Two methods for mitigating unacceptably high HCCI heat-release rates are investigated and compared in this combined experimental/CFD work. Retarding the combustion phasing by decreasing the intake temperature is found to have good potential for smoothing heat-release rates and reducing engine knock. There are at least three reasons for this: 1) lower combustion temperatures, 2) less pressure rise when the combustion is occurring during the expansion stroke, and 3) the natural thermal stratification increases around TDC. However, overly retarded combustion leads to unstable operation with partial-burn cycles resulting in high IMEPg variations and increased emissions.
Enhanced natural thermal stratification by increased heat-transfer rates was explored by lowering the coolant temperature from 100 to 50°C. This strategy substantially decreased the heat-release rates and lowered the knocking intensity under certain conditions. To further exploit the effect, the heat-transfer rates were further enhanced by increasing the in-cylinder air swirl. This led to even longer combustion durations. Unfortunately, the higher heat losses associated with high air swirl decreased the IMEPg. When the fueling rate was increased to compensate, most of the improvements on the heat-release rates were lost.
Overall, combustion phasing retard was found to have better potential for smoothing heat-release rates than enhancing the thermal stratification by the means considered in this work. However, operation with highly retarded combustion requires precise control of the ignition timing. Furthermore, it is found that the acceptable intake temperature range narrows rapidly with increasing equivalence ratio. Above a certain fueling rate a steady state operating point cannot be established by setting the intake temperature to a fixed value. This problem is caused by wall heating and the coupling between wall temperature and combustion phasing.