Early open loop injection
Example 3.5 V8 Range rover
The early injection systems work by monitoring the amount of air entering the engine via the airflow meter (AFM), and the throttle position via the throttle position sensor (TPS). The ECU reads this information and tells the petrol injectors how long to stay open. Thereby adjusting the fuel air mix.
On these systems there is normally a 'flap' or 'door' inside the AFM which is pushed open by the air entering the engine. The ECU reads the position of this flap.
These systems are Full Group Injection, meaning that all the petrol injectors fire at once. The fuel is then sucked into the cylinders from the manifold when the inlet valve opens.
The most common LPG system to be fitted to to this type of vehicle is an open loop induction kit, a mixer is fitted to the air intake of the engine, this is fed by the vaporiser/reducer which has a sensitive diaphram inside. As the air entering the engine rushes through the venturi of the mixer a depression is caused in the pipe from the vaporiser causing the diapham to be pulled, this in turn releases more gas from the vaporiser into the air intake of the engine. The petrol injectors are switched off, when the vehicle is running on gas, using an emulator.
A is the position of the mixer on a hot wire injection system ( 3.9 etc)
B this is the position of the ABV (anti blow-back valve) Normally used on flap type AFM
C is the plate mixer fitted to the front of the flap type air flow meter.
D is the flap type air flow meter
Closed loop fuel injection.
Most vehicles built from around 1993 onwards
This system works the same as the open loop described above, with some important changes. The flap inside the AFM has been replaced by a 'hot wire' which the air rushes over. The resistance is read to determine the rate of air flow. This is a much more precise and reliable system compared to the flap type. However the biggest inprovement comes from the Lambda probe or oxygen sensor fitted in the exhaust. The lambda probe reacts to the amount of oxygen present in the exhaust gasses. Most commonly by generating a voltage of between 0-1volt (some probes do vary - you must check which probe you have)
0 volt = weak mixture
1volt = rich mixture
So anything 0 - 0.5 volt is considered weak, 0.5 - 1 volt a rich mixture. In reality nothing is stable enough to stay a 0.5 of a volt. The vehicles ECU will increase the fuel until a rich mixture is sensed by the probe. It will then reduce the fuel until a lean mixture is sensed - then increase it again - and so on!
The ECU knows how much air is entering the engine, it knows what position the throttle is set to and it knows what the mixture of fuel and air is. So buy increasing or decreasing the injection timing ( that is the amount of time it tells the petrol injectors to stay open for). It has complete control of the mixture.
This means that if we fit an LPG kit to this vehicle, we can also use the signal from the oxygen sensor to control the mixture of air + LPG. The way this is done is to fit a control box that reads this signal. And a stepper motor to control the mixture. This fits in the main low pressure gas pipe between the vaporiser and the mixer. If the control box receives a rich signal it closes the stepper motor until it gets a weak signal and then it opens it again - and so on.
As with the petrol injection the control box, or ECU, also knows the throttle position, so if the throttle is pushed open, the systems allows the mixture to go rich, as more fuel is needed for the engine to accelerate.