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Abstract



Spark Advance Modeling and Control


The spark advance determines the efficiency of spark-ignited (SI) engines by positioning the combustion in relation to the piston motion. Today's spark-advance controllers are open-loop systems that measure parameters that effect the spark-advance setting and compensate for their effects. Several parameters influence the best spark-advance setting but it would be too expensive to measure and account for all of them. This results in a schedule that is a compromise since it has to guarantee 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. To cover this area two questions must be addressed: How to determine if the spark advance is optimal and how it can be measured? This is the scope of the present work.

One possible measurement is the in-cylinder pressure, which gives the torque, but also contains important information about the combustion. The cylinder pressure can accurately be modeled using well known single-zone thermodynamic models which include the loss mechanisms of heat transfer and crevice flows. A systematic procedure for identifying heat-release model parameters is presented.

Three well-known combustion descriptors have been presented in the literature that relate the phasing of the pressure signal to the optimal ignition timing. A parametric study was performed showing how changes in model parameters influence the combustion descriptors at optimum ignition timing.

Another possible measurement is the ionization current that uses the spark plug as a sensor, when it is not used for ignition. This is a direct in-cylinder measurement which is rich in information about the combustion. A novel approach to spark-advance control is presented, which uses the ionization current as a 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. A new method to increase engine efficiency is presented, by using the closed-loop spark-advance control strategy in combination with active water injection. However, the major result is that the controller maintains an optimal spark advance under various conditions and in the presence of environmental disturbances such as air humidity.

Lars Eriksson

1999

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