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Closed-Loop Spark-Advance Control using the Spark Plug as Ion Probe

The spark advance determines the efficiency of spark-ignited (SI) engines by positioning the combustion in relation to the crank shaft rotation. Today's spark-advance controllers are open-loop systems that measure parameters affecting the spark-advance setting and compensates for their effects. Several parameters affect the best spark-advance setting but it would be too expensive to measure and account for them all. This results in a schedule that is a compromise since it has to guaranty good performance over the range of all the non-measured parameters. A closed-loop scheme instead measures the result of the actual spark advance and maintains an optimal spark-advance setting in the presence of disturbances. One possible measurement is the in-cylinder pressure which gives the work produced by the combustion. Another measure is the ionization current that uses the spark plug as sensor, when it is not used for ignition. This is a direct in-cylinder measurement which is rich on information about the combustion.

A novel approach to spark-advance control is presented, which uses the ionization current as sensed variable. The feedback control scheme is closely related to schemes based on in-cylinder pressure measurements, that earlier have reported good results. A key idea in this approach is to fit a model to the measured ionization current signal, and extract information about the peak pressure position from the model parameters.

The control strategy is validated on an SI production engine, demonstrating that the spark-advance controller based on ionization current interpretation can control the peak pressure position to desired positions. The controller maintains an optimal spark advance under various conditions and in the presence of environmental disturbances such as air humidity. In addition, a new method to increase the engine efficiency is presented, by using the closed-loop spark-advance control strategy in combination with active water injection.

Lars Eriksson


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Last updated: 2021-11-10