Function mode of the Engine Control Unit (ECU)

The ECU consists of one or more small fast microcomputers, which measure, check and control the important engine functions. The microcomputer processes a program, which is stored in the chip memory (EPROM). For petrol engines the main task is the calculation of the requested and maximum possible amount of fuel to be injected. Since the air/fuel ratio must be correct and exact (the theoretical mixing proportion is approx. 1:14,8), to ensure the catalyst works properly, the amount of fuel injected is dependant upon the amount of air available. Furthermore the time must be determined, when the compressed mixture is to be ignited. If the ignition takes place too late, fuel consumption rises, when it is too early, the engine begins to knock. Additionally, the engine management system solves many other tasks, for example the automatic and gentle disengagement of the fuel injection system upon reaching the maximum engine rpm or upon reaching a given limitation of the maximum speed (e.g. M3, M5, E50, etc.).

With modern diesel engines the amount of fuel injected is determined by a function of the air mass, air pressure (atmospheric pressure), outside temperature, engine coolant, oil temperature, rpm and load. In addition measurements of boost and load volume for the turbocharger and speed-sensing must be determined accurately with turbo-vehicles. This operating and driving data is captured, by means of appropriate sensors, and the ECU processes them. Stored on a special chip (EPROM), is a data base with the appropriate characteristics diagrams. These include the characteristics for injection, ignition, boost and Lambda. The ECU calculates the amount of fuel to be injected based upon the demand required to reach a certain rpm while maintaining optimal ignition and the appropriate boost.