RPM Pickup
Overview
The ECU uses a sensor to determine the position of the crankshaft. The crankshaft position is used to determine the engine speed and the position of the engine’s pistons so that the ECU can control the ignition and fuel injection timing.
Most engines use a timing wheel with teeth that pass by a sensor. The timing wheel may have 1 or more missing teeth or an extra tooth.
The position(s) of the missing/extra teeth are detected by the ECU and used as a reference point for the engine’s position.
A full engine cycle on a 4 stroke engine is 720 degrees of crankshaft rotation. If sequential injection and/or ignition is required, then an additional sensor is required to determine the engine cycle position. Typically this is a camshaft sensor which rotates at half the speed of the crankshaft.
For port-injected engines that do not have the ability to detect the engine cycle position, it is possible to run the engine ‘wasted spark’ with batch injection. Batches of injectors are triggered simultaneously, once or twice per engine cycle. This is not ideal for emissions or fuel economy but may be acceptable for some applications.
In some cases there are no missing teeth on the timing wheel and an alternative synchronization strategy must be selected (e.g. cam sync).