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Homogeneous charge compression ignition (HCCI) combustion is widely regarded as an attractive option for future high efficiency gasoline engines. HCCI combustion permits operation with a highly dilute, well mixed charge, resulting in high thermal efficiency and extremely low NOx and soot emissions, two qualities essential for future propulsion system solutions. Because HCCI is a thermokinetically dominated process, full understanding of how combustion chamber boundary thermal conditions affect the combustion process are crucial. This includes the dynamics of the effective chamber wall surface temperature, as dictated by the formation of combustion chamber deposits (CCD). It has been demonstrated that, due to the combination of CCD thermal properties and the sensitivity of HCCI to wall temperature, the phasing of autoignition can vary significantly as CCD coverage in the chamber increases. In order to better characterize and quantify the influence of CCDs, a numerical methodology has been developed which permits calculation of the crank-angle resolved local temperature profile at the surface of a layer of combustion chamber deposits. This unique predictor-corrector methodology relies on experimental measurement of instantaneous temperature underneath the layer, i.e., at the metal-CCD interface, and known deposit layer thickness. A numerical method for validation of these calculations has also been devised. The resultant crank-angle resolved CCD surface temperature and heat flux profiles both on top and under the CCD layer provide valuable insight into the near wall phenomena, and shed light on the interplay between the dynamics of the heat transfer process and HCCI burn rates.

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.

A combined experimental and analytical study was carried out to understand the improvement in combustion performance of a 4-valve SIDI wall-guided engine operating at lean, stratified idle with enhanced in-cylinder charge motion by deactivating one of the two intake valves. A fully warmed-up engine was operated at low speed, light load by injecting the fuel from a pressure-swirl injector during the compression stroke to produce a stratified fuel cloud surrounding the spark plug at the time of ignition. Steady state flow-bench measurements and CFD calculations showed that valve deactivation primarily increased the in-cylinder swirl intensity as compared with opening both intake valves. Engine dynamometer measurements showed an increase in charge motion led to improved combustion stability, increased combustion efficiency, lower fuel consumption, and higher dilution tolerance. A CFD study was conducted using in-house models of spray and combustion to simulate the engine operating with and without valve deactivation. The computations demonstrated that the improved combustion was primarily driven by higher laminar flame speeds through enhanced mixing of internal residual gases, better containment of the fuel cloud within the piston bowl, and higher post-flame diffusion burn rates during the initial, main, and late stages of the combustion process, respectively.