Technical Note 101 Study on the performance and emissions of a compression ignition engine fuelled with dimethyl ether H W Wang, L B Zhou*, D M Jiang and Z H Huang Institute of Internal Combustion Engines, School of Energy and Power Engineering, Xi'an Jiaotong University, People's Republic of China Abstract: The paper presents the research results of a light-duty direct injection diesel engine operating on dimethyl ether (DME). The e ects of the main parameters of the combustion system, such as plunger diameter, nozzle type, fuel delivery advance angle, protruding distance of the nozzle tip from the bottom plane of the cylinder head and swirl ratio, on the performance of the DME engine are investigated. The indicator diagrams are taken after optimizing the combustion system and characteristics of combustion and emissions are measured for DME and diesel operation. The results show that, by adding a pressure pump in the fuel supply system, the vapour lock of DME in the fuel system is eliminated. The engine runs smoothly on DME over a wide range of speeds and loads. The e ective thermal e ciency of the DME engine is 3 per cent higher than that of the diesel engine, and a low rate of pressure rise, low combustion noise, smokeless combustion and low NO x emissions of the DME engine can be achieved. The results demonstrate good characteristics in reducing emissions for a diesel engine operating on DME. Keywords: performance, emissions, dimethyl ether, compression ignition engine 1 INTRODUCTION In response to the ever tightening standards for exhaust emissions and the problem of fuel shortage with the ever increasing number of automobiles, the search for an alternative fuel with high e ciency and low emissions for the automotive engine is becoming one of the key directions of engine development. The utilization of three-way catalysts in gasoline engines can control the engine emissions within the accepted level for these engines at the present time, whereas NO x and PM emissions, which are di cult to reduce simultaneously, have become the main factors preventing the development of diesel engines with low emissions. Therefore, simultaneous reduction in NO x and PM emissions has become one of the major challenges for diesel engines. One approach to solving this problem is the use of alternative fuels; in the past few years, a new type of clean fuel, dimethyl ether (DME), has attracted increasing attention in engine development worldwide owing to its excellent characteristics in reducing emissions. DME can be produced from either methanol or The MS was received on 28 January 1999 and was accepted after revision for publication on 7 April 1999. *Corresponding author: Institute of Internal Combustion Engines, School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, PR China. natural gas, or from coal, at an acceptable cost with the technology available. It will attract more interest in countries with abundant coal and natural gas resources, easing their fuel problems as well as helping them to tackle their pollution problems. In addition, DME can be handled without hazard to people, but should be pressurized in the fuel tank owing to its low boiling point. Some e cient methods to produce DME have been reported, and it is estimated that DME will have a comparable price with diesel if DME engines are widely used. Some preliminary studies have been conducted on diesel engines fuelled with DME, and it has been found that the fuel economy and performance of DME engines are equal to or better than those of diesel engines. The exhaust emissions are at a much lower level than those of baseline diesel engines, and smoke-free and particulate-free combustion can be realized [1±3]. Previous work has also revealed that the simultaneous cutting down of particulate matter and NO x could be achieved by operating the engine on DME together with exhaust gas recirculation (EGR) while keeping the other emissions (CO, HC) at a lower level [2, 4]. Although some preliminary work has been conducted on the performance and emissions of DME engines [1±3, 5], much work still needs to be done to elucidate the characteristics of DME diesel engines in order to make clear the in uence of parameters of the fuel injection and D01099 ß IMechE 2000 Proc Instn Mech Engrs Vol 214 Part D
