1ABSTRACTA recent breakthrough in understanding the origin of ion signals from operating combustion engines  led to a new approach in integrating advanced ion current sensing into a compact ignition system. Thus it is now possible to continuously monitor mixture, ignition and combustion properties through online ion current recordings via a novel AC technique. In this paper this AC technique is compared to the standard DC technique and its known drawbacks: expensive high voltage components, sensitivity to plug fouling and expensive electronics.The AC technique is based on the specific properties of the electrical field of spark plugs being characterized by a point source with an extreme inhomogeneity of the electrical field due to the small center electrode. This causes a distinct diode characteristic of the ion signal: very low signals for negative voltages and high signals for positive ion sensing voltages, respectively. Using an AC sensing voltage provides thus a very simple, robust and reliable ion current detection scheme.This AC technology is extremely simple to integrate into modern AC ignition systems since the sensing signal can be readily imposed on the primary ignition circuit without any need for additional components on the primary, or even more important, on the high voltage side. Additional advantages are: arbitrary high sensing voltages, simple control on the primary circuit, no separate sensing voltage source required and insensitivity to plug fouling.This AC technique has been implemented into a lab-type ignition system which was successfully applied to various S.I. engines. In this paper the fundamentals of the AC technique and it's implementation will be described in detail. Special emphasis will be given to the inherent robustness and insensitivity to plug fouling. Since the shunt resistor formed by plug fouling can be determined quantitatively, it can be also used for continuously monitoring and controlling the performance of long life ignition systems. Characteristic examples of engine results will be presented.