Naturally occurring thermal stratification significantly impacts the characteristics of HCCI combustion. The in-cylinder gas temperature distributions prior to combustion dictate the ignition phasing, burn rates, combustion efficiency, and unburned hydrocarbon and CO emissions associated with HCCI operation. Characterizing the gas temperature fields in an HCCI engine and correlating them to HCCI burn rates is a prerequisite for developing strategies to expand the HCCI operating range.
To study the development of thermal stratification in more detail, a new analysis methodology for post-processing experimental HCCI engine data is proposed. This analysis tool uses the autoignition integral in the context of the mass fraction burned curve to infer information about the distribution of temperature that exists in the cylinder prior to combustion. An assumption is made about the shape of the charge temperature profiles of the unburned gas during compression and after combustion starts elsewhere in the cylinder. Secondly, it is assumed that chemical reaction rates proceed very rapidly in comparison to the staggering of ignition phasing from thermal stratification. The autoignition integral is then coupled to the mass fraction burned curve to produce temperature-mass distributions that are representative of a particular combustion event. Due to the computational efficiency associated with this zero-dimensional calculation, a large number of zones can be simulated at very little computational expense.
The temperature-mass distributions are then studied over a coolant temperature sweep. The results show that very small changes to compression heat transfer can shift the distribution of mass and temperature in the cylinder enough to significantly affect HCCI burn rates and emissions.