Investigation of engine design parameters on the efficiency and performance of the high specific power downsized SI engine

Abstract

This study investigates the impact of employing the Miller cycle on a high specific power downsized gasoline engine by means of Early Intake Valve Closing (EIVC) and Late Intake Valve Closing (LIVC). This investigation assesses the potential for the Miller cycle to improve fuel economy at part load points, as well as high load points with significantly elevated boost pressures (Deep Miller) of up to 4 bar abs. The impact of geometric Compression Ratio (CR) and Exhaust Back Pressure (EBP) has also been investigated. The knock mitigating qualities of Deep Miller have been assessed, and its ability to increase maximum engine load explored. Low Speed Pre-ignition (LSPI) and autoignition tendencies with reduced coolant flow rates and with aged and new fuels have also been studied. This study comprises both experimental and analytical studies. A Ricardo Hydra single cylinder thermodynamic engine was developed and used for the experimental component of the study. This engine features a high specific power output (120kW/l) cylinder head from the Mahle 1.2l 3 cylinder aggressively downsized engine. The analytical component was carried out using a 1-dimensional GT-Power model based on the Ricardo Hydra experimental engine. A Design of Experiments (DoE) based test plan was adopted for this analytical study. The experimental study found that EIVC was the optimal strategy for improving fuel economy at both part-load and high-load conditions. LIVC yielded a fuel economy penalty at part-load operations and a fuel economy improvement at high-loads. The unexpected part-load LIVC result was attributed to the engine breathing dynamics of the experimental engine. The analytical study found moderate LIVC to be the optimal strategy at lower speeds, unless compensation for the increased degree of scavenging experienced with EIVC was compensated for, in which case EIVC was optimum. At higher speeds EIVC was found to be optimum regardless of whether or not compensation for scavenging was employed. It was generally found that less sensitivity to EBP was exhibited the more extreme the EIVC and LIVC. It was also found that a higher geometric CR could be tolerated with extreme EIVC and LIVC, and a fuel economy benefit could be obtained through the elevation of Geometric CR.