This engine works on Otto cycle and uses petrol (or gas) as a fuel. In this engine spark is produced to ignite the charge. This type of engines work on constant volume combustion cycle as combustion takes place nearly at constant volume with increase of pressure.
In a petrol engine, the petrol is evaporated and also it is mixed with correct proportion of air by the device which is known as carburetor.
The whole cycle is completed in four strokes, namely
- Suction stroke
- Compression stroke
- Power stroke
- Exhaust stroke
Suction stroke
- During this stroke, inlet valve remains open and exhaust valve is closed, the pressure in the cylinder will be atmospheric.
- The piston moves from TDC to BDC. So the volume in the cylinder increases, while simultaneously the pressure decreases. This creates a pressure difference between the atmosphere and inside of the cylinder. Due to this pressure difference the petrol and air mixture will enter into the cylinder through carburetor.
- At the end of this stroke piston reaches at BDC, the crank shaft rotates and the cylinder will be filled completely with petrol and air mixture called charge and inlet valve is closed.
Compressoin stroke
- During this stroke both the inlet valve and exhaust valve remains closed. Piston moves from BDC to TDC and the crank shaft rotates . As this stroke is being performed, the petrol and air mixture contained in the cylinder will be compressed, so pressure and temperature of mixture increases.
- Near the end of this stroke, the petrol and air mixture is ignited by electric spark given out by the spark plug. The combustion of the petrol releases the hot gases which will increase the pressure at constant volume.
Power stroke
- During this stroke both the inlet valve and exhaust valve remain closed, the piston moves from TDC to BDC. The high pressure and high temperature burnt gases force the piston to perform this stroke, called power stroke. This stroke is also known as expansion or working stroke. The engine produces mechanical work or power during this stroke.
- As the piston moves from TDC to BDC, the crank shaft rotates 1800 and the pressure of hot gases gradually decreases and volume increases.
- Near the end of this stroke, the exhaust valve opens which will release the burnt gases to the atmosphere. This will suddenly bring the cylinder pressure to the atmospheric pressure.
Exhust stroke
- During this stroke, the exhaust valve opens and the inlet valve remains closed. The piston moves from BDC to TDC, the crank shaft rotates and during this motion piston pushes the exhaust gases (combustion product) out of the cylinder at constant pressure.
- At the end of the exhaust stroke, the exhaust valve is closed and inlet valve will open. At the end of the four strokes crankshaft rotates or two complete revolutions. Then there will be again a suction stroke and the same cycle will be repeated.
