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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.
The fuel injection system in petrolengined cars is always indirect, petrol being injected into the inlet manifold or inlet port rather than directly into the combustion chambers . This ensures that the fuel is well mixed with the air before it enters the chamber.
Many diesel engines , however, use direct injection in which the diesel is injected directly into the cylinder filled with compressed air. Others use indirect injection in which the diesel fuel is injected into the specially shaped pre-combustion chamber which has a narrow passage connecting it to the cylinder head .
Only air is drawn into the cylinder. It is heated so much by compression that atomized fuel injected at the end of the compression stroke self-ignites.
All modern petrol injection systems use indirect injection. A special pump sends the fuel under pressure from the fuel tank to the engine bay where, still under pressure, it is distributed individually to each cylinder.
Depending on the particular system, the fuel is fired into either the inlet manifold or the inlet port via an injector . This works much like the spray nozzle of a hose , ensuring that the fuel comes out as a fine mist. The fuel mixes with the air passing through the inlet manifold or port and the fuel/air mixture enters the combustion chamber.
Some cars have multi-point fuel injection where each cylinder is fed by its own injector. This is complex and can be expensive. It's more common to have single-point injection where a single injector feeds all the cylinders, or to have one injector to every two cylinders.
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.