rb4912b35 term paper thermodynamics

8/7/2019 Rb4912b35 Term Paper Thermodynamics

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Phagwara, Punjab

TERM PAPER

Topic:-

OTTO CYCLE VS DIESEL CYCLE

Submitted To: - Submitted By:-

Mr. Tukesh Soni Namandeep SinghRB4912B3510904031


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ACKNOWLEDGEMENT

Namandeep Singh student of B.Tech (M.E.), RB4912B35.Iwould like to say thanks to Mr. Tukesh Soni. Without his help,would not able to complete my term paper which was onhe topic OTTO CYCLE VS DIESEL CYCLE

I hope that my term paper considered all the propernformation which would surely satisfy you. In my term paperhave mentioned all the current changes in climate andeasons behind it.


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OTTO CYCLEAn Otto cycle is an idealized thermodynamic cycle which describeshe functioning of a typical reciprocating piston engine. The Otto cycle

s constructed out of:

1. TOP and BOTTOM of the loop: a pair of quasi-parallel adiabatic processes2. LEFT and RIGHT sides of the loop: a pair ofparallel isochoric processes

The adiabatic processes are impermeable to heat: heat flows

nto the loop through the left pressurizing process and someof it flows back out through the right depressurizing process,and the heat which remains does the work.

The maximum theoretical thermal efficiency of the Otto cycles:

= 1 r(1 )

where r is the engine's compression ratio, and is

the heat capacity ratio for the working fluid. Because >1,the efficiency increases with increasing compression ratio.

The Otto cycle consists ofadiabatic compression, heataddition at constant volume, adiabatic expansion, andrejection of heat at constant volume. In the case of a four-stroke Otto cycle, technically there are two additionalprocesses: one for the exhaust of waste heat and

combustion products (by isobaric compression), and onefor the intake of cool oxygen-rich air (by isobaricexpansion); however, these are often omitted in asimplified analysis. Even though these two processes arecritical to the functioning of a real engine, wherein thedetails of heat transfer and combustion chemistry arerelevant, for the simplified analysis of the thermodynamiccycle, it is simpler and more convenient to assume that all

of the waste-heat is removed during a single volumechange.
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History

The four-stroke engine was first patented by AlphonseBeau de Rochas in 1861. Before, in about 185457, two

Italians (Eugenio Barsanti and Felice Matteucci) inventedan engine that was rumored to be very similar, but thepatent was lost.

"The request bears the no. 700 of Volume VII of the PatentOffice of the Reign of Piedmont. We do not have the textof the patent request, only a photo of the table whichcontains a drawing of the engine. We do not even know if

it was a new patent or an extension of the patent grantedthree days earlier, on December 30, 1857, at Turin."

f. Eugenio Barsanti and Felice Matteucci, June 4 1853

The first person to build a working four stroke engine, astationary engine using a coal gas-air mixture for fuel(a gas engine), was German engineer Nicolaus Otto. Thatis why the four-stroke principle today is commonly knownas the Otto cycle and four-stroke engines using spark

plugs often are called Otto engines.
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To move an airplane through the air, thrust is generated byome kind ofpropulsion system. Beginning with the Wright

brothers' first flight, many airplanes have used internalombustion engines to turn propellers to generate thrust.

Today, most general aviation or private airplanes arepowered by internal combustion (IC) engines, much like theengine in your family automobile. When discussing engines,we must consider both the mechanical operation of themachine and thethermodynamic processes that enable themachine to produce useful work. On this page we considerhe thermodynamics of a four-stroke IC engine.

To understand how a propulsion system works, we musttudy the basic thermodynamics ofgases. Gases have
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variousproperties that we can observe with our senses,ncluding the gas pressure p, temperature T, mass,and volume V that contains the gas. Careful, scientificobservation has determined that these variables are relatedo one another, and the values of these properties determinehe state of the gas. A thermodynamic process, such as

heating or compressing the gas, changes the values of thetate variables in a manner which is described by the laws ofhermodynamics. The workdone by a gas andhe heat transferred to a gas depend on the beginning and

ending states of the gas and on the process used to changehe state. It is possible to perform a series of processes, in

which the state is changed during each process, but the gaseventually returns to its original state. Such a series ofprocesses is called a cycle and forms the basis forunderstanding engine operation.

On this page we discuss the Otto ThermodynamicCycle which is used in all internal combustion engines. Theigure shows a p-V diagram of the Otto cycle. Using the

engine stage numbering system, we begin at the lower leftwith Stage 1being the beginning of the intake stroke of theengine. The pressure is near atmospheric pressure and thegas volume is at a minimum. Between Stage 1 and Stage 2he piston is pulled out of the cylinder with the intake valve

open. The pressure remains constant, and the gas volumencreases as fuel/air mixture is drawn into the cylinderhrough the intake valve.Stage 2 begins the compression

troke of the engine with the closing of the intake valve.Between Stage 2 and Stage 3, the piston moves back intohe cylinder, the gas volume decreases, and the pressurencreases because work is done on the gas by thepiston. Stage 3 is the beginning of the combustion of theuel/air mixture. The combustion occurs very quickly and the

volume remains constant. Heat is released duringombustion which increases both the temperature and the

pressure, according to the equation of state. Stage 4 begins
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he power stroke of the engine. Between Stage 4 and Stage5, the piston is driven towards the crankshaft, the volume inncreased, and the pressure falls as work is done by the gason the piston. At Stage 5 the exhaust valve is opened andhe residual heat in the gas is exchanged with theurroundings. The volume remains constant and the pressure

adjusts back to atmospheric conditions. Stage 6 beginshe exhaust strokeof the engine during which the piston

moves back into the cylinder, the volume decreases and thepressure remains constant. At the end of the exhaust stroke,onditions have returned to Stage 1 and the process repeatstself.

