A Model-Based Investigation of Electrically Split Turbocharger Systems Capabilities to Overcome the Drawbacks of High-Boost Downsized Engines

Engine downsizing is one the most common methods of coping with strict emission regulations. However, it must be coupled with complementary systems so that the engine performance would meet the standards. That is why new efficient solutions can pave the way toward this goal. The electric forced-induction system (EFIS) is the emerging replacement for conventional forced-induction systems (FIS), namely, turbochargers and superchargers. The reason behind this replacement is the drawbacks associated with FIS, among them are turbo lag and inefficiency in exhaust gas energy recycling. Electrically split turbocharger (EST) is a form of EFIS which offers a great potential for engine downsizing. In this paper, a new approach to EST utilization for lowering the fuel consumption (FC) without compromising performance has been introduced, through which the augmented degree of freedom enabled by an EST is used to optimize the air-charge boosting. To show the effectiveness of the proposed method, a model-based approach is used to compare two engines with and without EST technology; the performance of an already existing 1.6-l 4-cylinder turbocharged engine has been modeled based on the experimental data, and its performance indices are used as a benchmark for a downsized 1l 3-cylinder engine equipped with an EST. A comparison of these two engines in the dynamic drive cycles of the EPA Federal Test Procedure (FTP75) and Worldwide harmonized Light vehicles Test Cycles (WLTC) has shown a 28.87% and 25.35% reduction in FC, respectively, independent of the external electrical source. Furthermore, the downsized engine has shown superior performance through full-throttle acceleration in terms of torque transient response. Finally, the concept of coherence among gas-path components and its importance is presented, and knock precautions associated with air charging in this method are addressed.