Fuel Chemistry and Mixture Stratification in HCCI Combustion Control

Homogeneous charge compression ignition (HCCI) is an autoignition combustion process with a lean or dilute fuel/air mixture. It can provide both good fuel economy and very low emissions of nitrogen oxides (NOx) and particulates. Therefore, it is considered to be one of the most promising internal combustion engine concepts for the future. However, there are some obstacles that must be overcome before the potential benefits of HCCI combustion can be fully realized in commercialization, including combustion phasing control, operation range extending (high levels of noise, UHC and CO emissions), cold start, and homogeneous mixture preparation. All these HCCI characteristics have been summarized in Section 1. To overcome these obstacles, many effective technologies have been carried out and these technologies will be reviewed in Section 2 according to different fuel properties. HCCI can be applied to a variety of fuel types and the choice of fuel will have a significant impact on both engine design and control strategies. Some chemical components have the ability to inhibit or promote the heat release process associated with autoignition. Typical generalized diesel-fuelled HCCI combustion modes include: early direct injection HCCI, late direct injection HCCI, premixed/direct-injected HCCI combustion and low temperature combustion. Mixture control (mixture preparation), including charge components and temperature control in the whole combustion history and high pre-ignition mixing rate, is the key issue to achieve diesel HCCI combustion. The high octane numbers of gasoline fuels mean that such fuels need high ignition temperatures, which highlights the difficulty of autoignition. The main challenge for gasoline HCCI operation is focus on the obtaining sufficient thermal energy to trigger autoignition of mixtures late in the compression stroke, extending the operational range, and the transient control. In addition, alternative fuel can save the fossil fuel and reduce the CO2 emission, therefore it has been got more attention in recent years. And to understand fundamental theory of HCCI combustion process, the primary reference fuel is the best choice due to the better understood chemical kinetics. All these fuels will be also introduced in the Section 2. Advanced control strategies of fuel/air mixture are more important than simple “homogeneous charge’’ for the HCCI combustion control. Further, it is impossible to get an absolutely homogeneous mixture in the operation of practical HCCI engines. Modest inhomogeneity in fuel concentration or temperature appearing in mixing can affect the autoignition and combustion process. And stratification strategy also has the potential to extend the HCCI operation range to higher loads. The thermal stratification can be caused by wall heat transfer and turbulent mixing during the compression stroke for a low-residual engine. This thermal stratification causes the combustion to occur as a sequential autoignition of progressively cooler regions, slowing the rate of pressure rise. For engines with high levels of retained residuals, incomplete mixing between the fresh charge and hot residuals could also contribute to the thermal stratification. Apart from the thermal stratification, more researches are about the charge or compositional stratification. The charge stratification is focus on the different injection strategies, while the compositional stratification means that all the EGR, internal or external, changes the composition of the charge therefore forming the different compositional stratification. These stratification combustion characteristics have been reviewed in Section 3. Finally, a summary for the progress of HCCI combustion and future research direction has been shown in Section 4.

Keywords: Fuel, chemistry, mixture stratification, homogeneous charge compression ignition (HCCI), combustion control, auto-ignition, operation range, mixture preparation, gasoline, diesel, fuel surrogate, natural thermal stratification, charge and compositional stratification, low temperature combustion (LTC), combustion mechanism, chemical kinetics.