During the combustion of an air-fuel mixture ions are produced by thermo, photo, and chemical ionization. The process of ion neutralization in heavy hydrocarbons and particles is considerably slower than in gas molecules. Only exhaust gas particles and atoms are expected to carry an electric charge.The ions flowing in the exhaust manifold can be detected by an ion probe. The induced charge at the ion probe can measure up to 50 pC and can be directly recorded on an oscillograph.The signals from two diesel engines (DI and IDI) and as a double check also a spark-ignition engine were picked up by an ion probe. In the case of the diesel engine the signals increased with the smoke emission. Signals from the spark-ignition engine have a different shape to those of a diesel engine.