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Basic fuel quantity

Basic fuel quantity


The mass air flow sensor grounds the control module's input with a frequency which is dependent on the mass air flow. When the mass air flow increases, the frequency will also increase. The control module converts the frequency to grams air/second (g/s).

Most interesting is, however, the air mass that is drawn into each cylinder, as this is the air to which the petrol is to be added. The control module registers the air mass drawn in during one engine revolution. Since the engine is 4-cylinder 4-stroke, two cylinders must have drawn in air simultaneously during this engine revolution. The air mass passing the mass air flow sensor is divided by two so that the control module will know how much air each cylinder has drawn in. The unit has now changed to milligrams air/combustion (mg/c).

To achieve lambda = 1, there must be a specific fuel/air ratio, namely 1 kg fuel to 14.7 kg air. As we know how much air has been drawn into each cylinder per combustion, the control module can easily calculate how much fuel is to be injected into the cylinders on each occasion. The milligrams air/combustion figure is divided by 14.7 and the result is the number of mg fuel/combustion to be injected into the cylinder.

The following section explains why the basic fuel quantity must sometimes be adjusted to a leaner or, more frequently, a slightly richer mixture for the engine to function well and for emissions to be kept within prescribed limits.