During the cycle, work is done on the gas by the pistonbetween stages 2 and 3. Work is done by the gas on thepiston between stages 4 and 5. The difference between thework done by the gas and the work done on the gas is thearea enclosed by the cycle curve and is the work producedby the cycle. The work times the rate of the cycle (cycles perecond) is equal to the power produced by the engine.

The area enclosed by the cycle on a p-V diagram isproportional to the work produced by the cycle. On this pagewe have shown an ideal Otto cycle in which there is no heatentering (or leaving) the gas during the compression andpower strokes, no friction losses, and instantaneous burningoccurring at constant volume. In reality, the ideal cycle doesnot occur and there are many losses associated with each

process. These losses are normally accounted for byefficiency factors which multiply and modify the ideal result.For a real cycle, the shape of the p-V diagram is similar tohe ideal, but the area (work) is always less than the ideal

value.
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Diesel cycle

Thermodynamics

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[show]Materialproperties[show]Equations[show]Potentials[show]History andculture[show]Scientists

vde

The Diesel cycle is the thermodynamic cycle which

approximates the pressure and volume of the combustionhamber of the Diesel engine, invented by Rudolph Diesel in1897. It is assumed to have constant pressure during theirst part of the "combustion" phase (V2 to V3 in the diagram,

below). This is an idealized mathematical model: realphysical Diesels do have an increase in pressure during thisperiod, but it is less pronounced than in the Otto cycle. Thedealized Otto cycle of a gasoline engine approximates

onstant volume during that phase, generating more of apike in a p-V diagram.
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The Idealized Diesel Cycle

p-V Diagram for the Ideal Diesel cycle. The cycle follows thenumbers 1-4 in clockwise direction.

The image on the left shows a p-V diagram for the idealDiesel cycle; where p ispressure and v is specific volume. The

deal Diesel cycle follows the following four distinct processesThe color references refer to the color of the line on thediagram.):

Process 1 to 2 is isentropic compression (blue) Process 2 to 3 is reversible constant pressure heating(red) Process 3 to 4 is isentropic expansion (yellow)

Process 4 to 1 is reversible constant volume cooling(green)

The Diesel is a heat engine: it converts heat into work. Thesentropic processes are impermeable to heat: heat flowsnto the loop through the left expanding isobaric process andome of it flows back out through the right depressurizing

process, and the heat that remains does the work.
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Work in (Win) is done by the piston compressing theworking fluid Heat in (Qin) is done by the combustion of the fuel Work out (W

out) is done by the working fluid expanding

on to the piston (this produces usable torque) Heat out (Qout) is done by venting the air

Maximum thermal efficiency

The maximum thermal efficiency of a Diesel cycle isdependent on the compression ratio and the cut-off ratio. Ithas the following formula under cold air standard analysis:

where

th is thermal efficiency

is the cut-off ratio (ratio between the end and startvolume for the combustion phase)

r is the compression ratio

is ratio ofspecific heats (Cp/Cv)

The cut-off ratio can be expressed in terms oftemperature as shown below:

T3 can be approximated to the flame temperature of the fuelused. The flame temperature can be approximated tohe adiabatic flame temperature of the fuel with

orresponding air-to-fuel ratio and compression
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pressure, p3. T1 can be approximated to the inlet airemperature.

This formula only gives the ideal thermal efficiency. The

actual thermal efficiency will be significantly lower due toheat and friction losses. The formula is more complex thanhe Otto cycle (petrol/gasoline engine) relation that has theollowing formula;

The additional complexity for the Diesel formula comesaround since the heat addition is at constant pressure and

he heat rejection is at constant volume. The Otto cycle byomparison has both the heat addition and rejection atonstant volume.

Comparing the two formulae it can be seen that for a givenompression ratio (r), the ideal Otto cycle will be more

efficient. However, a Diesel engine will be more efficientoverall since it will have the ability to operate at higherompression ratios. If a petrol engine were to have the sameompression ratio, then knocking (self-ignition) would occur

and this would severely reduce the efficiency, whereas in aDiesel engine, the self ignition is the desired behavior.Additionally, both of these cycles are only idealizations, andhe actual behavior does not divide as clearly or sharply. Andhe ideal Otto cycle formula stated above does not includehrottling losses, which do not apply to Diesel engines.

The Diesel cycle is a combustion process of aeciprocating internal combustion engine. In it, fuel is ignitedby heat generated by compressing air in the combustionhamber, into which fuel is injected. This is in contrast togniting it with a spark plug as in the Otto cycle (four-troke/petrol) engine. Diesel engines (heat engines using the

Diesel cycle) are used inautomobiles, powergeneration, Diesel-electriclocomotives, and submarines.

Applications
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Diesel engines

The Diesel engine has the lowest specific fuel consumption of

any large internal combustion engine, 0.26 lb/hp.h0.16 kg/kWh) for very large marine engines. Two-stroke

Diesels with high pressure forced induction,particularly turbocharging, make up a large percentage ofhe very largest Diesel engines.

n North America, Diesel engines are primarily used in largerucks, where the low-stress, high-efficiency cycle leads to

much longer engine life and lower operational costs. Theseadvantages also make the Diesel engine ideal for use in theheavy-haul railroad environment.

