| 摘要 |
The invention relates to a method and system for controlling combustion engines by detection of the present air/fuel ratio within the cylinders of the combustion engine, using an analysis of the characteristics of the ionisation current, as detected via a measuring gap with a bias voltage applied being arranged in the combustion chamber, preferably using the spark plug gap in an Otto-engine. A measuring voltage corresponding to the degree of ionisation is detected during the flame ionisation phase and during a time- or crankshaft position dependent period A, B, C or D, which duration is dependent of the present air/fuel ratio, and at finally will be finished by an amplitude maximum PF during the flame ionisation phase. A parameter characteristic for the fundamental frequency of the measuring voltage during the period A, B, C or D is detected, which parameter indicates a tendency towards the rich direction of stochiometric when the fundamental frequency increases, and inversely indicates lean tendency when the fundamental frequency decreases. The fundamental frequency is preferably detected from the differential value of the measuring voltage during the period A, B, C or D, in respect of time t or crankshaft degrees VC, dUION/dt respectively dUION/dVC. The differential value multiplied with a constant is used at least partly when determining a relative or absolute air/fuel ratio.
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