model of piston for Aluminum Alloy 7475-T761

7.1.2 3D Model of Piston

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Fig-7.1.2 3D Model of Piston for Aluminum Alloy 7475-T761

7.2 Aluminum Alloy 60617.2.1 2 D model of piston

Fig-7.2.1 2D model of piston for aluminum alloy 6061

7.2.2 3D Model of Piston

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Fig-7.2.2 3D Model of piston for aluminum alloy 60617.3 2D Drafting of Piston7.3.1 Aluminum alloy 7475-T761

Fig-7.3.1 2D drafting of aluminum alloy 7475-t761 piston

7.3.2 Aluminum alloy 6061

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Fig-7.3.2 2D drafting of aluminum alloy 6061 piston

Chapter 8

INTRODUCTION TO FEA

8.1 Introduction

Finite Element Analysis FEA was first developed in 1943 by R. Courant, who utilized the Ritz method of numerical analysis and minimization of variational calculus to obtain approximate solutions to vibration systems. Shortly thereafter, a paper published in 1956 by M. J. Turner, R. W. Clough, H. C. Martin, and L. J. Topp established a broader definition of numerical analysis. The paper centered on the stiffness and deflection of complex structures.

By the early 70's, FEA was limited to expensive mainframe computers generally owned by the aeronautics, automotive, defense, and nuclear industries. Since the rapid decline in the cost of computers and the phenomenal increase in computing power, FEA has been developed to an incredible precision. Present day supercomputers are now able to produce accurate results for all kinds of parameters.

FEA consists of a computer model of a material or design that is stressed and analyzed for specific results. It is used in new product design, and existing product refinement. A company is able to verify a proposed design will be able to perform to the client's

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specifications prior to manufacturing or construction. Modifying an existing product or structure is utilized to qualify the product or structure for a new service condition. In case of structural failure, FEA may be used to help determine the design modifications to meet the new condition.

There are generally two types of analysis that are used in industry: 2-D modeling, and 3-D modeling. While 2-D modeling conserves simplicity and allows the analysis to be run on a relatively normal computer, it tends to yield less accurate results. 3-D modeling, however, produces more accurate results while sacrificing the ability to run on all but the fastest computers effectively. Within each of these modeling schemes, the programmer can insert numerous algorithms functions which may make the system behave linearly or non-linearly. Linear systems are far less complex and generally do not take into account plastic deformation. Non-linear systems do account for plastic deformation, and many also are capable of testing a material all the way to fracture.

FEA uses a complex system of points called nodes which make a grid called a mesh. This mesh is programmed to contain the material and structural properties which define how the structure will react to certain loading conditions. Nodes are assigned at a certain density throughout the material depending on the anticipated stress levels of a particular area. Regions which will receive large amounts of stress usually have a higher node density than those which experience little or no stress. Points of interest may consist of: fracture point of previously tested material, fillets, corners, complex detail, and high stress areas. The mesh acts like a spider web in that from each node, there extends a mesh element to each of the adjacent nodes. This web of vectors is what carries the material properties to the object, creating many elements.

A wide range of objective functions variables within the system are available for minimization or maximization:

Mass, volume, temperature Strain energy, stress strain Force, displacement, velocity, acceleration Synthetic User defined

There are multiple loading conditions which may be applied to a system. Some examples are shown: Point, pressure, thermal, gravity, and centrifugal static loads Thermal loads from solution of heat transfer analysis Enforced displacements Heat flux and convection Point, pressure and gravity dynamic loads

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Each FEA program may come with an element library, or one is constructed over time. Some sample elements are: Rod elements Beam elements Plate/Shell/Composite elements Shear panel Solid elements Spring elements Mass elements Rigid elements Viscous damping elements

Many FEA programs also are equipped with the capability to use multiple materials within the structure such as: Isotropic, identical throughout Orthotropic, identical at 90 degrees General anisotropic, different throughout 8.2 Types of Engineering Analysis

Structural analysis consists of linear and non-linear models. Linear models use simple parameters and assume that the material is not plastically deformed. Non-linear models consist of stressing the material past its elastic capabilities. The stresses in the material then vary with the amount of deformation as in.

