Study on Ultramultihole Nozzle Fuel Injection and Diesel Combustion


Feb 2, 2009 - A high-pressure common-rail fuel injection system was used in this ... Combustion and performance experiments with the UMH nozzle were ...
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Energy & Fuels 2009, 23, 740–743

Study on Ultramultihole Nozzle Fuel Injection and Diesel Combustion Xuelong Miao,*,†,‡ Xinqi Qiao,‡ Xianyong Wang,† Jianda Yu,† Jianhai Hong,† Jinbao Zheng,† Zhihong Fang,† and Zhen Huang‡ Wuxi Fuel Injection Equipment Research Institute, Jiangsu Wuxi 214063, China, and Key Laboratory for Power Machinery and Engineering of M.O.E., Shanghai Jiaotong UniVersity, Shanghai 200030, China ReceiVed October 15, 2008. ReVised Manuscript ReceiVed January 4, 2009

Lean premixed compression ignition combustion provides the potential for simultaneous reduction of NOx and PM, while imposing moderate penalties on CO and HC emissions. To overcome these drawbacks in existing premixed combustion modes of diesel engines, an ultramultihole (UMH) nozzle was developed in this study. The UMH nozzle has two layers of injection holes and a large flow area. Two sprays of the upper and lower layers meet in the space of the combustion chamber. A high-pressure common-rail fuel injection system was used in this experiment. The fuel injection rate of the UMH nozzle was measured using the constant volume method, and its spray pattern was recorded using high-speed digital photography. Combustion and performance experiments with the UMH nozzle were conducted on a turbocharged intercooled diesel engine. The results showed that the UMH nozzle exhibited a higher injection rate, shorter injection duration, shorter spray penetration, and bigger spray angle than those of the conventional nozzle. These characteristics facilitate better mixing of fuel and air prior to ignition, and thus NOx and PM emissions were simultaneously reduced with low CO and HC emissions by combining the UMH nozzle with EGR (exhaust gas recirculation).

1. Introduction Direct injection diesel engines are becoming increasingly popular in commercial vehicles, because of their superior fuel economy, durability, reliability, and high specific power output. However, it is a challenge to meet the increasingly stringent standards for exhaust emissions, particularly NOx and particulate matter (PM). The strategies for reducing NOx increase PM; for example, increasing injection pressure and swirl ratio reduce soot but increase NOx in the conventional diesel combustion. High levels of exhaust gas recirculation (EGR) also reduce NOx but increase soot. This is not the case in some new diesel combustion concepts.1-9 For example, low-temperature premixed compression ignition combustion can simultaneously * To whom correspondence should be addressed. E-mail: [email protected] sina.com. † Wuxi Fuel Injection Equipment Research Institute. ‡ Shanghai Jiaotong University. (1) Richards, K. J.; Subramaniam, M. N.; Reitz, R. D.; Lai, M.-C.; Henein, N. A.; Miles, P. Modeling the Effects of EGR and Injection Pressure on Emissions in a High-Speed Direct Injection Diesel Engine. SAE Tech. Pap. Ser. 2001, 2001-01-1004. (2) Kimura, S.; Aoki, O.; Kitahara, E.; Aiyoshizawa, E. Ultra-Clean Combustion Technology Combining a Low-Temperature and Premixed Combustion Concept for Meting Future Emission Standards. SAE Tech. Pap. Ser. 2001, 2001-01-0200. (3) Wagner, R. M.; Green, J. B.; Dam, T. Q.; Edwards, K. D.; Storet, J. M. Simultaneous Low Engine-Out NOx and Particulate Matter with Highly Diluted Diesel Combustion. SAE Tech. Pap. Ser. 2003, 2003-01026. (4) Pickett, L. M.; Siebers, D. L. Non-Sooting, Low Flame Temperature Mixing-Controlled DI Diesel Combustion. SAE Tech. Pap. Ser. 2004, 200401-1399. (5) Sluder, S. S.; Wagner, R. M.; Lewis, S. A.; Storey, J. M. Exhaust Chemistry of Low-NOx, Low-PM Diesel Combustion. SAE Tech. Pap. Ser. 2004, 2004-01-0114. (6) Kawamoto, K.; Araki, T.; Shinzawa, M.; Kimurra, S.; Koide, S.; Shibuya, S. Combination of Combustion Concept and Fuel Property for Ultra-Clean DI Diesel. SAE Tech. Pap. Ser. 2004, 2004-01-1868.