Other internal combustion engines without sparkplugs

Many model airplanes use very simple "glow" and "Diesel"engines. Glow engines use glow plugs. "Diesel" model

airplane engines have variable compression ratios. Bothypes depend on special fuels (easily obtainable in suchmited quantities) for their ignition timing.

Some 19th century or earlier experimental engines usedexternal flames, exposed by valves, for ignition, but thisbecomes less attractive with increasing compression. (Itwas the research ofNicolas Lonard Sadi Carnot that

established the thermodynamic value of compression.) Ahistorical implication of this is that the Diesel engine wouldeventually have been invented without the aid of electricity.See the development of the hot bulb engine and indirectinjection for historical significance.
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Rudolph DieselRudolph Diesel was born in Paris of Bavarianparents in 1858. As a budding mechanicalengineer at the Technical University in Munich, hebecame fascinated by the 2nd law ofthermodynamics and the maximum efficiency of aCarnot process and attempted to improve theexisting thermal engines of the day on the basisof purely theoretical considerations. His firstprototype engine was built in 1893, a year after

he applied for his initial patent, but it wasn't untilthe third prototype was built in 1897 that theorywas put into practice with the first 'Diesel' engine.

Diesel Cycle Operationhe Diesel cycle is the cycle used in the Diesel (compression-ignition)ngine. In this cycle the heat is transferred to the working fluid atonstant pressure. The process corresponds to the injection and burning

f the fuel in the actual engine. The cycle in an internal combustionngine consists of induction, compression, power and exhaust strokes.


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Induction Stroke

The induction stroke in a Diesel engine is used to

draw in a new volume of charge air into thecylinder. As the power generated in an engine isdependent on the quantity of fuel burnt duringcombustion and that in turn is determined by thevolume of air (oxygen) present, most dieselengines use turbochargers to force air into thecylinder during the induction stroke.

From a theoretical perspective, each of thestrokes in the cycle complete at Top Dead Centre(TDC) or Bottom Dead Centre (BDC), but inpracticality, in order to overcome mechanicalvalve delays and the inertia of the new charge air,and to take advantage of the momentum of theexhaust gases, each of the strokes invariablybegin and end outside the 0, 180, 360, 540 and

720 (0) degree crank positions (see valve timingchart).
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Compression Stroke

The compression stroke begins as the inlet valve

closes and the piston is driven upwards in thecylinder bore by the momentum of the crankshaftand flywheel.

The purpose of the compression stroke in a Dieselengine is to raise the temperature of the chargeair to the point where fuel injected into thecylinder spontaneously ignites. In this cycle, the

separation of fuel from the charge air eliminatesproblems with auto-ignition and therefore allowsDiesel engines to operate at much highercompression ratios than those currently inproduction with the Otto Cycle.

Compression Ignition

Compression ignition takes place when the fuelfrom the high pressure fuel injector spontaneouslignites in the cylinder.

In the theoretical cycle, fuel is injected at TDC, buas there is a finite time for the fuel to ignite(ignition lag) in practical engines, fuel is injected

into the cylinder before the piston reaches TDC toensure that maximum power can be achieved.This is synonymous with automatic spark ignitionadvance used in Otto cycle engines.
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Power Stroke

The power stroke begins as the injected fuel

spontaneously ignites with the air in the cylinder.As the rapidly burning mixture attempts toexpand within the cylinder walls, it generates ahigh pressure which forces the piston down thecylinder bore. The linear motion of the piston isconverted into rotary motion through thecrankshaft. The rotational energy is imparted asmomentum to the flywheel which not only

provides power for the end use, but alsoovercomes the work of compression andmechanical losses incurred in the cycle (valveopening and closing, alternator, fuel injectorpump, water pump, etc.).

Exhaust StrokeThe exhaust stroke is as critical to the smooth anefficient operation of the engine as that ofinduction. As the name suggests, it's the strokeduring which the gases formed during combustionare ejected from the cylinder. This needs to be ascomplete a process as possible, as any remaining

gases displace an equivalent volume of the newcharge air and leads to a reduction in themaximum possible power.
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Exhaust and Inlet Valve Overlap

Exhaust and inlet valve overlap is the transition

between the exhaust and inlet strokes and is apractical necessity for the efficient running of anyinternal combustion engine. Given the constraintsimposed by the operation of mechanical valvesand the inertia of the air in the inlet manifold, it isnecessary to begin opening the inlet valve beforethe piston reaches Top Dead Centre (TDC) on theexhaust stroke. Likewise, in order to effectively

remove all of the combustion gases, the exhaustvalve remains open until after TDC. Thus, there isa point in each full cycle when both exhaust andinlet valves are open. The number of degrees ovewhich this occurs and the proportional split across

TDC is very much dependent on the engine desigand the speed at which it operates.
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ntroduction

n these two articles we studied about Otto cycle and diesel

ycle and looked at their thermal efficiency. In this article wewill take a collective look at these two cycles in order toompare and contrast them, so that we can come to knowhe relative advantages and disadvantages of these cycles.

