Partial Fuel Stratification to Control HCCI Heat Release Rates: Fuel Composition and Other Factors Affecting Pre-Ignition Reactions of Two-Stage Ignition Fuels

Homogeneous charge compression ignition (HCCI) combustion with fully premixed charge is severely limited at high-load operation due to the rapid pressure-rise rates (PRR) which can lead to engine knock and potential engine damage. Recent studies have shown that two-stage ignition fuels possess a significant potential to reduce the combustion heat release rate, thus enabling higher load without knock.

This study focuses on three factors, engine speed, intake temperature, and fuel composition, that can affect the pre-ignition processes of two-stage fuels and consequently affect their performance with partial fuel stratification. A model fuel consisting of 73 vol.% isooctane and 27 vol.% of n-heptane (PRF73), which was previously compared against neat isooctane to demonstrate the superior performance of two-stage fuels over single-stage fuels with partial fuel stratification, was first used to study the effects of engine speed and intake temperature. The results for PRF73 show that increasing engine speed from 1200 to 1600 rpm causes almost no change in φ-sensitivity, which is defined by the advancement of combustion phasing for an increase in equivalence ratio. Consequently, the maximum combustion pressure rise rate (PRR_max) can be reduced substantially with partial fuel stratification at this higher speed as it was at 1200 rpm. In contrast, increasing intake temperature from 60°C to 174°C eliminates the low temperature heat release of PRF73. Despite the single-stage ignition at this temperature, PRF73 still shows a weak but definitive φ-sensitivity, likely due to the relatively strong intermediate temperature heat release before hot ignition. As a result, PRR_max was reduced modestly with partial fuel stratification. This PRF73 result is distinctively different from that of isooctane at the same intake temperature. To study the importance of fuel composition, PRF73 is compared with a low-octane, gasoline-like distillate fuel, termed Hydrobate, which could be readily produced from petroleum feedstocks. With the similar HCCI reactivity to PRF73, Hydrobate shows little difference in φ-sensitivity and performs similarly with partial fuel stratification compared to PRF73. This result indicates that it is the overall fuel HCCI reactivity, rather than the exact fuel composition, that determines the φ-sensitivity and the consequent performance with partial fuel stratification.