Internal combustion (IC) engine As the name suggests in such engines the combustion of the fuel takes place within the engine. More common IC engines are the four stroke gasoline engine, the Diesel engine and two stroke gasoline engine.
The four stroke gasoline engine are used in passenger cars aeroplanes etc. In each cycle of the engine four processes take place each associated with the movement of the piston called stroke. In fig. i5a-d the four strokes are illustrated.
(i) The intake stroke :- The intake valve in the left opens and the piston moves downwards drawing air and gasoline vapour in to the cylinder.
(ii) Compression stroke :- The intake valve closes and the piston moves upwards compressing the air gasoline mixture.
(iii) Power stroke :- The ignition system (spark plug) ignites the gasoline / air mixture which creates high pressure on the piston. The piston is pushed downwards.
(iv) The exhaust stroke :- The exhaust valve right, opens and piston moves upwards forcing the burned gases out of the cylinders.
The gasoline engine cycle is best approximated by the Otto cycle shown in fig. i6. The portion ab corresponds to the adiabatic compression, bc to the burning of the fuel, cd to the power stroke (adiabatic expansion), and da to the exhaust stroke.
The work output is the enclosed area abcd. Heat supplied to the engine is at constant volume V2 and heat rejected by the engine is at volume V1. The ratio, r = V1 / V2 is called the compression ratio which is the crucial parameter deciding the efficiency of the engine. The efficiency, h is given by,
h = 1 - (1 / rg -1)
where g is the ratio of specific heat at constant pressure to specific heat at constant volume (» 1.4). r has to be kept £ 10, because if r exceeds 10, the rise in temperature upon compression of the air gasoline mixture is large enough to cause an explosion.
The Diesel engine is more commonly used for trucks, tractors, earth movers, etc. In Diesel engine air is compressed adiabatically to a pressure high enough, so that the temperature exceeds ignition temperature of the fuel, which is sprayed inside the engine when the compression is complete. Compression ratio of 12:1 to 20:1 are used generally because there is no problem due to pre-ignition. The compression pressure attained is over 4 mega pascals.
The two stroke gasoline engine, which are generally used for small units where fuel economy is not as important. The engine does not have mechanically operated valves; the piston opens and closes the valve as it moves in the cylinder. It requires one revolution of the crankshaft for one complete combustion cycle.