Not counted in total): 228 Main text (intro, main body, conclusions, acknowledgments): 3420 Equations: 2 x (2 + 2) x (7.6) x 1 = 60.8 References: (29 + 2) x 2.3 x 7.6 = 541.88 AbstractSpark assisted compression ignition (SACI) combustion is capable of partially decoupling combustion phasing from peak heat release rate, overcoming one of the major challenges associated with low temperature combustion. Experimental SACI studies have shown that for a given combustion phasing, peak heat release rate can be modulated by trading the fraction of the fresh charge consumed by flame propagation with the fraction consumed by auto-ignition. The chemical and physical mechanisms controlling the changes in heat release rate are not well known. The current work uses computational simulation to explore these modes through the combined control of spark timing and charge temperature via changes in negative valve overlap. Open cycle CFD simulation results are compared with experimental data to validate a newly developed SACI model, and used to gain insight into the processes governing combustion behavior. The simulations predict a 43% reduction in the peak rate of heat release with increasing spark advance, while the unburned charge mass at the time of auto-ignition decreases by only 23%. Detailed analysis of the end-gas thermodynamic state prior to auto-ignition shows as spark is advanced, the end-gas thermal and compositional distributions vary with the fraction of the charge consumed by the flame. The reduction in the rate of peak rate of heat release during the auto-ignition process is therefore a function of both the mass of the end-gas and the end-gas reactivity.

h i g h l i g h t sThe effect of spark assist on air and EGR dilute HCCI combustion was examined. Spark had greater combustion advance effect on more air dilute (vs. EGR dilute) mixtures. Combustion advance was caused by an increased fraction of flame-based heat release. Flame quenching likely affected mixtures that showed a non-response to spark assist. a b s t r a c t Spark assisted compression ignition (SACI) is a practical mode for controlling the heat release rate of low temperature combustion (LTC). While flames are key phenomena in the SACI combustion process, they may not always be effective or viable under the mildly stratified and highly dilute low burned gas temperature conditions of LTC. To better understand the limits of flammability or flame effectiveness, this work explores combustion within a single cylinder direct injection engine near the high load limit of the HCCI combustion regime, where spark induced flame propagation has been seen to affect combustion phasing and heat release rate. Flame limiting conditions were identified using progressively more advanced spark timing, up to 120°before top dead center, for differing levels of air and EGR dilution while holding the chemical energy content of the charge constant. Under air dilute conditions, the measured combustion phasing advanced from 8°to 0°after top dead center with spark advance, while almost no effect was seen under EGR dilute conditions. Estimates of the experimental global and local state conditions were made at the time of spark using heat release analysis and a KIVA-3V engine model, respectively, and the flammability for each case was evaluated using the Karlovitz criterion. The results show that fuel rich stratification near the spark plug was likely responsible for the observed variations in the SACI flame behavior.