Journal of Mechanical Science and Technology 26 (10) (2012) 3323~3330 www.springerlink.com/content/1738-494x
DOI 10.1007/s12206-012-0806-8
An investigation on the spray characteristics of DME with variation of ambient pressure using the common rail fuel injection system† Sejun Lee1, Soojin Jeong2 and Ocktaeck Lim3,* 1
Graduate School of Mechanical and Automotive Engineering, Ulsan University, 102 Daehak-ro, Nma-gu, Ulsan, 680-749, Korea 2 Green Powertrain System R&D Center, Korea Automotive Technology Institute, 74 Yongjeong-ri, Pungse-myeon, Dongnam-gu, Cheonan-si, Cungnam 330-912, Korea 3 Department of Mechanical and Automotive Engineering, Ulsan University, 102 Daehak-ro, Nma-gu, Ulsan, 680-749, Korea (Manuscript Received August 22, 2011; Revised May 24, 2012; Accepted June 18, 2012)
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Abstract We investigated the DME spray characteristics about varied ambient pressure and fuel injection pressure using the common rail fuel injection system when the nozzle holes diameter is varied. The common rail fuel injection system and fuel cooling system were used since DME has compressibility and vaporization at atmospheric temperature. The fuel injection quantity and spray characteristics were measured. The spray was analyzed for spray shape, penetration length, and spray angle at the six nozzle holes. There are two types of injectors: 0.166 mm diameter and 0.250 mm diameter. The ambient pressure, which was based on gage pressure, was 0, 2.5, and 5 MPa. The fuel injection pressure was varied by 5 MPa from 35 to 70 MPa. By comparing with the common injector, using the converted injector it was shown that the DME injection quantity was increased 127% but it didn't have the same low heating value. Both the common and converted injectors had symmetric spray shapes. In case of converted injector, there were asymmetrical spray shapes until 1.2 ms, but after 1.2 ms the spray shapes were symmetric. Also, the converted injector had shorter penetration length and wider spray angle than the common injector. Keywords: Ambient pressure; DME (Di-methyl ether); Spray angle; Spray penetration ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1. Introduction The investigation of DME, which is an alternative for diesel fuel, is in progress to respond to environmental problems and the need for alternative fuel. Among these investigations, there are many advantages of DME. Oh et al. [1] studied the emission characteristics and fuel efficiency for heavy-duty DME buses; the dynamic characteristics of DME and diesel were the same. In addition, the exhaust characteristics were improved without after-treatment system. Seto et al. [2] found that when using the DME, CO2 was decreased. However, DME also has some disadvantages. In the study by Ishikawa et al. [3], a DME supply system was needed since its elastomeric attack did damage to the fuel supply system. In the investigation of the performance of a diesel engine with inline DME injection system by Yoshino et al. [5], increasing the injection quantity of DME was needed to get the same heating value of diesel. This means that the low heating value of DME is lower than for diesel. Thus, the injection quantity *
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[email protected] † Recommended by Associate Editor Kyoung Doug Min © KSME & Springer 2012
of DME is more needed than diesel to get the same power as diesel. There are various researches for solving the low heating value of DME. The first is increasing the nozzle holes diameter, the second is increasing the injection pressure, and the third is converting the needle tip. Among these, converting of the needle tip is difficult because it needs high technology. In contrast, both the enlargement of nozzle hole diameter and the increase of injection pressure are much easier than converting the needle tip shape. Therefore, this paper shows the solutions of the DME low heating value problem by increasing the nozzle hole diameter and injection pressure.
2. Experimental apparatus 2.1 Spray visualization and DME injection system Fig. 1 [6] is a schematic of the spray visualization, DME injection system and DME quantity measuring device. In Fig. 1, the left side is the DME injection system and the right side shows the spray visualization system. Nitrogen is compressed into a high pressure chamber located on the right side of Fig. 1. An injector controls the injection duration and the number of injections by the injector controller (TDA-3300). A strobe light, which is used as light source, is controlled by the pulse
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generator (DG535). The pressure is measured by the pressure sensor (Kistler 6056A) mounted in the high pressure chamber. The pressure sensor signal is amplified by the charge amplified (Kistler 5015). A camera (Nikon D90) which takes pictures of the spray is connected to the computer for capture and to save the images. The properties of DME and diesel are shown in Table 1. In the DME injection system, the common rail injection system is used for constant high pressure injection. DME is likely to leak and wear because of its low viscosity and bad lubricity [7-13]. For this, DME was added 1% of BDF (bio-diesel fuel) to increase lubricity; air compressor is used. DME exists in gas phase at atmospheric pressure by its low vaporization point. Thus DME needs to compress over 0.5 MPa to become liquid phase. DME is liquefied state with pressure 0.5~0.8 MPa and it is supplied to accumulator after passing from the fuel tank to low pressure pump. The liquefied DME can be gas state by getting heat from the accumula-
tor wall temperature. To prevent this, a compact cooling device is installed to keep cooling the accumulator; thus DME which is inside of the accumulator is liquefied. The liquefied DME passes the air pressure pump which activates the air compressor; it is compressed at 35 MPa and stored in the accumulator. DME is supplied to common rail that controls fuel pressure by a PCV (pressure control valve) driver. In the common rail system, DME is injected at constant pressure. The injector controls injection duration and the number of injections by injector driver; DME is injected when the injector driver passes the trigger signal to the injector. The circulating fuel from injector to common rail is re-supplied to the low pressure pump. 2.2 DME quantity measuring device The bottom right of Fig. 1 shows the schematic of DME injection quantity measuring device. At the front of the device, the liquefied DME is shown through quartz which marks the volume grid. After checking the quantity, the exhaust valve is opened to drain out the DME. The state of DME which is in the quantity measuring device is marked the point on Fig. 2. As this point, DME is compressed to 5.5 MPa by nitrogen gas; thus it can be maintained in the liquid phase.
Table 1. Properties of DME and diesel. Property [Unit] DME Diesel Chemical structure CH3OCH3 Boiling point at 1 atm [K] 248.1 450~643 467.13 Enthalpy of vaporization [kJ/kg] 300 Lower heating value [MJ/kg] 27.6 42.5 Gaseous specific heat capacity [kJ/kg K] 2.99 1.7 3.4/18.6 Ignition limits [vol% in air] 0.6/6.5 6.37E + 08 14.86E + 08 Modulus of elasticity [N/m2] Kinematic viscosity of liquid [cSt]
