Low Temperature Heat Release and φ-Sensitivity Characteristics of Iso-Octane/Air Mixtures

Chemical energy release from high octane number fuels via low temperature heat release (LTHR) can help develop high-efficiency gasoline engines by promoting ultra-lean combustion in spark ignition engines and improving combustion control in gasoline compression ignition engines. A recently developed experimental technique that permits isolated LTHR investigations in motored engines was used to characterize the LTHR behavior of iso-octane/air mixtures ranging in strength from phi approximate to 0:02 to 1.6 at multiple inlet temperature conditions (60 to 120 degrees C). LTHR changes were studied by observing variations in exhaust CO emissions and exhaust temperature increase. Observed heat release results were explained using cylinder mixture pressuretemperature histories alongside supporting chemical kinetics modeling estimates of mixture reactivity in the form of chemical ignition delay (ID) time. The effects of fuel enrichment on iso-octane/air mixture reactivity were found to be non-uniform and dependent on mixtures' thermal state trajectories in the LTHR ID peninsula. LTHR intensity measurements were used to discuss changes in mixture phi- sensitivity at different engine inlet conditions. It was shown that by appropriately adjusting mixture thermal conditions via charge cooling from direct fuel injection and intake air heating, reactivity enhancements could be exploited maximally; and strong, positive, linear phi- sensitivity of around 10 J per 0.1 increase in.phi could be realized across a wide range of equivalence ratios from 0.05-1.2. It was also found that dominance of charge cooling effects at rich conditions resulted in negative and zero phi sensitivity regions.