1 DME, it canbe used for compression ignition engines which have higherthermal efficiencies compared to spark ignition engines.Teng et al. [3] estimated the cetane number for DME tobe 68, a value that is much higher than that of conventionaldiesel fuel (40–56).
2 he higher cetane numberof DME offers mild engine operation with lower enginenoise because it reduces ignition delay and suppresses rapidpremixed burning
3 Many researchers [4–6] have reported lowerhydrocarbon (HC) and carbon monoxide (CO) exhaust emissions as well as soot free combustion for DME fueledengines. On the other hand, emissions of oxides of nitrogen(NOx) from the DME fueled engines are known to be significantcompared to other emissions [5,6].
4 Previous research [7] on the characteristics of DMEinjected through a high pressure common-rail injection systemindicates that the DME spray has a smaller dropletdiameter than that of diesel fuel under the same injectionconditions because of its lower surface tension.
5 Moreover, the rapid droplet evaporationof DME leads to the apparent turbulent-jet-like behaviorof the spray [3,8]..
6 alubrication improver (Lubrizol 539M, Lubrizol)
7 In order to determine the relationship between the pulsewidth of the injector and the mass of fuel supplied to theengine, the discharging mass flow rate was obtained bymeasuring the mass of the fuel tank. The accumulated massof fuel injected during 5000 cycles was weighed using anelectronic precision balancer (GP-30K, AND).
8 Fuel temperature (C) 20
9 the pulse width ofthe injector was determined from its relationship to themass of fuel supplied to the engine, which was obtainedfrom the previously described measurement of injectionmass.
10 a DME equivalent of11.9 mg due to its lower heating value.
11 Compression ratio 17.8
12 Spray angle (C) 156
13 Mass of fuel (mg/cycle) DME 11.9
14 Pulse duration (ls) DME 784
15 Although the pulse width was thesame for both fuels, DME was injected slightly faster (by0.02 ms) and ended later (by 0.1 ms) than diesel as shownin Fig. 2a. As a result, the actual injection duration ofDME was longer than that of diesel under the same injectioncondition.
16 start of injection pulse of 8
17 The rate of heat release also indicates DMEfuel was ignited faster and the premixed burn spike waslower than that of diesel fuel. The shorter ignition delayand lower energy supply rate explains the lower peak heat release of DME.shorter ignition delay because high centane number.
18 Although the mixing rate ofDME with air is lower than that of diesel fuel [4,8,9], areduced late combustion region was indicated due tothe faster evaporation of DME. shorter late combustion.
19 In the caseof DME fueling, the DME fueled engine presents ahigher premixed spike at an injection of 2 BTDC thanat 8 BTDC. This can be explained because the retardedinjection timing extended ignition delay, as a result, theaccumulated energy between the start of injection andstart of combustion was increased.
20 Due to the shorter ignitiondelay of DME fuel, excessive rise in pressure caused both (1) by chemical reactions occurring before combustion andby (2) the products produced during the ignition delay burning
21 the engine can beoperated under more delayed injection timing for a reductionof NOx emission compared to the engine operatingcondition with diesel fueling.
22 As the injection timingadvanced, IMEP decreased because the main heat releasewas created during the compression stroke; accordingly,negative work during compression stroke increased.
23 The higher IMEP of DMEindicates higher efficiency, and there are several reasonsfor this. First, contrary to the flame of diesel combustion,a non-luminous flame was found for DME [15]. Consequently,a lower heat radiation of the DME flame mightreduce cooling loss through the wall of the combustionchamber. Second, the combustion efficiency which canbe defined as a ratio of the fuel’s chemical energy andactual heat release during combustion is higher forDME due to the lower fraction of incomplete combustionproduct such as HC and CO.
24 In the range of injection timing between 15 and 2BTDC, lower COVIMEP of DME was observed as shownin Fig. 6a. This means that the DME engine operates morestably than the diesel fueled engine.
25 Higher NOx emissions throughout the injection timingwere observed and compared to diesel fuel. It can beexplained because the faster ignition of the DME mixtureleads to an increased charge temperature compared todiesel fuel.
26 lower exhaust gas temperature as seen in the figure.This result was affected by faster ignition and areduced late combustion regime compared with dieselfueling. Lower HC and CO emissions and lower exhausttemperatures
27 Considering the lower tip penetration and fasterevaporation and ignition of DME, reduced fuel wall wettingis expected to result in lower HC emission.
28 [4] Kim MY, Bang SH, Lee CS. Experimental investigation of spray andcombustion characteristics of dimethyl ether in a common-rail dieselengine. Energ Fuels 2007;21:780–93.
29 [7] Kajitani S, Oguma M, Mori T. DME fuel blends for low-emission,direct-injection diesel engine. SAE Tech Pap Ser; 2000. 2000-01-2004.
30 [8] Suh HK, Park SW, Lee CS. Atomization characteristics of dimethylether fuel as an alternative fuel injected through a common-railinjection system. Energ Fuels 2006;20:1471–81.
31 [9] Teng H, McCandless JC. Comparative study of characteristics ofdiesel-fuel and dimethyl-ether sprays in the engines. SAE Tech PapSer; 2005. 2005-01-1723.
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