Introduction to Two Stroke Engine

INTRODUCTION TO TWO STROKE GASOLINE ENGINE

Construction: A pist piston on reci recipr proc ocat ates es insi inside de the the cyli cylind nder er.. It is conn connec ecte ted d to the the crankshaft by means of a connecting rod and crank. There are no valves in two-stroke engines. Instead of valves, port (ways) is cut in the cylinder walls.  There are three ports namely inlet, exhaust and transfer ports. The closing and opening of the port is obtained by the movement of the piston. The crown of the piston is made of a particular shape. This is to deflect the fresh char charge ge (air(air-fu fuel el mixt mixtur ure) e) upwa upward rds s in the the cyli cylind nder er.. A sp spar ark k plug plug is also also provided in the cylinder. The events are given below.

Upward stroke of the piston (first stroke): a) Comp Compre ress ssio ion: n: 1. The pisto piston n moves moves up from from bottom bottom dead centre centre (BDC) (BDC) to top dead centre (TDC). 2. Both transfer transfer and and exhaust exhaust ports are covere covered d by the piston. piston. 3. AirAir- fuel fuel mixt mixtur ure e whic which h is tran transf sfer erre red d alre alread ady y into into the the engi engine ne cylinder is compressed by the moving piston.

4.  The pressure and temperature increases at the end of compression.

b) Ignition and inductance: 1. Piston almost reaches the top dead centre. 2. The air-fuel mixture inside the cylinder is ignited by means of an electric spark produced by a spark plug. 3. At the same time, the inlet port is uncovered by the piston.

4. Fresh air-fuel mixture enters the crankcase through the inlet port.

Downward stroke of the piston (second stroke): c) Expansion and crankcase compression: 1. The burning gases expand in the cylinder. 2. The burning gases force the piston to move down. Thus useful work is obtained. 3. When the piston moves down, the air-fuel mixture in the crankcase is partially compressed. This compression is known in crankcase compression. d) Exhaust and transference: 1. At the end of expansion, exhaust port is uncovered. 2. Burnt gases escape to the atmosphere. 3.   Transfer port is also opened. The partially compressed air-fuel

mixture enters the cylinder through transfer port. 4. The crown of the piston is made of a deflected shape. So the fresh charge entering the cylinder is deflected upwards in the cylinder.   Thus the escape of fresh charge along with exhaust gases is reduced. Again the piston moves up. The transfer port is first closed and then the exhaust port is closed. The compression of air-fuel mixture takes place inside the cylinder. When the piston is nearing top dead centre, the inlet port is uncovered. Fresh charge enters into the crankcase.

After compression, the ignition of air-fuel mixture takes place and the cycle is repeated.  Thus all the events (suction, compression, power and exhaust) are completed in two strokes of the piston. Hence this is known as two stroke cycle engine.

MODIFIED TWO STROKE GASOLINE ENGINE

 The schematic diagram of modified two stroke gasoline engine is shown in the above figure. In direct injection method the fuel is directly pumped into the cylinder at the time of compression stroke, when the exhaust port closed. Hence there is no wastage of fresh air fuel mixture. Complete combustion is possible, efficiency will be improved. Due to complete combustion unburnt gases are reduced. Due to this pollution also reduced.

Direct injection has considerable advantages in two-stroke engines, eliminating some of the waste and pollution caused by carbureted two-strokes where a proportion of the fuel/air mixture

entering the cylinder goes directly out, unburned, through the exhaust port. Two systems are in use, low-pressure air-assisted injection, and high pressure injection.

Port timing diagram for two stroke gasoline engine

 Two stroke cycle engines are provided with ports. The timing sequence of events opening and closing of various ports are shown graphically in terms of crank angles from dead centre positions. The figure shows that port timing diagram for two stroke cycle SI engines. The inlet port is opened by the piston 45º to 55º before TDC. The inlet port is closed by the piston 45º to 55º after TDC position.  The exhaust port is opened by the piston 65º to 75º before BDC. The exhaust port closed by the piston 65º to 75º after BDC position. The transfer port is opened by the piston 50º to 65º before BDC. The transfer port is closed by the piston 50º to 65º after BDC. Ignition occurs at 15º to 25º before TDC.

Modified port timing diagram