DME has lower energy content per unit volume than that of light oil (typical petroleum based diesel fuel). Roughly 1.8 times the quantity of DME is required to obtain equivalent content of light oil. DME also exhibits higher compressibility and much lower viscosity than light oil, so high pressure injection is not easy. Currently, DME engines have utilized a larger injection volume by enlarging the nozzle diameter with a relatively low injection pressure up to 60MPa. In order to obtain higher performance in future DME engines, high pressure fuel injection is considered essential, however the high pressure DME spray characteristics have not yet been understood.
In this research, DME spray characteristics of high injection pressure up to 140MPa were examined using a constant volume vessel under engine-like temperature/pressure conditions. For a spray observation, two methods were used: a shadowgraph method and a diffuse forward scatter method, in order to distinguish between gaseous and liquid phase fuel. A high-speed video camera with 20,000 frames per second and 304×304 pixel resolution was used to image the DME spray. The internal DME spray structure was investigated with numerical analysis using the KIVA-3V. Physical properties of DME required by the numerical analysis were carefully provided, and KIVA-3V was modified by calibrating the numerical model to the DME spray observation results.
Results showed that DME evaporation was overwhelmingly faster compared to light oil, and a more evenly distributed leaner spray was formed. Even when using a larger nozzle diameter at a lower injection pressure of about 60MPa, the overall average interior spray equivalence ratio was equivalent to a light oil spray with an injection pressure of 200MPa. When the fuel injection pressure was increased, air entrainment was further improved, which indicates that this may effectively improve DME engine performance in the high speed, high load region.