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TWO-STROKE ENGINE OPERATION
The two-stroke petrol engine is particularly suitable for motorcycles, having few
moving parts it is subjected to less wear and consequently is more reliable in
operation, its working action is accomplished in a single crankshaft revolution
(i.e. two piston strokes).
1. PISTON MOVES UPWARDS
The piston closes first the transfer ports, then the exhaust port and causes
compression of the mixture in the compression space of the cylinder head. A few
moments before the piston reaches its T.D.C. position the compressed mixture be
ignited by the electric spark from the sparking plug.
In the meantime a vacuum (underpressure) is created underneath the piston,
causing induction of fresh mixture from the carburetter to in the interior of the
crankcase.
2. PISTON MOVES DOWNWARDS
After ignition of the mixture the actual working stroke of the piston begins
(transmitting the power of the expanding gases by means of the crankshaft
mechanism and transmission to the motorcycle rear wheel). The top edge of the
piston opens first the exhaust port and the exhaust of the burnt gases takes place.
The top edge of the piston then opens the two transfer ports. Under the piston
and in the interior of the crankcase is fresh mixture, compressed by the piston
during its downward stroke. Through open transfer ports this fresh mixture will
now flow into the cylinder directed by the shape of the ports: the two streams
meet, reach the opposite cylinder wall, proceed towards the cylinder head which
directs them to the exhaust ports side. The fresh mixture fills the cylinder,
pressing at the same time the remaining burnt mixture out (scavenging).
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