DME and the Heavy Duty Diesel Engine

Licentiate thesis, 2008

Increasing awareness of adverse anthropogenic environmental effects, such as the effects of combusting fossil fuels on atmospheric carbon dioxide concentrations, and that combustion-related emissions from the transportation sector are going to approach sustainable levels in the near future has led to increased interest in fuels from renewable sources. These fuels offer possibilities to diversify the feedstocks that can be used for fuel production, as well as to reduce fossil carbon dioxide emissions, thereby reducing our dependence on a very limited range of sources. This thesis addresses the possibility of using an alternative, renewable fuel in the diesel engine, dimethyl ether or DME, a colourless gas with ethereal odour that becomes a crystal-clear liquid at pressures above 5 bar under standard conditions. DME is a chemical that can be, and is, used in various applications ranging from a chemical feedstock to fuel for domestic applications. Further, and most importantly in the context of this thesis, it can be used in diesel engines, and can perform large amounts of transportation work relative to the energy inputs required to produce it from a feedstock. The properties of DME differ from those of standard transportation fuels in several respects. First, DME as a chemical is environmentally benign, posing minor risks to animal and human health since it does not cause cancer and has low toxicity. Second, due to its volatility, DME is handled as a liquefied gas and since its heating value, viscosity and lubricity are all relatively low, it requires a devoted fuel injection system. Third, it is a simple and uncomplicated molecule with several advantageous chemical properties, most notably its combustion results in negligible soot emissions and potentially low emissions of other species, such as hydrocarbons. Due to the large differences which those properties result in (no soot particles and different injection strategy) it is anticipated that there is much to gain by developing a combustion system for DME. The objectives of the project this thesis is based upon are to investigate, understand and elucidate the requirements of a DME combustion system, and initial steps towards fulfilling these objectives are presented in the thesis. In order to investigate the practical implications of using DME in a diesel engine, simulations and experiments have been performed, using a single cylinder research engine and a six cylinder engine, both heavy duty engines with standard diesel combustion systems. The main findings are that DME combustion is diesel-like, and it reacts to changes in injection parameters in a similar fashion to diesel combustion, but it is slower, especially under low NOx conditions, consequently yielding high emissions of CO, and that particle size and number were below the soot size range and were low, respectively. The slow combustion is attributed to the low injection pressures used and the absence of soot particles to the chemical properties of the DME molecule. Improving the mixing by using higher injection pressures and the use of advanced injection timing improves combustion and reduces CO emissions, but the mixing rate needs to be further enhanced in order to reduce CO emissions to satisfactory levels and improve engine efficiency.