Haltech F10 - Manual (Page 9)

4
How It Works
While the technology involved with electronic fuel injection is complex, the underlying
principles of its operation 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 opens and fuel is
allowed to flow. As long as the pressure 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 controlling
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 range, we consider the load on the engine, using 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 fuel 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 you 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 variables affect the density of air. Extra injection 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 injection time is determined: the time for which the injectors are
actually held open.
Injection pulses usually occur one or more times per engine cycle. The ECU uses a trigger
signal locked to engine speed in order to determine when to inject. When it receives an
appropriate trigger, the ECU applies a magnetising current to the injector coils for precisely as
long as the final computed injection time, providing an extremely accurate delivery of fuel
that will exactly suit the engine's needs.