Combustion control and operating range expansion in an homogeneous charge compression ignition engine with suppression of low-temperature oxidation by methanol: Influence of compression ratio and octane number of main fuel

Abstract

Abstract

Characteristics of combustion control with direct methanol injection in homogeneous charge compression ignition (HCCI) combustion were investigated over a wide operating range under several conditions including different compression ratios and three main fuels with different octane numbers. The operating range of HCCI combustion is limited by knocking at one extreme and misfire at the other. On the knocking side, moderate methanol injection is shown to be effective at preventing knock. In addition, a lower compression ratio can delay the onset of both low- and high-temperature oxidations and suppress rapid combustion due to the lower in-cylinder gas temperature while not directly affecting the quantity of heat released during low-temperature oxidation. To extend the misfire limit, a lower octane number fuel yields a greater low-temperature heat release, increasing the temperature in the interval between the low- and high-temperature oxidation, which prevents misfiring. Therefore, a combination of lower compression ratio and lower octane number fuel assisted by a direct methanol injection can expand the operating load range envelope as the low-temperature oxidation can be controlled flexibly by varying the quantity of methanol injection with the operating condition. It was concluded that the ignition suppression effect of methanol in HCCI combustion is much stronger than those of other fuels with a similar octane number.