Interaction of Components

The pistons moving up and down while the engine is running draw in air. If you fully depress the accelerator pedal, the engine draws in the maximum possible amount of air, because both throttle valves are fully open. Accordingly, with the throttle valves closed or partially open, the air quantity/mass is lower. For clean engine operation, the intake air must be mixed with fuel in a precisely defined ratio. To determine the fuel/air ratio, the MPI system uses the intake air mass. The air flow meter serves as the source of intake air mass information for the MPI control unit. The comparable value in the MPFI system is the intake manifold pressure and temperature information.

The control unit can now send the correct pulses to open and close the injectors. A longer opening time is required when a large amount of fuel is needed; a short opening time is sufficient when only a small amount of fuel is required. The injector 'injection rate' thus remains constant. The quantity is reduced by shortening the 'injection time'.