The 2 Cycles Compared

n the last article we compared the three cycles their

ompression ratio and heat rejection were kept constant forall cycles. In this article we will focus on peak pressure, peakemperature and heat rejection. The P-V and T-S diagrams ofhese three cycles for such a situation are drawnimultaneously as described below.

n the above diagrams the following are the cycles

Otto cycle: 1 2 3 4 1 Diesel cycle: 1 2 3 4 1

Remember that we are assuming the same peak pressuredenoted by Pmax on the P-V diagram. And from the T-Sdiagram we know that T3 is the highest of the peakemperature which is again same for all three cycles underonsideration. Heat rejection given by the area under 4 1

5 6 in the T-S diagram is also same for each case.

n this case the compression ratio is different for each cycleand can be found by dividing V1 with the respective V2
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volumes of each cycle from the P-V diagram. The heatupplied or added in each cycle is given by the areas asollows from the T-S diagram

Otto cycle: Area under 2 3 6 5 say q1 Diesel cycle: Area under 2 3 6 5 say q3

t can also be seen from the same diagram that q3>q2>q1

We know that thermal efficiency is given by 1 heatejected/heat supplied

Since heat rejected is same and we know the order ofmagnitude of heat supplied, we can combine this informationo conclude that

Thermal efficiency of these engines under givenircumstances is of the following order

Diesel>Otto

Hence in this case it is the diesel cycle which shows greaterhermal efficiency.

Four-stroke engineFour-stroke cycle used in gasoline/petrol engines. The right

blue side is the intake and the left yellow side is the exhaust.The cylinder wall is a thin sleeve surrounded by cooling

water.

Today, internal combustionengines in cars, trucks, motorcycles, aircraft, constructionmachinery and many others, most commonly use afour-troke cycle. The four strokes refer to intake, compression,ombustion (power), and exhaust strokes that occur during
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wo crankshaft rotations per working cycle of the gasolineengine and diesel engine.

The cycle begins atTop Dead Center (TDC), when the piston

s farthest away from the axis of the crankshaft. A strokeefers to the full travel of the piston from Top Dead CenterTDC) to Bottom Dead Center (BDC). (See Dead centre.)

1. INTAKE stroke: On the intake or induction stroke of thepiston , the piston descends from the top of the cylinder tohe bottom of the cylinder, reducing the pressure inside theylinder. A mixture offuel and air is forced by atmosphericor greater) pressure into the cylinder through the intake

port. The intake valve(s) then close.2. COMPRESSION stroke: With both intake and exhaustvalves closed, the piston returns to the top of the cylinderompressing the fuel-air mixture. This is known ashe compression stroke.

3. POWER stroke.: While the piston is close to Top DeadCenter, the compressed airfuel mixture is ignited, usually by

a spark plug (for agasoline or Otto cycle engine) or by theheat and pressure of compression (for a dieselycle or compression ignition engine). The resulting massive

pressure from the combustion of the compressed fuel-airmixture drives the piston back down toward bottom deadenter with tremendous force. This is known ashe power stroke, which is the main source of the

engine's torque and power.

4. EXHAUST stroke.: During the exhaust stroke, the pistononce again returns to top dead center while the exhaustvalve is open. This action evacuates the products ofombustion from the cylinder by pushing the spent fuel-air

mixture through the exhaust valve(s).
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History

The Otto cycleThe four-stroke engine was first patented by Alphonse Beaude Rochas in 1861. Before, in about 185457, two ItaliansEugenio Barsanti and Felice Matteucci) invented an enginehat was rumored to be very similar, but the patent was lost.

The request bears the no. 700 of Volume VII of the PatentOffice of the Reign of Piedmont. We do not have the text of

he patent request, only a photo of the table which containsa drawing of the engine. We do not even know if it was a newpatent or an extension of the patent granted three daysearlier, on December 30, 1857, at Turin."

The first person to actually build a car with this enginewas German engineer Nikolaus Otto. That is why the four-troke principle today is commonly known as the Otto cycle

and four-stroke engines using spark plugs often are called

Otto engines. The Otto cycle consistsofadiabatic compression, heat addition at constant volume,adiabatic expansion and rejection of heat at constantvolume. In the case of a four-stroke Otto cycle, there are alsoan isobaric compression and an isobaric expansion, usuallygnored since in an idealized process those do not play anyole in the heat intake or work output.

Design and engineering principles

Fuel octane ratingnternal combustion engine power primarily originates fromhe expansion of gases in the power stroke. Compressing theuel and air into a very small space increases the efficiency

of the power stroke, but increasing the cylinder compressionatio also increases the heating of the fuel as the mixture isompressed (following Charles's law).
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A highly flammable fuel with a low self-ignition temperaturean combust before the piston reaches top-dead-centerTDC), potentially forcing the piston backwards againstotation. Alternately, a fuel which self-ignites at TDC but

before the piston has started downwards can damage thepiston and cylinder due to the extreme thermal energyoncentrated into a very small space with no relief. This

damage is often referred to as engine knocking and can leado permanent engine damage if it occurs frequently.

The octane rating is a measure of the fuel's resistance toelf-ignition, by increasing the temperature at which it will

elf-ignite. A fuel with a greater octane rating allows for amuch higher compression ratio, virtually eliminating the riskof damage due to self-ignition.

Diesel engines rely on self-ignition for the engine to function.The premature ignition problem is solved by separatelynjecting high-pressure fuel into the cylinder shortly beforehe piston has reached TDC. Air without fuel can beompressed to a very high degree without concern for self-

gnition, and the highly pressurized fuel in the fuelnjection system cannot ignite without the presence of air.

