Misfire Detection

As part of CARB/OBD regulations, engine control module must determine if misfire is occurring and also identify the specific cylinder(s) and the severity of the misfire event, and whether it is emissions relevant or catalyst damaging. In order to accomplish these tasks the control module monitors the crankshaft for acceleration losses during firing segments of each cylinder based on firing order.

Misfire/engine roughness calculation is derived from the differences in the period duration of individual increment gear segments. Each segment period consists of an angular range of 120 degree crank angle that starts 78 degrees before Top Dead Center (TDC). If the expected period duration is greater than the permissible value, a misfire fault for the particular cylinder is stored in the fault memory of the DME. Depending on the level of misfire rate measured, the control unit will illuminate the CHECK ENGINE light, may cut-off fuel to the particular cylinder and may switch lambda operation to open loop. All misfire faults are weighted to determine if the misfire is emissions relevant or catalyst damaging.