Study of combustion characteristics of a compression ignition engine ...

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Technical Note

Study of combustion characteristics of a compression ignition engine fuelled with dimethyl ether Z H Huang*, H W Wang, H Y Chen, L B Zhou and D M Jiang Institute of Internal Combustion Engines, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, People’s Republic of China

Abstract: This paper presents the combustion characteristics of a light-duty direct-injection diesel engine operating on dimethyl ether (DME). The indicated pressure diagrams and injector needle lifts are recorded and the combustion characteristics are demonstrated and compared with those of an engine operated on diesel fuel. The experimental and calculated results show that the DME engine has a longer delay of injection and duration of injection, a lower maximum cylinder pressure and rate of pressure rise, as well as a shorter ignition delay compared with those of a diesel engine. The DME engine has a low mechanical load and combustion noise, a fast rate of diffusion combustion and a shorter combustion duration than that of a diesel engine. It has the ideal pattern of compression ignition engine heat release. Keywords: combustion, characteristics, dimethyl ether, compression ignition engine

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INTRODUCTION

In response to the ever-tightening standards for exhaust emissions, especially NOx and particulate matter (PM) emissions, for which standards are becoming severe for the diesel engine, the search for high-efficiency ultralow-emission alternative fuels 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 engine emissions within an acceptable level at the present time, but the NOx and PM emissions, which are difficult to reduce simultaneously, have become the main restricting factors for ultralow-emission engines. Therefore, simultaneous reduction of NOx and PM is one of the major challenges for diesel engines. One of the effective approaches to solving this problem is the use of alternative fuels. Alcohol fuels such as methanol and ethanol have previously been widely studied in diesel engine application [1– 3] and have demonstrated advantages in reduction of NOx and PM, but the low cetane number restricts The MS was recei6ed on 7 January 1999 and was accepted after re6ision for publication on 23 March 1999. *Corresponding author: Institute of Internal Combustion Engines, School of Energy and Power Engineering, Xi’an Jiaotong Uni6ersity, Xi’an, 710049, People’s Republic of China. D00399

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their application in compression ignition engines as spark ignition is necessary to ensure normal combustion in this case. Recently, dimethyl ether (DME) with its high cetane number and superior combustion and emission characteristics has attracted increasing attention in engine development worldwide. The DME can be handled safely, but it should be pressurized in a fuel tank owing to its low boiling point. Some efficient methods for producing DME have been reported, and it is estimated that DME will have a comparative price to 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 the DME engine are equal to or better than those of the diesel engine, the exhaust emissions exhibit a much lower level than those of the baseline diesel engine and also smokefree and particulate-free combustion can be realized [4 –6]. Previous work has also revealed that a simultaneous decrease in particulate matter and NOx can be achieved when the engine is operated on DME combined with exhaust gas recycled (EGR) while keeping the other emissions (CO, HC) at a lower level [2, 7]. Although some preliminary work has been conducted on the performance and emissions of the DME engine [1–3] and [8], much work is still needed in order fully to Proc Instn Mech Engrs Vol 213 Part D

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Z H HUANG, H W WANG, H Y CHEN, L B ZHOU AND D M JIANG

understand the combustion characteristics of the DME engine. The objectives of this paper are to investigate the combustion characteristics, such as the ignition delay and heat release, of the DME engine as well as to compare the combustion parameters with those of an engine operating on diesel fuel in order to demonstrate the advantages of this alternative fuel engine in the coming decades.

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FUEL CHARACTERISTICS

Dimethyl ether (chemical structural formula CH3OCH3) 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 fuel supply is needed to ensure the same engine power output. 2. The cetane number of DME is higher and the auto-ignition temperature is lower than those of diesel, and therefore a shorter period of ignition delay and

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3 Table 1

Physical and chemical properties of DME and diesel

Properties

DME

Diesel

Chemical formula Molecular weight (g) Boiling point (°C) Reid vapour pressure (MPa) Liquid density (g/cm3) Liquid viscosity (cP) Low heat value (MJ/kg) Explosion limit in air (vol %) Ignition temperature (°C) Cetane number Stoichiometric air–fuel ratio (kg/kg) Latent heat of evaporation (kJ/kg) Carbon content (wt %) Hydrogen content (wt %) Oxygen content (wt %)

CH3OCH3 46.07 −24.9 0.51 (20 °C) 0.668 0.15 28.43 3.4–17 235 55–60 9.0 460 (−20 °C) 52.2 13.0 34.8

Cx Hy 190–220 180–360

Table 2

0.84 4.4–5.4 42.5 0.6–6.5 250 40–55 14.6 290 86 14 0

Engine specifications

Bore×stroke (mm×mm) Cylinder number Displacement (cm3) Compression ratio Rated power (kW) Rated speed (r/min) Swirl ratio Fuel delivery advance angle (°CA BTDC) Injector open pressure (MPa) Plunger diameter (mm) Protruding distance of nozzle tip into cylinder (mm) Nozzle number×orifice diameter (mm)