Power output limitThe maximum amount of power generated by an engine isdetermined by the maximum amount of air ingested. The

amount of power generated by a piston engine is related to

its size (cylinder volume), whether it is a two-stroke or four-stroke design,volumetric efficiency, losses, air-to-fuel ratio,the calorific value of the fuel, oxygen content of the air and

speed (RPM). The speed is ultimately limited by materialstrength and lubrication. Valves, pistons and connecting

rods suffer severe acceleration forces. At high engine speed,physical breakage and piston ring flutter can occur, resultingin power loss or even engine destruction. Piston ring flutter

occurs when the rings oscillate vertically within the piston
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grooves they reside in. Ring flutter compromises the sealbetween the ring and the cylinder wall which results in a loss

of cylinder pressure and power. If an engine spins tooquickly, valve springs cannot act quickly enough to close thevalves. This is commonly referred to as 'valve float', and it

can result in piston to valve contact, severely damaging theengine. At high speeds the lubrication of piston cylinder wallinterface tends to break down. This limits the piston speed

for industrial engines to about 10 m/s.

ntake/exhaust port flow

The output power of an engine is dependent on the ability ofntake (airfuel mixture) and exhaust matter to move quicklyhrough valve ports, typically located in the cylinder head. Toncrease an engine's output power, irregularities in the intakeand exhaust paths, such as casting flaws, can be removed,and, with the aid of an air flow bench, the radii of valve porturns and valve seat configuration can be modified to reduceesistance. This process is called porting, and it can be done

by hand or with a CNC machine.

SuperchargingOne way to increase engine power is to force more air intohe cylinder so that more power can be produced from each

power stroke. This can be done using some type of airompression device known as a supercharger, which can be

powered by the engine crankshaft.

Supercharging increases the power output limits of annternal combustion engine relative to its displacement. Mostommonly, the supercharger is always running, but there

have been designs that allow it to be cut out or run atvarying speeds (relative to engine speed). Mechanicallydriven supercharging has the disadvantage that some of the

output power is used to drive the supercharger, while powers wasted in the high pressure exhaust, as the air has been
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ompressed twice and then gains more potential volume inhe combustion but it is only expanded in one stage.

TurbochargingA turbocharger is a supercharger that is driven by theengine's exhaust gases, by means of a turbine. It consists ofa two piece, high-speed turbine assembly with one side thatompresses the intake air, and the other side that is powered

by the exhaust gas outflow.

When idling, and at low-to-moderate speeds, the turbine

produces little power from the small exhaust volume, theurbocharger has little effect and the engine operates nearlyn a naturally-aspirated manner. When much more poweroutput is required, the engine speed and throttle opening arencreased until the exhaust gases are sufficient to 'spin up'he turbocharger's turbine to start compressing much more

air than normal into the intake manifold.

Turbocharging allows for more efficient engine operation

because it is driven by exhaust pressure that wouldotherwise be (mostly) wasted, but there is a design limitationknown as turbo lag. The increased engine power is notmmediately available, due to the need to sharply increaseengine RPM, to build up pressure and to spin up the turbo,before the turbo starts to do any useful air compression. Thencreased intake volume causes increased exhaust and spinshe turbo faster, and so forth until steady high power

operation is reached. Another difficulty is that the higherexhaust pressure causes the exhaust gas to transfer more ofts heat to the mechanical parts of the engine.

Rod and piston-to-stroke ratioThe rod-to-stroke ratio is the ratio of the length ofhe connecting rod to the length of the piston stroke. Aonger rod will reduce the sidewise pressure of the piston on

he cylinder wall and the stress forces, hence increasing
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engine life. It also increases the cost and engine height andweight.

A "square engine" is an engine with a bore diameter equal to

ts stroke length. An engine where the bore diameter isarger than its stroke length is an oversquare engine,onversely, an engine with a bore diameter that is smallerhan its stroke length is an undersquare engine.

ValvetrainThe valves are typically operated by a camshaft rotating at

half the speed of the crankshaft. It has a series ofcams alongts length, each designed to open a valve during theappropriate part of an intake or exhaust stroke.A tappet between valve and cam is a contact surface onwhich the cam slides to open the valve. Many engines useone or more camshafts above a row (or each row) ofylinders, as in the illustration, in which each cam directly

actuates a valve through a flat tappet. In other engine

designs the camshaft is in the crankcase, in which case eacham contacts a push rod, which contacts a rocker arm which

opens a valve. The overhead cam design typically allowshigher engine speeds because it provides the most directpath between cam and valve.

Valve clearance

Valve clearance refers to the small gap between a valve lifterand a valve stem that ensures that the valve completelyloses. On engines with mechanical valve adjustment

excessive clearance will cause noise from the valve train.Typically the clearance has to be readjusted each20,000 miles (32,000 km) with a feeler gauge.

Most modern production engines use hydraulic lifters toautomatically compensate for valve train component wear.Dirty engine oil may cause lifter failure.
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Energy balanceOtto engines are about 35% efficient in other words, 35% of

he energy generated by combustion is converted into usefulotational energy at the output shaft of the engine, while theemainder appears as waste heat.By contrast, a six-stroke

engine may convert more than 50% of the energy ofombustion into useful rotational energy.

