S.I ENGINE

S.I ENGINE

Theory:

In two engine the cycle is completed in 2- strokes of the piston or in one revolution of crankshaft. The preparatory strokes (suction and exhaust) are combined with the working strokes (compression and expansion). In 2- stroke engines, instead of valves ports are used for suction and exhaust purposes. As the moving parts are less; mechanical efficiency is high. But thermal efficiency of the engine is lower due to some charge escaping without burning, through the exhaust port. Also, in 2 –stroke engine s, as the power is available once in every revolution of the crankshaft, flywheel required is less in weight and small in size.

Two Stroke Petrol Engine:

Fig. 2.1 shows a two stroke petrol engine. It has no valves but consists of inlet or induction port (IP), exhaust port (EP), and a third port called the transfer port (TP). Referring to the fig. 2.1 (a) let the piston be nearing the completion of its compression stroke. The ignition starts due the spark given by the spark plug and the piston is pushed down (fig.

(b) and (c) ) performing the working strokes and in doing so the air fuel mixture already drawn from the inlet port in the previous stroke is compressed to a pressure of about 1.4 bar. When about 4/5th of this stroke is completed the exhaust port (EP) is uncovered slightly and some of the burning gases escape to the atmosphere. Immediately afterwards as the exhaust port is uncovered by the further downward movement of the piston, the transfer port which is only very slightly lower than exhaust port is also uncovered as shown in fig.(d) and a charge of compressed fuel air mixture enters the cylinder and further pushes out the burnt gases out of the exhaust port. The top of the piston is made of a particular shape that facilitates the deflection of fresh charge upwards and thus avoids its escape along with the exhaust gases. This process is known as scavenging.

After reaching the bottom dead center when the piston moves up, it first closes the inlet port, then transfer port and then exhaust port. The charge of fuel which previously entered the cylinder is now compressed. Simultaneously there is a fall of pressure in the crank case, creating a partial vacuum. When the piston is nearing the upward movement, the inlet port opens and a fresh charge of air fuel mixture from the carburetor enters the crank case. After the ignition of the charge, the piston moves down for the power stroke and the cycle is repeated as before.