Experimental Investigation On Combustion And Emissions Performance Of A DME-Enriched Spark-Ignited Engine

Concerning environmental pollution and energy shortage, researching and developing (R&D) green and renewable alternative fuels have attracted more and more attentions in internal combustion (IC) engine fields. Among all alternative fuels, dimethyl ether (DME) and methanol possess many good properties, such as large reserves, easy preparation, low pollution, environmentally friendly, renewable, and convenient storage and transportation, which make them become the most promising alternative fuels for IC engines. Meanwhile, previous investigations also have pointed out the good prospects for adopting DME and methanol in IC engines.IC engines always run at complex and variable conditions. However, most of the traditional spark-ignited (SI) engines use gasoline as the mono-fuel, which is difficult to satisfy requirements for fuel properties, especially for octane number, at different operating conditions. Because of the high cetane number of DME and high octane number of methanol, vehicle engines fueled with pure DME or methanol alone tend to encounter the decreased operating range, incomplete combustion, knocking and other problems. Comparatively, since the fuel's physicochemical and combustion properties could be optimized adapting to engine operating conditions through adjusting the blending ratio of the two fuels, the blended fuel-powered engine fueled with the mixture of DME and methanol could gain the improved economic and emissions performance.In the experiment, a self-developed hybrid electronic control unit was used to govern fuel injection duration. And air-to-fuel ratio, blending ratio, etc, are adjusted on-line by varying the injection durations of the two fuels. Because of the high residual gas fraction and low combustion temperature, SI engines always gain the dropped combustion and emissions performance at low speed and load conditions. Thus, the experiments in this paper were carried out at low operating conditions to investigate the effect of applying the blended fuel for improving engine performance at low speed and load conditions.The effect of DME addition on performance of a gasoline engine is firstly investigated. The test results showed that total fuel energy flow rate, indicated mean effective pressure (Imep) and peak cylinder pressure were reduced whereas engine indicated thermal efficiency was improved with the increase of DME addition fraction. Because of the high diffusion and laminar flame speeds of DME, engine flame development period was shortened and cyclic variation was eased after DME addition. At lean conditions, residual gas fraction and cooling loss were decreased, and HC emissions were reduced by nearly 50% after DME addition.To further investigate the combustion characteristics of the blended fuel consisting of high cetane number and octane number fuels, the performance of a methanol/DME-fueled engine was experimentally studied on the test bench. The test results demonstrated that engine indicated thermal efficiency was enhanced by 20-30%, compared with the original one after DME enrichment. Particularly at idle conditions, the addition of DME was effective on shortening both engine flame development and propagation durations, reducing the coefficient of variation in indicated mean effective pressure (COVimep) and decreasing HC emissions.According to experimental results from the gasoline/DME and methanol/DME engines, the blended fuel-powered engine fueled with high cetane number and octane number fuels could gain improved combustion characteristics and reduced HC emissions at low speed and load conditions, compared with the mono fuel-powered engine. The operating ranges of methanol and DME-fueled engines were extended by adopting the blended fuel combustion strategy.