PETROL ENGINES

Classification of Petrol Engines

•         Two Stroke cycle Petrol Engines 

•         Four Stroke cycle petrol Engines

TWO STROKE CYCLE PETROL ENGINES:

Two Stroke Cycle Petrol Engine – Construction

Construction :

•         A piston reciprocates inside the cylinder 

•         It is connected to the crankshaft by means of connecting rod and crank 

•         There are no valves in two stroke engines, instead of valves ports are cut on the cylinder walls.

•         There are three ports, namely inlet, exhaust and transfer ports. 

•         The closing and opening of the ports are obtained by the movement of piston. The crown of piston is made in to a shape to perform this.

•         A spark plug is also provided.

First Stroke : (Compression, ignition and inductance) (Upward stroke of piston)

(a) compression: 

•         The piston moves up from Bottom Dead Centre (BDC) to Top Dead Centre (TDC) 

•         Both transfer and exhaust ports are covered  by the piston. 

•         Air fuel mixture which is transferred already into the engine cylinder is compressed by moving piston.

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

First Stroke : (Compression, ignition and inductance) (Upward stroke of piston)

(b) Ignition and Inductance:

•                      Piston almost reaches the top dead centre 

•                     The air fuel mixture inside the cylinder is ignited by means of an electric spark 

•                       produced by a spark plug 

•                     At the same time, the inlet port is uncovered by the plane. 

•                     Fresh air fuel mixture enters the crankcase through the inlet port.

Second Stroke: (Downward Stroke of the engine) : (c)Expansion and Crankcase compression

•         The burning gases expand in the cylinder 

•         The burning gases force the piston to move down. Thus useful work is obtained. 

•         When the piston moves down, the air fuel mixture in the crankcase is partially compressed. This compression is known as Crank case compression.

Second Stroke: (Downward Stroke of the engine) :

(d) Exhaust and transfer: 

•         At the end of expansion, exhaust port is uncovered. 

•         Burnt gases escape to the atmosphere. 

•         Transfer port is also opened. The partially compressed air fuel mixture enters the cylinder through the transfer port. 

•         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 the exhaust gases is reduced.