A Study of HCCI Combustion Using Spectroscopic Measurements and Chemical Kinetic Simulations: Effects of Fuel Composition, Engine Speed and Cylinder Pressure on Low-temperature Oxidation Reactions and Autoignition

The Homogenous Charge Compression Ignition (HCCI) engine is positioned as a next-generation internal combustion engine and has been the focus of extensive research in recent years to develop a practical system. One reason is that this new combustion system achieves lower fuel consumption and simultaneous reductions of nitrogen oxide (NOx) and particulate matter (PM) emissions, which are major issues of internal combustion engines today. However, the characteristics of HCCI combustion can prevent suitable engine operation owing to the rapid combustion process that occurs accompanied by a steep pressure rise when the amount of fuel injected is increased to obtain higher power output. A major issue of HCCI is to control this rapid combustion so that the quantity of fuel injected can be increased for greater power. Controlling the ignition timing is also an issue because it is substantially influenced by the chemical reactions of the fuel. Various approaches are being researched for expanding the range of stable engine operation, including the application of turbocharging, the use of residual combustion gas, stratification of the fuel concentration, and the use of a blend of two types of fuel having widely different ignition characteristics.

Experiments and chemical kinetic simulations were conducted using a blended fuel of n-heptane (0 RON), which autoignites easily, and iso-octane (100 RON) that is not conducive to autoignition. The results revealed the possibility of increasing the fuel injection quantity while controlling the ignition timing. It was also found that varying the engine speed influenced the cool flame activity level, with the result that the ignition timing differed. These results provide insights for controlling the autoignition process leading to HCCI combustion.