How Spark Ignition Engine Works?

What is Spark Ignition Engine ?

It is an internal combustion engine in which the ignition of the air-fuel mixture takes place by the spark. The spark is generated with the help of spark plug. Since in this engine, the spark is responsible for the ignition of the fuel, it is named as spark ignition engine (SI engine). This engine uses petrol as a fuel for its working. It works on the principle of otto cycle. The fuel in this engine is injected through carburetor during suction stroke. The compression ratio of this engine is usually 6 to 10. It has light weight engine and used in light duty vehicles like motorcycle, cars etc.

Main Components

Spark Ignition Engine (Si Engine)

The main components of spark ignition engine are as follows

  1. Inlet Valve: Air-fuel mixture enters into cylinder through inlet valve.
  2. Exhaust Valve: The burnt or exhaust gases produced in the power stroke escapes out through exhaust valve.
  3. Spark Plug: It produces spark at the end of the compression stroke, which ignites the compressed air-fuel mixture.
  4. Cylinder: It is a hollow cylinder in which the piston reciprocates.
  5. Piston: It is moving part of the engine that performs reciprocating motion and transmits the power generated during power stroke to the crankshaft through connecting rod.
  6. Connecting Rod: It is that part of the engine which connects the piston to the crankshaft.
  7. Crankshaft: It is used to convert the reciprocating motion of the engine into rotary motion.


Here we will discuss the working four stroke engine. The working of 4 stroke SI engine can be divided into four strokes.

1. Suction Stroke: Air-fuel mixture enters into the cylinder.
2. Compression Stroke: Compression of air fuel mixture takes place.
3. Power Stroke: Combustion of Fuel and Power Generation.
4. Exhaust Stroke: Escaping of burnt gases out of the engine.

How Spark Ignition Engine Works?

Lets us discuss each stroke in detail.

1. Suction Stroke

In this stroke, the piston moves downward and the air-fuel mixture from the carburetor enters into the cylinder through inlet valve. During this stroke inlet valve opens and exhaust valve remains closed.

2. Compression Stroke

In this stroke, the piston moves upward and compresses the air-fuel mixture. The compression strokes completes as the piston moves at TDC. During this stroke Both the inlet and exhaust valve remains closed.

3. Power Stroke

At the end of compression stroke, a spark is produced by the spark plug. This spark ignites the air-fuel mixture and combustion takes place in the combustion chamber. Due to combustion, a very high thrust force is generated which pushes the piston downward rapidly and makes the crankshaft to rotate. This stroke is called as power stroke because we get power in it. Both inlet and exhaust valve remains closed in this stroke.

4. Exhaust Stroke

In this stroke, the piston moves upward and burnt or exhaust gases produced in the power stroke escapes out of the cylinder through exhaust valve. In this stroke, the exhaust valve gets open and inlet valve remains closed.

After the completion of exhaust stroke, again all the four stroke repeats itself. The most commonly used spark ignition engine are of two stroke engine and four stroke engine. In two stroke engine we have inlet and exhaust port instead of valve.

Also Read: Difference Between Turbocharger and Supercharger

The position of inlet and exhaust valve and operation performed during all the four strokes of the SI engine are given in the table below:
Inlet Valve
Exhaust Valve
Operation Performed
Suction stroke
Suction of fuel
Compression stroke
Compression of fuel
Power stroke
Combustion of fuel
Exhaust stroke
Escaping of burnt gases


The spark ignition engine are used in automobiles (motorcycle, Scooters, cars etc), aircraft, motorboats and in small engines such as chainsaws, lawn mowers etc.

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