102 H W WANG, L B ZHOU, D M JIANG AND Z H HUANG Table 1 Physical and chemical properties of DME and diesel Property DME Diesel Chemical formula CH 3 ±O±CH 3 C x H y Mole weight (g) 46.07 190±220 Boiling point ( C) 24.9 180±360 Reid vapour pressure (MPa) 0.51 (20 C) Liquid density (g/cm 3 ) 0.668 0.84 Liquid viscosity (cp) 0.15 4.4±5.4 Low heat value (MJ/kg) 28.43 42.5 Explosion limit in air (vol %) 3.4±17 0.6±6.5 Auto-ignition temperature ( C) 235 250 Cetane number 55±60 40±55 Stoichiometric air/fuel ratio (kg/kg) 9.0 14.6 Latent heat of evaporation (kj/kg) 460 (-20 C) 290 Carbon content (wt %) 52.2 86 Hydrogen content (wt %) 13.0 14 Oxygen content (wt %) 34.8 0 combustion system on engine performance and combustion. These parameters are important to the engine when operating on DME fuel. The study of the in uence of these parameters on engine combustion will help to optimize the DME engine to achieve the targets of high e ciency and low emissions. 2 FUEL CHARACTERISTICS DME (chemical formula CH 3 ±O±CH 3 ) is one of the simplest ether compounds. Its physical and chemical properties compared with diesel are shown in Table 1. The properties of DME can be summarized as follows: 1. The low heat value of DME is only 64.7 per cent of that of diesel, and therefore a larger amount of fuel supply is needed to ensure the same engine power output. 2. The cetane number of DME is higher and the autoignition temperature is lower than in the case of diesel, and therefore a shorter period of ignition delay and a lower combustion noise are expected compared with those of diesel operation. 3. DME has only C±H and C±O bonds, no C±C bond, and moreover contains about 34.8 per cent oxygen, and therefore combustion-produced emissions such as CO, HC, CO 2, smoke and PM are expected to be lower than those of diesel operation and to tolerate a higher EGRratio to reduce NO x. 4. The latent heat of evaporation of DME is much higher than that of diesel and will be bene cial to NO x reduction owing to the larger temperature drop of the mixture in the cylinder. 5. DME will be in the gaseous state even at 20 C and ambient pressure. Its vapour pressure varies with temperature, so DME must be pressurized to over 0.5 MPa to keep it in the liquid state under ambient conditions (25 C). The fuel delivery pressure should be increased to 1.7±2.0 MPa under engine operating conditions to prevent vapour lock in the fuel system. 6. Lubricant is needed to ensure reliability and durability of the parts of the fuel system, due to the low viscosity of DME. Neat DME with 2 per cent castor oil is used for engine operation and neat DME without castor oil is only used for emission measurement in order to prevent the in uence of castor oil additive on emission level. 3 EXPERIMENTAL APPARATUS The test is carried out on a water-cooled, single-cylinder, four-stroke, naturally aspirated, direct injection diesel engine. The speci cations of the engine and the schematic block of the fuel supply system of the DME engine are shown in Table 2 and Fig. 1. The fuel supply system is pressurized by compressed nitrogen and a supply pump is installed before the fuel injection pump to prevent vapour lock in the fuel owmeter and the fuel injection system; a pressure regulator and a bu er are used to keep the pressure constant and eliminate pressure pulsation. Fig. 1 Schematic block of the fuel supply system for a DME engine Proc Instn Mech Engrs Vol 214 Part D D01099 ß IMechE 2000
STUDY ON THE PERFORMANCE AND EMISSIONS OF A COMPRESSION IGNITION ENGINE 103 Table 2 Engine speci cations Diesel engine DME engine Borestroke (mmmm) 100115 Cylinder number Single Displacement (cm 3 ) 903 Compression ratio 18.4 Rated power (kw) 11 Rated speed (r/min) 2300 Swirl ratio 2.3 1.4±1.8 Fuel delivery advance angle ( CA BTDC) 25 19 (high speed) 15 (low speed) Injector open pressure (MPa) 18 15 Plunger diameter (mm) 8.5 9.5 Nozzle numberori ce diameter (mm) 40.32 50.32 4 OPTIMIZATION OF THE PARAMETERS OF THE COMBUSTION SYSTEM 4.1 E ect of the fuel delivery advance angle Figure 2 shows the e ect of fuel delivery advance angle on engine e ective thermal e ciency. It can be seen that the optimum fuel delivery advance angle for maximum thermal e ciency is about 19 CA BTDC at high speed (2300 r/min) and 15 CA BTDC at low speed (1400 r/min). These values of the DME engine are lower than those of the corresponding diesel engine, it is thought because of the shorter ignition delay of DME. from 1.4 to 1.8, while the optimum value for the baseline diesel engine is 2.3. 