Internal Combustion

Engine

The development of the internal combustion engine was made possible by the earlier development of the STEAM ENGINE. Both types of engines burn fuel, releasing energy from it in the form of heat which is then used to do useful work. The steam engine, however, is an external combustion engine, because the fuel is burned in a separate part of the engine from the cylinder containing the piston. Anything combustible can be used as a fuel in the steam engine, such as wood, coal or petroleum products, and the liberated energy is used to heat a fluid, usually water. The hot water vapour expands in a confined place (the cylinder) to push the piston. In the internal combustion engine, the burning of the fuel takes place in the combustion chamber (the top of the cylinder). The combustion is very sudden, amounting to an explosion which pushes the piston.

Introduction

During the eighteenth and nineteenth centuries, as the steam engine was made more efficient, advancements were made in engineering and metallurgy which made possible the first successful internal combustion engines. The operation of steam engines was not fully understood at first; the French physicist Sadi CARNOT published in 1824 his theories which led to the science of heat exchange (THERMODYNAMICS). Fifty years before that James WATT had already begun to develop packings and piston rings to prevent the escape of energy past the piston in his steam engines. By 1800, the British engineer Henry Maudslay was making improvements to the lathe which led to machinery capable of producing precision-made parts for engines. In the 1850s more volatile fuels were being refined from petroleum.   In 1860 J J E Lenoir, a French engineer, built a successful engine which was essentially a modified steam engine, using illuminating gas as the fuel. In 1867 the firm of Otto and Langen began producing an engine which transmitted the power of a freely moving piston to a shaft and a heavy flywheel by means of a rack-and-gear device, using a FREEWHEELING clutch in the gear, so that it

History

Meanwhile, in 1865 Alphonse Beau de Rochas had published in Paris his theory of a four-stroke engine of the type used in the modern car. While de Rochas never built any engines, his theory included compression of the fuel mixture in order to raise its temperature, and he also realized that a four-stroke design would be more efficient at scavenging (intake of fuel mixture and exhaust of burned gases) than the two-stroke.   A two-stroke engine provides for intake of fuel, combustion and exhaust of burned gases with each back-and-forth motion of the piston (that is, with each revolution of the crankshaft). A four-stroke engine requires four strokes, that is, two complete back-and-forth movements of the piston (two revolutions of the crankshaft). The two-stroke engine delivers twice as many power impulses as the four-stroke engine to the cranks haft, but the four-stroke is much more efficient at scavenging, if all other things are equal. The two-stroke design is also wasteful because unburned fuel is exhausted with the burned gases.   In 1876, Otto and Langen began building the Otto ‘silent’ engine (it was a good deal quieter than their earlier model). It was the first modern internal combustion engine, a four-stroke design which compressed the fuel mixture before combustion. After 1878, it was also manufactured in the United States, where it was an inspiration to Henry FORD in his early research.

History

PARTS Cutaway view of a typical four cylinder small-car engine with conventional pushrod-operated overhead valves.

Parts

The four-stroke cycle operates as follows: on the first downstroke of the piston, the intake valve opens and the fuel mixture is pulled into the combustion chamber. On the following upstroke, all valves are closed and the fuel mixture is compressed. At the beginning of the second downstroke, combustion takes place; the fuel mixture is ignited by a spark from the SPARK PLUG and the expanding gases drive the piston downwards. On the second upstroke, the exhaust valve opens and the burned gases are expelled. Thus the four parts of the cycle are intake, compression, combustion and exhaust.   The fuel mixture is a mixture of fuel and air in the form of a vapour which is prepared by the carburettor. The fuel is usually petrol [gasoline], but internal combustion engines of various types can be designed to run on anything from paraffin [kerosene] to high-test aviation fuel. TI e carburettor must be adjusted properly; if the mixture is too lean (does not contain enough fuel), the engine will not run properly; if it is too rich, the result will be carbon deposits fouling the spark plugs, the valves and the inside of the combustion chamber, wasting fuel and affecting the performance of the engine.

Four-Stroke

How a four-stroke engine of this type works. This one is shown with twin overhead camshafts for clarity. Camshafts turn at half engine speed, opening each valve once in the cycle. Intake: the rotating crankshaft pulls the piston down, creating low pressure in the cylinder. The inlet valve opens and mixture enters. Compression: the crankshaft raises the piston, compressing the mixture. At the top of the stroke, the spark plug fires. Power: the mixture, ignited by the spark, expands, forcing the piston down and turning the crankshaft to give power. Exhaust: the piston rises again. The exhaust valve opens, so that the burned gases are forced out of the cylinder.