Vibrational analysis is used to test a material against random vibrations, shock, and impact. Each of these incidences may act on the natural vibrational frequency of the material which, in turn, may cause resonance and subsequent failure.Fatigue analysis helps designers to predict the life of a material or structure by showing the effects of cyclic loading on the specimen. Such analysis can show the areas where crack propagation is most likely to occur. Failure due to fatigue may also show the damage tolerance of the material.

Heat Transfer analysis models the conductivity or thermal fluid dynamics of the material or structure. This may consist of a steady-state or transient transfer. Steady-state transfer refers to constant thermo properties in the material that yield linear heat diffusion. 8.3 Results of Finite Element Analysis

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FEA has become a solution to the task of predicting failure due to unknown stresses by showing problem areas in a material and allowing designers to see all of the theoretical stresses within. This method of product design and testing is far superior to the manufacturing costs which would accrue if each sample was actually built and tested. In practice, a finite element analysis usually consists of three principal steps:

8.3.1 Preprocessing

The user constructs a model of the part to be analyzed in which the geometry is divided into a number of discrete sub regions, or elements, connected at discrete points called nodes. Certain of these nodes will have fixed displacements, and others will have prescribed loads. These models can be extremely time consuming to prepare, and commercial codes vie with one another to have the most user-friendly graphical preprocessor to assist in this rather tedious chore. Some of these preprocessors can overlay a mesh on a preexisting CAD file, so that finite element analysis can be done conveniently as part of the computerized drafting-and-design process.

8.3.2 Analysis

The dataset prepared by the preprocessor is used as input to the finite element code itself, which constructs and solves a system of linear or nonlinear algebraic equations Kijuj = fi. Where u and f are the displacements and externally applied forces at the nodal points. The formation of the K matrix is dependent on the type of problem being attacked, and this module will outline the approach for truss and linear elastic stress analyses. Commercial codes may have very large element libraries, with elements appropriate to a wide range of problem types. One of FEA's principal advantages is that many problem types can be addressed with the same code, merely by specifying the appropriate element types from the library.

8.3.3 Postprocessing

In the earlier days of finite element analysis, the user would pore through reams of numbers generated by the code, listing displacements and stresses at discrete positions within the model. It is easy to miss important trends and hot spots this way, and modern codes use graphical displays to assist in visualizing the results.

8.4 Introduction to ANSYS

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ANSYS is general-purpose finite element analysis FEA software package.  Finite Element Analysis is a numerical method of deconstructing a complex system into very small pieces of user-designated size called elements. The software implements equations that govern the behaviour of these elements and solves them all; creating a comprehensive explanation of how the system acts as a whole. These results then can be presented in tabulated or graphical forms.  This type of analysis is typically used for the design and optimization of a system far too complex to analyze by hand.  Systems that may fit into this category are too complex due to their geometry, scale, or governing equations. ANSYS is the standard FEA teaching tool within the Mechanical Engineering Department at many colleges. ANSYS is also used in Civil and Electrical Engineering, as well as the Physics and Chemistry departments. 

ANSYS provides a cost-effective way to explore the performance of products or processes in a virtual environment. This type of product development is termed virtual prototyping.With virtual prototyping techniques, users can iterate various scenarios to optimize the product long before the manufacturing is started. This enables a reduction in the level of risk, and in the cost of ineffective designs. The multifaceted nature of ANSYS also provides a means to ensure that users are able to see the effect of a design on the whole behavior of the product, be it electromagnetic, thermal, mechanical etc.8.5 Generic Steps to Solving any Problem in ANSYS 

Like solving any problem analytically, you need to define solution domain, The physical model, Boundary conditions and The physical properties.

