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Engin
Four-stroke engin
The internal combustion engine is a machine that develops power from th
combustion of fuel within a cylinder. The cycle of operation is as follows
a) Suction Stroke - Just before the piston reaches the inner dead – centre,
valve, usually of the "poppet" type, is opened
On its outward stroke, the piston draws into the cylinder an explosiv
mixture of air and fuel vapour
Shortly before the piston reaches the outer dead - centre, the inlet valve i
closed and the compression stroke begins
1. Compression stroke- During this stroke, the whole of the gas in th
cylinder is compressed into the free space at the head of the cylinder
2. Explosion, or Working Stroke - When the compression stroke is almos
complete, the explosive mixture is ignited by an electric spark or by othe
suitable means. The gases reach their maximum pressure almost immediately
and work is done until the piston has again reached a position just before th
outer dead - centre, when a second exhaust valve is opened
If the ignition takes place too late, the piston begins its outward journe
before the maximum pressure is developed and power is lost
If, on the other hand, the explosive mixture is ignited too early, th
maximum pressure is reached before the piston has completed its inwar
journey, and engine is slowed down or even stopped
d) Exhaust Stroke- The burnt gases are driven out in front of the and it
return until the inlet valve again opens, when the cycle is repeated
Two-Stroke engine
This engine differs from the four-stroke engine in having one workin
stroke during each complete revolution of the crankshaft
During the inward stroke of the piston a port leading to the crankcase i
uncovered and the explosive mixture of air and fuel is drawn into the crankcase
On beginning its outward stroke, the inlet port is soon covered, and the mixtur
is compressed in the crankcase to pressure of about 20 lb. per sq. in. As the oute
dead - centre is approached the exhaust port is opened, and shortly afterward
another port opens, which allows the partially compressed charge of gas to ente
the cylinder, driving out the exhaust gas before it. A baffle in the piston to
directs the unburnt charge to the top of the cylinder, thus preventing undue los
through mixing with the exhaust gas
The gas is compressed on the inward stroke and is exploded as the pisto
reaches inner dead-centre.