Diesel engine

DME engine

100×115 single 903 18.4 11 2300 2.3 25

1.8 19

18 8.5 3

15 9.5 5

4×0.32

5×0.32

Proc Instn Mech Engrs Vol 213 Part D

lower combustion noise are expected to be realized compared with those of diesel operation. DME has only CH and CO bonds, no CC bond, and moreover contains about 34.8 per cent oxygen, and therefore combustion-produced emissions such as CO, HC, CO2, smoke and PM are expected to be lower than those of diesel operation and tolerate a higher EGR ratio to reduce NOx. The latent heat of evaporation of DME is much higher than that of diesel, which will be beneficial to the NOx reduction owing to the larger temperature drop of the mixture in the cylinder. DME will be in the gaseous state even at −20 °C and ambient pressure, and its vapour pressure varies with temperature, so that 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. DME has a low viscosity. To ensure reliability and durability of the parts of the fuel system, 2 wt % castor oil is added to DME.

EXPERIMENTAL APPARATUS

The test is carried out on a water-cooled, single-cylinder, four-stroke, naturally aspirated, direct-injection diesel engine. The specifications of the engines operating on diesel and DME are listed in Table 2. Some modifications to the fuel system are made in order to satisfy the requirements of DME fuel: the plunger diameter is enlarged from 8.5 mm for the diesel engine to 9.5 mm for the DME engine to maintain the equivalent amount of heat of fuel, a nozzle with five holes of 0.32 mm diameter is adopted to ensure a sufficient flow area, the swirl ratio is reduced from 2.3 to 1.8 as DME has a good atomization property, the protruding distance of the nozzle tip into the cylinder is increased from 3 to 5 mm to prevent the fuel from being sprayed on to the surface of the piston crown, the fuel delivery advance angle is reduced from 25° crank angle (CA) before top dead centre (BTDC) to 19°CA BTDC and a low injector opening pressure (15 MPa) is used for DME engine operation. Injector needle lifts and indicated pressure diagrams are recorded in the experiments. Based on the recorded data, the rate of heat release, the injection delay and the combustion duration are obtained.

4 4.1

COMBUSTION CHARACTERISTICS Fuel injection delay

The fuel injection delay for the DME and diesel engines versus the brake mean effective pressure (b.m.e.p.) is D00399

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STUDY OF COMBUSTION CHARACTERISTICS OF A COMPRESSION IGNITION ENGINE

Fig. 1

Comparison of injection delay

illustrated in Fig. 1, in which the two engines have the same fuel delivery advance angle. It can be seen that the fuel injection delay, in crank angle, increases with increasing engine speed for both kind of engine, this delay angle reflecting pressure wave propagation in the fuel pipe. It is well known that the period of propagation is mainly dominated by the length of the fuel pipe and fuel compressibility, and therefore the fuel injection delay remains almost unchanged in time but increases in crank angle with increasing engine speed. It can also be seen that the DME engine shows a longer fuel injection delay than that of the diesel engine at the same engine speed. A previous study revealed that the velocity of sound in DME fuel is 980 m/s, which is lower than that in diesel fuel (1330 m/s) [6], which means that pressure wave propagation will take longer in the DME engine compared with the diesel engine, and consequently the DME engine will have a longer fuel injection delay than the diesel engine. Little variation in fuel injection delay is showed with brake mean effective pressure when the fuel and engine speed are fixed for the above reasons. 4.2

Fig. 2

Comparison of injector needle lift

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Comparison of ignition delay

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Injection duration

The injector needle lifts of the DME and diesel engines under the same engine speeds and brake mean effective pressures are shown in Fig. 2. The injection duration is 27°CA for the DME engine and 22°CA for the diesel engine. A large amount of DME is injected in order to maintain the same power output as that of the diesel engine owing to the low thermal density of the fuel. A large amount of DME can be realized by using a plunger of large diameter or long stroke; an increase in plunger stroke increases the duration of injection while an increase in plunger diameter can shorten the duration of injection. In this study, an increase in plunger diameter from 8.5 to 9.5 mm while keeping the plunger stroke unchanged will make the injection duration of the DME engine longer than that of the diesel engine. 4.3

Fig. 3

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Ignition delay

The ignition delay is obtained (and shown in Fig. 3) by analysing the fuel injection curves and pressure diagrams. From the figure it can be seen that the ignition delays of the two fuels show similar trends with engine speed and load, a significant difference at 1200 r/min and very similar values at 1800 r/min. It is obvious that DME has a greater sensitivity of ignition delay to engine speed. The DME engine demonstrates a shorter ignition delay compared with the diesel engine, the reason being that DME has a high cetane number, a low auto-ignition temperature and good atomization and ignition properties. In addition, the long injection delay of the DME results in the fuel injected into the cylinder being at higher pressure and temperature. All Proc Instn Mech Engrs Vol 213 Part D