Then solve the problem and present the results. In numerical methods, the main difference is an extra step called mesh generation. This is the step that divides the complex model into small elements that become solvable in an otherwise too complex situation. Below describes the processes in terminology slightly more attune to the software. 

8.5.1 Build Geometry

Construct a two or three dimensional representation of the object to be modeled and tested using the work plane coordinates system within ANSYS. 

8.5.2 Define Material Properties

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Now that the part exists, define a library of the necessary materials that compose the object or project being modeled.  This includes thermal and mechanical properties.

8.5.3 Generate Mesh

At this point ANSYS understands the makeup of the part.  Now define how the modeled system should be broken down into finite pieces. 

8.5.4 Apply Loads

Once the system is fully designed, the last task is to burden the system with constraints, such as physical loadings or boundary conditions.           

8.5.5 Obtain Solution

This is actually a step, because ANSYS needs to understand within what state steady state, transient etc the problem must be solved. 

8.5.6 Present the Results

After the solution has been obtained, there are many ways to present ANSYS’ results, choose from many options such as tables, graphs, and contour plots. 

8.6 Specific Capabilities of ANSYS 8.6.1 Structural

Fig-8.6.1 Structural analysis

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Structural analysis is probably the most common application of the finite element method as it implies bridges and buildings, naval, aeronautical, and mechanical structures such as ship hulls, aircraft bodies, and machine housings, as well as mechanical components such as pistons, machine parts, and tools.  8.6.1.1 Static Analysis

Used to determine displacements, stresses, etc. under static loading conditions. ANSYS can compute both linear and nonlinear static analyses. Nonlinearities can include plasticity, stress stiffening, large deflection, large strain, hyper elasticity, contact surfaces, and creep. 

8.6.1.2 Transient Dynamic Analysis

Used to determine the response of a structure to arbitrarily time-varying loads. All nonlinearities mentioned under Static Analysis above are allowed. 

 8.6.1.3 Buckling Analysis

Used to calculate the buckling loads and determine the buckling mode shape. Both linear eigen value buckling and nonlinear buckling analyses are possible.   In addition to the above analysis types, several special-purpose features are available such as Fracture mechanics, Composite material analysis, Fatigue, and both p-Method and Beam analyses.  

8.6.2 Thermal 

Fig-8.6.2 Thermal analysis

ANSYS is capable of both steady state and transient analysis of any solid with thermal boundary conditions. 

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Steady-state thermal analyses calculate the effects of steady thermal loads on a system or component. Users often perform a steady-state analysis before doing a transient thermal analysis, to help establish initial conditions. A steady-state analysis also can be the last step of a transient thermal analysis; performed after all transient effects have diminished. ANSYS can be used to determine temperatures, thermal gradients, heat flow rates, and heat fluxes in an object that are caused by thermal loads that do not vary over time. Such loads include the following: 

Convection Radiation Heat flow rates Heat fluxes (heat flow per unit area) Heat generation rates (heat flow per unit volume) Constant temperature boundaries 

A steady-state thermal analysis may be either linear, with constant material properties; or nonlinear, with material properties that depend on temperature. The thermal properties of most material vary with temperature. This temperature dependency being appreciable, the analysis becomes nonlinear. Radiation boundary conditions also make the analysis nonlinear. Transient calculations are time dependent and ANSYS can both solve distributions as well as create video for time incremental displays of models. 8.6.3 Fluid Flow

Fig-8.6.3 Fluid flow analysis

The ANSYS/FLOTRAN CFD Computational Fluid Dynamics offers comprehensive tools for analyzing two-dimensional and three-dimensional fluid flow fields.  ANSYS is capable of modeling a vast range of analysis types such as: airfoils for pressure analysis of airplane wings lift and drag, flow in supersonic nozzles, and complex, three-dimensional flow patterns in a pipe bend.  In addition, ANSYS/FLOTRAN could be used to perform tasks including:

Calculating the gas pressure and temperature distributions in an engine exhaust manifold Studying the thermal stratification and breakup in piping systems

