Most downsized gasoline engines currently in the market place appear to have a ‘downsizing factor’ of approximately 30% to 40%. However, as concerns regarding fuel efficiency and emission legislations increase, more aggressive downsizing may have to be introduced. This, although can improve fuel efficiency and enhance power density of a gasoline engine further, has some challenges that must be addressed.Large backpressure is one of the most important aspects that needs to be improved for a highly downsized gasoline engine, especially if a Regulated 2-stage system is considered. The Divided Exhaust Period (DEP) concept is an alternative gas exchange process, where two exhaust valves from each cylinder separately function, with one valve leading the blow-down pulse into the turbocharger turbine, while the remainder of the exhaust mass flow bypasses the turbine through the other valve. The simulation results suggest that by adopting the DEP concept, the full-load Brake Specific Fuel Consumption (BSFC) and the stability of the engine were all improved due to the fact that the DEP concept features a better gas exchange process and improved combustion phasing. The part-load BSFC could also be reduced by using the scavenging valve to extend the ‘duration’ of the exhaust valve (thus reducing the re-compression effect) and to achieve internal exhaust gas recirculation (EGR) through reverse flow. However, this depends on the authorities of the scavenging valve timing (and piston clearance) and the combustion stability (EGR tolerance).Driveability issue and poor fuel efficiency in some engine operating regions of a conventional fixed-ratio positive-displacement supercharging system also need to be mitigated. A continuously variable transmission (CVT) driven supercharging solution, with the capability to decouple the supercharger speed from the engine speed, has the potential to provide the full-load BSFC improvement and to enhance the driveability performance, with a minor penalty in part-load BSFC. Both the simulation and experimental results have demonstrated its advantages over its fixed-ratio positive-displacement counterpart. The high-load fuel consumption can be improved by as much as 17.5%, and the time-to-torque performance can be improved by up to 37%. The low-load BSFC was only degraded by up to 2%, however, given that there was no clutch fitted for the CVT driven supercharging system, a better transient performance is anticipated.
|Date of Award||9 Jun 2016|
|Supervisor||Sam Akehurst (Supervisor), Chris Brace (Supervisor) & Colin Copeland (Supervisor)|