Modern engines are often intentionally built to be slightlyess efficient than they could otherwise be. This is necessary

or emission controls such as exhaust gasecirculation andcatalytic converters that reduce smog andother atmospheric pollutants. Reductions in efficiency maybe counteracted with an engine control unit using lean burnechniques.

OTTO CYCLE VS DIESEL CYCLE

One cannot be non-fascinated by the performance of a carengine. This engine works. If it doesnt start running at themoment the key is turned, we instantly get shocked,paralyzed and aroused by disbelief.

Sure, the reason it works so well is simple - years ofperfecting.

Yet, there are so many basic things about gasoline and dieselengines that I learned only recently. I listed some of themhere, hoping that you will find something interesting also.
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Fuels

All engines are engineered around fuels, so first we have todiscuss fuels. Both, the gasoline (petrol) and the diesel arehydrocarbons. The only difference being the (average)ength of the hydrocarbon chain - diesel being longer. Thus,hey have different properties:

- gasoline is lighter (less dense) and less viscose thandiesel- gasoline vaporizes more readily than diesel at roomtemperature. This makes gasoline a bit more dangerous

because it easily creates enough-concentrated vaporsthat are easy to ignite into fire.- diesel is somewhat better in lubricating things thangasoline

n the engine, fuel is well mixed with air and burned.Therefore we have to say something about air-fuel mixtureproperties: the diesel-air mixture will ignite at lower

emperature than the gasoline-air mixture. The gasoline-airmixture can simply withstand higher temperature before self-gnition. This is a very important thing to remember. It is alsoa bit confusing.

n everyday experience, it seems that the gasoline will mucheasier engage into flame then the diesel. But remember thathis is only because the gasoline vaporizes more readily at

oom temperature. It means that, at room temperature,here is much higher concentration of gasoline vapors than

diesel vapors and therefore the gasoline will ignite moreeasily. However, when we consider same concentrations ofgasoline and diesel vapors, then the diesel will ignite easierlower temperature is needed to activate combustion). Insidehe engine, the temperature is high enough for both, the

gasoline and the diesel to vaporize completely and create

ich-enough (flammable) concentrations of vapor.
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Now we know something about fuels. Lets talk aboutengines.

Engine basics

Mister Rudolph Diesel was aware of the gasoline engine (Ottoycle) problems and wanted to improve it. The gasoline

engine inherently has problems with efficiency and/or fuel.

n order to improve the efficiency one must increase theompression ratio of an internal-combustion engine (see the

bonus section at bottom of this article). However, in the

gasoline engine there is a limit the gasoline-air mixture willelf ignite once the compression gets too high (because

every compression drives temperature increase). So, eitheryou can have a low-efficient, low-compression engine thatuses a cheap fuel, or you can have a high-efficient, high-ompression engine that uses expensive, high-refined fuelhat wont self-ignite even at high compression levels (a 120

octane gasoline?).

n diesel engine this problem is solved. The diesel engine canuse much higher compression levels than the gasolineengine reaching higher efficiency. In addition, the dieselengine can use fuel that is not nearly as refined as the high-octane gasoline fuel (thus cheaper).

To make this possible, Rudolph changed the Otto cycle and

reated the diesel cycle. The difference is that duringompression phase, no fuel is present in the cylinder andhus no self-ignition can happen. The fuel is only injected athe moment the ignition is wanted when injected into the

hot pressurized air the diesel fuel self-ignites immediatelythe diesel-air mixture, as we said already, is happy to ignite

even at relatively low temperatures).

The diesel fuel is better for a diesel motor because it self-gnites more readily, and this is desirable in the diesel cycle.
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f we try to inject gasoline instead of diesel into the dieselengine, this may not work because gasoline, having muchhigher self-ignition temperature, may not ignite at all. Ofourse, if we build the diesel engine to have a really, really

high compression ratio then even the 120 octane gasolinewill self-ignite and our engine will be able to work withalmost anything we put in our tank.

However, remember that the diesel fuel is better inubricating things than the gasoline, and diesel-enginemanufacturers use this property heavily so using even low-octane gasoline in your diesel engine may fail because fuel

upplying parts will not be lubricated enough. Dont kill yourengine!)

How about using diesel in a gasoline engine? Maybe it wouldbe possible to build an Otto-cycle engine that runs on dieseluel, but it would have to have very low compression ratio to

avoid diesel-air mixture self ignition problems. In turn, suchan engine would have very poor efficiency (the efficiency is

directly connected to the compression ratio). There isadditional problem with diesel it doesnt vaporize so readilyas the gasoline and because of it, such an engine would haveo use complicated system of high-pressure injection nozzleso generate rich-enough fuel-air mixtures (as in the diesel

engine).

Yes, the diesel engine has benefits (high efficiency, lower

uel restrictions), but it is much more complicated to buildhan the gasoline engine. The gasoline engine has sparkplugs to ignite gasoline-air mixture while the diesel engineneeds to have complicated system of high pressure injectionnozzles that need to inject controlled amount of fine fuel mistnto the cylinder at exactly right moment a quite difficultob (luckily, the diesel fuel is quite lubricating and non-abrasive so it is not an impossible job.)


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The diesel engine is much heavier than the gasoline engine his is because the higher compression ratio produces highertress on materials. In the diesel engine you will find thickylinders, heavy pistons, rods and valves. All this moving

mass restricts the speed a diesel engine can turn. A typicalar diesel engine doesnt turn faster than 4000rpm, while a

gasoline engine goes up to 7000rpm. (Also notice that thediesel fuel vaporizes quite slowly and, at high speed, therewon't be enough time for all the fuel to burn out, thus dieselengine efficiency will be reduced.)

