Modeling of HCCI Combustion and Emissions Using Detailed Chemistry 2001-01-1029

To help guide the design of homogeneous charge compression ignition (HCCI) engines, single and multi-zone models of the concept are developed by coupling the first law of thermodynamics with detailed chemistry of hydrocarbon fuel oxidation and NO_x formation. These models are used in parametric studies to determine the effect of heat loss, crevice volume, temperature stratification, fuel-air equivalence ratio, engine speed, and boosting on HCCI engine operation.

In the single-zone model, the cylinder is assumed to be adiabatic and its contents homogeneous. Start of combustion and bottom dead center temperatures required for ignition to occur at top dead center are reported for methane, n-heptane, isooctane, and a mixture of 87% isooctane and 13% n-heptane by volume (simulated gasoline) for a variety of operating conditions. Detailed chemistry of NO_x formation is coupled with multi-step chemical kinetics of hydrocarbon fuel oxidation to gain insight into the ignition and NO_x formation processes.

A more detailed multi-zone model is also developed in an effort to explore the effects of temperature stratification, heat loss, and crevice volume on the combustion process and HC, CO, and NO_x emissions. The combustion chamber volume is divided into four types of zones: crevice, boundary layer, inner core, and outer core. The inner core zones are considered to be adiabatic and constant mass, while the outer core, boundary layer, and crevice zones are allowed to exchange mass and energy. The multi-zone model is used to conduct parametric studies to investigate the effect of changing engine operating conditions, crevice volume, and boundary layer thickness on engine performance and emissions. It is shown that CO emissions primarily arise from fuel flowing out of the crevices and boundary layer during expansion and being partially oxidized. The multi-zone model is hence shown to provide a much more physical representation of the ignition and emission formation processes in HCCI engines than the single-zone model.