Introduction To Internal Combustion Engines

1. Definition of Internal Combustion Engine

An Internal Combustion Engine (I.C. Engine) is a heat engine in which the combustion of fuel takes place inside the engine cylinder, and the heat energy released during combustion is directly converted into mechanical work. The working fluid (air–fuel mixture or air) remains inside the engine system.

2. Basic Principle of I.C. Engines

The working of an internal combustion engine is based on the following principle:

1. Fuel is mixed with air
2. Combustion occurs inside the cylinder
3. High-pressure and high-temperature gases are produced
4. These gases exert force on the piston
5. The piston motion is converted into rotary motion of the crankshaft
6. Useful mechanical power is obtained

3. Main Components of an I.C. Engine

The important parts of an internal combustion engine are:

1. Cylinder – Provides space for piston movement and combustion
2. Piston – Receives force from combustion gases
3. Piston Rings – Prevent gas leakage and control lubrication
4. Connecting Rod – Connects piston to crankshaft
5. Crankshaft – Converts reciprocating motion into rotary motion
6. Inlet Valve – Admits air or air–fuel mixture
7. Exhaust Valve – Releases burnt gases
8. Spark Plug (Petrol Engine) – Ignites air–fuel mixture
9. Fuel Injector (Diesel Engine) – Injects fuel into compressed air
10. Flywheel – Stores energy and smoothens engine operation

4. Classification of Internal Combustion Engines

4.1 Based on Type of Fuel

• Petrol Engine
• Diesel Engine
• Gas Engine (CNG, LPG)

4.2 Based on Method of Ignition

• Spark Ignition (SI) Engine – Uses spark plug (Petrol engine)
• Compression Ignition (CI) Engine – Self-ignition due to high compression (Diesel engine)

4.3 Based on Number of Strokes

• Two-Stroke Engine
• Four-Stroke Engine

4.4 Based on Cooling System

• Air-Cooled Engine
• Water-Cooled Engine

4.5 Based on Number of Cylinders

• Single-Cylinder Engine
• Multi-Cylinder Engine

5. Working of Four-Stroke Petrol Engine

The four-stroke petrol engine completes one cycle in four strokes of piston (two revolutions of crankshaft):

1. Suction Stroke
   • Inlet valve opens
   • Air–fuel mixture enters cylinder
2. Compression Stroke
   • Both valves closed
   • Mixture compressed
   • Spark plug ignites mixture at end
3. Power Stroke
   • Combustion produces high pressure
   • Piston moves downward
   • Power is produced
4. Exhaust Stroke
   • Exhaust valve opens
   • Burnt gases expelled

6. Working of Four-Stroke Diesel Engine

Steps are similar but with important differences:

1. Suction Stroke – Only air is taken in
2. Compression Stroke – Air is highly compressed
3. Power Stroke – Fuel injected, self-ignition occurs
4. Exhaust Stroke – Burnt gases removed

7. Two-Stroke Engine

In a two-stroke engine, the cycle is completed in two strokes (one crankshaft revolution).

Characteristics

• No separate suction and exhaust strokes
• Ports are used instead of valves
• Higher power-to-weight ratio
• Less efficient compared to four-stroke engines

8. Comparison Between Two-Stroke and Four-Stroke Engines

Two-Stroke Engine	Four-Stroke Engine
One power stroke per revolution	One power stroke per two revolutions
Simple construction	Complex construction
Higher fuel consumption	Better fuel efficiency
More pollution	Less pollution

9. Advantages of Internal Combustion Engines

• High thermal efficiency
• Compact and light in weight
• Quick starting
• Lower installation cost
• Suitable for automobiles and portable machines

10. Disadvantages of Internal Combustion Engines

• Produces noise and vibration
• Causes air pollution
• Limited power output compared to power plants
• Requires high-quality fuel

11. Applications of I.C. Engines

• Automobiles (cars, bikes, trucks)
• Agricultural machinery (tractors, pumps)
• Power generators
• Marine engines
• Construction equipment

12. Comparison Between I.C. Engine and External Combustion Engine

Internal Combustion Engine	External Combustion Engine
Combustion inside cylinder	Combustion outside engine
Higher efficiency	Lower efficiency
Compact size	Large size
Quick start	Slow start

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