Intake air dynamics on a turbocharged SI-engine with wastegate

University dissertation from Linköping : Linköpings universitet

Abstract: On turbocharged spark-ignited (SI) engines with wastegate the position of the wastegate changes the exhaust manifold pressure. A secondary effect of this is that the residual gas mass trapped inside the cylinder at exhaust valve closing changes and causes the volumetric efficiency to change. The volumetricefficiency is used to estimate air-mass-to-cylinder which is important for good air/fuel ratio control.Air-mass to-cylinder is not directly measurable so observers for air-mass flow to the cylinder are therefore often proposed. For observers with one state for intake manifold pressure and proportional feed-back from measured state, there is a tradeoff whether to estimate intake manifold pressure or air-mass-to-cylinder. A new nonlinear air-mass-to-cylinder observer is suggested with two states: one for intake manifold pressure and one for the in-cylinder air-mass offset compared to expected using the volumetric efficiency.The exhaust manifold pressure can change rapidly in an engine with wastegate. A method to estimate the exhaust manifold pressure is presented for diagnosis of wastegate and turbocharger on SI-engines. It does not use any extra sensors in the exhaust system after the calibration. The exhaust manifold pressure estimator is validated using a series of wastegate steps. The exhaust pressure estimation is designed for steady-state conditions and the validation shows that it works well and converges within 1 to 4 seconds.Finally a method to detect leakages in the exhaust manifold is suggested. Leakage detection before the three way catalyst is important since untreated emissions leak out and since, due to standing waves in the exhaust system, air can leak in and disturb the air/fuel ratio controller. To extend the operating region for the detection, the proposed method utilizes both information on leaks out of the manifold and information on presence of oxygen in the exhaust manifold.

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