reduce NOx and PM emissions.10 Some obstacles, however, have to be overcome before they are practically applied to internal combustion (IC) engines.11,12 First, combustion that is too early and too rapid must be avoided to realize a perfect combustion. Second, preparation of a homogeneous mixture of fuel and air is critical. Finally, high levels of HC and CO emissions should be avoided. The purpose of this investigation is to develop a new premixed combustion mode using the ultramultihole (UMH) nozzle, which partially overcomes the above-mentioned drawbacks of the existing premixed combustion. The UMH nozzle facilitates better mixing of fuel and air prior to ignition and the resultant realization of premixed combustion, because of its shortened injection duration and improved atomization as compared to that of the conventional nozzle. The experimental results of fuel injection and combustion with the UMH nozzles in a diesel engine are reported in this Article. The present Article (7) Stanglmaier, R. H.; Roberts, C. E. Homogeneous Charge Compression Ignition(HCCI) Benefit, Compromises, and Future Engine Applications. SAE Tech. Pap. Ser. 1999, 1999-01-3682. (8) Hisakazu, S.; Noriyuki, K.; Matsuo, O. Combustion Control Method of Homogeneous Charge Diesel Engine. SAE Tech. Pap. Ser. 1998, 980509. (9) Takeshi, H.; Takeshi, M. Combustion and Emission Characteristics of Multiple Stage Diesel Combustion. SAE Tech. Pap. Ser. 1998, 980505. (10) Jacobs, T. J.; Stanislav, V. B.; Assanis, D. N. Lean and Premixed Compression Ignition Combustion in a Light-Duty Diesel Engine. SAE Tech. Pap. Ser. 2005, 2005-01-0166. (11) Junjun, M.; Xingcai, L.; Libin, J.; Zhen, H. An Experimental Study of HCCI-DI Combustion and Emissions in a Diesel Engine With Dual Fuel. Int. J. Therm. Sci. 2008, 47, 1235–1242. (12) Myung, Y. K.; Chang, S. L. Effect of a Narrow Fuel Spray Angle and a Dual Injection Configuration on the Improvement of Exhaust Emissions in a HCCI Diesel Engine. Fuel 2007, 86, 2871–2880. (13) Xuelong, M. New Type Injection Nozzle for Premixed Combustion of Internal Combustion Engine. ZL 2005, 2, 0007457.1.

10.1021/ef800890d CCC: $40.75  2009 American Chemical Society Published on Web 02/02/2009

Ultramultihole Nozzle Fuel Injection

Energy & Fuels, Vol. 23, 2009 741 Table 1. Specifications of the Test Nozzles

type conventional nozzle UMH nozzle

hole number

hole diameter (mm)

flow rate (L/min) @ 10 MPa

8 16 16

0.16 0.16 0.165

1.2 2 2.2

Table 2. Specifications of the Test Engine

Figure 1. Schematic of the UMH nozzle.

model type cylinder number-bore × stroke (mm) rated power/speed (kW/r/min) maximum torque/speed (N · m/r/min) minimum torque/speed (N · m/r/min) minimum BSFCa (g/kW · h) emissions standard combustion chamber compression ratio Ricardo swirl ratio of inlet port a

CA6DF2 in-line, supercharged, intercooled 6-110 × 115 155/2300 680/1400 580-620/1000 205 Euro II reentrant 17 2.8

BSFC is an acronym for “brake specific fuel consumption”.

Figure 2. Schematic comparison between the conventional and UMH nozzle.

gives results at low loads, and results at high load will be published in future work. 2. Experimental Apparatus 2.1. UMH Nozzle Structure. Figure 1 shows the schematic of the UMH nozzle.13 It mainly consists of a needle and a body and has the following characteristics: (1) There are two layers of injection holes in the front part of body. Any injection hole of the upper layer and the corresponding injection hole of the lower layer are positioned in a vertical plane. (2) The injection holes cone angle (here defined as the angle of cone consisting of all of the axes of injection holes on the same layer) of the lower-layer holes is larger than that of the upper-layer holes (R2 > R1). (3) It has a large enough flow area of holes such that cyclic fuel can be completely injected into the combustion chamber prior to ignition, which is a prerequisite for premixed combustion. Figure 2 shows the schematic comparison between the existing conventional nozzle and the UMH nozzle. There is only one layer of holes on the conventional nozzle, so sprays possibly impinge on the wall of the combustion chamber or the cylinder liner to cause high HC and CO emissions. Sprays might impinge at the point A as shown in Figure 2a. The UMH nozzle, however, has two layers of holes, and the cone angle of the lower-layer injection holes is larger than that of the upper-layer holes. Two sprays of upper and lower layer meet in the space of the combustion chamber, for example, at the point B as shown in Figure 2b. This not only avoids fuel sprays impingement on the wall of the combustion chamber or the cylinder liner, but also strengthens sprays turbulence, which promotes fuel-air mixing. Therefore, the result is a more homogeneous mixture required to perform the premixed combustion in diesel engines. Table 1 lists the specifications of the test nozzles. 2.2. Fuel Injection Measuring Device. The UMH nozzle is fixed to a common-rail fuel injection system mounted on a test bench (manufactured by EFS, France) for measuring the fuel injection process. Its fuel injection rate is measured using the constant volume method, and the spray pattern is recorded

