Study on ignition and combustion of dimethyl ether : air pre-mixture(CNG and Alternative Fuels, Oxygenated Fuels)

Abstract

Over the last several years there has been much interest in dimethyl ether (DME) as an alternative fuel for diesel engines. DME combines advantages of a high cetane number with no soot combustion, which makes it highly suitable for compression ignition engines. On the other hand, a homogeneous charge compression ignition (HCCI) engine has been known to have high thermal efficiency, no soot and low NO_X emissions. DME for the HCCI engine has also good adaptability. But its ignition and combustion characteristics are not completely understood. The objective of this study is to investigate the ignition delay of DME as homogeneous mixtures with air by using a rapid compression machine. The investigated fuels are ethane and propane in addition to DME. When the air is used as the working gas, the maximum compressed gas temperature reaches 730K. This temperature can't realize the ignition of ethane or propane. Therefore, a mixture gas composition of 53% argon, 26% helium and 21% oxygen is used for realizing temperatures above 730K. Equivalence ratios are 0.2, 0.3 and 0.4. Ignition delays of each fuel are measured at the constant gas density with changing the compressed mixture temperature at the TDC. So far, the representative temperature at the ignition of fuel tended to use the mean gas temperature calculated by the perfect gas law using both of the measured cylinder pressure and the charged gas amount. But the temperature at the center of the combustion chamber measured by a thin Pt resistance wire thermometer having a diameter of 15 micron is used in this study. The experimental results under the constant piston displacement show that the ignition timings of ethane and propane are advanced with increasing in an equivalence ratio, on the contrary, the ignition timing of DME is late. Arrhenius plots of ignition delay on each fuel were drawn up from a series of combustion history data. The results show that ignition delay times of ethane and propane depend on both of the equivalence ratio and the gas temperature, whereas that of DME dose not depends on equivalence ratio, but only temperature. Also, the combustion histories of DME indicate two-stage combustion. The first combustion is known as the heat release by the low temperature oxidation reaction. The heat release amount during this reaction period is also independent of equivalence ratios. Furthermore, the numerical simulation of the DME combustion by means of CHEMKIN II was carried out. In the computation, detailed reaction scheme of DME proposed by Curran et al. was used in this study. Also, the computation results were compared with those from a rapid compression machine. Generally, a comparison of experiment results with computation results indicates a good agreement.