Gunson COLORTUNE G4074 Mode D'emploi page 33

attempt to measure flow directly with a moving vane in the air flow, or a hot wire
system which senses the cooling effect of the air flow. Others use a prediction from the
throttle position, engine rpm and the manifold pressure. All have various means of
compensating for engine and air temperature, atmospheric pressure etc. Early systems
use more mechanical parts, later systems still require a mechanical pump to maintain a
steady high pressure but control is almost entirely electronic.
Fuel injection systems with adjustable idle mixture
Single point injection.
This system has one injector and one throttle plate through which all air and fuel passes
into the engine manifold. Despite this simplicity, the same control systems are required
to achieve accurate fuel metering. Idle mixture and idle speed, are the two adjustments
that may be provided on early systems.
Multipoint injection with one throttle plate.
Separate injectors are placed close to the inlet valves rather than in the main throttle
body. The manifold system is designed with a suitable inlet tract length to enhance
engine performance. This feeds from a larger chamber and throttle body which does not
have the same dimensional restrictions as the single point system because it feeds air
only. Idle mixture and idle speed, are the two adjustments that may be provided on
early systems.
Multipoint injection multiple throttle.
Some early systems which were mainly mechanical in operation had this arrangement.
Balancing the air flow through each throttle plate is crucial to effective operation.
Injectors are set to deliver equal quantities of fuel and any error in air flow then has a
dramatic effect on mixture. Testing and fine tuning of the balance between throttles
requires patience even with a Colortune in each cylinder. It is almost impossible without
Colortune. A Lucas system of this type was fitted to a few high specification vehicles in
the early 70's and a few other road vehicles and racing systems fit this description.
Fuel injection systems with oxygen sensor feedback
The development of sophisticated electronic control systems plus a robust exhaust
oxygen sensor allowed the three way exhaust catalyst to be introduced. This had a
dramatic effect on exhaust emissions and was introduced into most vehicle markets
from the early 1990's.
The oxygen sensor gives no output when there is oxygen present in the exhaust (both
sides of the sensor are exposed to oxygen) and it gives an output (usually 1.0 volts) when
there is no oxygen present in the exhaust (when there is a difference across the sensor).
It is therefore an excellent detector of small percentages of oxygen and the electronic
control unit (ECU) is able to continually adjust above and below the point at which the
sensor has an output. The ability of the ECU to learn and adjust the programmed map of
information stored in its memory, adds a further precision of control.
14