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these factors favour a reduction in ignition delay. The short period of ignition delay of the DME engine reduces the amount of fuel injected into the cylinder during this period and results in a reduction in peak pressure and rate of pressure rise, therefore reducing the engine mechanical load and combustion noise. 4.4

Pressure and rate of pressure rise

Figure 4 shows the cylinder pressure diagram, injector needle lift and rate of pressure rise for the DME and diesel engines, and here the cylinder pressure curves are the average of 50 cycles. It is found that, at the same

Fig. 4 Proc Instn Mech Engrs Vol 213 Part D

engine speed and load, the peak pressure and rate of pressure rise show great differences from those of the diesel engine both in value and in crank angle. For the diesel engine the peak pressure is 8.38–9.35 MPa and the crank angle at peak pressure is 8–10°CA, while for the DME engine the peak pressure is 7.37–8.41 MPa, which is 1–1.3 MPa lower than that of the diesel engine and the crank angle at peak pressure is 10–14°CA, which is 2–4°CA higher than that of the diesel engine. From the figure it can also be seen that the pressure variation of the DME engine in the high-pressure region will not change sharply as with the diesel engine, but the duration of this high-pressure period is longer

Comparison of pressure and rate of pressure rise D00399

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STUDY OF COMBUSTION CHARACTERISTICS OF A COMPRESSION IGNITION ENGINE

Fig. 5

Comparison of heat release

than that of the diesel engine. The low peak pressure of the DME engine results in a low mechanical load, high mechanical efficiency and high thermal efficiency. The rate of pressure rise of the DME engine is 0.28–0.47 MPa/°CA, which is at the level of a gasoline engine, and the crank angle at maximum rate of pressure rise exceeds that after top dead centre. This low rate of pressure rise results in a reduction in combustion and mechanical noise and realizes the smooth combustion of the compression ignition engine. 4.5

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3. 4.

Rate of heat release

Figure 5 gives the rate of heat release for the DME and diesel engines under the same load. It can be seen that the heat release curves of the DME and diesel engines show a similar curve pattern, although the rate of heat release for the DME engine starts late owing to its long injection delay. However, its end point of combustion arrives earlier than that of the diesel engine. The periods of premixed combustion of the two engines show no difference, but the rate of diffusion combustion of the DME engine is much faster than that of the diesel engine. The good atomization and evaporation of DME promotes rapid mixing of DME with the surrounding air, and excessive oxygen in the combustion chamber during the diffusion combustion period also increases the rate of diffusion combustion. It can be concluded that a rapid rate of diffusion combustion of DME is realized by the fast rate of mixing processes compared with the poor mixing processes of diesel fuel. Consequently, the DME engine has a controlled rate of premixed combustion and a fast rate of diffusion combustion, with a shorter overall combustion duration than that of the diesel engine.

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CONCLUSIONS

The main conclusions of this study are as follows: 1. The DME ending has a longer injection delay comD00399

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pared with the diesel engine at the same engine speed and load. The DME engine demonstrates a lower peak pressure and rate of pressure rise compared with the diesel engine at the same engine speed and load, and so the DME engine has a low mechanical load and combustion noise. The DME engine has a shorter ignition delay compared with the diesel engine. The rate of diffusion combustion of DME is much faster than that of the diesel engine and the DME engine has a shorter combustion duration that that of the diesel engine. The pattern of heat release of the DME engine is the ideal pattern that would be expected in a compression ignition engine.

ACKNOWLEDGEMENTS This work is supported by Ford China Research and Development Fund (No. 9715613). The authors thank experts of the 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 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 fueled 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 Proc Instn Mech Engrs Vol 213 Part D

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fueled with neat dimethyl ether. SAE Trans., 1995, 104(4), 80–90. 4 Zhou, L. B. and Zhang, Y. Y. An experimental study on the working stability and combustion characteristics of a direct injection diesel engine operating on multifuels. SAE Trans., 1990, 99(4), 503–512. 5 Egeback, K. E. Experiences from the use of ethanol for heavy-duty compression ignition engines. SAE paper 931630, 1993. 6 Saeed, M. N. and Henein, N. A. Combustion phenomena of alcohols in CI engine. ASME Trans., J. Engng for Gas

Turbines and Power, 1989, 111, 439 – 447. 7 Huang, Z., Wang, H., Zhou, L. and Jiang, D. A new type of alternative fuel for high efficiency ultra low emission diesel engine — dimethyl ether. In Proceedings of Sino– Korea International Conference on Internal Combustion Engines, 6 – 10 August 1998, Xi’an, China, pp. 77–83. 8 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, 6 – 10 August 1998, Xi’an China, pp. 84–88.

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