F2008-06-121
Effect of Ethanol Stratification on the HCCI Combustion
Homogeneous Charge Compression Ignition (HCCI) technology has attracted interest over recent years because it could improve NOx emissions and fuel consumption simultaneously. However, it has some problems such as difficulty of controlling auto-ignition timing and the limitation of the operating range within the part loads. Meanwhile, bio-ethanol is one of the most promising alternative fuels because of its productivity, lower CO2 emissions and plentiful availability. It is important for both the further application of ethanol and the practical use of HCCI combustion to utilize the ethanol for HCCI combustion control. The objective of this study is to address the potential of ethanol fueled HCCI.
In this study, the characteristics of dual fueled HCCI were investigated by engine tests. Ethanol and n-heptane that have different ignitability were applied with two independent injectors. Also, the mechanisms of combustion were investigated by chemical kinetics analysis.
At first, basic method to control the ignition timing with changing the fraction of fuels was applied. As a result, ignition timing could be retarded by using ethanol because the cool flame of n-heptane was inhibited by ethanol. However, the high load operation limit was restricted to a range of less than 500kPa IMEP due to high pressure rise and noise in this case.
Next, another method of the ignition timing control was investigated. Ignition timing was significantly affected by stratification level of ethanol that could be changed by the timing of ethanol direct injection, which was changed from -90deg to -25deg CA ATDC. As the result, late direct injection of ethanol made more stratified distribution in the cylinder. Thus, cool flame of n-heptane was less inhibited, so that the ignition timing was advanced.
Furthermore, the combination of two methods enabled not only the control of ignition timing but also the control of combustion duration. Ignition timing was retarded by increase of ethanol fraction under the ethanol-stratified condition. In this condition, combustion duration became longer with steady ignition because the stratification of ethanol would make the distribution of temperature rise by the cool flame in the cylinder. As the ignition occurred from the part of high temperature to the part of low temperature, combustion duration would be elongated.
The high load operation of HCCI with low pressure rise and low IMEP variation rate was accomplished using these technologies. In this case, maximum load was 850kPa IMEP with the pressure rise less than 500kPa/deg.
Session: HCCI Combustion