For the engine to run smoothly and efficiently it needs to be provided with the right quantity of fuel/air mixture according to its wide range of demands.

Traditionally, the fuel/air mixture is controlled by the carburettor, an instrument that is by no means perfect.

Its major disadvantage is that a single carburettor supplying a four-cylinder engine cannot give each cylinder precisely the same fuel/air mixture because some of the cylinders are further away from the carburettor than others.

One solution is to fit twin-carburettors, but these are difficult to tune correctly. Instead, many cars are now being fitted with fuel-injected engines where the fuel is delivered in precise bursts. Engines so equipped are usually more efficient and more powerful than carburetted ones, and they can also be more economical, as well as having less poisonous emissions.

Injectors

The injectors through which the fuel is sprayed are screwed, nozzle-first, into either the inlet manifold or the cylinder head and are angled so that the spray of fuel is fired towards the inlet valve.

The injectors are one of two types, depending on the injection system. The first system uses continuous injection where the fuel is squirted into the inlet port all the time the engine is running. The injector simply acts as a spray nozzle to break up the fuel into a fine spray - it doesn't actually control the fuel flow. The amount of fuel sprayed is increased or decreased by a mechanical or electrical control unit - in other words, it is just like turning a tap on and off.

The other popular system is timed injection (pulsed injection) where the fuel is delivered in bursts to coincide with the induction stroke of the cylinder. As with continuous injection, timed injection can also be controlled either mechanically or electronically.

The earliest systems were mechanically controlled. They are often called petrol injection (PI for short) and the fuel flow is controlled by a mechanical regulator assembly. These systems suffer from the drawbacks of being mechanically complex and having poor response to backing off the throttle.

Mechanical systems have now been largely superseded by electronic fuel injection (known as EFi for short). This is thanks to the increasing reliability and decreasing costs of electronic control systems.

Types of fuel injector

Two main types of injector can be fitted, depending on whether the injection system is mechanically or electronically controlled.

In a mechanical system, the injector is spring-loaded into the closed position and is opened by fuel pressure.

The injector in an electronic system is also held closed by a spring, but is opened by an electromagnet built into the injector body. The electronic control unit determines how long the injector stays open.

Check the video and learn how a fuel injection system works: