A Computational Analysis of Direct Fuel Injection During the Negative Valve Overlap Period in an Iso-Octane Fueled HCCI Engine 2007-01-0227

This computational study compares predictions and experimental results for the use of direct injected iso-octane fuel during the negative valve overlap (NVO) period to achieve HCCI combustion. The total fuel injection was altered in two ways. First the pre-DI percent, (the ratio of direct injected fuel during the NVO period “pre-DI” to the secondary fuel supplied at the intake manifold “PI”), was varied at a fixed pre-DI injection timing, Secondly the timing of the pre-DI injection was varied while all of the fuel was supplied during the NVO period. A multi-zone, two-dimensional CFD simulation with chemistry was performed using KIVA-3V release 2 implemented with the CHEMKIN solver. The simulations were performed during the NVO period only.

Our simulated results show that when there is a very small change in the amount of heat released during the NVO period with a change in the pre-DI injection, it is the fuel reformation that produces the chemical effect which can alter the main combustion timing. This was observed when varying the pre-DI percent at a constant lean A/F of 17.0. The thermal effects produced by the energy release from the NVO fuel injection under this engine condition were not significant because the amount of heat release during the NVO period and its change with the percentage of pre-DI fuel injection were small. On the other hand, as the amount of heat release during the NVO period became greater, the thermal effect became more pronounced. This was observed in our experiments of varying the start of fuel injection timing during the NVO period. The combination of thermal and chemical effects showed a stronger impact on the main combustion than the chemical effect alone. The findings in this study indicate that the use of NVO fuel injection has the ability to expand the engine operating range and improve the HCCI combustion.