Passenger car turbochargers are contributing to further improvement of fuel efficiency by recovering discarded thermal energy and engine downsizing. This turbocharging process is a positive feedback system. The structure of this system was determined according to the principle equation of thermodynamics, and the parameters were determined through experiments to derive a reduced order turbocharging process model. It is feasible to obtain further fuel economy with EGR that increases combustion gas at stoichiometric air-fuel ratio. The fuel economy merit is limited within partial load in a naturally aspirated engine. However, the fuel economy range can be extended in an engine with a turbocharging system. A suitable turbocharged engine model is required to evaluate fuel economy strategies or to design an engine controller. A smaller model is preferable considering model construction work and calculation time. A turbocharged engine consists of many sub-systems and each sub-system has different cutoff frequency that determines calculation sample time, e.g. cut off frequency of turbocharger is much smaller than that of a vehicle because of lower inertia. Since one cycle work of the gasoline engine is determined at the time of intake valve close, the mean value engine model is useful in terms of accuracy and convenience if combustion fluctuation is not considered. Turbocharging process is a feedback system with a positive gain, i.e. increasing exhaust work leads to increasing a cycle work. The feedback path consists of a turbine – compressor system. Behavior of the system was studied with turbine speed measurement on a test engine. The gain of the feedback system is determined mainly by exhaust work ratio in a cycle and inertia of the turbine. Each stroke work ratio to indicated mean effective work in a cycle was also investigated with EGR based on the engine test. A turbocharging process model was obtained using the work ratio in a cycle and theoretical equations. The model is applied to investigate manifold absolute pressure strategies with EGR.
Dr. Eng. Kazuhide Togai, Osaka Sangyo University, JAPAN Mr. Naoto Fujinaga, Mitsubishi Motors Corporation, JAPAN