Modelling of gasoline injection process and its application to the development of a new GDI engine

Abstract

This thesis details an experimentally validated and simplified spray modelling approach and its application to the development of low emission, high efficiency modern GDI engines. A detailed literature review was carried out to describe the underlying process of spray atomisation. In this modelling approach, the fundamental nozzle parameters, such as nozzle diameter and static flow rate, are used as simulation inputs. The effect of modelling constants in resolving the secondary droplet breakup mechanisms was studied. A set of modelling constants was obtained for injection pressures ranging from 150 bar to 350 bar. There is a good correlation with the penetration depth and the Sauter-mean diameter (SMD) with the experimental data. In-cylinder simulation was then carried out to evaluate and optimise the injection strategy for faster catalyst light-off during the cold-start operations. Simulation shows that earlier second injection prevents charge motion decay and fuel wall wettings. Equivalence ratio and turbulent kinetic energy around the spark plug show a qualitative agreement with the measured engine combustion stability differentiating the fuel injection timing. Further studies were carried out to understand the benefit of air-guided piston in comparison to the wall-guided piston. The air-guided piston is shown to decrease the wall wetting of fuel by 14% in comparison to the wall-guided piston. Engine data show that the PN (#/cm3) with air-guided strategy decreased by an order of magnitude (19 times lower) during the catalyst light-off condition. This enables to meet the emission standards over the WLTC driving cycle. Effects of injection timing and injection quantity on the charge motion were studied. The charge motion improvements achieved with the side-mounted injector were provided. Effect of spray patterns on the charge motion, wall wetting and mixing were also analysed. The outward pointing sprays benefit the charge motion/tumble ratio by 60 to 70%. A detailed study was carried out to understand the difference in charge motion between the early inlet valve closing (EIVC) and late inlet valve closing (LIVC) approach adopted for improving thermal efficiency of engines. For the LIVC CAMs, under all operating conditions, the tumble ratio is 40 to 50% higher in comparison to the tumble ratio obtained for EIVC CAMs. This results in higher turbulent kinetic energy (TKE) for LIVC engines, which will benefit combustion and emission. However, the residual gas fraction shows to be higher for the LIVC CAMs. Based on the initial understanding of the spray on charge motion, a multiple injection strategy was adopted to improve the charge motion for EIVC CAMs for a low speed high load condition. Simulations show that the TKE and mixing could be improved significantly for faster combustion with multiple injection strategy for EIVC CAMs. The triple injection strategy increased the mixing and TKE resulting in 36% decrease in burn duration. Spray input parameters were further improved using a simplified nozzle flow model to recalculate the effective injection velocity by considering the nozzle flow contraction for a given injection pressure, L/D and static flow rates. This improvement requires further evaluation for future work.