Browse Publications Technical Papers 2020-01-0959
2020-04-14

Investigation of homogeneous lean SI combustion in high load operating conditions 2020-01-0959

Homogeneous lean combustion (HLC) can be utilized to substantially improve SI ICE efficiency which is vital in the process of enabling clean, efficient and affordable propulsion for the next generation light duty vehicles. More research is however necessary to ensure robustness, fuel efficiency/ NOx trade-off and utilization of HLC. Utilization can be improved by expanding the HLC operating window to higher engine torque domains which increases impact on real driving. The authors have earlier assessed dedicated HLC boosting in a downsized 2lt engine but it was found that HLC operation could not be achieved above 15 bar IMEP due to instability and knock. The observation led to the conclusion that there exists a lean load limit. Therefore, further experiments have been conducted in a single cylinder research SI GDI engine to increase understanding of high load lean operation. HLC is known to suppress end-gas autoignition (knock) by decreasing reactivity and temperatures, but during the experiments knock was observed to be prominent and increasing in severity when engine load was increased despite operating ultra-lean close to lambda 2. Knock is normally mitigated by spark retardation but to achieve stable combustion of CoV IMEP <2% at lambda 2 the combustion phasing had to be kept close to TDC which implies higher cylinder peak pressures. Therefore, the combustion event had to be balanced in a window where early combustion promoted knock and late resulted in misfire. A tumble flap was introduced to increase in-cylinder tumble which reduced knock and improved combustion stability. The heat release dispersion (cycle-to-cycle CA50) increased proportionally to the level of dilution despite IMEP CoV <2%. Main combustion (CA10-90) dispersion was reduced with increased tumble but the initial flame development (CA0-10) still displayed high dispersion indicating that the increased tumble did not improve initial flame kernel development.

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