Comparative Study of Characteristics of Diesel-Fuel and Dimethyl-Ether Sprays in the Engine

A comparative study of characteristics of diesel fuel and dimethyl ether sprays was conducted on the basis of momentum conservation. The analysis reveals that the DME spray in the diesel combustion system may not develop as well as that of diesel fuel at high engine loads and speeds due primarily to the following reasons. (1) Because 42% more fuel volume must be injected into the engine to reach the diesel-fuel equivalent and because the DME injection pressure is lower than that of diesel fuel, longer injection duration for DME is needed even if with the enlarged orifice diameters. As a result, the penetration of the DME spray tends to be longer than that of diesel fuel. (2) Due to the enlarged diameter, the DME orifice becomes hydrodynamically short and the contracted fuel flow at the orifice entrance is not recovered fully; the still-diverging orifice flow at the orifice exit results in an increased angle for the subsequent spray. (3) Being less dense and more volatile than diesel fuel, DME has a higher gas-to-liquid density ratio in the spray, which also tends to increase the spray angle. Because of the large spray angle, the fuel-rich boundaries (due to the rapid fuel evaporation) of the DME sprays from different orifices of the injector tend to overlap under long injection duration and a strong swirl, which is unfavorable to complete combustion.