Four-Stroke Engine Working Principle
The four-stroke engine is one of the most widely used internal combustion engine designs in the world. It is commonly found in automobiles, motorcycles, generators, agricultural machinery, construction equipment, marine engines and many industrial applications.
A four-stroke engine completes one complete operating cycle in four piston strokes: intake, compression, power and exhaust. During this cycle, the crankshaft rotates through two complete revolutions, or approximately 720 degrees.
What Is a Four-Stroke Engine?
A four-stroke engine is an internal combustion engine in which the piston completes four separate strokes to produce one complete combustion cycle.
The four strokes are:
- Intake Stroke
- Compression Stroke
- Power Stroke
- Exhaust Stroke
The engine uses valves to control the movement of air, fuel and exhaust gases into and out of the cylinder.
Main Components of a Four-Stroke Engine
| Component | Function |
|---|---|
| Cylinder | Provides the space in which the piston moves and combustion takes place. |
| Piston | Moves up and down inside the cylinder and transfers combustion force to the connecting rod. |
| Connecting Rod | Connects the piston to the crankshaft. |
| Crankshaft | Converts the reciprocating motion of the piston into rotary motion. |
| Inlet Valve | Allows fresh air or air-fuel mixture to enter the cylinder. |
| Exhaust Valve | Allows combustion gases to leave the cylinder. |
| Spark Plug | Ignites the compressed air-fuel mixture in a spark-ignition engine. |
| Fuel Injector | Delivers fuel into the combustion chamber in a controlled manner. |
| Cylinder Head | Forms the upper section of the combustion chamber and houses valves and other components. |
| Crankcase | Houses and supports the crankshaft and provides an enclosure for the lower engine assembly. |
How Does a Four-Stroke Engine Work?
The operation of a four-stroke engine can be understood by following the movement of the piston and the opening and closing of the valves during each stroke.
1. Intake Stroke
During the intake stroke, the piston moves from the Top Dead Centre (TDC) toward the Bottom Dead Centre (BDC).
The inlet valve opens while the exhaust valve remains closed. As the piston moves downward, it creates a pressure difference that draws fresh air or an air-fuel mixture into the cylinder, depending on the engine design.
At the end of the intake stroke, the cylinder contains the charge required for combustion.
Valve condition: Inlet valve open; exhaust valve closed.2. Compression Stroke
The piston now moves from BDC toward TDC. Both the inlet and exhaust valves are closed.
The piston compresses the contents of the cylinder, increasing the pressure and temperature of the charge.
In a petrol engine, the compressed air-fuel mixture is subsequently ignited by a spark plug. In a diesel engine, air is compressed to a high temperature and fuel is injected near the end of the compression stroke.
Valve condition: Inlet valve closed; exhaust valve closed.3. Power Stroke
The power stroke is the stage in which useful mechanical work is produced.
Combustion causes a rapid increase in cylinder pressure. The high-pressure gases push the piston downward from TDC toward BDC.
The connecting rod transfers this force to the crankshaft, causing the crankshaft to rotate.
Both valves remain closed during most of the power stroke.
Valve condition: Inlet valve closed; exhaust valve closed.4. Exhaust Stroke
After the power stroke, the piston moves from BDC toward TDC again.
The exhaust valve opens, allowing the burned gases to leave the cylinder. The inlet valve remains closed during most of this stroke.
When the piston reaches TDC, most of the exhaust gases have been expelled and the engine is ready to begin another intake stroke.
Valve condition: Inlet valve closed; exhaust valve open.Four-Stroke Engine Cycle Summary
| Stroke | Piston Movement | Inlet Valve | Exhaust Valve | Main Process |
|---|---|---|---|---|
| Intake | TDC → BDC | Open | Closed | Fresh charge enters cylinder |
| Compression | BDC → TDC | Closed | Closed | Charge is compressed |
| Power | TDC → BDC | Closed | Closed | Combustion produces useful work |
| Exhaust | BDC → TDC | Closed | Open | Exhaust gases leave cylinder |
Four-Stroke Engine Cycle Diagram
FOUR-STROKE ENGINE CYCLE
┌─────────────┐
│ INTAKE │
│ ↓ Piston │
└──────┬──────┘
↓
┌─────────────┐
│ COMPRESSION │
│ ↑ Piston │
└──────┬──────┘
↓
┌─────────────┐
│ POWER │
│ ↓ Piston │
└──────┬──────┘
↓
┌─────────────┐
│ EXHAUST │
│ ↑ Piston │
└──────┬──────┘
│
└──────→ New Cycle
4 Piston Strokes = 2 Crankshaft Revolutions
= 720° Crankshaft Rotation
Petrol vs Diesel Four-Stroke Engine
Both petrol and diesel engines can operate using the four-stroke cycle, but the method of fuel ignition differs.
