Stochastic limit control and its application to spark limit control using ionization feedback

Spark timing of an internal combustion (IC) engine is often limited by engine knock in advanced direction and by partial burn and misfire in retard direction. The ability to operate the engine at either its advanced (borderline knock) spark limit or its retard (partial-burn or misfire) spark limit is the key for improving emissions and fuel economy. Due to combustion cycle-to-cycle variations, IC engine combustion behaves similar to a random process. For example, the combustion stability metric covariance of indicated mean effective pressure (IMEP) is calculated from in-cylinder pressure signals, assuming that IMEP is a random process. Presently, the spark limit control of IC engines is deterministic in nature. The controller does not utilize stochastic information associated with control variables such as knock intensity for advanced limit control and combustion stability for retard limit control. This paper proposes a stochastic limit control strategy to maintain engine operation in a normal region. It also presents a simple stochastic model for evaluating the proposed stochastic controller. The stochastic limit control is applied to both borderline knock limit control and combustion stability retard limit control on a 3.0L V6 engine.