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Air Charge Estimation in Turbocharged Spark Ignition Engines

Turbocharged (TC) spark-ignited (SI) engines are popular as they combine high power output with good fuel economy. Furthermore, their emissions can be successfully reduced using a three way catalyst (TWC) provided that the air-fuel ratio is precisely controlled. As the air-fuel ratio depends on the precision of the air charge estimate, this thesis is devoted to the systematic improvement of the cylinder air charge (CAC) estimation on TC SI engines. A second objective is to provide the design engineer with a flexible framework for CAC estimation that easily can be adapted to various engines.

The CAC is not measurable and it is therefore estimated using a model. Part I concentrates on stationary conditions and examines existing air charge estimation methods using engine experiments where the wastegate is opened and closed. Measurements show that the existing methods are insufficient for TC SI engines since the CAC depends on exhaust backpressure and charge cooling from evaporating fuel. A new 2-parameter CAC model which accounts for these effects is developed and the validation shows that the error is reduced from 10% to 3%.

Part II deals with transient conditions and a dynamical component-based model is developed for the gas flow systems of TC SI engines. The physical structure of different TC SI engines is similar and these similarities are exploited in the developed model. The division into components provides the basis for a flexible framework that enables a straightforward adaption to various engines. It is described how the model parameters are systematically fitted using an engine map and maps from the turbocharger manufacturer. The accuracy of the model is good and the stationary error is less than 10% on the intake side.

An observer that estimates the CAC, given available measurements, is suggested. It is shown that the system is locally structurally observable from arbitrarily measured model states. Further, a specific combination of signals that is most suitable for CAC estimation is pointed out. The developed observer is based on the constant gain extended Kalman filter (CGEKF) and a systematic method for selecting the design parameters in CGEKF filters is proposed. The method only requires an engine map and the variance of the signals considered for observer feedback. Several different combinations of observer feedback signals are studied and it is shown that the observer is capable of estimating the model states. The design method is successfully tested on two different engines.

Finally, the developed model and observer is used for model-based air-fuel ratio control. A TC SI engine is controlled by the proposed controller in real-time and the transient deviations from λ=1 are less than 7% in very rapid throttle transients.

Per Andersson


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Last updated: 2021-11-10