Working Principles of Combustion Engine

1. Introduction

A combustion engine, commonly called an Internal Combustion Engine (IC Engine), is a heat engine in which the combustion of fuel takes place inside the engine cylinder.

The chemical energy of the fuel is converted into:

Chemical Energy → Heat Energy → Pressure Energy → Mechanical Energy → Rotary Motion

IC engines are widely used in automobiles, motorcycles, tractors, generators, construction equipment, ships and many industrial applications.

2. Basic Principle of an IC Engine

The basic working principle is based on the burning of fuel inside a cylinder.

When fuel burns with air, a large amount of heat is released. This produces high-pressure gases, which act on the piston.

The piston moves up and down inside the cylinder. This reciprocating motion is converted into rotary motion by the:

Piston → Connecting Rod → Crankshaft

The crankshaft then supplies useful mechanical power to the vehicle or machine.

Energy Conversion

Fuel + Air
↓
Combustion
↓
Heat and High-Pressure Gas
↓
Piston Movement
↓
Connecting Rod
↓
Crankshaft Rotation
↓
Useful Mechanical Power

3. Main Parts of a Combustion Engine

Part	Function
Cylinder	Provides space for combustion and piston movement
Piston	Receives force from high-pressure gases
Connecting Rod	Connects piston to crankshaft
Crankshaft	Converts reciprocating motion into rotary motion
Inlet Valve	Allows air or air-fuel mixture into cylinder
Exhaust Valve	Allows burnt gases to leave the cylinder
Spark Plug	Produces spark in petrol/SI engines
Fuel Injector	Injects fuel in diesel/CI engines
Flywheel	Maintains smooth rotation of the crankshaft
Cylinder Head	Closes the upper portion of the cylinder
Crankcase	Houses and supports the crankshaft

4. Types of Combustion Engines

Combustion engines can be classified in several ways.

Based on ignition

1. Spark Ignition Engine (SI Engine)

Generally used in petrol engines.
Air-fuel mixture is ignited using a spark plug.

2. Compression Ignition Engine (CI Engine)

Generally used in diesel engines.
Air is highly compressed.
Fuel is injected into the hot compressed air.
Fuel self-ignites due to the high temperature.

5. Four-Stroke Engine

The most commonly studied IC engine is the four-stroke engine.

It completes one operating cycle in four strokes:

Suction / Intake
Compression
Power / Expansion
Exhaust

One complete four-stroke cycle requires:

4 piston strokes = 2 crankshaft revolutions = 720°

6. Stroke 1 – Suction / Intake Stroke

Working

During the intake stroke:

The piston moves from TDC to BDC.
The inlet valve opens.
The exhaust valve remains closed.
Fresh charge enters the cylinder.

In an SI engine, an air-fuel mixture enters the cylinder.

In a CI engine, generally only air enters the cylinder.

Direction

Piston: TDC → BDC

Result

The cylinder is filled with fresh charge.

7. Stroke 2 – Compression Stroke

During the compression stroke:

Both inlet and exhaust valves are closed.
The piston moves from BDC to TDC.
The charge is compressed into a small volume.
Pressure and temperature increase.

SI Engine

The compressed air-fuel mixture is prepared for ignition by the spark plug.

CI Engine

Only air is compressed to a high pressure and temperature.

Direction

Piston: BDC → TDC

8. Stroke 3 – Power Stroke

This is the main working stroke of the engine.

At the end of compression:

SI Engine

The spark plug produces a spark.

Spark → Ignition → Combustion → High-pressure gases

CI Engine

Fuel is injected into the hot compressed air.

Fuel Injection → Self-Ignition → Combustion → High-pressure gases

The high-pressure gases push the piston downward.

Direction

Piston: TDC → BDC

The connecting rod transfers this force to the crankshaft, causing the crankshaft to rotate.

This is the stroke that produces useful power.

9. Stroke 4 – Exhaust Stroke

During the exhaust stroke:

The exhaust valve opens.
The inlet valve remains closed.
The piston moves from BDC to TDC.
Burnt gases are pushed out of the cylinder.

Direction

Piston: BDC → TDC

At the end of the exhaust stroke, the cylinder is ready for the next cycle.

10. Four-Stroke Cycle – Summary

Stroke	Piston Movement	Inlet Valve	Exhaust Valve	Main Process
1. Intake	TDC → BDC	Open	Closed	Fresh charge enters
2. Compression	BDC → TDC	Closed	Closed	Charge is compressed
3. Power	TDC → BDC	Closed	Closed	Combustion produces power
4. Exhaust	BDC → TDC	Closed	Open	Burnt gases leave

Easy Memory Method

I – C – P – E

Intake → Compression → Power → Exhaust

11. Two-Stroke Engine

A two-stroke engine completes one operating cycle in two strokes of the piston.

