1. Intake Stroke (Suction):
- The piston moves downward from Top Dead Center (TDC) towards Bottom Dead Center (BDC).
- The intake valve opens, allowing an air-fuel mixture (gasoline engines) or fresh air (diesel engines) to be drawn into the cylinder by the increasing volume created by the descending piston.
- The exhaust valve remains closed throughout the intake stroke.
2. Compression Stroke:
- The piston moves upward from BDC towards TDC.
- Both the intake and exhaust valves are closed.
- The trapped air-fuel mixture (or air in diesel engines) is compressed by the rising piston, significantly increasing its pressure and temperature.
3. Combustion Stroke (Power Stroke):
- Shortly before TDC on the compression stroke, a spark plug ignites the compressed air-fuel mixture in gasoline engines. In diesel engines, the high pressure and temperature cause the injected fuel to spontaneously combust.
- The combustion process rapidly expands the hot gases, forcing the piston down with significant force, creating the power output of the engine.
- Both the intake and exhaust valves remain closed throughout the combustion stroke.
4. Exhaust Stroke:
- The piston moves upward from BDC towards TDC.
- The exhaust valve opens.
- The burned exhaust gases are pushed out of the cylinder by the rising piston.
- The intake valve remains closed throughout the exhaust stroke.
Repeating Cycle:
- Once the exhaust stroke is complete, the piston reaches TDC again, and the cycle repeats, continuously drawing in fresh air/fuel mixture, compressing it, combusting it for power generation, and expelling the exhaust gases.
Key Points:
- The crankshaft is connected to the piston by a connecting rod and converts the reciprocating motion of the piston into rotary motion for driving the wheels or other applications.
- The four strokes occur in two complete revolutions of the crankshaft.
- Precise timing of valve opening and closing is crucial for efficient engine operation and is controlled by a camshaft or other valve actuation mechanisms.