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Using flow mixing studies to evaluate potential for thermal shock Doing natural convection analyses to evaluate the thermal performance of chips in electronic enclosures Conducting heat exchanger studies involving different fluids separated by solid regions

FLOTRAN analysis provides an accurate way to calculate the effects of fluid flows in complex solids without having to use the typical heat transfer analogy of heat flux as fluid flow.  Types of FLOTRAN analysis that ANSYS is able to perform include:

Laminar or Turbulent Flows Thermal Fluid Analysis Adiabatic Conditions Free surface Flow Compressible or incompressible Flows Newtonian or Non-Newtonian Fluids Multiple species transport

These types of analyses are not mutually exclusive. For example, a laminar analysis can be thermal or adiabatic. A turbulent analysis can be compressible or incompressible. 

8.6.4 Magnetic 

Fig-8.6.4 Magnetic analysis

Magnetic analyses, available in the ANSYS/Multiphysics and ANSYS/Emag programs, calculate the magnetic field in devices such as:

Power generators Magnetic tape/disk drives Transformers Waveguides

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Solenoid actuators Resonant cavities Electric motors Connectors Magnetic imaging systems Antenna radiation Video display device sensors Filters Cyclotrons

Typical quantities of interest in a magnetic analysis are:

Magnetic flux density Power loss Magnetic field intensity Flux leakage Magnetic forces and torques S-parameters Impedance Quality factor Inductance Return loss Eddy currents Eigen frequencies

Magnetic fields may exist as a result of an electric current, a permanent magnet, or an applied external field.

8.6.5 Acoustics / Vibration 

Fig-8.6.5 Noise analysis and optimization of a Craftsman Table Saw Blade

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ANSYS is capable of modeling and analyzing vibrating systems in order to that vibrate in order to analyze Acoustics is the study of the generation, propagation, absorption, and reflection of pressure waves in a fluid medium. Applications for acoustics include the following:

Sonar - the acoustic counterpart of radar Design of concert halls, where an even distribution of sound pressure is desired Noise minimization in machine shops Noise cancellation in automobiles Underwater acoustics Design of speakers, speaker housings, acoustic filters, mufflers, and many other similar devices. Geophysical exploration

Within ANSYS, an acoustic analysis usually involves modeling a fluid medium and the surrounding structure. Characteristics in question include pressure distribution in the fluid at different frequencies, pressure gradient, and particle velocity, the sound pressure level, as well as, scattering, diffraction, transmission, radiation, attenuation, and dispersion of acoustic waves. A coupled acoustic analysis takes the fluid-structure interaction into account. An uncoupled acoustic analysis models only the fluid and ignores any fluid-structure interaction.

The ANSYS program assumes that the fluid is compressible, but allows only relatively small pressure changes with respect to the mean pressure. Also, the fluid is assumed to be non-flowing and inviscid that is, viscosity causes no dissipative effects. Uniform mean density and mean pressure are assumed, with the pressure solution being the deviation from the mean pressure, not the absolute pressure.

8.6.6 Coupled Fields Analysis

A coupled-field analysis is an analysis that takes into account the interaction coupling between two or more disciplines fields of engineering. A piezoelectric analysis, for example, handles the interaction between the structural and electric fields: it solves for the voltage distribution due to applied displacements, or vice versa. Other examples of coupled-field analysis are thermal-stress analysis, thermal-electric analysis, and fluid-structure analysis.

Some of the applications in which coupled-field analysis may be required are pressure vessels thermal-stress analysis, fluid flow constrictions fluid-structure analysis, induction heating magnetic-thermal analysis, ultrasonic transducers

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piezoelectric analysis, magnetic forming magneto-structural analysis, and micro-electro mechanical systems MEMS.

8.7 Modal Analysis

A modal analysis is typically used to determine the vibration characteristics natural frequencies and mode shapes of a structure or a machine component while it is being designed. It can also serve as a starting point for another, more detailed, dynamic analysis, such as a harmonic response or full transient dynamic analysis.

