Engine performance and emission ratio. The thermal efficiency η of the dimethyl ether engine is higher than that of the diesel engine under medium and low loads, while the thermal efficiency is slightly lower under high load. This is due to the increase in circulating fuel injection at high load, and the long fuel injection duration and combustion duration. In future research, it is necessary to further increase the plunger diameter to improve engine performance. Dimethyl ether engine can achieve smokeless combustion (smoke R is 0) under any operating conditions. This is because dimethyl ether contains oxygen and its rapid diffusion combustion rate inhibits the generation of soot. Due to the low temperature in the cylinder of the dimethyl ether engine, NO emissions are significantly reduced (only half of that of diesel engines). The carbon monoxide (CO) and unburned hydrocarbons (HC) of the dimethyl ether engine are equivalent to that of the diesel engine (HC is slightly lower), but the emission of formaldehyde (HCHO) is higher than that of diesel.
Combustion characteristics analysis, comparison of fuel injection delay period and flame retardation period. The injection delay angle of dimethyl ether is larger than that of diesel. This is because the speed of sound is low in liquid DME, and the corresponding pressure wave propagation speed is lower than that in diesel. The propagation speed of sound wave in dimethyl ether is 980 compared with the change of average effective pressure P. The flame retardation period of dimethyl ether is significantly shorter than that of 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 fuel injection are beneficial to shorten the flame retardation period. The shorter flame retardation period reduces the combustible mixture formed during the flame retardation period, and reduces the maximum explosion pressure, maximum pressure rise rate, mechanical load, combustion noise and NO emissions.
The maximum pressure rise rate of the dimethyl ether engine is only 0.3～0.4MPa/(°), which is at the level of the gasoline engine and at the position after the top dead center, thus reducing the combustion noise of the engine. The heat release rate dQ/dφ varies with the crank angle φ. The engine is somewhat similar, both of which have two peak heat release rates of premixed and diffuse combustion, but the peak heat release rate of premixed combustion of dimethyl ether engine is lower than that of diesel engines. The crank angle between the two peaks of heat release of the dimethyl ether engine is smaller than that of the diesel engine. All these factors indicate that the premixed combustion amount of the dimethyl ether engine is small, the diffusion combustion amount is large, and the diffusion combustion speed is fast.
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