1. ABSTRACTThe combustion of fuel inside an engine cylinder produces ions. By applying a small DC voltage across the spark gap after ignition, ion current is produced. The ion current wave form contains combustion information and is called Ion-Gap sense. Ion-Gap sense is described and can be used to detect misfire, control knock, obtain cam phasing, and detect pre-ignition. In the future, it will provide air/fuel information.Test results on nine different engine types are summarized for misfire detection. Knock testing is reviewed for eight engines. Engine tests associated with engine control techniques have been included.