Haltech F10 User Manual download pdf (Page 9)

How It Works

While the technology involved with electronic fuel injection is complex, the underlying

principles of its op eration are really quite straightforward. The object of any fuel delivery

system in a gasoline engine is to determine the amount of air being drawn by the engine, and

supply the appropriate quantity of fuel to "burn" all the oxygen in that mass of air.

A carburettor uses primarily only one parameter to determine fuel metering: air speed. Higher

air speeds through the carburettor result in larger pressure drops across the venturis, and thus

more fuel is sucked through the jets.

Electronic fuel injection revolves around the use of solenoid-actuated injectors. These devices

employ a coil attached to a valve. When the coil is energised, the valve op ens and fuel is

allowed to flow. As long as the p ressure between the fuel and the air in front of the injector

nozzle is held constant, the rate of fuel flow will remain the same. By accurately controllin g

the length of time the injector remains open, precise quantities of fuel can be metered to the

engine.

Since we have no convenient means of directly measuring the amount of air entering the

engine to determine the amount of fuel to deliver, we use a number of engine parameters to

determine an injection opening time. We build a table that breaks the engine's operation into a

series of rpm ranges. At each ran ge, we consider the load on the en gin e, usin g either the

position of the throttle or the manifold pressure as a reference to the load on the engine.

Collectively, the ranges in this table (also called a look-up table), form a map of the

volumetric efficiency for the engine. Our standing assumption, therefore, is that for any

combination of engine speed and load, we have a direct reference to the amount of air that is

being drawn into the engine by means of this map.

The Haltech F10 uses a digital microcomputer to measure engine speed and load, and uses

them to access the base fu el map. The base fuel map is a look-up table of injector opening

times stored in non-volatile memory i.e. when power is switched off, the contents of the

memory are retained. By using the programming software, the contents of this memory can be

changed so that y ou can match injector opening times to the injectors you are using, and to

suit the requirements of your engine.

Having determined the base injection time, the microcomputer then performs a number of

adjustments to this value. Corrections for air temperature and barometric pressure are applied,

since these variab les affect the density of air. Extra in jection time is also added, when

necessary, for transient throttle movement and the temperature of the engine. At the end of all

these calculations, the final in jection time is determined: the time for which the injectors are

actually held open.

Injection p ulses usually occur on e or more times per en gine cy cle. The ECU uses a trigger

signal locked to en gine speed in order to determin e when to inject. When it receives an

appropriate trigger, the ECU ap p lies a magnetising current to the injector coils for p recisely as

long as the final computed injection time, providing an extremely accur ate delivery of fuel

that will exactly suit the engine's needs.

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Haltech F10 User Manual Page 9