Four Stroke Petrol Engine (Spark Ignition Engine)
A four-stroke petrol engine operates on the Otto Cycle, which is a constant volume cycle. The engine completes a power cycle in four piston strokes, requiring two full rotations of the crankshaft (720°).
The charge is a mixture of air and petrol, typically prepared by a carburettor, and the ignition is initiated by a spark plug.
Main Components:
- Cylinder
- Piston
- Connecting Rod
- Crankshaft
- Inlet & Exhaust Valves
- Spark Plug
- Camshaft (valve control)
Working Principle – Four Strokes
1. Suction Stroke (Intake Stroke)
- 🔹 Valves: Inlet valve open, exhaust valve closed
- 🔹 Piston movement: TDC ➝ BDC
- 🔹 Action: As piston moves down, it creates suction, drawing the air-fuel mixture into the cylinder.
- 🔹 Crank rotation: 0° ➝ 180°
- 🔹 PV Diagram: Line AB
2. Compression Stroke
- 🔹 Valves: Both valves closed
- 🔹 Piston movement: BDC ➝ TDC
- 🔹 Action: The charge is compressed, raising its pressure and temperature (Compression ratio ~7:1 to 11:1).
- 🔹 Crank rotation: 180° ➝ 360°
- 🔹 PV Diagram: Curve BC
- 🔹 At the end of this stroke, spark plug fires, igniting the mixture.
- 🔹 Heat addition at constant volume: Line CD
3. Power Stroke (Expansion or Working Stroke)
- 🔹 Valves: Both valves closed
- 🔹 Piston movement: TDC ➝ BDC
- 🔹 Action: The combusted gases expand rapidly, pushing the piston down—this is the only power-producing stroke.
- 🔹 Crank rotation: 360° ➝ 540°
- 🔹 PV Diagram: Curve DE
- 🔹 At the end of this stroke, exhaust valve opens
- 🔹 Sudden pressure drop at constant volume: Line EB
4. Exhaust Stroke
- 🔹 Valves: Inlet valve closed, exhaust valve open
- 🔹 Piston movement: BDC ➝ TDC
- 🔹 Action: Spent gases are expelled out of the cylinder through the exhaust valve.
- 🔹 Crank rotation: 540° ➝ 720°
- 🔹 PV Diagram: Curve BA
Cycle Completion:
At the end of the exhaust stroke, the inlet valve opens again, starting the next cycle.