Optimized PFI+DI Operation For Minimizing DI Gasoline Engine Particulates 2018-01-1415
Direct Injection (DI) fueled gasoline engines provide higher efficiency than port fueled injected (PFI) engines. However, emission of small particulates is greatly increased when DI is used. Particulate mass emission is increased by more than a factor of 10 and particulate number is increased by a factor of 10-100 relative to PFI engines leading to health concerns and to implementation and consideration of new regulations.
Optimized combinations of PFI and DI can greatly reduce DI-generated particulate emissions without compromising efficiency and performance. A DI enhanced PFI mode of engine operation is employed where PFI is the dominant means in dual-injection fueling over a drive cycle, and the fuel fraction that is directly injected is reduced/minimized while still preventing knock at high loads. Further reduction can be obtained by optimal use of spark retard. The already low particulate emissions are further reduced by decreasing the percentage of DI fuel that results in particulate generation from wall wetting; this is accomplished by adjustment of injection timing, injection rate and pulse length.
We have developed a computational model of DI-generated particulates over the torque and speed map for gasoline engine operation with combined PFI and DI. The computational model includes models for knock suppression and for particulate generation from wall wetting. These models are calibrated using experimental data. We have used this computational approach to determine illustrative reductions in DI generated particulates for turbocharged gasoline engines various drive cycles. These reductions in DI-generated particulates for the US06 and UDDS cycles are greater than 20 and 50 times, respectively relative to the use of DI alone. An optimized PFI+DI system could be used in combination with a GPF for even further particulate reduction while also reducing GPF cost and the efficiency loss from GPF back pressure.