Engine performance and emissions ratio. The thermal efficiency η of the dimethyl ether engine is lower than that of the diesel engine under medium and low load, and the thermal efficiency is slightly lower at high load. This is due to the increase of the cycle fuel injection at high load, the duration of the injection and the long duration of the combustion. In future studies, it is necessary to further increase the diameter of the plunger to improve engine performance. Dimethyl ether engines can achieve smokeless combustion (flux R is 0) under any operating conditions. This is because dimethyl ether contains oxygen, and its rapid diffusion combustion rate inhibits the formation of soot. Due to the low temperature inside the dimethyl ether engine, NO emissions are significantly reduced (only half of the diesel engine). The carbon monoxide (CO) and unburned hydrocarbon (HC) of the dimethyl ether engine are equivalent to the diesel engine (the HC is slightly reduced), but the emission of formaldehyde (HCHO) is higher than that of diesel.
Combustion characteristics analysis, injection delay period and flame retardation comparison. Dimethyl ether has a larger injection delay than diesel. This is due to the low speed of sound in liquid DME, the corresponding pressure wave propagation speed is lower than in diesel fuel, and the propagation speed of sound waves in dimethyl ether is a comparison of the variation of 980 with the mean effective pressure P. Dimethyl ether has a shorter ignition delay than diesel. This is due to the high cetane number, low auto-ignition temperature, and good vaporization performance of dimethyl ether. At the same time, the advance angle of dimethyl ether injection is small, and the higher temperature and pressure in the cylinder during injection can help to shorten the ignition delay. The shorter flame retardation period reduces the formation of combustible mixtures during the post-ignition period and reduces the maximum burst pressure, maximum pressure rise rate, mechanical load, combustion noise, and NO emissions.
The maximum pressure rise rate of dimethyl ether engine is only 0.3-0.4 MPa/(°), which is at the level of the gasoline engine and is located after top dead center, thus reducing engine combustion noise. The heat release rate dQ/dφ varies with the crank angle φ. The machine is somewhat similar. Both have preheating and diffusion combustion peaks for two heat release rates, but the premixed combustion heat release rate of the DME engine is lower than that of the diesel engine. The two exothermic peaks of the dimethyl ether engine have a smaller crank angle than the diesel engine. All these factors indicate that the DME engine has less premixed combustion, large diffusion combustion, and fast diffusion and combustion.