Ion current sensors have been considered for the feedback electronic control of gasoline and diesel engines and for onboard vehicles powered by both engines, while operating on their conventional cycles or on the HCCI mode. The characteristics of the ion current signal depend on the progression of the combustion process and the properties of the combustion products in each engine. There are large differences in the properties of the combustible mixture, ignition process and combustion in both engines, when they operate on their conventional cycles. In SI engines, the charge is homogeneous with an equivalence ratio close to unity, ignition is initiated by an electric spark and combustion is through a flame propagating from the spark plug into the rest of the charge. In diesel engines, the charge is heterogeneous with equivalence ratios that vary from zero to infinity, gas pressures are much higher than the pressures in gasoline engines, autoignition starts the combustion process in a premixed charge followed by combustion controlled by mixing and diffusion processes.This paper examines the sources of ionization in hydrocarbons-air flames and the characteristics of the ion current signals in S I and C I engines under their conventional and HCCI operating modes. Some concepts have been introduced on the ionization in diesel engines, based on the limited publications in this area and the experimental results of this paper. Experiments are obtained on three engines. The first is a single cylinder gasoline engine equipped with different types of spark plugs used as ion current sensors. The second is a single-cylinder diesel engine equipped with a glow plug converted to a glow-ion current sensor. The third is an optically accessible diesel engine where images are taken to show the interaction between the different sprays and their impact on the ion current. The similarities and differences between the ion current signals in SI, HCCI and CI engines are explained.