Four Stroke Diesel Engine (CI Engine / Diesel Cycle)
A 4-stroke diesel engine works on the Diesel Cycle, also known as the constant pressure combustion cycle. It is widely used in heavy vehicles, generators, and industries.
Key Components
- Cylinder
- Piston
- Connecting rod
- Crankshaft
- Inlet and exhaust valves
- Fuel injector (instead of spark plug)
- Flywheel
Diesel engines are Compression Ignition (CI) Engines, which means they do not use a spark plug. Instead, they compress air to a very high pressure and temperature, and then inject diesel, which self-ignites due to the high temperature.
Working – Four Strokes
Each stroke = 180° crankshaft rotation
Complete cycle = 4 strokes = 720° crankshaft rotation
1️⃣ Suction Stroke (0°–180°)
PV Diagram Line: A → B
- Inlet valve: Open
- Exhaust valve: Closed
- Piston movement: TDC → BDC
- Action: Fresh air only is drawn into the cylinder.
- Result: Cylinder fills with atmospheric air.
2️⃣ Compression Stroke (180°–360°)
PV Diagram Curve: B → C
- Both valves: Closed
- Piston movement: BDC → TDC
- Action: Air is compressed to very high pressure and temperature (Compression ratio: 20:1 to 22:1)
- Result: Temperature rises enough to ignite fuel.
Fuel Injection and Combustion
PV Diagram Line: C → D (Heat added at constant pressure)
- At the end of compression, diesel is injected at high pressure by the fuel injector.
- The fuel ignites due to high temperature of compressed air.
- Pressure remains nearly constant during this process (isobaric combustion).
3️⃣ Power Stroke (360°–540°)
PV Diagram Curve: D → E
- Both valves: Closed
- Piston movement: TDC → BDC
- Action: Expanding gases force the piston downwards.
- Result: Useful mechanical work is produced, energy stored in flywheel.
Pressure Drop (End of Power Stroke)
PV Diagram Line: E → B
- Exhaust valve opens, causing instant pressure drop at constant volume.
4️⃣ Exhaust Stroke (540°–720°)
PV Diagram Curve: B → A
- Inlet valve: Closed
- Exhaust valve: Open
- Piston movement: BDC → TDC
- Action: Burnt gases are pushed out of the cylinder.
- Result: Cylinder is cleared for the next suction stroke.
Summary of the PV Diagram (Diesel Cycle)
- A → B: Suction (Intake of air)
- B → C: Compression (Adiabatic)
- C → D: Combustion (Heat addition at constant pressure)
- D → E: Expansion (Power stroke – adiabatic)
- E → B: Heat rejection (at constant volume)
- B → A: Exhaust stroke (clearing gases)