Four-Stroke

The two-stroke engine must accomplish intake, combustion and exhaust in one back-and- forth movement of the piston. Since scavenging is incomplete and inefficient, the proper mixture is difficult to obtain. Small two-stroke engines such as are still used in some motorcycles, lawn mowers and small cars must have oil added to the petrol, and constitute an air pollution problem; the blue smoke from the exhaust pipe of these engines is one of their familiar characteristics. One way of improving the scavenging of the two-stroke engine is to build opposing pistons, which reciprocate in opposite directions and share a common combustion chamber. This design was chosen by Henry Ford for his first car, which was built in 1896. A big disadvantage is that each piston must drive a separate crankshaft, and the motion of the two crank- shafts must then be combined through a system of gearing. Another way of improving the scavenging of a two-stroke engine is to use a turbocharger, which is a SUPERCHARGER driven by the energy of the exhaust gases, and resembles a pump for blowing air into the cylinder. This is combined with a FUEL INJECTION SYSTEM instead of the usual carburettor. The modern DIESEL ENGINE pulls in only air on the intake, and compresses it to between a twelfth and a twenty-fifth of its original volume, compared to a sixth to a tenth for compression in a petrol engine. This raises the temperature of the air to over 1000°F ( 38°C). At this point the fuel is injected and ignites spontaneously, without the need for a spark plug. Diesel engines may be of two- or four-stroke design (both types can be turbo- charged), though most road-going diesels are four-stroke.

Two-Stroke

A two-stroke engine, as used on small motorcycles, which uses both the cylinder and the crankcase below the piston in its operation. While the piston rises to compress the mixture above it, it creates low pressure below it, drawing mixture into the crankcase through the inlet port, which is uncovered by the piston (there are no valves). When the piston is at the top, the mixture in the cylinder is ignited, driving the piston down to open the exhaust port. While the exhaust gases leave, mixture, compressed by the falling piston, is forced up the transfer port into the cylinder. A shaped piston improves separation of mixture and exhaust.

Two-Stroke

The block, the head and the crankshaft are all castings that require extensive machining before the engine can be assembled (some larger crankshafts are forgings, for extra strength). The cylinders must be bored and finished precisely in the block. The top of the block and underside of the head are planed or milled to fit smoothly together, and the tops of the compression chambers are also machined on the underside of the head—except in ‘bowl in piston’ designs, where the head is flat and the tops of the pistons are recessed. Both the block and the head must have numerous surfaces machined and holes drilled and tapped where various components will be mounted. Where the head is bolted to, the block, a head gasket is included in order to prevent escape of compression in the assembled engine.   The underside of the block is open; at the bottom of each cylinder wall, bearing surfaces are machined to accept the main bearings of the crankshaft. Bearing caps are screwed down to hold the crankshaft in place. The pistons slide up and down in the cylinders and are connected to the crankshaft by means of connecting rods which pivot in the pistons and turn on the throws of the crankshaft. A sump [oil pan] made of sheet metal or cast light alloy is screwed to the bottom of the block, covering the crankshaft; a gasket is included to prevent leakage of oil, and there is also an oil seal at each end of the crankshaft where it protrudes from the block.

Design Aspects

Above: A 1928 Bugatti straight 8 engine. The eight exhaust ports converge into two exhaust pipes.

Design AspectsThe crankshaft itself is a mechanical adaptation of the hand crank, used for centuries to operate simple machines such as early lathes. For each cylinder in the engine there is a separate throw (offset section forming a crank) which revolves around the axis of the crankshaft, pushed by the operation of the piston when the engine is running. Opposite each throw on the crank-shaft is a web(a mass of metal) to balance it. The throws in a multi-cylinder engine are arranged equidistant around the circle described by their revolution, and the f ir ing sequence of the combustion chambers, which depends on the crank position, is timed in such a way as to balance the engine and provide for smooth running. Internal combustion engines have been built with as many as sixteen cylinders or more, in several configurations: opposed, radial, V-formation, and in-line (all in a row). The most common type of engine today is the in-line four or six cylinder engine used in cars; V4 engines are also common, especially in American cars.

On the front of the crankshaft, where it protrudes from the engine, is mourned a pulley wheel from which are operated, by means of a belt, the DYNAMO (generator or alternator, and the water pump, if the engine is water-cooled. The crankshaft also drives the oil pump (for lubrication of the engine) by means of a skew gear. Also mounted on the crankshaft is the timing gear. The timing gear is a pair of gearwheels, or a sprocket linked by a chain to a smaller sprocket, which turns the camshaft, which is generally located in the block, at half crankshaft speed. The camshaft in turn operates the valves, and also drives the DISTRIBUTOR to spark the mixture at the correct moment. If the valves are located in the block, the engine is a side-valve, valve-in-hand, flat head or L-head design. In this case the valve stems ride directly on the h. If the engine is an overhead valve design, the valves are operated from the camshaft by means of an assembly of pushrods and rocker arms, and access to the valves for repairs is more easily obtained by removing a sheet metal cover on top of the head, instead of having to remove the head itself. In yet another variation, the overhead cam, the camshaft is also located on top.   In all cases, the valves are operated against spring pressure; there are at least two valves for each cylinder (intake and exhaust); and the adjustment of the timing gear is vital to the performance of the engine. Some high performance engines have four valves to a cylinder; some aero engines have sleeve valves, in which a tubular sleeve with holes in it covers and uncovers the ports.

Design Aspects