A New Turboexpansion Concept in a Twin-Charged Engine System

Engines equipped with pressure charging systems are more prone to knock partly due the increased intake temperature. Meanwhile, turbocharged engines when operating at high engine speeds and loads cannot fully utilize the exhaust energy as the wastegate is opened to prevent overboost. The turboexpansion concept thus is conceived to reduce the intake temperature by utilizing some otherwise unexploited exhaust energy. This concept can be applied to any turbocharged engines equipped with both a compressor and a turbine-like expander on the intake loop. The turbocharging system is designed to achieve maximum utilization of the exhaust energy, from which the intake charge is over-boosted. After the intercooler, the turbine-like expander expands the over-compressed intake charge to the required plenum pressure and reduces its temperature whilst recovering some energy through the connection to the crankshaft. It is anticipated that such a concept has benefits for knock resistance and energy recovery despite suffering higher pumping losses. This paper, for the first time, will investigate the net fuel efficiency benefit from this concept using a super-turbo twin-charger 1-D simulation model. By the operation of a switch between compressor and expander mode, the supercharger could provide boost at low engine speed whilst behaving like a turbine presenting an indirect means to recover exhaust gas energy at high engine speed and meanwhile reducing the intake temperature. The results showed that the BSFC improvement depended on the efficiency of the supercharger as an expander.