Spark assist (SA) has been demonstrated to extend the operating limits of homogeneous charge compression ignition (HCCI) modes of engine operation. This experimental investigation focuses on the effects of 100% indolene and 70% indolene/30% ethanol blends on the ignition and combustion properties during SA HCCI operation. The spark assist effects are compared to baseline HCCI operation for each blend by varying spark timing at different fuel/air equivalence ratios ranging from ϕ = 0.4–0.5. High speed imaging is used to understand connections between spark initiated flame propagation and heat release rates. Ethanol generally improves engine performance with higher IMEPn and higher stability compared to 100% indolene. SA advances phasing within a range of ∼5 CAD at lower engine speeds (700 RPM) and ∼11 CAD at higher engine speeds (1200 RPM). SA does not affect heat release rates until immediately (within ∼5 CAD) prior to autoignition. Unlike previous SA HCCI studies of indolene fuel in the same engine, flames were not observed for all SA conditions.

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