Browse Publications Technical Papers 2019-01-1145
2019-04-02

Understanding Fuel Stratification Effects on Partially Premixed Compression Ignition (PPCI) Combustion and Emissions Behaviors 2019-01-1145

Fuel stratification effects on the combustion and emissions behaviors for partially premixed compression ignition (PPCI) combustion of a high reactivity RON80 gasoline was investigated using the third generation Gasoline Direct-Injection Compression Ignition (Gen3 GDCI) multi-cylinder engine. The PPCI combustion mode was achieved through a double injection strategy with the first fuel injection event occurring during the intake stroke while the second injection taking place in the compression stroke. Well-premixed fuel-air mixture across the combustion chamber was achieved via the first fuel injection event. The second fuel injection during the later compression stroke was used to trigger the main combustion event. The extent of in-cylinder fuel stratification was tailored by varying the start of second fuel injection timing (SOIsecond = -70 °ATDC to -6 °ATDC), while the first fuel injection timing in the intake stroke was maintained constant (SOIfirst = -280 °ATDC). The fuel split ratio, defined as a ratio between the intake stroke fuel injection quantity and the compression stroke fuel injection quantity, was fixed at 70:30. The PPCI investigation was conducted at 1500 rpm, 11 bar gross indicated mean effective pressure (IMEP) using two sets of injectors of different spray inclusion angles (130° vs. 100°). Based on the experimental results, three combustion characteristic zones were identified in terms of the SOIsecond - CA50 relationship: (I) no response zone: the SOIsecond had no authority to control the combustion event (i.e., HCCI-like combustion); (II) negative CA50 slope zone: CA50 advanced as the SOIsecond was retarded due to the enhanced fuel stratification (i.e., early PPCI mode); and (III) positive CA50 slope zone: CA50 retarded as SOIsecond was retarded (i.e., late PPCI mode). Across the three zones, Zone II produced the best overall performance results in terms of fuel efficiency, emissions (BSNOx: <0.05 g/kWh, smoke: <0.1 FSN), engine noise, and CoV of IMEP. In addition, a wider spray inclusion angle was found to allow for later SOIsecond, thereby resulting in stronger fuel stratification and thus enhanced control authority over the combustion phasing. Finally, closed-cycle combustion CFD analysis was performed to gain insight into the in-cylinder fuel-air mixing and combustion process for both injector sets covering all three combustion zones

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