05/10/2020
The 4 Strokes of the Engine:
1. Intake Stroke: The combustion starts with the piston at the top dead center or TDC position. The piston now starts to move down. Just before the piston begins its downward motion, the intake valve opens up. As the piston moves down, it sucks in the fresh air-fuel mixture from the manifold. As the piston reaches bottom dead center or BDC, the chamber fills up with air-fuel mixture.
2. Compression Stroke: Once the piston has reached BDC, the compression stroke begins. Just before the piston reaches the lowermost position, the intake valve closes. Now the piston moves upwards. As it moves up, it compresses the air-fuel mixture as it has no place to escape with the closed valves.
3. Power Stroke: Just before the piston reaches the topmost position in the compression stroke, the spark plug mounted on the cylinder head lets off a very tiny spark. When this spark comes into contact with the compressed air-fuel mixture, it ignites the mixture. Once ignited, the flame rapidly expands. Since the valves are still closed, the flame has no place to escape and pushes the piston downwards. This is the power stroke where usable power is generated by the motion of the piston.
*Note– Diesel Engines don’t have spark plugs. Instead, the fuel injector sits in that position. On diesel engines, the combustion mechanism is slightly different. Only hot air is sent to the combustion chamber during the intake stroke. This air is then compressed which causes it to heat up even more. During the power stroke, the injector sprays the fuel which on coming into contact with the hot air catches fire and starts the combustion. The remaining cycle is the same as a petrol engine.
4. Exhaust Stroke: Last comes the exhaust stroke. The piston with the momentum gained from the previous stroke starts to move back upwards. As it begins to move, the exhaust valve opens up. The leftover gases from the combustion process are pushed out. With this one 4-stroke cycle is completed. After this, the piston again moves from TDC to BDC and the cycle restarts.