Figure 3. Combustion experimental apparatus.

using the high-speed digital photography. The rail pressure is set at 65 MPa in the spray experiment. The diesel fuel is injected into quiescent surroundings at atmospheric pressure to visualize clearly the spray difference between the UMH nozzle and the conventional nozzle. 2.3. Combustion Experimental Apparatus. The specifications of the test engine are shown in Table 2. The test engine is operated at the speed of 1000 r/min and brake mean effective pressure (BMEP) of 0.118 MPa, on the commercially available diesel fuel with the cetane number of 62 in all of the test cases. The coolant temperature is set to 80 ( 3 °C. Figure 3 gives the combustion experimental apparatus. The EGR cooler is connected to the EGR pipe running from the exhaust turbine inlet to the intercooler exit downstream from the intake air compressor. The EGR cooler and the intercooler are water-cooled, with water circulation volume and water temperature that are adjustable. The inlet air temperature after the intercooler is maintained at 40 ( 3 °C during the whole experiment. The fuel injection is performed by a high-pressure common-rail electriccontrolled system on the engine. The exhaust gas emissions are measured using a HORIBA MEXA-7100 gas analyzer, smoke density (SOOT) measured using an AVL 415s smoke meter, and particle matter (PM) measured using an AVL 472 partialflow particulate sampler, which allows double particulate filters to be exposed. In-cylinder pressure is acquired using a KISTLER cylinder pressure sensor. Table 3 lists the test conditions. 3. Experimental Results and Discussion 3.1. Fuel Injection Rate. Figure 4 compares the fuel injection rate at 50 mm3 per cycle of injection amount between

742 Energy & Fuels, Vol. 23, 2009

Miao et al. Table 3. Test Conditions

parameter

case 1

case 2

case 3

case 4

case 5

nozzle injection hole number-diameter (mm) sac volume (mm3) flow rate (L/min) @ 10 MPa rail pressure (MPa) injection timing (°CA BTDCb) EGR (%)a

8-Φ0.16 0.2 1.2 70 1.42 10.1

8-Φ0.16 0.2 1.2 65 1.44 15.8

16-Φ0.165 0.6 2.2 65 0.2 19.6

16-Φ0.165 0.6 2.2 65 1.2 25.3

16-Φ0.16 0.4 2 65 3.1 21.4

a EGR(%) ) CO b 2,I - CO2,A/CO2,E - CO2,I, where E, I, and A denote exhaust gas, inlet gas, and atmosphere, respectively. BTDC is an acronym for “before top dead center”.

Table 4. Performance Results

Figure 4. Fuel injection rate.

Figure 5. Comparison of spray side photos.

the UMH nozzle and the conventional nozzle. It can be easily found that the UMH nozzle (16 × Φ0.16) has a higher fuel injection rate and shorter injection duration than those of the conventional nozzle (8 × Φ0.16), which makes it possible that fuel injection finishes before ignition. 3.2. Spray Characteristics. Figure 5 compares the spray photos between the UMH nozzle and the conventional nozzle. Two sprays of the UMH nozzle meet near the injection holes as shown in Figure 5a. These two sprays of an upper injection hole and its corresponding lower injection hole meet to form a spray. Its spray angle is defined as the angle formed by two tangential lines to the circumference of the spray from the original point in the bottom view. The measured spray penetration and spray angle are given in Figure 6. The spray angle of the UMH nozzle is larger than that of the conventional nozzle, which results in more air entrainment into the spray. Therefore,

Figure 6. Comparison of spray characteristics.

performance parameter

case 1

case 2

case 3

case 4

case 5

exhaust gas temperature smoke density (FSN) BSFC (g/kW · h) HC (10-6) CO (10-6) NOx (10-6) PM (g/kW · h)

