Haltech E6GMX - Manual (Page 10)

E6GMX Manual
10
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 of 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 generally only one parameter to determine fuel metering: air speed. Higher
air speeds through the carburettor result in larger pressure drops across the venturis, resulting
in more fuel being drawn through the jets.
Electronic fuel injection is based on 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 difference 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
delivered to the engine.
Since there is no convenient means of directly measuring the amount of air entering the
engine to determine the amount of fuel to deliver, the injection opening time can be calculated
using a number of engine operating conditions. The ECU uses a table that breaks the engine's
operation into a series of rpm ranges, each range has a series of points that represents the
different loads on the engine, using either the position of the throttle or the manifold pressure
as a load reference.
The ranges in this 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 ECU 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.
Corrections for air temperature and barometric pressure are applied to the base fuel value,
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. Once these
corrections have been applied the ECU knows the amount of fuel the engine requires.
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 ECU has a table
configured for ignition instead of fuel and applies corrections in a similar way.