Four strokes of the petrol engine

Working of a four Stroke Petrol Engine and PV Diagram Part 1

This article gives a detailed description of the principle and working of a four Stroke Petrol Engine. The illustration is with diagrams and PV diagram.


Four Stroke Petrol Engine

What is an Engine?

The word Engine comes from the Latin word ingenium which means ‘ability’. Engine is machinery. Generally a combustion engine is defined as a machine that produces power(mechanical) by the combustion of substance like water or a fuel.
Engines are classified into various categories based on the type of cycle they use, the layout, the energy source used, the cooling mechanism employed or its use.

Based on the layout, there are two types of Engines

Internal Combustion Engines
An engine is called an Internal Combustion Engine, If the combustion of a fuel like petrol takes place inside it, in a chamber. The oxidizer used for combustion is usually air. The gases produced as a result of combustion are high temperature and pressure. These gases exert pressure on components such as a piston which moves over and produces energy (mechanical). E.g. Petrol Engine

External Combustion Engines
An External Combustion engine is one in which the energy is externally applied on a non combustible fluid like pressurized/hot water, liquid sodium etc. Here these fluids are heated in an external chamber like boiler and steam is used to drive the engine. E.g. Steam Engine

Based on the source of energy, internal combustion engines are classified as
Petrol Engines
Diesel Engines

Petrol Engine is again categorized into
1.2 stroke engine
2.4 stroke engine

Here we will discuss about a Four Stroke Petrol Engine and its working.

Principle of a Four Stroke Petrol Engine

The principle used in a four stroke petrol engine is commonly known as Otto Cycle. It states that there would be one power stroke for every four strokes. Such engines use a spark plug which is used for the ignition of the combustible fuel used in the engine. Most of the cars, bikes and trucks use a 4 stroke engines.

In every Otto cycle there is an adiabatic compression, addition of heat at constant volume, an adiabatic expansion and the release of heat at constant volume. The P-V diagram for a 4 stroke engine is as follows:

Four Stroke Petrol Engine

Working of a Four Stroke Petrol Engine

A stroke is the movement of the piston from the top, to the bottom of the cylinder.
Watch to get more clarification.


As the name suggest the Four Stroke Petrol Engine uses a cycle of four strokes and petrol as the fuel. Each cycle includes 2 rotations of the crankshaft and four strokes, namely:
1.An Intake Stroke
2.A Compression Stroke
3.A Combustion Stroke also called Power Stroke
4.An Exhaust Stroke

The steps involved are as follows:

intake stroke

1.Intake Stroke: As the name suggests in this stroke the intake of fuel takes place. When the engine starts, the piston descends to the cylinder’s bottom from the top. Thus the pressure inside the cylinder reduces. Now the intake valve opens and the fuel and air mixture enters the cylinder. The valve then closes.

2.Compression Stroke: This stroke is known as compression stroke because the compression of the fuel mixture takes place at this stage. When the intake valve closes (exhaust valve is already closed), the piston forced back to the top of the cylinder and the fuel mixture gets compressed. The compression is around 1/8th of the original volume. An engine is considered more efficient if its compression ratio is higher.

compression stroke

3.Combustion/Power Stroke: Now in case of petrol engine when the fuel mixture compresses to the maximum value the spark plug produces spark which ignites the fuel mixture. The combustion leads to the production of high pressure gases. Due to this tremendous force the piston is driven back to the bottom of the cylinder. As the piston moves downwards, the crankshaft rotates which rotates the wheels of the vehicle.

power stroke

4.Exhaust Stroke: As the wheel moves to the bottom the exhaust valve opens up and due to the momentum gained by the wheel the piston is pushed back to the top of the cylinder. The gases due to combustion are hence expelled out of the cylinder into the atmosphere through the exhaust valve.

exhaust stroke

The exhaust valve closes after the exhaust stroke and again the intake valve opens and the four strokes are repeated.

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