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How Does a Petrol Engine Work

PETROL ENGINE WORKING: Information on how a petrol engine works. The details include carburetor and injection systems.

This page has the basic workings of a petrol engine explained in simple terms. It includes some technical data on induction, compression, and other information.

Here is a very simple way to remember how a petrol engine works.

It uses a cycle of 4 strokes commonly referred to as 'suck, squeeze, bang, and blow' - similar to diesel engine working.

A mixture of petrol and air gets drawn into the cylinder (suck). The mixture gets compressed by the movement of the piston (squeeze).

The compressed mixture is then ignited by a spark (bang). Finally, the waste gases get forced out of the cylinder (blow) and into the exhaust system.

Stroke 1: Induction

Stroke 2: Compression

Stroke 3: Ignition

Stroke 4: Exhaust

This cycle is better known as the OTTO cycle. Eugenio Barsanti and Felice Matteucci first patented the OTTO Cycle engine in 1854. They made the first prototype in 1860.

The cycle is most likely named after Nicolaus Otto, a German engineer. In 1876, it seems he had the final ideas for the design that is still used today.

It uses quite a few components, actuators and sensors, but they are all set up to work in accordance with each other. Everything works together to produce effective power output, performance and economy.

Petrol engine blocks are usually made from cast iron. Even so, some performance engine blocks are aluminium (with cast iron cylinder sleeves).

The Engine runs at a compression ratio of 8:1 - 10:1. The air/fuel mixture ratio changes depending on running conditions:

Manufacturers use 3 different types of fuelling systems to deliver fuel to the engine:

Single and multipoint systems run at an injection pressure of approximately 3 bar.

Multipoint Fuel Injection

The multi point injection system delivers fuel into the inlet manifold. It takes place near to where the manifold connects to the cylinder head.

This system uses one injector for each cylinder. This is often the most effective as it provides more precise fuelling. It also produces better fuel economy, better power output, and lower exhaust emissions.

The multi-point fuel injection system is more expensive to manufacture. It has more individual parts if a problem arises. But, it can be cheaper to repair. You can replace individual components and not have to replace the whole system.

Single Point Injection

The single point injection system uses one injector as part of the throttle body unit. Fuel gets injected into the throat of the throttle body. It then travels through the inlet manifold to all the cylinders. This system is cheaper to manufacture. But, if mechanical problems occur the whole system will need replacing.

Carburetor System

In a carburetor system, the air and fuel get mixed within the carburetor. The air gets drawn through the carburetor (through a venturi) covered and controlled by a butterfly. It occurs by vacuum pressure from the inlet manifold.

The butterfly controls the amount of air entering the carburetor. The throttle cable controls the butterfly. The air/fuel mixture is then delivered to the cylinders through the inlet manifold.

This is the general operation of a carburetor system. Even so, there are several variations in the operation of carburetor system. This system can be expensive to run and is not as economical as the multi point injection system.


There are two types of camshaft operating systems; over head valve and over head cam. The camshaft has a system of cams which operate the opening and closing of the valves. It's controlled by the shape of the cam's lobe through a system of rockers.

The camshaft runs at half - engine RPM (half crankshaft speed) and connects to the crankshaft via a chain, belt, or gears. Some engines will have more than one camshaft (twin camshaft).

Actuator: A device that receives output signals from the ECU (Electronic Control Unit). It converts them into an action or motion. For example, an injector receives electrical current from the ECU. This tells the injector when to deliver fuel.

Sensor: Used to collect information for the ECU so it can deliver the correct information about running conditions, temperatures and speed to the actuators so they can perform accurately. Example: Crankshaft Position Sensor detects the position of the crankshaft within the stroke to let the injectors know when to deliver fuel.

Compression Ratio: This is the difference in the capacity of the cylinder when the piston is at TDC and BDC. At TDC, the capacity of the cylinder is 8 times smaller than when the piston is at BDC; this is a compression ratio of 8:1.

Throttle Body Unit: Part of the air intake system that controls the amount of air flowing into the engine. It gets controlled by the throttle cable. The throttle cable responds to the driver's use of the accelerator pedal.

Venturi: Part of the carburetor and cylindrical shaped. Air will flow through and gets controlled by a throttle butterfly. This restricts or increases the quantity of air that flows to the cylinder.

Bottom Dead Centre: Occurs when the piston is at the bottom of the cylinder and at its lowest position in the stroke.

Top Dead Centre: Occurs when the piston is at the top of the cylinder and at its highest position in the stroke.

Note: This is a small section taken from the glossary of automotive terms and mechanical phrases.