Effect Of Dimethyl Ether Addition On Combustion And Emissions Charcteristics Of The Spark-ignition Ethanol Enging

With the stronger pressure of environmental protection and energy crisis, people pay more attention on seeking clean alternative fuels. Among the current alternative fuels for internal combustion engines, dimethyl ether and ethanol could be made from coal, natural gas and renewable biomass, which combustion produce lower toxic emissions than gasoline. Thus, these two fuels are thought to be the most promising alternative fuels for the application in spark-ignition engines. However, for mono fuel engines, it is difficult for the fuel-air mixtures to meet the required fuel properties under different engine operating conditions. Dimethyl ether and ethanol belong to high cetane number fuel and high octane number fuel, respectively. Thus, by combusting the mixtures of ethanol and dimethyl ether and changing the dimethyl ether fraction in dimethyl ether-ethanol blends, the global cetane and octane numbers of the binary fuel could be adjusted according to engine operating conditions. This enables the dimethyl ether-ethanol blends to satisfy the demand of fuel octane number for different combustion conditions.The testing engine used in this study is an electronically controlled four-cylinder multipoint-injection gasoline engine produced by Beijing Hyundai Motors. Before the experiment, the engine was modified to be fueled with dimethyl ether and ethanol. An engine electronic control system which could control the injections of dimethyl ether and ethanol was developed to enable the engine to be fueled with dimethyl ether-ethanol blends. By using this self-developed engine electronic control unit, the global air-to-fuel ratio,spark advance and dimethyl ether fraction in the binary fuel could be adjusted online through changing the injection durations of the two fuels. A dimethyl ether-blended ethanol engine testing bench was accomplished.The effect of dimethyl ether addition on the combustion and emissions characteristics of the ethanol engines at two typical engine operating conditions of idle and part load were investigated on the engine test bench. Due to the lean burning is an effective method to improve the combustion and emission characteristics, so the experiments were carried out at idle and lean conditions. The effect of dimethyl ether addition on the combustion and emissions characteristics of the ethanol engines under the excess air ratio of 1.0 and dimethyl ether energy fractions in the intake of 0 to 40% were carried out. The effect of dimethyl ether addition on the combustion and emissions characteristics of the ethanol engines were studied when dimethyl ether volume fraction in the intake was 0 and 5.5%. The excess air ratio was raised from 1.04 to 1.08, 1.12, 1.16 and 1.20, respectively. The effect of excess air ratio on the combustion and emissions performance for the ethanol engine were investigated. Part load experiments were carried out under an engine speed of 1800r/min, a manifold absolute pressure of 61.5 k Pa and the maximum brake torque ignition-timing. At excess air ratios of 1.0 and 1.1, the dimethyl ether volume fraction was increased from 0 to 3.5% for the ethanol engine. The dimethyl ether addition on the combustion and emissions characteristics of the ethanol engines was studied.The results showed that the addition of dimethyl ether could improve the engine indicated thermal efficiency and reducing the cyclic variation of ethanol engine through enhancing the combustion under both stoichiometric and lean conditions. At stoichiometric conditions, HC emissions decrease with dimethyl ether energy fraction increase, NOx emissions increase with dimethyl ether energy fraction increase, CO emissions increase then decrease with dimethyl ether energy fraction increase. At lean conditions, NOx and CO emissions decrease with excess air ratios increase, HC emissions increase with excess air ratios increase. The results also demonstrated that the addition of dimethyl ether could also improve the engine indicated thermal efficiency at part load conditions. At an excess air ratio of 1.0, indicated thermal efficiency was elevated by 6.2% when the dimethyl ether addition fraction was increased from 0 to 3.15%. For lean conditions, at an excess air ratio of 1.1, indicated thermal efficiency was increased by 5.7% when the dimethyl ether fraction in the intake was raised from 0 to 3.07%. Besides, the engine cyclic variation under lean conditions was obviously eased by the dimethyl ether enrichment. Moreover, HC and CO emissions were reduced after the blending of dimethyl ether under part load and lean conditions. NOx emissions from the dimethyl ether-enriched ethanol engine could be controlled by adopting lean combustion.