Most interesting, however, is the air mass that is drawn into each cylinder, as it is to this air the petrol is to be added. The control module registers the air mass drawn in during one engine revolution. As 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 be aware how much 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 each time. The milligrams air/combustion is divided by 14.7 and the result is the number of mg fuel/combustion to be injected into the cylinder.
The following text is an account of why the basic fuel quantity must sometimes be adjusted to a leaner, or most often, a slightly richer mixture so that the engine will run smoothly and emissions will be kept within required limits.