Citation: Arcoumanis, C., Bae, C., Crookes, R. and Kinoshita, E. (2008). The potential of di-methyl ether (DME) as an alternative fuel for compression-ignition engines: A review. Fuel, 87(7), pp. 1014-1030. doi: 10.1016/j.fuel.2007.06.007 This is the accepted version of the paper.This version of the publication may differ from the final published version. Permanent AbstractThis paper reviews the properties and application of di-methyl ether (DME) as a candidate fuel for compression-ignition engines. DME is produced by the conversion of various feedstock such as natural gas, coal, oil residues and bio-mass. To determine the technical feasibility of DME, the review compares its key properties with those of diesel fuel that are relevant to this application. DME's diesel engine-compatible properties are its high cetane number and low auto-ignition temperature. In addition, its simple chemical structure and high oxygen content result in soot-free combustion in engines. Fuel injection of DME can be achieved through both conventional mechanical and current common-rail systems but requires slight modification of the standard system to prevent corrosion and overcome low lubricity. The spray characteristics of DME enable its application to compression-ignition engines despite some differences in its properties such as easier evaporation and lower density. Overall, the low particulate matter production of DME provides adequate justification for its consideration as a candidate fuel in compression-ignition engines. Recent research and development shows comparable output performance to a diesel fuel led engine but with lower particulate emissions. NO x emissions from DME-fuelled engines can meet future regulations with high exhaust gas recirculation in combination with a lean NO x trap. Although more development work has focused on medium or heavy-duty engines, this paper provides a comprehensive review of the technical feasibility of DME as a candidate fuel for environmentally-friendly compression-ignition engines independent of size or application.

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The effects of charge dilution on low-temperature diesel combustion and emissions were investigated in a smallbore single-cylinder diesel engine over a wide range of injection timing. The fresh air was diluted with additional N 2 and CO 2 , simulating 0 to 65% exhaust gas recirculation in an engine. Diluting the intake charge lowers the flame temperature T due to the reactant being replaced by inert gases with increased heat capacity. In addition, charge dilution is anticipated to influence the local charge equivalence ratio φ prior to ignition due to the lower O 2 concentration and longer ignition delay periods. By influencing both φ and T, charge dilution impacts the path representing the progress of the combustion process in the φ-T plane, and offers the potential of avoiding both soot and NO x formation.In-cylinder pressure measurements, exhaust-gas emissions, and imaging of combustion luminosity were performed to clarify the path of the combustion process and the effects of charge dilution and injection timing on combustion and fuel conversion efficiency. Based on the findings, a postulated combustion process in the φ-T plane is presented for different dilution levels and injection timings. Although the ignition delay increased with high dilution and early injection, the heat release analysis indicated that a large portion of the combustion and emissions formation processes was still dominated by the mixing-controlled phase rather than the premixed phase. Because of the incomplete premixing, and the need to mix a greater volume of charge with unburned or partially-burned fuel to complete combustion, the diluted mixtures increased CO emissions. Injecting the fuel at earlier timings to extend the ignition delay helped alleviate this problem, but did not eliminate it. Fuel conversion efficiencies calculated for each dilution level and start of injection provide guidance as to the appropriate combustion phasing and practical levels of charge dilution for this low-temperature diesel combustion regime.

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