Experimental Investigation On Combustion And Emissions Performance Of A DME-blended Spark-ignited Engine

With the tremendous increase of transportation energy consumption, theenvironmental pollution and energy shortage have emerged as a main societalproblem and a focus for more and more people. In order to protect the environmentand realize the sustainable development, finding better substitutes for fossil fuels hasbecome a major work for most of the researchers studying internal combustion (IC)engines. Due to the low emissions, environmental friendliness, alternative fuels havebeen considered as one of the best choices for future IC engines.Nowadays, SI engines use a fixed octane rating fuel such as#93gasoline under awide range of torques and speeds. However, to produce the possibly highest thermalefficiency and lowest emissions, SI engines should be fueled with the variable octanerating fuels for various engine operating conditions, e.g. at cold start, the low octanerating fuel is used to facilitate the engine start, and at high loads, the high octanerating fuel is utilized to avoid the engine knock. Thus, for the sake of improving theperformance of SI engines, the fuel and operating condition coordinative strategy isproposed in this paper.As a high cetane number fuel, DME is introduced into the SI engine to widen theoperational range and improve the performance of SI engines. DME can be producedfrom a variety of feedstocks such as natural gas, coal, crude oil, residual oil, wasteproducts and biomass. The high cetane number and low boiling point of DMEsymbolize the short ignition delay, low auto-ignition temperature and almostinstantaneous vaporization. Moreover, as DME is non-toxic and environmentallybenign, whenever at low or high mole fractions (percent by volume) in air, it hardlyhas any odor and causes no negative health effects. The DME has a lowcarbon-to-hydrogen ratio (C:H), a high oxygen content (around35%by mass) and noC-C bonds in its molecular structure, which help realize the smoke-free combustion.A four-cylinder SI engine is used in this experiment. The engine intake manifoldsis modified with four DME injectors mounted near the intake port of each cylinder, sothat DME and different high octane rating fuels can be injected and mixed in theintake ports online. A hybrid electronic control unit (HECU) is developed and adoptedto govern the spark timings, injection timings and durations of DME and differenthigh octane rating fuels. The HECU acquires the engine sensor data from the originalECU (OECU) and communicates with a calibration computer. The injection durationsof fuels can be adjusted freely through the commands from the calibration computer.The exhaust emissions of NOx, HC and CO from the test engine are measured andanalyzed by a Horiba MEXA-7100DEGR emissions analyzer. Cylinder pressure andcrank angle signals are sampled and treated via a DEWE-CA combustion analysissoftware embedded in the DEWETRON combustion analyzer to obtain profiles of cylinder pressure, heat release rate, indicated thermal efficiency, CA0-10, CA10-90,COVimep, COVnand so on.The tests are carried out at cold start, idle and1400r/min conditions. Differenthigh octane number fuels blended with DME under different addition levels are tested.The investigated mixture conditions include stoichiometric excess fuel air ratio andlean combustion. The main conclusions are as follows.At the cold start condition, the SI DME engine could be started successfully andrealize the stabile running. Meanwhile, compared with the original SI gasoline engine,the operating performance and emissions level of the SI DME engine are improvedunder the cold start condition. About the emissions performance, HC emissions of theSI DME engine are reduced by75%compared with the original SI gasoline engine,NOxand CO emissions of the SI DME engine obviously drop to only about30%ofthose of the SI gasoline engine after20s. Furthermore, the SI DME engine only needsabout50%fuel energy flow rate of the SI gasoline engine. Thus, based on the testresults, adding DME is found to be a quite innovative application way, as well as aneffective and economical measure for improving the cold start performance oftraditional gasoline engines.Under idle conditions, the performances of SI engines fueled with four fuels,including pure DME, DME/gasoline, DME/methanol and DME/ethanol mixtures, aretested. DME can be used independently as well as blended with other high octanerating fuels to enhance the idle performance for SI engines. DME addition to the SIengine could improve the indicated thermal efficiency, shorten the flame developmentperiod and meliorate the running stability, especially when it is used independently.The indicated thermal efficiency of the SI DME engine is increased by about30%compared with that of the SI gasoline engine. Under the stoichiometric condition, theNOxemissions of the SI DME engine are20310-6and only40%of those of thegasoline engine. The CO emission of the DME engine is also less than that of thegasoline engine when fuel/air ratio is less than1.2. In conclusion, whenever the SIengine is fueled with pure DME or DME-blended fuel, it is helpful for realizing thestable operation, decreasing the fuel consumption and reducing the emissions, whichcould effectively improvethe overall performance of SI engines at idle.Experiments on the SI engines fueled with DME/gasoline, DME/methanol, andDME/ethanol mixtures are performed to investigate the performance ofDME-enriched SI engines under steady operating conditions. The results of the enginefueled with DME/ethanol blends show that, under the stoichiometric condition, theindicated thermal efficiency is firstly increased and then decreased with the increaseof engine load. Meanwhile, DME addition obviously increases indicated thermalefficiency at all tested engine loads. The indicated thermal efficiency after DMEenrichment is averagely enhanced by about5.5%in comparison with that of the original engine at αDME=2%. Furthermore, the enrichment of DME also avails thereduction of COVimepand the shortened combustion duration. Moreover, DMEaddition could obviously decrease HC emissions at all test engine loads, which aredecreased by nearly40%on average. All in all, the test results indicate that DMEblending to high octane number fuels is a suitable way to overcome the shortcomingsof single fuel SI engines at low to mid engine speeds and loads.