Homogeneous Charge Compression Ignition (HCCI) combustion is a process dominated by chemical kinetics of the fuel-air mixture. The hottest part of the mixture ignites first, and compresses the rest of the charge, which then ignites after a short time lag. Crevices and boundary layers generally remain too cold to react, and result in substantial hydrocarbon and carbon monoxide emissions. Turbulence has little effect on HCCI combustion, and may be most important as a factor in determining temperature gradients and boundary layer thickness inside the cylinder.
The importance of thermal gradients inside the cylinder makes it necessary to use an integrated fluid mechanics-chemical kinetics code for accurate predictions of HCCI combustion. However, the use of a fluid mechanics code with detailed chemical kinetics is too computationally intensive for today's computers.
This paper presents a hybrid procedure that uses a fluid mechanics code (KIVA) and a detailed chemical kinetics code (HCT; Hydrodynamics, Chemistry and Transport). First, the fluid mechanics code is run to evaluate the temperature distribution inside the cylinder without combustion, and the information is then fed into the chemical kinetics code. This two-step procedure takes full account of the effects of temperature gradients inside the cylinder, with a much-reduced computational intensity that makes it accessible to current computers.
The predictions have been compared to recent experimental results. The computations do an excellent job at predicting maximum pressure, burn duration, indicated efficiency and combustion efficiency. Agreement in HC and CO emissions is not as good, and some possible explanations for the disagreement are discussed. The paper also includes a discussion of the chemical kinetics of HCCI combustion.