| Feature | Petrol Engine | Diesel Engine |
|---|---|---|
| Ignition | Spark ignition | Compression ignition |
| Air Intake | Air-fuel mixture or air depending on injection system | Primarily air during intake |
| Fuel Injection | Fuel is metered by the fuel system | Fuel is injected into highly compressed hot air |
| Compression Ratio | Generally lower | Generally higher |
| Typical Applications | Cars, motorcycles, small generators | Trucks, buses, generators, heavy equipment |
Why Does the Crankshaft Rotate Continuously?
A four-stroke engine produces useful power during only one of the four strokes—the power stroke. However, the crankshaft continues rotating through the other three strokes because of the energy stored in its rotating mass, particularly the flywheel, and because multiple cylinders may be operating at different stages of the cycle.
In a multi-cylinder engine, the power strokes are distributed between cylinders to provide smoother and more continuous crankshaft rotation.
Role of the Flywheel
The flywheel stores rotational energy generated during the power stroke and releases some of that energy during the intake, compression and exhaust strokes.
It helps maintain relatively uniform crankshaft speed and smooth engine operation.
Advantages of Four-Stroke Engines
- Good fuel efficiency.
- Lower emissions compared with many equivalent two-stroke designs.
- Longer service life when properly maintained.
- Lower lubricating-oil consumption.
- Suitable for a wide range of applications.
- Generally quieter and smoother operation.
- Better suited to modern emission-control systems.
Disadvantages of Four-Stroke Engines
- More complex valve and timing mechanisms.
- Higher manufacturing cost than simpler engine designs.
- More moving components.
- Lower power-to-weight ratio than some comparable two-stroke engines.
- Requires regular maintenance of lubrication, cooling, fuel and valve systems.
Common Applications
- Passenger vehicles
- Heavy-duty trucks
- Diesel generators
- Construction equipment
- Agricultural machinery
- Marine engines
- Industrial engines
- Motorcycles
- Water pumps
- Emergency power systems
Common Four-Stroke Engine Problems
| Problem | Possible Causes |
|---|---|
| Engine Overheating | Low coolant level, blocked radiator, faulty thermostat, poor lubrication or excessive load. |
| Loss of Power | Restricted air filter, poor fuel delivery, injector problems, low compression or valve issues. |
| Excessive Smoke | Incorrect fuel-air mixture, worn piston rings, injector problems or excessive oil consumption. |
| Hard Starting | Weak battery, fuel system problems, low compression, faulty glow plugs or ignition problems. |
| Abnormal Noise | Worn bearings, valve train problems, piston clearance or lubrication failure. |
Four-Stroke Engine Maintenance Tips
- Change engine oil according to the manufacturer's recommended interval.
- Use the correct grade of lubricating oil.
- Replace air and fuel filters when required.
- Inspect cooling system components regularly.
- Check coolant level and condition.
- Inspect belts, hoses and pulleys.
- Maintain correct valve clearance where applicable.
- Inspect fuel injectors or spark plugs as applicable.
- Monitor engine temperature and oil pressure.
- Investigate abnormal vibration, noise or smoke immediately.
Frequently Asked Questions
What are the four strokes of an engine?
The four strokes are intake, compression, power and exhaust.
How many crankshaft revolutions are required to complete a four-stroke cycle?
A four-stroke engine requires two complete crankshaft revolutions, equivalent to 720 degrees of crankshaft rotation, to complete one cycle.
Which stroke produces power?
The power stroke produces useful mechanical work. Combustion increases cylinder pressure and forces the piston downward.
What is the difference between a petrol and diesel engine?
A petrol engine normally uses spark ignition, while a diesel engine uses compression ignition. In a diesel engine, fuel is injected into highly compressed air, causing ignition because of the high temperature of the compressed air.
Why is the compression stroke important?
Compression increases the pressure and temperature of the charge before combustion, helping the engine produce useful power efficiently.
Conclusion
The four-stroke engine is a fundamental example of how thermal energy can be converted into mechanical work. Its operation consists of four sequential strokes: intake, compression, power and exhaust. The piston moves through these four stages while the crankshaft converts the piston's reciprocating motion into useful rotary motion.
Understanding the four-stroke engine working principle is essential for mechanical engineers, technicians, automobile professionals and anyone involved in the maintenance of engines and rotating equipment.
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