Controlling power

There is one profound difference between diesel and gasolineengine. This is about the amount of air that comes into theylinder during the first phase of the cycle (intake).

n the gasoline engine, there is a special valve (throttle) that

estricts the amount of air-fuel mixture that is filled into theylinder. When the engine is idling, only a limited amount of

air-fuel mixture is allowed in. At full power, the valve is fullyopened allowing the cylinder to suck the maximum amountof the air-fuel mixture in. It means that the final pressureevel at the end of compression stroke highly variesdepending on how much air-fuel mixture was allowed intoylinder during the intake stroke.

Also, note that the concentration of the fuel in the air-fuelmixture is always the same, no mater what is the power levelof the engine. Thus, the mixture is always aimed at fullombustion (or more or less so read further about it). Theoncentration of fuel in this mixture is, of course, always high

enough to make self-sustained burning.

The diesel engine is another story. Here, the amount of airucked into the cylinder is always the same, invariable of the


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power level. However the power is controlled by the amountof the fuel that is injected into the cylinder. The good thing ishat the final pressure of the air (just before the fuel isnjected) is always the same and so the injected fuel will findnice temperature to burn even if the engine is running at lowpower level. (Even if a very small amount of fuel is injected maller than otherwise needed for self-sustained burning it

will all burn out because the surrounding air temperature ishigh enough).

Now we know how power is controlled in both engines. Sowhat can we do if we want to get more power?

We can improve efficiency. This is the greenest way. But thats for ladies. What we want is creating a monster.

The only other way is to burn more fuel in the same amountof time. The fuel is where the power comes from and if wewant power we have to burn it burn it fast. However, notonly that we have to increase supply of fuel, but we also

have to increase supply of air. Supply of air is the majorproblem (the air is bulky) and there are several ways to do it.

The first one is to make larger cylinders or to increasenumber of cylinders. So, in each cycle more air will come in,and we will be able to burn more fuel producing more powern each engine cycle. This approach works well for both,diesel and gasoline engines.

Second, we can put more air into the cylinder than it isylinders volume we simply force it inside by turbo-harging or super-charging. If we force two liters of air into

one litter cylinder we could get twice the power in eachtroke. But of course, as we already started with pressurized

air, at the end of the compression stroke we will end up atmuch higher pressure-level than in non-force-chargedengine. In the gasoline engine this will cause premature self-

gnition of the air-fuel mixture and our engine will break
http://en.wikipedia.org/wiki/Turbohttp://en.wikipedia.org/wiki/Turbohttp://en.wikipedia.org/wiki/Superchargerhttp://en.wikipedia.org/wiki/Turbohttp://en.wikipedia.org/wiki/Turbohttp://en.wikipedia.org/wiki/Supercharger


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down (except if we use a high-octane fuel that wont self-gnite). The diesel engine, however, doesnt have thisproblem. That is why turbo-charging or super-charging ismore popular way for boosting power on diesel engines thann gasoline engines. (In addition, the diesel engine is by itsnature built more robustly than gasoline so increasedpressure wont generate major problems to materials.)

Third way for getting more power would be to make theengine turn faster. Although the same amount of fuel will beburnt in each cycle, the number of cycles in unit of time willncrease and thus we get more power. Unfortunately, diesel

engines are very limited because of its heavy high-pressure-proof design. Increasing turning speed of a diesel enginewould break the engine apart (except if you use stronger,more expensive materials but there are not many thingsbetter than steel). However, the gasoline engine is muchghter every shaft, cam or rod inside a gasoline engine isght and can move very fast without self-breaking. Thusncreasing turning speed is the preferred method of gaining

power in the gasoline engine.

Emission control

When a pure hydrocarbon burns in oxygen only water andarbon dioxide is produced. Unfortunately, in the real lifehings are more complicated.

Neither gasoline nor diesel are pure and they both containonsiderable amounts of impurities. Especially diesel is

known for its high impurity level (maybe partially becausehe diesel engine can cope with quite low-refined fuels so no

high purification is needed in the first place).

Even more bad stuff happens when a hydrocarbon burns inhe air (nitrogen and oxygen mixture). If the combustion

akes its place at too high temperature the nitrogen will reactwith oxygen producing various nitro-oxides that are nasty to


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environment. So, in the internal-combustion engine it isessential to control the combustion temperature at theoptimum level.

n the gasoline engine it is fairly easy. As we told already, theoncentration of the air-fuel mixture is always the same (no

mater of the power level of the engine) and so we only needo keep this concentration somewhat below optimal to

decrease the combustion temperature. This ensures for cleanexhaust.

The diesel engine is more problematic. As the air-fuel

oncentration varies greatly depending on the power level ofhe engine, we cannot easily control the combustionemperature. Various tricks are used like restricting theombustion area to only a part of the cylinder or injecting theuel in several delayed intervals One regularly used trickeems very charming the EGR (Exhaust Gas Recirculation):

The combustion temperature will be lowered if a part of

oxygen in the air is replaced with some inert gas. Todecrease the concentration of the oxygen, some of theengine exhaust (concentrated with inert gases; carbon-dioxyde and water wapor) is supplied back at engine airnput. This controls the peek combustion temperature, butalso reduces the amount of excesive oxygen available forombining with nitrogen. The EGR is very useful, and is used

on both, diesel and gasoline engines.

