Haltech E8 - Manual (Page 149)

Haltech E11/E8 Instruction Manual
Appendix A: Electronic Engine Management Overview
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 ECU uses a digital microcomputer to measure engine speed and load, and uses them to
index to the base fuel map. The base fuel map is a look-up table of injector opening times stored in non-
volatile memory. Non-volatile memory simply means when power is switched off, the contents of the
memory are retained. By using the Halwin 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 programmed the base injection times, the microcomputer is able to perform a number of
adjustments to this value. Corrections for air temperature and barometric pressure can be 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.
The ignition timing is determined in a similar way to the fuel needs. The Haltech ECU has a look-up table
configured in the same way as for the fuel, but instead of the fuel delivery in the table the Ignition Map
contains the Ignition Advance for that point. This means that the ignition point can be controlled with much
greater accuracy then ever possible with bob-weights and vacuum advance in a distributor.
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