Most people understand the basic principle of how a carburettor works, mixing petrol and air into an explosive mixture which is then sucked into the cylinders as the valves open and which, at the right point in the 4 stroke cycle, is ignited to produce a burst of energy driving the pistons down and rotating the crankshaft. 

But the carburettor - one of the very oldest parts of the century old internal combustion engine - has almost completely disappeared into obsolescence being replaced by that little "i" now found on most boot lids. Carburettors were, at best, inefficient mixers of fuel and air and have been overtaken by fuel injection systems as the need to reduce exhaust emissions has increased.

So what is fuel injection and in what way is it different to the "carb"? By measuring very precisely the fuel/air mixture fuel injection - itself controlled by the computerised engine "black box" management system - injection more efficiently manages the combustion process.
Essentially,   fuel injection, as it's name suggests, directly injects a very precisely measured amount of fuel, in a fine spray, into the inlet manifold or combustion chamber where it mixes with a precise volume of air at exactly the right moment to be ignited for the biggest possible bang with the least residue.

Injection, being more efficient than a carburettor, can provide more power for an equal amount of fuel or, at lower loadings, use less fuel for the same amount of power. Either way, less fuel is wasted consequently there are much lower toxic emissions.

This is mono-point injection but higher performance cars often have multi-point injection where the mixing process occurs in each combustion chamber for still more efficiency and greater power.

The timing of the injection of this combined fuel/air mixture is controlled by a microprocessor - a computer - which, every few milliseconds, measures air pressure and temperature in the inlet manifold, engine temperature and rotational speed and accelerator position to assess the optimum mixture for the amount of power required.

Article © Graham Benge 2007

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