Therefore:

2 piston strokes = 1 crankshaft revolution = 360°

Advantages

Simple construction
Fewer moving parts
High power-to-weight ratio
Power stroke occurs once every crankshaft revolution

Applications

Small motorcycles
Chainsaws
Portable machines
Small marine engines
Some older scooters and equipment

12. SI Engine Working Principle

The Spark Ignition (SI) engine is generally associated with petrol engines.

Working sequence

Air + Fuel → Compression → Spark → Combustion → Expansion → Exhaust

The spark plug ignites the compressed air-fuel mixture.

Key Features

Uses spark plug
Lower compression ratio compared with diesel engines
Generally uses petrol/gasoline
Smooth and relatively quiet operation
Commonly used in cars and motorcycles

13. CI Engine Working Principle

The Compression Ignition (CI) engine is generally associated with diesel engines.

Working sequence

Air Intake → High Compression → Fuel Injection → Self-Ignition → Combustion → Power

The fuel is injected into highly compressed and heated air.

Key Features

Uses fuel injector
Does not normally use a spark plug for ignition
Higher compression ratio
Generally higher thermal efficiency
Commonly used in trucks, buses, tractors, generators and heavy machinery

14. SI Engine vs CI Engine

Parameter	SI Engine	CI Engine
Common fuel	Petrol	Diesel
Ignition	Spark ignition	Compression ignition
Ignition device	Spark plug	Fuel injector
Charge during intake	Air-fuel mixture	Mainly air
Compression ratio	Lower	Higher
Fuel injection	Usually port/direct injection depending on design	High-pressure injection
Efficiency	Generally lower	Generally higher
Applications	Cars, motorcycles	Trucks, buses, tractors, generators

15. TDC and BDC

Two important piston positions are:

TDC – Top Dead Centre

The position where the piston is nearest to the cylinder head.

BDC – Bottom Dead Centre

The position where the piston is farthest from the cylinder head.

16. How Reciprocating Motion Becomes Rotary Motion

The piston moves in a straight reciprocating motion.

The connecting rod transfers this motion to the crankshaft.

The crank mechanism converts:

Reciprocating Motion ↔ Rotary Motion

Sequence

Piston
↓
Connecting Rod
↓
Crankshaft
↓
Rotary Motion
↓
Flywheel / Transmission
↓
Vehicle or Machine

This principle makes it possible to use the reciprocating motion of the piston to drive wheels, pumps, generators and other machinery.

17. Combustion Process

Combustion is the rapid chemical reaction between fuel and oxygen that releases heat.

For example, in simplified form:

Fuel + Oxygen → Carbon Dioxide + Water + Heat

The released heat increases the temperature and pressure of the gases inside the cylinder.

The high-pressure gases exert force on the piston.

Force on piston

F = P \times A

Where:

F = Force acting on piston
P = Gas pressure
A = Area of piston

This force produces useful mechanical work.

18. Important Terms

Bore

The inside diameter of the engine cylinder.

Stroke

The distance travelled by the piston between TDC and BDC.

Swept Volume

Volume displaced by the piston during one stroke.

V_{s} = \frac{π}{4} D^{2} L

Where:

D = Cylinder bore
L = Stroke length

Clearance Volume

The volume remaining above the piston when it is at TDC.

Compression Ratio

CR = \frac{V _{s} + V _{c}}{V _{c}}

where Vc is the clearance volume.

19. Advantages of IC Engines

Compact size
High power-to-weight ratio
Quick starting
Suitable for mobile applications
High power output
Widely available fuels
Easy integration with vehicles and machinery

20. Limitations of IC Engines

Produces exhaust emissions
Generates noise and vibration
Requires regular maintenance
Fuel combustion produces pollutants
Thermal efficiency is limited
Depends largely on fossil fuels in conventional applications

21. Applications

IC engines are used in:

🚗 Automobiles
🏍️ Motorcycles
🚌 Buses
🚚 Trucks
🚜 Tractors
🚢 Ships
⚡ Diesel generators
🏗️ Construction equipment
🌾 Agricultural machinery
🏭 Industrial equipment

22. Conclusion

The Internal Combustion Engine is one of the most important power-producing machines in mechanical engineering. Its operation is based on the combustion of fuel inside a cylinder, producing high-pressure gases that move the piston.

In a four-stroke engine, the complete cycle consists of: