The Effects of Injector Targeting and Fuel Volatility on Fuel Dynamics in a PFI Engine During Warm-up: Part II - Modeling Results

The effects of injector targeting and fuel volatility on transient fuel dynamics were studied with a comprehensive quasi-dimensional model and compared with experimental results from Part I of this report (1). The model includes the transient, convective vaporization of four multi-component fuel films coupled with a transient thermal warm-up model for realistic valve, port and cylinder temperatures (2, 3). Two injector targetings were analyzed, first with the fuel impacting the intake valve and in addition, the fuel impacting the port floor directly in front of the intake valve. The model demonstrates the importance of both component temperature and fuel impaction area on fuel vaporization, transient air fuel ratio (AFR) response and the amount of liquid fuel entering the cylinder. Generally, a smaller injector footprint area will lead to more liquid fuel entering the cylinder even if the spray is targeted at the back of the intake valve. The model results confirm that the lighter fuel components are preferentially vaporized and burned during a cold start and the heavier fuel components have a longer time response during a transient and are preferentially stored in both the port and cylinder. The model predicted AFR response as a function of both fuel volatility and injector targeting closely matches that found experimentally and the model provides additional insight into the physical processes important in PFI fuel preparation.