As we know, a petrol engine operates by the explosion,
in its combustion chambers, of a mixture of air and petrol, compressed
by the pistons.
Good combustion depends upon the precision of the
blending of the air and the petrol. The theoretically perfect
proportioning to obtain complete combustion is 1 gram of petrol for 15
grams of air. This is the stoichiometric ratio: 1/15.
To obtain more power, it is necessary to enrich the mixture with fuel
(1/12.5):for better efficiency (burn less at equal output), it is
necessary to make the mixture leaner (1/18).
This gaseous mixture can be made by a carburettor or by injection.
In an injection engine, each cylinder has an injector
which directly sprays the petrol into the intake valve, at the exact
instant and in the exact quantity necessary.
This is why injection is the fuel supply system best fitted for
optimized proportioning, homogeneous proportioning with equal
distribution among the cylinders, cold start-up with fast turnover,
good filling at high engine speed.
Injection provides increased engine performance by improving the
filling with air (elimination of the choke formed by the carburettor
nozzle), a reduction in the specific consumption of fuel and in the
exhaust gas proportion of carbon monoxide and unburnt hydrocarbons due
to more accurate proportioning of the fuel, quieter engine operation
and increased flexibility due to uniform combustion in the various
cylinders, and automation of the activating of the engine when cold and
In the type of injection provided by the BX 19 GTi's Jetronic LE-3 ,
the quantity of air sucked in is the most accurate proportioning
reference for specifying the quantity of petrol necessary for good
An electronic computer receives accurate and instantaneous information
concerning the quantity and temperature of the air sucked in, the
turning rate and the engine load condition, the cooling water
temperature and the beginning of injection. By processing this data,
the computer accurately determines the quantity of petrol which is
strictly necessary for the engine at a given moment and it actuates and
controls the precise duration of opening of the injectors.
Electronic injection comprises three separate circuits: - petrol, air, electronics.
The petrol circuit:
The petrol is routed from the tank to an injector array by an electric
pump. In this array, a regulator keeps the petrol pressure
substantially constant. The electronically controlled injectors spray
the petrol upline from the intake valves. Their flow rate depends only
upon their opening time. When the engine is cold, the mixture is
enriched by a longer injector opening time.
The air circuit:
The air sucked in, coming from the air filter, crosses the airflow
meter and the butterfly valve and then comes to the intake manifold
each conduit of which is connected to a cylinder. The airflow meter
sends a voltage signal to the computer (which will determine the
opening time of the injectors, therefore the quantity of petrol), and
this signal varies as a function of the quantity of air sucked in by
It has an air temperature probe on a by-pass circuit for regulating the
mixture on idle. It brings the petrol pump into the circuit. Based upon
the temperature of the engine, an additional air command supplies extra
air during the engine heat-up phase.
The electronic circuit:
The electronic computer receives information concerning the quantity of
air sucked in by the engine, the cooling water temperature, the
position of the air valve, the start-up phase, plus information
concerning engine speed and the beginning of injection. It processes
this data and instantly and accurately determines the quantity of
petrol necessary in each cylinder at moment M, then, by electric
pulses, controls the exact opening time of the solenoid injectors.