Dimethyl ether appears to be a better choice among various Diesel alternatives due to its high cetane number and soot-less combustion. However, the physical and chemical properties of dimethyl ether are very different from Diesel. The physical properties influence spray formation and atomization characteristics, while chemical properties determine combustion and emission formation characteristics. Thus, fuel's physical and chemical properties significantly determine engine performance and emissions. In the present work, spray combustion and emission formation characteristics of n-heptane, dimethyl ether, and their blends (10, 25, and 50% dimethyl ether in n-heptane) were numerically studied in a constant volume chamber. Results show that the n-heptane spray combustion has the highest heat release rate with an intense premix combustion phase, whereas dimethyl ether spray combustion has the lowest heat release rate and shortest premix combustion phase. The magnitude of the premixed phase and heat release rate decreases with the increase in dimethyl ether mass fraction in the blends. Soot, carbon monoxide (CO), unburned hydrocarbon (UHC), and nitric oxide (NO) emissions decreased with the increase in the dimethyl ether mass fraction in the blends and were lowest for the dimethyl ether.
The lower calorific value of dimethyl ether (DME) is approximately 65% of that of diesel; therefore, a higher quantity of DME must be supplied per cycle to generate the same magnitude of power. A retrofitted DME-fueled engine generally uses a longer fuel injection duration to provide the excess fuel mass. The present work seeks to find the most appropriate way to increase the mass flow rate of a DME-fueled direct-injected compression ignition engine by varying the number of nozzle holes, nozzle hole diameter, and injection pressure. The results show that increasing mass flow rate increases peak combustion pressure and heat release rate but decreases combustion duration. Increasing the number of injector holes increases indicated specific fuel consumption (ISFC) and exhaust emissions (NOx, HC, and CO) due to mixing between unburned fuel spray and neighboring combustion products.Increasing the nozzle hole diameter or fuel injection pressure increases spray tip penetration and improves fuel-air mixing before combustion initiates. Increasing the nozzle hole diameter by merely 22 μm than the baseline injector reduces ISFC and exhaust emissions at a slightly retarded start of injection (SOI). However, the lowest ISFC was found at the 30 MPa injection pressure at the most advanced SOI, at the expense of higher NOx.
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