Modal analyses, while being one of the most basic dynamic analysis types available in ANSYS, can also be more computationally time consuming than a typical static analysis.  A reduced solver, utilizing automatically or manually selected master degrees of freedom is used to drastically reduce the problem size and solution time.

8.8 Harmonic Analysis

Used extensively by companies who produce rotating machinery, ANSYS Harmonic analysis is used to predict the sustained dynamic behavior of structures to consistent cyclic loading.  Examples of rotating machines which produced or are subjected to harmonic loading are:

Turbines Gas Turbines for Aircraft and Power Generation Steam Turbines Wind Turbine Water Turbines Turbo pumps Internal Combustion engines Electric motors and generators Gas and fluid pumps Disc drives

A harmonic analysis can be used to verify whether or not a machine design will successfully overcome resonance, fatigue, and other harmful effects of forced vibrations.

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Chapter 9

RESULTS AND INTERPRETATION

9.1 Sequence of steps for Aluminum Alloy 6061for Model-1

Importing the piston model from Pro/Engineer Defining the Thermal Environment. Defining the Structural Environment. Solution phase-assigning loads and solving. Post processing and viewing the results.

Boundary Conditions-In a piston under static conditions it is supported by the gudgeon pin region. So, the areas corresponding to these have to be constrained in all degrees of freedom. The working pressure is 15.454 Mpa. Pressures applied at the top of the piston.

9.1.1 Importing the piston model

Utility menu > file > Import > browse >Pro/Engineer part modelANSYS Utility menu > plot controls > style > solid model facets> normal faceting.

63

Before going into the later part of the analysis a little bit of description, regarding the type of analysis and the method used appears to be mandatory.

Fig-9.1.1 Model of piston in ANSYS9.1.2 Defining the thermal environment:

Give the analysis title:

Utility menu > file >change title>optimization of piston.

Give the preferences:

ANSYS main menu>preferences>thermal.

Define the type of Element:

Preprocessor > elemet type > add/edit/del > add element > add > solid > solid 20 node 90.

The element type that has been selected for the thermal parts of the coupled field analysis is 20 node90.It is a ten nodded tetrahedron element.

Define the element material properties:Preprocessor> material props> material models> thermal conductivity> isotropic.

In the window that appears, enter the following geometric properties for 6061

64

Thermal conductivity (kxx) =180W/mKSpecific heat(c) = 0.896KJ/KgKDensity = 0.0000027Kg/mm3

Meshing:

Giving element length:

Preprocessor > meshing > size controls > manual size > lines > all lines > element edge length > 5 > ok.

Meshing:

Preprocessor > Meshing > Mesh > Volumes > free > pick all > ok

Fig-9.1.2 Meshed model

Write environment:

Preprocessor > Physics > Environment > Write.

In the window that appears, enter the Title Thermal and click ok.

Clear Environment:

65

Preprocessor > Physics Environment > Clear > Ok.

Doing this, clears all the information prescribed for the geometry such as the element type, material props etc. It does not clear the geometry however so it can be used in the next stage which is defining the structural environment.

9.13 Defining the Structural Environment: Switch Element Type:Preprocessor > Element Type > Switch Element Type > Thermal To Structural > Ok.Choose Thermal To Structural from the scroll down the list. This will switch to the complimentary structural element automatically from solid 20 node 90 to solid 20 node 95 .Reason is structural analysis uses solid 95 element and thermal analysis uses solid 95 types.

Define Element Material Properties

Preprocessor > Material props > Material Models > Structural > Liner > Elastic > Isotropic.

In the window that appears enter the following material properties:

Young′s Modulus (EX) : 68.9e3N/mm2

Poisson’s Ratio (PRXY) : 0.33Density : 2700kg/m3

Write Environment

The structural Environment is now fully described. Preprocessor > Physics > Environment > Write >Struct.In the window that appears enter the Title Struct.

