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Boosted HCCI Combustion Using Low-Octane Gasoline with Fully Premixed and Partially Stratified Charges

High-load HCCI combustion has recently been demonstrated with conventional gasoline using intake pressure boosting. The key is to control the high combustion heat release rates (HRR) by using combustion timing retard and mixture stratification. However, at naturally aspirated and moderately boosted conditions, these techniques did not work well due to the low autoignition reactivity of conventional gasoline at these conditions. This work studies a low-octane distillate fuel with similar volatility to gasoline, termed Hydrobate, for its potential in HCCI engine combustion at naturally aspirated and low-range boosted conditions. The HCCI combustion with fully premixed and partially stratified charges was examined at intake pressures (Pin) from 100 to 180 kPa and constant intake temperature (60?C) and engine speed (1200 rpm).

First, the key parameters for high-load HCCI operation were investigated, including 1) intermediate temperature heat release (ITHR), which determines the potential of using combustion retard to control the HRR, and 2) ?-sensitivity, which determines the effectiveness of mixture stratification for controlling the HRR. It is found that consistent with the low octane number, Hydrobate shows strong ITHR and ?-sensitivity even at Pin = 100 kPa. However, the ITHR of Hydrobate is not enhanced by increasing intake pressure, which is different from gasoline at Pin = 100 ? 180 kPa, but similar to gasoline at Pin ? 180 kPa. For high-load operation, it is found that with a fully premixed charge, Hydrobate allowed higher loads than conventional gasoline for Pin < 160 kPa, mainly due to the lower intake temperature allowing a higher fueling rate. However, for Pin ? 160 kPa, the maximum load reached by Hydrobate was lower than gasoline due to the higher level of EGR required, which limited the oxygen available for combustion. On the other hand, higher thermal efficiency was consistently observed with Hydrobate at all Pin tested, mainly due to the lower intake temperature and more advanced combustion phasing. Applying partial fuel stratification to Hydrobate combustion further improved thermal efficiency compared to the fully premixed case, since it allowed even more advanced combustion phasing; however, it only slightly extended the high-load limit, again due to the limited oxygen availability. This work indicates that low-octane gasoline is a superior HCCI fuel for naturally aspirated and modestly boosted operations.

Yi Yang, Sandia National Laboratories

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