Engine Animation diagram

Petrol Engine: How A 4 Stroke Petrol Engine Or Spark Ignition Cycle Works?

Petrol Engine Principle and Working Cycle Explained:

What is an Engine?

An engine is a power generating machine which converts potential energy of the fuel into heat energy and then into motion. It produces power and also runs on its own power. The engine's power is generated by burning the fuel in a self-regulated and controlled ‘Combustion’ process. The combustion process involves many sub-processes; which burn the fuel efficiently to effect the smooth running of the engine.

These processes include:

  1. Suction of air (also known as breathing or aspiration)
  2. Mixing of the fuel with air after breaking the liquid fuel into highly atomised / mist form
  3. Igniting the air-fuel mixture either with a spark (petrol engine) or by self-ignition after raising the temperature of the air by compressing it (diesel engine)
  4. Burning of highly atomised fuel particles; which results in releasing / ejection of heat energy.

How an Engine Works?

The Heat Energy is converted into Kinetic Energy in the form of ‘Reciprocating Motion’. The expansion of heated gases and their forces act on the engine pistons, pushing them downwards; resulting in reciprocating motion of pistons. The reciprocating motion of the piston enables the crank-shaft to rotate and finally gets converted into the 'Rotary motion' and passed on to wheels.




Operation / Working Principle:

The conventional internal combustion engine operates on two basic principals –

  1. Otto Cycle &
  2. Diesel Cycle

What is 'Otto Cycle'? How Petrol Engine Works?

Otto Cycle is also known as Four-Stroke Spark-Ignition Cycle. It was named after German engineer Nikolaus Otto, who invented, developed and patented first Four-Stroke petrol engine. The Four-Stroke petrol engine works on the following cycle –




1. Suction Stroke – with pistons moving downwards and opening of the inlet valve creates suction of the air-fuel mixture.

Petrol Suction Stroke
Petrol Suction Stroke

2. Compression Stroke – With closing of Inlet valve, the area above the piston gets closed. The piston moves up resulting in compression of the air-fuel mixture in a confined space.

Petrol Compression Stroke
Petrol Compression Stroke

Combustion Process - At this stage a spark is fired by the spark-plug resulting in instantaneous burning of petrol resulting in an explosion. This causes heat to release resulting in generation of expanding forces known as power.

Petrol Combustion process diagram
Petrol Combustion process diagram

3. Power Stroke – These forces again push the pistons downwards resulting in their reciprocating motion.

Petrol Power Stroke
Petrol Power Stroke

4. Exhaust Stroke – On their way up, the pistons push the exhaust gases above them thru’ the exhaust valve which opens during the exhaust stroke.

Petrol Exhaust Stroke
Petrol Exhaust Stroke

This cycle repeats itself until the engine is turned off, resulting in continuance of its running.

Watch 4-Stroke Petrol Engine Working Animation Here:

4 Stroke Petrol Engine Animation
4 Stroke Petrol Engine Animation

The earlier generation petrol engine used 'Carburettor' to supply petrol to the engine. However, the newer generation petrol engines use sophisticated 'Fuel-Injection' technology (just like diesel engines) with an 'Engine Management System'  for improved performance and lower emissions. However, it still uses the spark-plug for the ignition of petrol; as was the case in the earlier generation petrol engines.

Keep reading: Diesel engine principle and working explained >>

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