What is a Compression Ignition Engine? – Complete Guide

What is a Compression Ignition (CI) Engine?

A compression ignition engine or CI engine is an internal combustion engine in which the 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 the suction stroke and then this air is compressed while the 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 the 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 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.

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Working of CI Engine

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

1. Suction stroke: The 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 up to that level where the combustion of the diesel takes place.

3. Power Stroke

At just before the end of the 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 are produced that put a very high thrust force on the piston and it moves downward. The piston rotates the crankshaft with the help of a connecting rod. It is called a 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 the exhaust valve.

After the Exhaust stroke, all the strokes repeat themselves. In two stroke engine we get one power stroke in every single rotation of the crankshaft. But in a four stroke engine, we get one power stroke, in every two rotations of the crankshaft.

The position of valve, and various operation performed in four strokes of a compression Ignition engine is given below.

S.noStrokeInlet ValveExhaust valveOperation performed
1.Suction strokeOpenClosedSuction of air
2.Compression strokeClosedClosedCompression of air
3.Power strokeClosedClosedFuel injection, ignition and combustion
4.Exhaust strokeClosedOpenEscaping of burnt gases

For a better explanation of compression ignition engines watch the video given below:

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Compression ignition 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.


Here we have discussed what is a Compression Ignition Engine, its main parts, working, and application with video tutorial. I hope you have understood about it more clearly. If you found this piece of information useful and valuable then don’t forget to like and share it.

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