9.14 Solution phase: Assuming Loads and solving

Define Analysis Type Solution > Analysis Type > New Analysis > Static > Ok.

Read in the thermal environmentPreprocessor > Physics > Environment > Read > Thermal > OkChoose thermal and click ok.

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If the physics option is not available under solution. Unabridged Menu at the bottom of the solution menu .This should make it visible.

Define state:Solution > Analysis Type > New Analysis > Steady State > Ok.

Apply Temperature load:Solution > Define Loads > Apply > Thermal > Temperature >On areas

The temperature load is applied by taking different areas and applying the load according to the area of the piston. Temperature is 573K. Apply convection on surfaces :Solution > Define Loads > Apply > Thermal > Convection > on areas

Convection at various surfaces is applied based on the experimental values.The values are taken from SAE papers. On the surface of the piston head film coefficient of 222 W/mk and bulk temperature 313K is applied.

Apply heat flow Solution > Define Load > Apply > Thermal > Heat Flow > On Nodes> Pick All.

Heat flow that occurs through the piston head is calculated making use of the formulae from the data book. This heat flow corresponds to that which flows through the piston head Solve the SystemSolution > Solve > Current LS

67

Fig-9.1.4.1 Nodal temperature

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Fig-9.1.4.2 Thermal gradient

Fig-9.1.4.3 Thermal flux

69

Close the solution Menu, Main Menu > Finish It is very important to click Finish as it closes that environment and allows a new one to be opened without contamination. If this is not done, you will get error message.

This information is saved in a file labeled job name.rth,wererth is the results file . Since the job name was not changed at the beginning of the analysis, this data can be found as file .rth, we will use these results in determining the structural effects.

Read in the Structural EnvironmentSolution > Physics > Environment > Read >Struct> OkChoose struct and click OK

Analysis typeSolution >Analysis type > New analysis > Static >Ok

Apply Boundary ConditionsSolution > Define Loads >Apply > structural > Displacement >On areas

In a piston under static conditions is closely observed for where to apply the boundary conditions. It can be plotted out that it is supported by the gudgeon pin region. So, the areas corresponding to these have to be constrained in all degrees of freedom.

Apply pressure load: Solution > Define Loads >Apply > Structural > Pressure >On areas

The working pressure is 15.454 Mpa. Pressures applied at the top of the piston and choose ok.

Fig-9.1.4.4 solution

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Include Thermal Effects Solution > Define Loads > Apply > Structural > Temperature > From > Thermal Analysis > Browse > Ok

As shown below enter the file name File.rth. This couple the results from the solution of the thermal environment to the information prescribed in the structural environment and uses it during the analysis.

Define reference temperaturePreprocessor > Loads > Define Loads > Settings > Reference TempFor this example set the reference temperature to 273 degrees Kelvin.Solve the system, Solution > Solve > Current Ls

When this command is used. We get windows appearing on the screen this includes the element matrices and sparse solver .At last we can see an out put window appearing on the screen which implies that the solution is done.

The solver writes output to the file job name OUT and the results file. If you run the solution interactively the output file is actually your screen window . By using one of the following before issuing SOLVE. you can divert the output to a file instead of the screen.

9.1.5 Post processing: Viewing the result

The following steps show the various results that have been observed.

Deformed shape: General post processing>plot results>deformed shape

This will result in the deformed shape of the piston in the output widow.

Nodal Results:General post processing > Plot Results > Contour Plot > Nodal solution.

