Understanding the fuel injection system

By Barry Lake

CARBURETORS are history. Once they mixed and measured air and gasoline, turning fuel into a fine mist and spraying it into the combustion chamber.

But as carburetors wore, they became less precise at measuring the right amount of fuel and less able to mix the right amount of gasoline and air.

Also, to draw enough air and fuel into all the cylinders fast enough to keep up with increasing power and torque demands, carburetors were arranged in banks, but had to be carefully balanced so they all pulled together to keep the engine running smoothly.

As its name implies, the fuel-injection system injects fuel into the engine.

Most car engines run on gasoline, but gasoline will not burn without oxygen.

It is imperative to mix the gasoline with air before burning it inside an engine's cylinders. And this has to be done in a precise ratio - roughly 14 parts of air (by weight) to one part of gasoline.

Even when the ratio of air to fuel is correct, the fuel needs to be broken up into the smallest droplets possible and spread evenly throughout the air-fuel mixture.

In the very earliest carburetors, gasoline was dripped slowly on to a gauze patch where it evaporated as it passed over the gauze on its way to the engine.

The mixture was altered by the rate at which the fuel was dripped on to the gauze.

The next great advance was the float-bowl carburetor.

Fuel was held in a bowl at the side of the carburetor and fed to a carefully calibrated opening called a jet.

This jet was in a narrowed section of the intake port, called a venturi.

As the air passed through the venturi it sped up, the pressure dropped, and the fuel was drawn into the air flow.

But this, in itself, was not enough to cover all aspects of engine operation.

There had to be a separate system for when the engine was idling, another to provide a richer mixture at high revolutions per minute and full throttle, as well as systems for enrichment (more fuel) when the engine was cold and when it was accelerated suddenly.

When strict exhaust emission regulations began to be introduced in the 1970s, carburetors became even more complicated.

More efficient mechanical fuel-injection had become common for high-performance engines, but were similarly hard-pressed to provide clean exhaust emissions and were expensive to make.

The biggest move forward came with the invention of the computer chip.

Now, the amount of fuel injected into the air stream could be precisely metered to suit a wide variety of conditions.

There is a high-pressure fuel pump, and the injectors that spray fuel into the inlet ports leading to each cylinder to provide a very fine mist of fuel.

But the big factor is that an on-board computer can read several conditions in the engine like air temperature, engine temperature, revolutions per minute, load, throttle opening, intake manifold pressure to name just a few. It is also programmed to provide exactly the right amount of fuel for all conditions at all times.

The amount of fuel injected is adjusted by how many milliseconds each injector stays open during each cycle and this can be varied in small increments over a wide range.

It cannot be pulled apart and fixed on the roadside like carburetors could, but electronic fuel injection is extremely reliable.

The injectors can be cleaned regularly by adding a cleaning agent while the engine is running (best done by your service workshop) and electronic faults - which are rare - can be traced by the service workshop's diagnostic computer in the same way as other engine functions are monitored.