Viscosity and Lubricity of (Liquid) Dimethyl Ether - An Alternative Fuel for Compression-Ignition Engines

In this paper, dependence of liquid-DME viscosity on temperature and pressure was studied theoretically. It was found that in the saturated-liquid state, the DME viscosity is 0.37 cSt at - 40 ° C and it drops to 0.17 cSt when temperature increases to 80 ° C. In the subcooled-liquid state, viscosity varies linearly with pressure at a given temperature; at 20 ° C, viscosity of the subcooled liquid is 0.23 cSt at 5.3 bar and it increases to 0.33 cSt at 500 bar. The predicted liquid-DME viscosity and its pressure dependence agree with those obtained by measurement. Lubricity of liquid DME also was studied. Polar-headed, long-chain alcohols and fatty acids with chain length of C₁₅ ∼ C₂₂ were found to be candidates of lubricity additives for DME. Castor oil (chemically, it is basically a C₁₈ fatty acid) was found to be a good additive for improving the DME lubricity. Fuel-system tests on the bench as well as on engines had shown that by adding 1-wt% castor oil into DME, the DME lubricity was improved significantly in comparison to that of pure DME, and no unusual wear of the sliding parts of the fuel system was noticed in a >500-hour test period.