Numerical Investigation Of Spray Characterization Of Heater-GDI System

Abstract

Gasoline engines require fuel enrichment at low temperature cranking (cold start) conditions for efficient engine operation. Since the amount of fuel injected is high at cold start to compensate low fuel evaporation, fuel sprays tend to impinge on the cold surfaces of the piston and cylinder walls leading to the formation of excessive unburned hydrocarbons. One of the ways to ensure reliable cold start performance and to reduce UHC emissions is to have the fuel subjected to preheating. The objective of this study is to investigate the effects of fuel preheating on Gasoline Direct Injection sprays to improve the mixture preparation during the cold start conditions. Injection of fuel sprays of neat gasoline and gasoline-ethanol blends (E10 and E85) from the heater-injector into a constant volume combustion chamber was studied. Computational Fluid Dynamics (CFD) simulations of the fuel flow through the injector were performed to understand the impact of the heater in improving the mixture preparation quality. The gasoline was modelled with 14 component surrogate fuel model to capture its physical properties and distillation characteristics. Fuel spray processes in the Constant Volume Combustion Chamber (CVCC) were simulated using an in-house CFD code, MTU-KIVA. Parametric simulations were performed at different injection pressures and at a wide range of fuel temperature ranging from -6°C to 250°C. The simulation of injector internal flow could improve the spray simulations in the CVCC by providing accurate velocity and temperature distributions of fuel sprays at the exit of individual nozzles. The results show that the injector with the preheating system performs reliably at cold start conditions to increase the fuel temperature from -6°C to 75°C in less than a second. The spray were in good agreement between the measurements and predictions. For the given operating range, the spray changes from normal evaporation to flash boiling regime. The model captures the spray collapsing behaviour for the flash boiling conditions. However, the model tends to over-predict the spray penetration for fuel temperatures in the higher range of boiling/flash boiling, regardless of the injection pressure variation.