Many of my friends with gasoline-powerd cars are puzzledwith the questions "why do they measure oxygen (O2) atmadatory emission tests?" and "why is it bad to have highoxygen level in the exhaust?". The fact is, as we discussedabove, that in the gasoline engine the oxygen to fuel ratio isalways almost-ideal and aimed for full combustion, invariableo the throtle level. Therefore, if there is much oxygen left, it

s a strong indication that the engine is sub-optimaly tuned.)
http://en.wikipedia.org/wiki/EGRhttp://en.wikipedia.org/wiki/EGR


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Bonus explanation: Why is diesel engine

more efficient than gasolines it because of a higher compression? Well, yes, it is, butonly indirectly... read below.

Some amount of heat energy will be generated duringombustion (aka expansion, aka power) stroke in an engine.

But not all of the generated energy will be transformed tomechanical form. Large part of the heat energy (maybe morehan half) will be expelled through the exhaust pipe andherefore lost. Because it uses higher compression ratio, the

diesel engine will be able to extract more mechanical energyrom heat (that is, the diesel will attain higher efficiency).

f you look at the engine in largely simplified way, you canee that the combustion stroke is a simple adiabatic process.

During the combustion stroke, you start with high pressureP1) and high temperature (T1) of the combustion gas in theylinder. Then the piston travels downward for some lengtho its bottom position, and now you have lower pressure (P2)

and lower temperature (T2) of the gas. Obviously, the energyontained in gas is now lower (it has lower temperature andower pressure, while its molar amount is the same). Thisdifference is converted to mechanical energy of piston.

You can also look at the above process following way: at thebeginning of the expansion stroke, the gas is compressed,hot and energetic and it pushes the piston downwardorcefully. Doing this, the gas becomes more and more tiredcolder and depressurized because it gives its energy tohe piston. Longer it pushes the piston, more of its energy is

given to the piston, but at the same time the gas becomes

more and more tired and pushes the piston less and lessenthusiastically that is, with weaker force. After some


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ime you simply have to give up and let the exhausted gasout because it would take the entire eternity for it to give allof its energy to the piston.)

Now, clearly it is possible to convert more heat energy tomechanical if the difference between P1,T1 and P2,T2 is

larger. The equation in the picture above also tells the same the difference between volumes V1 and V2 is what gives us

the mechanical work (energy). That is why diesel beatsgasoline the compression ratio in diesel engine is larger,therefore the volume difference between top and bottomposition is also larger, therefore more mechanical work is

extracted from heat during combustion (expansion) stroke.

But, during the compression stroke the diesel spends moreenergy for compression than gasoline? Yes, but it doesntmatter overall. All that energy that was put into the gasduring compression stroke will be recovered duringexpansion stroke (plus the energy that comes from fuel).magine an engine that simply doesnt fire the fuel as the

engine turns it neither spends nor produces energy. Duringompression it takes some, but during expansion it gives theame amount. No matter what the compression ratio is. The

higher compression ratio of the diesel really doesnt cost theenergy (in first approximation).

Why then there are no engines with compression ration1:zillion? Sure, such an engine would be efficient. In gasoline

engine the limitation is because of self-ignition of thegasoline-air mixture. In diesel engines the limitation is only inmechanical strength of materials (pistons, cylinders, valves,ods, crankshaft...).

What does engine power has to do with efficiency? Well, nothat much. Efficiency and power are largely unconnected.

However, in general, a less efficient internal combustion

machine can be made more powerful. How come? Well, itakes time for gas to give its energy to the piston and the


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gas gives it slower and slower as it gets colder. If you need apowerful engine, you dont have time to waste. You have toexpel the half-used gas and get the fresh one in quickly.

What is Atkinson-cycle? This is a way to improve inherentlyow efficiency of a gasoline engine. In gasoline engine youare limited about compression ratio. However if you had readhe above text more carefully, you can see that theompression ratio doesnt really matter what is importantor efficiency is the decompression ratio. Certainly, in

normal gasoline and diesel engines those two are the same.But in Atkinson-cycle, the compression stroke doesnt start at

he bottom point of the piston. No no. The valve only closeswhen the piston is already half-way up... At the top position,he gasoline-air mixture is compressed to its maximumactually to the same pressure level as in an ordinary

gasoline engine ). But during expansion stroke, it goes all theway to the bottom. However the disadvantage is that onlymaller amount of gasoline-air mixture is used in every cyclesmaller than in ordinary gasoline engine and so the power

of Atkinson cycle engine will be lower than the power of Otto-ycle engine of the same volume

Refrences:-

1. ^ Air pollution from motor vehicles By Asif Faiz,Christopher S. Weaver, Michael P. Walsh

Hardenberg, Horst O., The Middle Ages of the Internalcombustion Engine, Society of Automotive Engineers(SAE), 1999
http://en.wikipedia.org/wiki/Four-stroke_engine#cite_ref-0http://en.wikipedia.org/wiki/Four-stroke_engine#cite_ref-0


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. scienceworld.wolfram.com/physics/OttoCycle.html

. P.K.NAG ( TEXT BOOK )

. SCIENCE.NET ( OTTO CYCLE, DIESEL CYCLE )

rb4912b35 term paper thermodynamics

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