An Investigation into the Effects of Variable Valve Actuation in a Heavy-Duty Gasoline Compression Ignition Engine

The application of variable valve actuation (VVA) has been well demonstrated for improvements in fuel economy and reduced emissions in spark-ignited (SI) and diesel engine applications. The current research numerically investigates effects of VVA in a prototype heavy-duty Gasoline Compression Ignition (GCI) engine modified from a MY2013 Cummins ISX15 heavy-duty diesel engine. For the GCI engine system, the geometric compression ratio was modified to 15.7, and the RON92 gasoline was assumed as a fuel . In a sister paper, a 3-D CFD analysis was conducted to characterize effects of reduced effective compression ratios on the fuel efficiency improvements and reduced soot & NOx emissions for RON92 GCI combustion at mid-to-high engine load conditions. As a follow-up, the current research conducted a 1-D system level analysis to evaluate the effects of VVA on the boost system requirements for the RON92 GCI combustion. Reduction of effective compression ratio by means of early intake valve closing (EIVC) and late intake valve closing (LIVC) is a well-known methodology. However, the LIVC and EIVC strategies severely affect the volumetric efficiency due to flow losses and reduce the engine power density. At high load conditions, a high capability boost system is desired to compensate for the flow loss and enable VVA as a practical strategy. To identify a capable air-handling system, a prototype single-stage and a prototype two-stage air handling system with combinations of exhaust gas recirculation (EGR) delivery routes (high pressure and lower pressure) were analyzed. The stock Cummins ISX15 air-handling system consisting of a single-stage turbocharger and a high-pressure EGR system was also analyzed and used as a baseline for the study. The analysis focused on the mid-to-high load operating conditions such as B50, B75 and A100 from the heavy-duty Supplemental Emission (SET) cycle. At a fixed effective compression ratio, the analysis showed much reduced flow restriction losses for the LIVC strategy. A single-stage boost system showed much deteriorated turbocharger operation at high load conditions. Near peak load conditions, a 2-Stage boost system showed significantly improved air-handling system performance. Finally, the performance of each air-handling system was compared in terms of incurred pumping losses. The effectiveness of both LIVC and EIVC to complement the GCI boundary conditions were also discussed in both 1-Stage and 2-stage boost systems.