Four-Stroke Cycle Diesel Engine:
In four-stroke cycle engines there are four strokes completing two revolutions of the crankshaft. These are in that order, the suction, compression, power and exhaust strokes. In Fig.1, the piston is shown descending on its suction stroke. Only pure air is drawn into the cylinder during this stroke through the inlet valve, while the exhaust valve is closed. These valves are operated by the following elements Viz; cam, push rod and rocker arm. The next stroke is the compression stroke in which the piston moves up with both the valves remaining closed. The air, which has been drawn into the cylinder during the suction stroke, is progressively compressed as the piston ascends. The compression ratio generally varies from 14:1 to 22:1.
The pressure at the end of the compression stroke ranges from 30 to 45 kg/cm2. As the air is progressively compressed inside the cylinder, its temperature increases rapidly, until when near the end of the compression stroke, it becomes sufficiently high (6500C-8000C) to instantly ignite any fuel that is injected into the cylinder. When the piston is close to the top of its compression stroke, a liquid hydrocarbon fuel, such as diesel oil, is injected into the combustion chamber under high pressure (140-160 kg/cm2), higher than that existing pressure inside the cylinder itself. This fuel then ignites, being burnt with the oxygen of the highly compressed air.
During the fuel injection time, the piston reaches the end of its compression stroke and commences to return on its third consecutive stroke, viz., power stroke. During this stroke the hot products of combustion consisting chiefly of carbon dioxide, together with the nitrogen left from the compressed air expand, as a result forcing the piston downward. This is only the working stroke of the cylinder. During the power stroke the pressure falls from its maximum combustion value (47-55 kg/cm2), which is usually higher than the greater value of the compression pressure (45 kg/cm2), to about 3.5-5 kg/cm2 near the end of the stroke. The exhaust valve then opens, usually a little earlier than when the piston reaches its lowest point of travel. The exhaust gases are swept out on the following upward stroke of the piston. The exhaust valve remains open throughout the whole stroke and closes at the top of the stroke.
Fig.1 Principle of four-stroke engine
Two-stroke cycle diesel engine:
The cycle of the four-stroke of the piston is completed only in two strokes in the case of a two-stroke engine. The air is pinched into the crankcase due to the suction created by the upward stroke of the piston. On the down stroke of the piston it is compressed in the crankcase, The compression pressure is usually very low, being just sufficient to enable the air to flow into the cylinder through the transfer port when the piston reaches near the bottom of its down stroke. The air thus flows into the cylinder, at the area where the piston compresses it as it ascends, till the piston is nearly at the top of its stroke. The compression pressure is enlarged sufficiently high to raise the temperature of the air above the self-ignition point of the fuel used. The fuel is injected into the cylinder head just before the completion of the compression stroke and only for a short period. The burnt gases expand throughout the next downward stroke of the piston.
Fig.2 Principle of two-stroke cycle diesel engine
Four-Stroke Spark Ignition Engine:
In Four-Stroke Spark Ignition Engine gasoline is mixed with air, derelict up into a mist and partially vaporized in a carburettor. The mixture is then taken into the cylinder by suction process. There it is compressed by the upward movement of the piston and is ignited by an electric spark. When the mixture is burned, the resulting heat causes the gases to expand. The expanding gases exert a pressure on the piston (power stroke). The exhaust gases run off in the next upward movement of the piston. The strokes are similar to four-stroke diesel engines.
Fig.3 Principle of operation of four-stroke petrol engine
Two-Stroke Cycle Petrol Engine
The two-cycle carburettor type engine makes use of an airtight crankcase for incompletely compressing the air-fuel mixture. As the piston travels down, the mixture previously drawn into the crankcase is incompletely compressed. As the piston nears the bottom of the stroke, it uncovers the exhaust and intake ports. The exhaust flows out, dropping the pressure in the cylinder. once the pressure in the combustion chamber is lesser than the pressure in the crankcase through the port openings to the combustion chamber, the incoming mixture is deflected upward by a baffle on the piston. When the piston moves up, it compresses the mixture above and draws into the crankcase below a new air-fuel mixture. The, two-stroke cycle engine can be easily identified by the air-fuel mixture valve attached to the crankcase and the exhaust Port located at the bottom of the cylinder.
SI Engine CI Engine