Fuel Injection - Why and How

Fuel Injection - Why and How

Though fuel injection has been used in diesel engines for more than 80 years, it was not until about 30 years ago that it began to make serious inroads against the reign of the carburettor on petrol engines.

Since that time, fuel injection systems have gone through various phases of development. The current systems all use electronic controls, integrated with an overall engine control unit. The reasons why the carburettor had to yield to fuel injection in petrol engines can be summarised in two words: precision and reliability.

By injecting precisely metered amounts of fuel into the air, entering each cylinder during its inlet stroke, one can ensure that the mixture is always exactly right for the conditions of the moment - slightly rich when really necessary (when starting from cold, for instance), otherwise in the perfect ratio for complete combustion which will leave no unburnt fuel to be blown out of the exhaust pipe. (A catalytic converter can only do its job if the mixture is maintained at this perfect ratio, and a carburettor could never achieve such precision. Thus fuel injection became compulsory when catalytic converters appeared on the scene).

Cylinder-to-cylinder variations in mixture ratio, unavoidable with a carburettor, are eliminated with fuel injection, and this also leads to a smoother-running engine, less wear and less oil contamination caused by some cylinders receiving an overly rich mixture.

As for reliability, those of us whose daily transport is still fitted with a carburettor will know that it can be a pesky thing, either too rich or too lean, messing you around with "flat spots", hard starting when hot, flooding when cold. When a carburettor gets older, wear develops around the spindles of the butterfly, allowing extra air to be sucked in which upsets the air-fuel ratio.

The only good thing that can be said about a carburettor is that its problems can usually either be lived with or fixed at the roadside with a combination of patience and the help of knowledgable passers-by. On the current generation of injection systems, by contrast, both the electronics and the few remaining mechanical components are very reliable, though they also have their foibles.

At the heart of a multipoint injection system are the injectors, one on each cylinder. They can be seen, screwed into the inlet tracts just upstream of the inlet valves.

Each one has an electrical wire going to it, as well as a connection to a high-pressure fuel line which feeds fuel into a passage running the length of the injector.

The passage is normally blocked off by a spring-loaded plunger, but at the right moment, as determined by the engine control unit, a pulse of current is supplied to a solenoid coil inside the injector, which pulls back the plunger (by less than one-tenth of a millimetre), allowing fuel under high pressure to be squirted out at the spray tip in the form of a fine mist.

The duration of this operation, called the pulse width, determines the amount of fuel injected. To give an idea of the precision, at 3000rpm this happens 25 times a second on each cylinder, and each injection event takes less than the blink of an eye. The engine control unit, the brain of the whole system, calculates the fuel mass to be injected at each injection event.

It bases its calculation on information received from a variety of sensors, including a mass air-flow sensor which measures the mass of air breathed in by the engine during the inlet strokes, a throttle position sensor, an oxygen sensor in the exhaust path, temperature sensors, and a pressure sensor in the inlet manifold.