1. Introduction
Four stroke engines is an internal combustion engine in which the complete working cycle is completed in four distinct piston strokes, requiring two revolutions of the crankshaft. These engines are widely used in automobiles, motorcycles, generators, and industrial applications due to their higher efficiency, durability, and cleaner exhaust compared to two-stroke engines.
2. Principle of Operation
The engine converts chemical energy of fuel into mechanical energy through four piston movements:
- Suction (Intake) stroke
- Compression stroke
- Power (Expansion) stroke
- Exhaust stroke
Each stroke corresponds to 180° of crankshaft rotation, totaling 720° for one complete cycle.
3. Main Components of a Four-Stroke Engine
- Cylinder and cylinder head
- Piston, piston rings
- Connecting rod
- Crankshaft
- Inlet and exhaust valves
- Camshaft and valve mechanism
- Spark plug (SI engine) or fuel injector (CI engine)
4. Working of a Four-Stroke Engine
1. Suction (Intake) Stroke
- Piston moves from Top Dead Center (TDC) to Bottom Dead Center (BDC)
- Inlet valve opens, exhaust valve closed
- Fresh charge enters the cylinder:
- SI engine: air–fuel mixture
- CI engine: air only
- Slight vacuum is created inside the cylinder
2. Compression Stroke
- Piston moves from BDC to TDC
- Both valves remain closed
- Charge is compressed, raising pressure and temperature
- Near TDC:
- SI engine: spark plug ignites mixture
- CI engine: fuel is injected
3. Power (Expansion) Stroke
- Combustion occurs rapidly
- High-pressure gases push piston from TDC to BDC
- Both valves closed
- This is the only stroke that produces useful work
4. Exhaust Stroke
- Piston moves from BDC to TDC
- Exhaust valve opens, inlet valve closed
- Burnt gases are expelled from the cylinder
- Cycle repeats
5. Valve Timing in Four-Stroke Engines
In actual engines, valves do not open and close exactly at TDC or BDC due to:
- Gas inertia
- Time required for gas flow
Valve timing features:
- Inlet valve opens before TDC
- Inlet valve closes after BDC
- Exhaust valve opens before BDC
- Exhaust valve closes after TDC
- Valve overlap improves scavenging
6. Four-Stroke SI and CI Engines
| Feature | Four-Stroke SI Engine | Four-Stroke CI Engine |
|---|---|---|
| Ignition | Spark plug | Self-ignition |
| Fuel | Petrol | Diesel |
| Compression ratio | 6:1 – 10:1 | 14:1 – 25:1 |
| Combustion | Homogeneous | Heterogeneous |
| Speed | High | Medium |
| Efficiency | Lower | Higher |
7. Advantages of Four-Stroke Engines
- Higher thermal efficiency
- Better fuel economy
- Lower exhaust emissions
- Less lubrication oil consumption
- Longer engine life
- Smoother operation
8. Disadvantages
- Heavier and more complex
- Higher initial cost
- Lower power-to-weight ratio than two-stroke engines
- More moving parts
9. Applications
- Cars and motorcycles
- Trucks and buses
- Power generators
- Agricultural machinery
- Marine engines
- Industrial engines
10. Comparison: Four-Stroke vs Two-Stroke Engine
| Aspect | Four-Stroke Engine | Two-Stroke Engine |
|---|---|---|
| Power stroke | Once every 2 revolutions | Once every revolution |
| Fuel efficiency | High | Low |
| Emissions | Low | High |
| Engine life | Long | Short |
| Complexity | More | Less |
11. Efficiency and Performance
- Indicated power is produced inside the cylinder
- Brake power is available at crankshaft
- Mechanical efficiency depends on friction losses
- Volumetric efficiency influenced by valve timing
12. Conclusion
Four-stroke engines are the most commonly used IC engines due to their high efficiency, reliability, and environmentally friendly operation. Though heavier and more complex than two-stroke engines, their advantages make them ideal for both light- and heavy-duty applications.