180 0.03 415.9 1057 1519 153 1.27

176 0.04 400.5 896 1529 140 1.06

181 0.19 385.1 648 990 134 0.76

182 0.3 385 570 935 127 0.62

178 0.06 402.8 258 577 83 0.51

it can promote fuel-air mixing and help prepare a more homogeneous mixture. Additionally, the spray penetration of the UMH nozzle is shorter than that of the conventional nozzle. So impingement on the wall can be reduced with the UMH nozzle, which is beneficial to HC emission control. 3.3. Combustion Characteristics and Performance. The minimum values of NOx and PM emissions are obtained by adjusting the injection pressure, injection timing, and EGR rate in every test case. Meanwhile, the brake specific fuel consumption (BSFC) and the exhaust gas temperature are kept from being deteriorated. The results are shown in Table 4 and Figure 7 corresponding to the test conditions in Table 3. It can be seen that NOx and PM emissions simultaneously decrease from case 1 to 5. The NOx emission is reduced from 153 g/kW · h in case 1 to 83 g/kW · h in case 5 by 45.8%. Meanwhile, PM is decreased from 1.27 g/kW · h in case 1 to 0.51 g/kW · h in case 5 by up to 59.9%. Therefore, the tradeoff between reduction of NOx and PM emissions observed in traditional diesel engines is eliminated. Table 4 indicates that HC and CO emissions are also reduced by 75.6% and 62%, respectively, from case 1 to 5. Thus, the use of the UMH nozzle overcomes the drawback of excessive HC and CO emissions with the existing premixed combustion. Figure 8 compares the heat release rates among different test cases. Each curve has only a single peak and no diffusion period is observed in conventional sprays combustion, which means that the combustion is premixed. The premixed duration ∆θ (here defined as ignition delay θ1 minus fuel injection duration θ2), however, is different as shown in Table 5. The homogeneity of the mixture tends to improve with the increase of ∆θ, and accordingly NOx and PM emissions tend to decrease simulta-

Ultramultihole Nozzle Fuel Injection

Energy & Fuels, Vol. 23, 2009 743

injection pressure produces less local overlean mixture. Therefore, HC emission is lower in cases 2, 3, 4, and 5 than in case 1. NOx is also reduced from case 1 to case 2 due to the increase of EGR rate, while CO emission remains nearly unchanged. HC and CO emissions of the UMH nozzle are much lower than those of the conventional nozzle. This is because the UMH nozzle weakens fuel wall-impingement and strengthens sprays turbulence. Additionally, Table 5 indicates that the premixed duration ∆θ of the UMH nozzle is larger than that of the conventional nozzle; the UMH nozzle has much time for fuel and air to mix prior to ignition. Therefore, it can promote more homogeneous fuel-air mixing. Smoke density in cases 3, 4, and 5 (16-hole UMH nozzle) is higher than that in cases 1 and 2 (8-hole conventional nozzle) due to the increase of EGR rate. Especially, smoke density in cases 3 and 4 is much higher than that in case 5 due to the later injection timing. The exhaust gas temperature remains nearly unchanged in all test cases. The BSFC in cases 3 and 4 is lower than that in cases 1, 2, and 5 as shown in Table 5, because of the shorter combustion duration regardless of the later ignition. The BSFC in case 5, however, is still lower than that in case 1.

Figure 7. NOx-PM curve.

4. Conclusions Figure 8. Comparison of the heat release rates. Table 5. Comparison of Combustion Parameters combustion parameter injection timing (°CA BTDC) ignition timing (°CA ATDC) ignition delay θ1 (°CA) fuel injection duration θ2 (°CA) premixing duration ∆θ ()θ1 - θ2 °CA) combustion duration (from 10% cumulative heat release to 95%) (°CA)

case 1 case 2 case 3 case 4 case 5 1.42 8.7 10.12 6.6 3.52

1.44 8.8 10.24 6.6 3.64

0.2 10.3 10.5 3.4 7.1

1.2 9.3 10.5 3.4 7.1

3.1 7.7 10.8 4.5 6.3

46.5

42.5

36

31

44.5

neously as identified in Figure 7. Table 5 indicates that the premixed duration ∆θ increases from 3.52-3.64 °CA in case 1,2 (8-hole conventional nozzle) to 6.3-7.1 °CA in case 3,4,5 (16-hole UMH nozzle). So the UMH nozzle results in more time for fuel and air to mix prior to ignition with the aid of EGR. This is beneficial to the formation of a more homogeneous mixture. Therefore, the UMH nozzles can improve the premixed combustion in diesel engines. The injection timing is different, but the ignition delay is almost the same for all five test cases. The HC emission is reduced with the reduction of the rail pressure from 70 MPa in case 1 to 65 MPa in case 2 as indicated in Tables 3 and 4. This is because the spray penetration shortens and thus wall-impingement weakens. Additionally, the lower

(1) The UMH nozzle has a large flow area of holes, which is beneficial to homogeneous mixture preparation for the premixed combustion in a diesel engine. (2) The UMH nozzle shows higher fuel injection rate and shorter injection duration than does the conventional nozzle, which helps fuel and air mix better prior to ignition. (3) The UMH nozzle exhibits shorter spray penetration and bigger spray angle resulting from sprays interaction than does the conventional nozzle, which helps prepare a more homogeneous mixture. (4) NOx and PM emissions are simultaneously reduced by combining the UMH nozzle with EGR. (5) CO and HC emissions with the UMH nozzle are very low due to weakened wall-impingement and strengthened sprays turbulence. Acknowledgment. This study was financially supported by the Major State Basic Research Development Program of China (973 Program) (No. 2007CB210000) and the Research Fund for the Doctoral Program of Higher Education (No. 20050248013). We also wish to express our gratitude to the Executive of the Wuxi Fuel Injection Equipment Research Institute. EF800890D