Abstract: The invention relates to a sensor 9 for detecting the degree of ionisation in a combustion chamber 2 in combustion engines with direct injection, and to a combustion engine equipped with such a sensor. The sensor is arranged around and preferably concentrically to the injector 6 in the latter's duct 5 in the cylinder head 1. The measuring electrode of the sensor 9 is substantially sleeve-shaped and is clamped firmly and centered by electrically insulating washers 11, 12 between a shoulder 7 in the duct 5 and a corresponding shoulder on the injector 6. The result is an inexpensive sensor which may easily be replaced without major modifications of the cylinder head or simultaneous replacement of the injector.