An investigation into high-load SFI EGR boosted operation for downsized GTDI engine with valve-overlap reduction

Abstract

Downsized gasoline turbocharged direct injection (GTDI) engines deliver superior fuel economy by operating the engine at higher loads but become prone to knocking combustion at boosted operations, which requires the application of knock mitigation strategies, such as the retarded spark timing, with a negative impact on the engine performance and efficiency. Furthermore, the use of wide valve-overlaps to maximise positive scavenging by elevated intake pressure at low and medium engine speeds leads to greater tailpipe NOx emissions. In light of increased use of Real Driving Emissions (RDE) test where higher load operations are far more prominent, there is a strong need to further explore the approaches to improve engine efficiency and lower harmful emissions at knock limited operations. This project investigates the use of stratified flame ignition (SFI) combustion with exhaust gas recirculation (EGR) on a downsized GTDI engine. EGR dilution is added to control the knocking combustion to replace the traditional knock mitigation strategies. The subsequent combustion is further improved by stratified fuel injection and uprated ignition system. The valve-overlap duration is shortened to avoid the air short-circuiting and hence reduced tailpipe NOx emission through greater conversion efficiency of the 3-way catalyst, but with trade-off with lower volumetric efficiency and knock onset. The novelty of the study is identified as the combination and optimisation of these strategies to improve operational efficiency and reduce harmful tailpipe emissions at knock limited loads. The results showed that the EGR dilution lowered knock tendency and high energy (HE) ignition accelerated the combustion and recovered stability exclusively at boosted operations. Split injection strategy showed fuel consumption benefit at very limited cases, and most cases led to the loss of efficiency from slower less efficient combustion. Through computational fluid dynamics (CFD) analysis, this was found to be caused by the unfavourable mixture preparation of the multi-hole injectors due to high spray penetration and insufficient mixture preparation time. However, the combined use of EGR dilution and reduced valve-overlap improved mixture preparation due to increased charge temperatures and induced turbulence. Indicated specific fuel consumption (ISFC) improvements of 4.8% and 5.2% were achieved at 13.7bar and 16.4bar Net_IMEP at 2000rpm, respectively. The reduction of valve-overlap also improved the combustion efficiency and reduced emissions of tailpipe NOx and particulates due to eliminated short-circuit air and enhanced turbulence for faster mixture preparation. Hence, a synergy between valve-overlap reduction, split injection, and EGR dilution was found, and the proposed strategy successfully lowered fuel consumption and harmful emissions from this combined synergy effect.