In this engine the cycle is completed in two strokes of the piston or one revolution of the crankshaft (in four stroke engine crank rotates through two revolution in one cycle).
(a) Ports:
This engine has no valves but there are three ports inlet, exhaust and transfer ports as shown. These ports get opened and closed due to the position of the piston.
(b) Piston:
The piston has a deflector on its crown which prevents loss of fresh charge while allowing the burnt gases to be exhausted.
(c) Crank Case:
Charge is compressed in the crank case when the piston is coming down and is transferred to cylinder via. transfer port.
Working:
Its working is shown in Fig. 24.4.
(a) Starting from top dead centre position (TDC), as the piston goes downwards first the exhaust port gets uncovered. The burnt gases are going out through this port.
(b) With further downward movement of the piston the inlet port is opened. Fresh air fuel mixture is admitted into the crank case. The downward movement of the piston acts as pump and slightly compresses the fresh charge in the crank case.
(c) Further movement of the piston uncovers the transfer port allowing the air—fuel mixture charge to be admitted into the cylinder.
(d) When the piston is going upwards both the transfer and exhaust ports are covered and the charge is compressed. Just before the TDC, the spark plug gives spark to initiate combustion and the combustion develops fully when the piston is in the TDC. Due to high pressure and temperature of the gases the piston is pushed downwards.
(e) Points to be noted are that as the fresh charge is being admitted burnt gases are going out. Specific shape of the piston crown prevents to some extent that the fresh charge being let out along with the exhaust but total prevention is difficult.
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