This paper presents an original approach to estimate the air-fuel ratio (AFR) of the mixture that burned inside a given cylinder of a spark-ignited (SI) internal combustion engine, using the information hidden in the corresponding in-cylinder pressure signal. In modern closed-loop fuel injection control strategies, the feedback signal is usually given by one (or more) heated exhaust gas oxygen (HEGO) sensor(s), mounted in the exhaust manifold(s). The information that such sensors give is related to the stoichiometry of the mixture that burned inside the cylinders. The HEGO sensor is not able to evaluate the AFR value precisely, being only able to determine whether the mixture was rich or lean. This information is sufficient to allow the implementation of a closed-loop strategy for injection time control. Generally speaking, such strategy could be improved in terms of readiness and precision by directly measuring (or by estimating) the actual AFR. Universal exhaust gas oxygen (UEGO) sensors are still considered expensive and their use is mostly limited to laboratory and racing applications, even if some automotive manufacturers have started installing such sensors on board passenger cars, as part of an effort to comply with ULEV (ultra low emission vehicles) regulations. For this reason the idea of estimating AFR values from other signals has received great attention in the past few years. A new approach based on in-cylinder pressure frequency analysis is presented here.

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