DME and the Heavy Duty Diesel Engine
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
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.
fuel injection system