Enhancing Light Load HCCI Combustion in a Direct Injection Gasoline Engine by Fuel Reforming During Recompression 2009-01-0923

Homogeneous charge compression ignition (HCCI) engines have the potential for high fuel efficiency and low NO_x emissions. The major disadvantage of HCCI remains the narrow operating range. One way to extend the operating range of HCCI combustion to lower load is to inject part of the total fuel mass into the hot gas during recompression. With even lower engine load, part of the fuel can also be injected late in the main compression and ignited by a spark. The propagating flame further compresses the remaining fuel-air mixture until auto-ignition occurs (spark-assisted HCCI).

In this study we investigated the effect of fuel reforming and spark assist in a gasoline engine with direct fuel injection and negative valve overlap. We performed experiments with different injection quantities and varying injection timings during recompression. Besides understanding the effect of reforming on engine performance, combustion stability and NO_x emissions we focused on methods that can be used to help monitor and control the amount of fuel reforming in a vehicle. It was found that fuel reforming reduces NO_x emissions but increases cyclic variability. Therefore the amount of fuel reforming should be closely monitored to achieve the desired trade-off between stability and emissions. The study shows that combustion phasing does not show a good correlation and should therefore not be used to monitor the amount fuel reforming. It was found, however, that PMEP shows a strong linear correlation with the fuel mass that is burned during recompression. Finally it is shown that the amount of fuel reforming can be controlled by injection timing and split ratio.