4.3 E ect of plunger diameter Three diameters of plunger were tested: 8.5, 9.0 and 9.5 mm. It can be seen from Fig. 4 that the e ective 4.2 E ect of swirl intensity The e ect of the inlet swirl ratio on the e ective thermal e ciency of the DME engine is shown in Fig. 3. As DME readily evaporates and mixes with the surrounding air after being injected into the cylinder, the mixing of fuel with air is not so dependent on the air motion as for diesel fuel, so a lower swirl intensity is needed to achieve the maximum thermal e ciency. In this study, the optimum inlet swirl ratio for the DME engine ranges Fig. 3 E ect of inlet swirl ratio on e ective thermal e ciency: n=1400 r/min; b.m.e.p.=0.691 MPa Fig. 2 E ect of fuel delivery advance angle on e ective thermal e ciency: 1400 r/min; ~ 2300 r/min Fig. 4 E ect of plunger diameter on e ective thermal e ciency: n=1400 r/min; b.m.e.p.=0.483 MPa D01099 ß IMechE 2000 Proc Instn Mech Engrs Vol 214 Part D
104 H W WANG, L B ZHOU, D M JIANG AND Z H HUANG Fig. 5 E ect of plunger diameter on e ective thermal e ciency: n=1400 r/min; b.m.e.p.=0.580 MPa Fig. 7 E ect of injector open pressure on e ective thermal e ciency: n=2300 r/min; b.m.e.p.=0.642 MPa thermal e ciency is increased with increasing plunger diameter. This is because the heating value and density of DME are respectively only 64.7 and 80 per cent of the values for diesel. A large-diameter plunger will shorten the injection duration to improve the engine performance of the DME engine. 4.4 E ect of the nozzle Five nozzles were tested in the study. It was found that a 50.32 mm arrangement can achieve the best thermal e ciency (see Fig. 5). The increased number and the increased total ow area of the nozzle ori ces match well with the lower air swirl ratio and large fuel delivery per cycle, which will be bene cial to mixture formation and combustion for DME. 4.5 E ect of the protruding distance of the nozzle tip to the bottom plane of the cylinder head Figure 6 shows the e ect of the protruding distance of the nozzle tip to the bottom plane of the cylinder head on e ective thermal e ciency. The optimum protruding distance of the nozzle tip for DME operation is 5 mm, which is deeper than that of diesel operation (3 mm). This is because DME has good evaporation characteristics, and the spray angle becomes much larger soon after being injected into the cylinder. It is necessary to increase the protruding distance of the nozzle tip to prevent the spray from touching the bottom plane of the cylinder head, which will worsen combustion. 4.6 E ect of injector open pressure As DME readily evaporates, a high injector open pressure is not necessary, as in the diesel engine, to promote better fuel atomization and mixing. Thus, lowering of the injector open pressure will reduce the power for driving the fuel pump and increase the engine thermal e ciency, but further decrease in the injector open pressure will bring about the phenomenon of longer injection duration, and this, as mentioned above, will be detrimental to engine thermal e ciency. In this study, the optimum injector open pressure for the DME engine is 15 MPa, which is lower than that of the diesel engine (18 MPa) (see Fig. 7). 5 ENGINE PERFORMANCE AND EMISSIONS Fig. 6 E ect of the protruding distance of the nozzle tip into the cylinder on the e ective thermal e ciency: n=1400 r/min; b.m.e.p.=0.725 MPa Figure 8 shows the indicator diagrams and rates of pressure rise of DME and diesel engines. The diagrams were obtained for the same power output or mean e ective pressure for DME and diesel. It can be seen that the DME engine has a lower maximum cylinder pressure and a much lower rate of pressure rise compared with those of the diesel engine. The reason for this is that the temperature and pressure in the cylinder decrease owing to the temperature drop of the mixture as the fuel evaporates, and, in addition, the short igni- Proc Instn Mech Engrs Vol 214 Part D D01099 ß IMechE 2000
