A New De-throttling Concept in a Twin-Charged Gasoline Engine System

Throttling loss of downsized gasoline engines is significantly smaller than that of naturally aspirated counterparts. However, even the extremely downsized gasoline engine can still suffer a relatively large throttling loss when operating under part load conditions. Various de-throttling concepts have been proposed recently, such as using a FGT or VGT turbine on the intake as a de-throttling mechanism or applying valve throttling to control the charge airflow. Although they all can adjust the mass air flow without a throttle in regular use, an extra component or complicated control strategies have to be adopted. This paper will, for the first time, propose a de-throttling concept in a twin-charged gasoline engine with minimum modification of the existing system. The research engine model which this paper is based on is a 60% downsized 2.0L four cylinder gasoline demonstrator engine with both a supercharger and turbocharger on the intake. The idea is to use a CVT driven supercharger to ‘throttle’ the intake mass flow. By the adoption of a CVT, the supercharger outlet pressure could be controllable. Depending on whether the outlet pressure is larger than the inlet, the supercharger could supply boost at high load consuming engine power or behave like an expander under part load presenting a means to recover the throttling loss to provide all the necessary need. A 1-D simulation model was used for this research with some experimental data of the supercharger functioned as an expander from a test rig. The results showed that at part load, by recovering some throttling loss through the supercharger, up to 3% BSFC improvement could be achieved compared to the throttled counterpart depending on the engine operating points. The effect of the reduced supercharger outlet temperature on the combustion efficiency was also discussed. It showed in the end that by the speed control, extended working range of the supercharger can be achievable which could push the fuel efficiency of the downsized engine further.