Application of a Supercharger in a Two-Stage Boosting System for a Gasoline HCCI Engine: A Simulation Study

Recently, a number of studies have demonstrated that boosting can extend the high load limit of HCCI. This paper compares two two-stage boosting systems for a 4-cylinder, 2.0 liter engine, within the framework of a 1D engine simulation. A series two-stage boosting system wherein both high and low pressure stages are turbochargers (TCTC) is compared with another series two-stage system, where the high pressure stage from TCTC is replaced with a small supercharger (TCSC). The engine model in these configurations is operated in steady state at high load boosted HCCI points (∼ 6.5 bar BMEP) over a range of engine speeds. The comparison has been carried out by two methods: in Method I the intake pressure to the engine has been matched for both TCTC and TCSC; and in Method II, the amount of fresh charge into the engine has been matched for both systems. A detailed energy balance shows that the performance in terms of BSFC for the TCSC system is worse for Method I. However, this changes for Method II, and the TCSC system is comparable or even better than the TCTC system. This is achieved by greatly reducing the pumping losses associated with the TCTC system, while the parasitic losses of the supercharger are minimized by having to boost to lower intake pressures due to lowered back pressures in the TCSC system.