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Cylinder Air Charge Estimator in Turbocharged SI-Engines

Mean value cylinder air charge (CAC) estimation models for control and diagnosis are investigated on turbocharged SI-engines. Two topics are studied; Firstly CAC changes due to fuel enrichment and secondly CAC sensitivity to exhaust manifold pressure changes. The objective is to find a CAC model suitable for control and diagnosis.

Measurements show that CAC models based on volumetric efficiency gives up to 10% error during fuel enrichment. The error is caused by the cooling effect that the fuel has as it evaporates and thus increases the charge density. To better describe the CAC during fuel enrichment a simple one parameter model is proposed which reduces the CAC estimation error on experimental data from 10% to 3%.

With active wastegate control, the pressure changes in the exhaust manifold influences the CAC. The magnitude of this influence is investigated using sensitivity analysis on an exhaust manifold pressure dependent CAC-model. From the sensitivity analysis it can be concluded, that the CAC is most sensitive to exhaust manifold pressure changes for low intake manifold pressures (part load). Without taking the exhaust manifold pressure into account the CAC error is approximately 5% when the wastegate is opened at part load.

The exhaust manifold pressure dependent CAC model is then augmented with the charge cooling model and the total model gives precise agreement on experimental data. The resulting model is thus highly suitable for CAC estimation for control and diagnosis of turbocharged SI-engines.

Per Andersson and Lars Eriksson

SAE Technical paper series SP-1822, 2004

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Last updated: 2021-11-10