Search Results

Three-dimensional combustion simulation tools together with the Universal Coherent Flamelet Model (UCFM), a flame propagation model, have been applied to SI lean-burn combustion to study the influence of the equivalence ratio on the amount of unburned hydrocarbons (HCs). Unburned HCs from piston-cylinder crevices were taken into the consideration by using a calculation grid incorporating the actual crevice volume and shape and by applying an autoignition model to post-flame phenomena. The calculation results show the general tendencies for the total amount of unburned HCs and their distribution in the combustion chamber.

A transient knock prediction technique has been developed by coupling a zero-dimensional knocking simulation with chemical kinetics and a one-dimensional gas exchange engine model to study the occurrence of transient knock in SI engines. A mixed chemical reaction mechanism of the primary reference fuels was implemented in the two-zone combustion chamber model as the auto-ignition model of the end-gas. An empirical correlation between end-gas auto-ignition and knock intensity obtained through intensive analysis of experimental data has been applied to the knocking simulation with the aim of obtaining better prediction accuracy. The results of calculations made under various engine operating parameters show good agreement with experimental data for trace knock sensitivity to spark advance.

A new auto-ignition combustion model for performing multi-zone engine cycle simulations has been developed to investigate the characteristics of compression ignition combustion in gasoline engines. In this combustion model, the auto-ignition timing is predicted with a modified shell model and combustion speed is calculated with a three-region (burned, ignited and unburned) model. Engine cycle simulations performed with this model were used to analyze the effect of engine operating parameters, i.e., temperature and air-fuel distributions in the cylinder, on combustion characteristics. It was found that the air-fuel distribution in the cylinder has a large impact on combustion characteristics and knocking was prevented by creating a fuel-rich zone at the center of the cylinder under high load conditions. The fuel-rich zone works as an ignition source to ignite the surrounding fuel-lean zone. In this way, two-step combustion is accomplished through two separate auto-ignitions.