STUDY ON THE PERFORMANCE AND EMISSIONS OF A COMPRESSION IGNITION ENGINE 105 Fig. 8 Pressure and rate of pressure rise: n=1800 r/min; b.m.e.p.=0.642 MPa tion delay will decrease the amount of fuel being injected into the cylinder prior to combustion. All these factors bring about a low maximum pressure and a low rate of pressure rise, and also give the DME engine superior low mechanical load and low combustion noise compared with the diesel engine. Figures 9 and 10 show the load characteristics of the DME and diesel engine. It can be seen that the DME engine has a higher thermal e ciency than the diesel engine in the range of medium and low loads, but a slightly lower e ciency at high load owing to its longer injection and combustion duration. However, it can be improved by adopting a much larger plunger diameter in the next step. Smokeless combustion can be realized for DME operation, NO x is only half that of the diesel engine, and the HC, CO and HCHO emission levels are Fig. 9 Comparison of the load characteristics of diesel and DME: DME; ~ diesel; n=1800 r/min comparable with those of the diesel engine. The cause of higher CO emission levels at higher powers on DME is that, with increasing power, more DME fuel has to be injected, causing the injection duration of DME to be longer, as the injection advance angle is reduced for the DME engine. A greater volume of DME will be injected compared with diesel fuel at the same power, and so a large amount of DME will be injected in a later period, which obviously may cause a rise in CO and a drop in e ciency owing to the temperature drop and late combustion. 6 CONCLUSIONS The main conclusions of this study are: 1. Adding a supply pump, a regulator and a bu er can eliminate vapour lock in the injection system of the DME engine. 2. The test results show that e ective thermal e ciency and power output of the DME engine can be improved by adjusting parameters such as the plunger diameter, swirl intensity and fuel delivery advance angle. Using larger-diameter plungers, a larger ow area of the nozzle holes, a lower inlet swirl ratio, a smaller fuel delivery advance angle and a lower injector open pressure will result in a better DME engine performance compared with the baseline diesel engine. 3. DME engines can realize smoke-free combustion and lower combustion noise and NO x emissions, while the HC, CO and HCHO emission levels are comparable with those of diesel engines. D01099 ß IMechE 2000 Proc Instn Mech Engrs Vol 214 Part D
106 H W WANG, L B ZHOU, D M JIANG AND Z H HUANG Fig. 10 Emissions of DME and diesel engines: DME; ~ diesel; n=1800 r/min ACKNOWLEDGEMENTS This work is supported by Ford China Research and Development Fund (No. 9715613) and Doctor Dissertation Fund of Xi'an Jiaotong University. The authors thank the experts of Ford Motor Company for their constructive suggestions and necessary technical materials and papers. The authors also gratefully acknowledge the National Natural Science Foundation of China (NSFC) for its joint support, as well as their professional colleagues for their technical contributions to this work. REFERENCES 1 Kapus, P. and Ofner, H. Development of fuel injection equipment and combustion system for DI diesels operated on dimethyl ether. SAE Trans., 1995, 104(4), 54±69. 2 Fieisch, T., McCarthy, C., Basu, A. and Udovich, C. A new clean diesel technology: demonstration of ULEV emissions on a Navistar diesel engine fuelled with dimethyl ether. SAE Trans., 1995, 104(4), 42±53. 3 Sorenson, S. C. and Mikkelsen, S. E. Performance and emissions of a 0.273 liter direct injection diesel engine fueled with neat dimethyl ether. SAE Trans., 1995, 104(4), 80±90. 4 Huang, Z., Wang, H., Zhou, L. and Jiang, D. A new type of alternative fuel for high e ciency ultra low emission diesel engineðdimethyl ether. In Proceedings of Sino±Korea International Conference on Internal Combustion Engines, Xi'an, People's Republic of China, 6±10 August 1998, pp. 77±83. 5 Wang, H., Chen H., Huang, Z., Zhou, L. and Jiang, D. An investigation on the performance of a direct injection diesel engine fueled with dimethyl ether. In Proceedings of Sino± Korea International Conference on Internal Combustion Engines, Xi'an, People's Republic of China, 6±10 August 1998, pp. 84±88. Proc Instn Mech Engrs Vol 214 Part D D01099 ß IMechE 2000