Compression Ignition Engine - Definition, Main Components, Working with Application

What is Compression Ignition (CI) Engine?

Compression ignition engine or CI engine is an internal combustion engine in which ignition of the fuel takes place with the help of hot compressed air. As the air is compressed, it gets hot and its heat is used for the ignition and burning of the fuel. In this engine the air is sucked during suction stroke and then this air is compressed while compression stroke. At the end of the compression stroke, fuel is injected into the cylinder and it gets ignited from the heat of compressed air and burning process begins. Diesel is used as fuel for the working of this engine. It works on the principle of Diesel Cycle. The compression ratio of this type of engine is usually ranges from 14:1 to 22:1. It is used in heavy duty vehicles like buses, trucks, ships, etc.

Main Components of Compression Ignition Engine

Compression ignition engine or CI engine

The main components of compression ignition (CI) engine are
  1. Injector: It is used to inject the fuel into the cylinder during compression of air.
  2. Inlet valve: The air inside the cylinder is sucked through inlet valve during suction stroke.
  3. Exhaust Valve: The whole burnt or exhaust from the cylinder thrown out through exhaust valve.
  4. Combustion chamber: It is a chamber where the combustion of fuel takes place.
  5. Piston: It is reciprocating part of CI engine which  does reciprocating motion inside the cylinder. Its main function is to transmit the thrust force generated during power stroke to the crankshaft through connecting rod.
  6. Connecting rod: It connects piston to the crankshaft.
  7. Crankshaft: It is used to convert the reciprocating motion of the piston into rotary motion.


Same as spark ignition engine, compression ignition engine also works. Its working is also divided into 4 strokes. Here we are discussing the working of four stroke CI engine.

1. Suction stroke: Suction of air into the cylinder takes place.
2. Compression stroke: The compression of air happens in this stroke.
3. Power stroke: Ignition and combustion of fuel.
4. Exhaust stroke: Escaping of burnt or exhaust gases.

Compression ignition engine or CI engine

Lets discuss these strokes in detail.

1. Suction Stroke

In this stroke the piston moves from TDC to BDC ( i.e downward) and the suction of air takes place through the inlet valve.

2. Compression Stroke

This stroke compresses the air that is taken into the cylinder in the suction stroke. As the air gets compressed, the temperature of the air increases and reaches upto that level where the combustion of the diesel takes place.

3. Power Stroke

At just before the end of compression stroke, the injector injects the fuel into the cylinder. Due to the heat of the air, the ignition of the fuel begins and combustion takes place. Due to the combustion of the fuel, hot exhaust gases produced that puts a very high thrust force on the piston and it moves downward. The piston rotates the crankshaft with the help of connecting rod. It is called as power stroke because power is produced in this stroke.

4. Exhaust Stroke

In this stroke the piston moves upward (i.e. BDC to TDC) and pushes the burnt gases out of the engine cylinder through exhaust valve.

After Exhaust stroke, again all the strokes repeats itself. In two stroke engine we get one power stroke in every single rotation of the crankshaft. But in four stroke engine, we get one power stroke, in every two rotation of the crankshaft.

The position of valve, and various operation performed in four stroke of a compression Ignition engine is given below.
Inlet Valve
Exhaust valve
Operation performed
Suction stroke
Suction of air
Compression stroke
Compression of air
Power stroke
Fuel injection, ignition and combustion
Exhaust stroke
Escaping of burnt gases

For better explanation watch the video given below:


CI engine is used in heavy duty vehicle where more power is required. They are used in automobiles ( like cars, buses, trucks etc), Aircraft, marine, and in various production industries.

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