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Fig-9.1.5.1 Displacement

Fig-9.1.5.2 Vonmises stres

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9.2 Aluminum Alloy 6061 for Model-2

Fig-9.2.1 Nodal temperature

Fig-9.2.2 Thermal gradient

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Fig-9.2.3 Thermal flux

Fig-9.2.4 Displacement74

Fig-9.2.5 Vonmisses stress

9.3 Aluminum 7475 T761 Piston Model-1

Thermal conductivity (kxx) = 147w/mkSpecific heat(c) = 0.88kj/kgkYoungs Modulus (EX) = 70300N/mm2

Poissons Ratio (PRXY) = 0.33Density = 2810kg/m3

Temperature = 563KFilm Coefficient = 222W/mkBulk Temperature = 313K

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Fig-9.3.1 Nodal temperature

Fig-9.3.2 Thermal gradient76

Fig-9.3.3 Thermal flux

Fig-9.3.4 Displacement77

sFig-9.3.5 Vonmisses stress

9.4 Aluminum 7475 T761 Piston Model-2

Fig-9.4.1 Nodal temperature

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Fig-9.4.2 Thermal gradient

Fig-9.4.3 Thermal flux

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Fig-9.4.4 Displacement

Fig-9.4.5 Von-misses stress

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9.5 LM 25 Piston Model-1

Thermal conductivity (kxx) = 134w/mkSpecific heat(c) = 0.963 kj/kgkYoung’s Modulus (EX) = 70000N/mm2

Poisson’s Ratio (PRXY) = 0.32Density = 2680kg/m3

Fig-9.5.1 Nodal temperature

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Fig-9.5.2 Thermal gradient

Fig-9.5.3 Thermal flux

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Fig-9.5.4 Displacement

Fig-9.5.5 Vonmisses stress

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9.6 LM 25 Piston Model 2

Fig-9.6.1 Nodal temperature

Fig-9.6.2 Thermal gradien

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Fig-9.6.3 Thermal flux

Fig-9.6.4 Displacement85

Fig-9.6.5 Von-misses stress

9.7 As per the analysis images for Model 1

Material Displacement(mm)

Von-Misses Stress(N/mm2)

NodalTemp (K)

Thermal Gradient (K/mm)

Thermal Flux (W/mm2)

Aluminum alloy7475-761

0.016593 123.902 563 21.601 3.175

Aluminum alloy 6061

0.01693 123.902 563 21.422 3.856

LM25 0.016677 122.637 563 22.101 2.962

Table 9.7 Result as per the analysis images for Model 1

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9.8 Permissible Yield Stress Values for Different Aluminum Alloys

LM25 = 180N/mm2

7475-T761 = 448 N/mm2

6061 = 310 N/mm2

9.9 As per the analysis images for Model2

Material Displacement(mm)

Von-Misses Stress(N/mm2)

NodalTemp (K)

Thermal Gradient (K/mm)

Thermal Flux (W/mm2)

Aluminum alloy7475-761

0.016734 123.092 563 20.988 3.085Aluminum alloy

60610.01707 127.174 563 19.038 3.427

LM250.01602 127.174 563 21.681 2.905

Table 9.9 Result As per the analysis images for Model2

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Chapter 10

CONCLUSION AND FUTURE SCOPE

10.1 Conclusion

1. In my project I have designed a piston used in a two wheeler. The present used material for piston is Aluminum alloy LM25. I am replacing with different aluminum alloys 7475-T761 and 6061. I am replacing with above materials, since they have more strength than the Aluminum alloy LM25.

2. It was found that aluminum alloy 7475-T761 has around 150% more permissible yield stress values when compared to that of LM25. Whereas it was found around 73% for aluminium alloy 6061 when compared to the LM25.

3. Two models of piston are designed for two materials- aluminum alloy 7475-T761 and 6061. Coupled field analysis is done on the models to validate structural and thermal properties like displacement, stress, thermal gradient, thermal flux.

4. By observing the analysis results, von-misses stress values are around 3.9% less for material 7475-T761 when compare to 6061 & LM25.

5. By observing the analysis results, its thermal gradient value is around 0.10% more when compare to 6061.

6. The main disadvantage of this material 7475-T761 when compared to 6061, it is denser around 0.040% and when compare to LM25 it is denser around 0.048%, so weight of the piston increases.

10.2 Future Scope

It should be noted that analysis of the piston pin is beyond the scope of this work which can be however achieved by an appropriate material model for the piston pin and examinig the stresses in the cross section of the pin.

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