Four-stroke engine petrol SI engine
In four-stroke cycle engines, one working cycle is completed in four-stroke of the piston movements or two revolution of the crankshaft. Hence it is called four-stroke engine. In four-stroke engine, 2 valves are placed instead of port in two-stroke engines. These are called as inlet and exhaust valves.
Four-stroke engines are classified as
(a) four stroke petrol (SI) Engine
(b) four-stroke diesel (CI) Engine
Working of four stroke petrol SI engine
It consists of the following four strokes
1. Suction stroke
2.compression stroke
3.power stroke
4.Exhaust stroke
Suction stroke
Suction stroke starts when piston is at top dead center and about to move downwards.
The inlet valve is open at this time and the exhaust valve is closed
. Due to the suction created by the motion of the piston towards the BDC, Air alone is inducted during the suction stroke.
Compression stroke
Engine pushes piston into cylinder Air is compressed to high pressure and temperature (700psi, 540°C). Fuel injected as piston reaches top of stroke Engine does work on the gases during this stroke
Power stroke
• At the end of compression stroke, the fuel is injected into the cylinder in the form of fine spray through the nozzle and is ignited by the temperature of hot compressed air in the chamber.
So that combustion process is started at the end of this stroke.
• The combustion of gases expands inside the cylinder so that piston start to move towards BDC.
• Both the valves remain closed during this stroke
Exhaust stroke
In this stroke exhaust valve is opened when piston reaches to BDC & moves to upward. Piston pushes out the burnt gases to the atmosphere through the exhaust valve. Hence called exhaust stroke & the engine is ready to begin the cycle again.