Understanding Optimal Engine Operating Strategies for Gasoline-Fueled HCCI Engines using Crank-Angle Resolved Exergy Analysis
This study couples a crank-angle resolved exergy analysis methodology with a multi-zone chemical kinetic model of a gasoline-fueled HCCI engine to quantify exergy loss mechanisms and understand how the losses change with different HCCI engine operating conditions. The in-cylinder exergy loss mechanisms are identified as losses to combustion, heat loss, unburned species, and physical exergy lost to exhaust gases. These loss mechanisms and their effect on overall operating efficiency are studied over a range of engine intake pressures, equivalence ratios, engine speeds and for different engine sizes.
Prior studies have demonstrated that optimal efficiency is achieved in HCCI engines at intermediate combustion timings, with this optimal combustion timing being later for higher load conditions. This exergy analysis study provides a quantitative explanation for this experimental observation by demonstrating that exergy losses to heat loss decrease with delayed combustion timing, and exergy losses to unburned species increase sharply at later combustion timings. The optimal exergy efficiency combustion timing typically occurs at the combustion timing when unburned species losses surpass heat losses, and with higher load conditions these unburned species losses take effect at later combustion timings. The insights from this study also provide guidance towards an optimal efficiency operating strategy to control load in an HCCI engine. From the perspective of in-cylinder exergy losses, the results suggest that equivalence ratio should be maintained at relatively high values across most operating conditions while intake pressure is used to vary engine load. Only at lower load conditions should equivalence ratio begin to be changed for load control, and combustion timing should always be maintained at a value just earlier than the sharp increase of unburned species losses.