Experimental Investigation Of Combustion And Emissions On A Turbocharged DME Engine

The development of China's automotive industry is facing two major constraints: energy security and environment protection. It is an imperative task for China to develop alternative fuels for automobiles with due consideration to the country's structure of energy supply. DME, with high thermal efficiency and low emission due to its special properties, has in recent years attracted increasingly wide attention and has been considered as an alternative fuel which will contribute to the rationalization of China's energy structure and the improvement of the environment. Funded by the"Clean Auto Action", a project sponsored by the Ministry of Science and Technology of China, this paper investigates the characteristics of combustion and emissions of a turbocharged DME engine with special focus on the emission control strategy.Based on the D6114ZLQB diesel engine, a DME fuel supply and injection system was developed to fit in with the special properties of DME. The effect of fuel supply system and operating parameters on the DME engine was investigated. It was found that DME temperature has a great effect on the power of the DME engine. The output power of the DME engine would be reduced with the increase of DME temperature. The fuel temperature should be controlled in order to keep steady power of the DME engine in practical use.Combustion, performance and emissions characteristics between the DME engine and a diesel engine were compared. Compared with the diesel engine, the injection delay of DME is longer than that of diesel while ignition delay is shorter than that of diesel. However, the maximum cylinder pressure, maximum pressure rise rate and combustion noises of the DME engine are lower than those of diesel. The combustion velocity of DME is faster than that of diesel, resulting in shorter combustion duration of DME. The maximum torque and power of the DME engine at full load are greater than those of the diesel engine, particularly at low engine speeds. The fuel consumption at low engine speeds is lower than diesel. NOX emissions of the DME engine is decreased remarkably, which is more obvious at high engine speeds. HC emission is reduced while CO emission is increased slightly. The DME engine is smoke free throughout all the operating points of the engine. The emissions of the DME engine are far below the limits of EURO-III.The effect of methanol content on performance and emissions characteristic of the DME engine was investigated. HCHO emission of the engine was tested. The experimental results show that fuel consumption and exhaust gas temperature are improved with the increase of methanol content. HC and CO emissions are increased while NOX emission is reduced. HCHO emission is increased with methanol, but it is lower than 15×10-6. It can be found that HC and CO emissions would be increased greatly as the methanol content is higher than 1.5%. Taking into consideration of the performance and emissions of the DME engine, it is recommended that the methanol content should be lower than 1.5%. This will be a scientific basis for setting the DME standard for vehicle in future.In order to further reduce the NOX emission of the DME engine, a"low pressure routine"EGR system was developed specifically. The effect of EGR rate on combustion and emissions was investigated. It is found that: At all engine operating points, with the increase of EGR rate, the cylinder pressure is reduced; the position of peak pressure is delayed. Ignition timing is prolonged; the peak of pre-mixed combustion ROHR is increased while the peak of scatter combustion ROHR is reduced. The combustion duration is enlarged and the exhaust gas temperature is increased with EGR rate. The effect of EGR rate on fuel consumption is more complex; the fuel consumption is decreased at low load with EGR while it is increased at medium and high loads with EGR. With the increase of EGR rate, NOX emission of DME engine is significantly reduced, especially at high load, while HC emission is increased. When the EGR rate is lower than a certain value, EGR has little effect on CO emission. When EGR rate is lager than a certain value, it would lead to a sharp increase of CO emission. EGR has little effect on smoke, the smoke of DME engine is still zero with EGR.The oxidation catalyst technology was used to solve the problem of HC and CO emissions caused by EGR. It is found that HC and CO emissions are reduced greatly by oxidation catalyst,CO emission is reduced by 90%. The HC and CO emissions of DME engine achieve a very low level. The strategy for ultra-low emission is proposed: EGR technology is taken to reduce NOX emission. Oxidation catalyst technology is taken to reduce HC and CO emissions.The effect of LPG content on DME engine was investigated. With the increase of LPG content, ignition delay is prolonged; the combustion duration of blend fuel is reduced especially at low load. LPG had two effects on fuel consumption: fuel consumption would be increased with LPG content for increase of ignition delay. On the other hand, fuel consumption would be decreased with LPG content for shorting combustion duration. The fuel consumption would be reduced firstly then increased for the two effects. It was found that the DME engine achieved better efficiency when the content of LPG in blend fuel was 30%. With the increase of LPG rate, NOX emission is increased at low load, while NOX emission is increased firstly and decreased secondly at medium and high load. HC emission is increased slightly while CO emission is increased greatly with LPG content.The strategy of pre-mixed combustion for DME was proposed. Taking advantage of the characteristic of high vapor pressure and good spraying of DME, Methods such as enlarging plunger diameter and blending a certain LPG in DME were taken to shorten combustion duration. Methods such as cooled-EGR technology, lagging fuel supply timing and blending LPG were taken to increase ignition delay. All those measures were taken to achieve premixed combustion. More DME was injected into cylinder in ignition delay, forming mixed gas before ignition, enlarging the scope of pre-mixed combustion. The concept of DME engine with high performance and ultra-low emission was accomplished. It is found that the DME engine kept high efficiency and NOX emission is significantly decreased. NOX emission is reduced by 90% at the speed of 800r/min.