A Computational Study of the Effects of Low Fuel Loading and EGR on Heat Release Rates and Combustion Limits in HCCI Engines 2002-01-1309
Two fundamental aspects of HCCI engine combustion have been investigated using a single-zone model with time-varying compression and the full chemical-kinetic mechanisms for iso-octane, a representative liquid-phase fuel. This approach allows effects of the kinetics and thermodynamics to be isolated and evaluated in a well-characterized manner, providing an understanding of the selected fundamental processes. The computations were made using the CHEMKIN-III kinetic-rate code for an 1800 rpm operating condition.
The study consists of two parts. First, low-load HCCI operation was investigated to determine the role of bulk-gas reactions as a source for HC and CO emissions. The computations show that as fueling is reduced to equivalence ratios of 0.15 and lower (very light load and idle), the bulk-gas reactions do not go to completion, leading to inefficient combustion and high emissions of HC and CO. Thus, these results indicate a fundamental low-load limit for complete combustion for well-mixed HCCI with typical fuels, since real-engine effects of heat transfer and boundary-layer cooling would only drive the onset of this behavior to higher loads. These computations also show that emissions of oxygenated hydrocarbons are equal to or greater than the HC emissions for the conditions studied. Second, the main effects of EGR on the combustion process were isolated and evaluated. The results show that the largest effect of EGR is to reduce the compressed-gas temperature, which retards the ignition timing and slows the combustion rate; however, it is shown that this effect could also be accomplished without EGR. EGR is also found to significantly slow the kinetic rates at a fixed ignition timing, and a combination of the two effects provides maximum smoothing of the heat release. Finally, an additional slowing of the kinetics is found to occur when all dilution is provided by EGR, with the fuel/air mixture being stoichiometric.