Haltech E6A - Manual (Page 10)

10
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 E6A 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.
The ignition timing is determined in a similar way to the fuel needs. The Haltech E6A 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.