| Two classes of oxygenated compounds, ethers and glycols, have been identified as possible alternative fuels for Diesel engine combustion. These compounds offer the possibility for significantly lower Diesel engine emissions and improved driveability for two reasons: (1) because they are oxygenated, they appear to produce almost no smoke or particulate matter when burned in a Diesel engine, and (2) they have excellent autoignition quality. Excellent autoignition quality, defined as having short autoignition delay times, leads to improved cold start performance, lower combustion noise, and lower levels of NOx. The fact that the fuels produce almost no smoke also offers the possibility to use increased levels of exhaust gas recirculation in order to further lower NOx emissions. Finally, all of these synthetic Diesel engine fuels can be produced from natural gas, coal, biomass, or other hydrocarbons feedstocks making them viable replacements for traditional, crude oil-derived fuels.; Experiments were performed in a unique apparatus, the externally heated constant volume combustion apparatus (CVCA-II) designed specifically to study the pyrolysis and autoignition of fuels. For pyrolysis, the fuels were injected into an inert volume of helium, and extractive samples were taken to measure product yields. For autoignition, the fuels were injected into a heated, high pressure volume of oxidizing gas (usually air). Autoigntion delay times were measured based on the time-pressure history of the gases in the CVCA-II.; A chemical kinetic mechanism consisting of 44 species and 148 reversible reactions was developed to describe the autoignition, oxidation, and pyrolysis of methane, methanol, diemethyl ether, and dimethoxy methane. For some species, thermodynamic properties were estimated using a program developed by the author. Hydrogen abstraction reactions were estimated based on principles of analogy with other known reactions. Uni-molecular decomposition reactions, assumed to be at the high pressure limit, were estimated using transition state theory. For dimethyl ether and dimethoxy methane, sensitivity analysis showed that at lower temperatures ({dollar}<{dollar}800 K) peroxide chemistry plays an important role in determining the rate of autoignition, while at higher temperatures ({dollar}>{dollar}1000 K) uni-molecular decomposition and hydrogen abstraction control the rate of autoigntion. At intermediate temperatures (800-1000K), a negative temperature coefficient (NTC) behavior was observed. For methanol and methane, peroxide chemistry did not play a role in the autoignition delay times calculated for temperatures between 700 K and 1100 K, and no NTC behavior was observed.; A multi-zone adiabatic mixing approach was used to model the non-homogeneous autoigntion of a fuel injected into a hot quiescent environment. The model assumes that fuel and air are present in zones of varying stoichiometry. Because of the heat transfer required to vaporize and heat the fuel, the zones vary in temperature which is a function of stoichiometry. A trade-off exists between the leaner-hotter zones and the cooler-richer zones, with autoignition occurring at an optimum point of stoichiometry and temperature. The model predicts the autoigntion delay times of methane, methanol, DME, and DMM measured in the experiments. The model also describes the physical and chemical effects of methane mixed homogeneously with the oxidizing gas before injection of ignition fuel. Finally, the autoignition delay times of fuel mixtures such as methane and methanol blended with dimethyl ether and dimethoxy methane were studied to examine the use of oxygenated fuels as autoignition improvers. |
