Combustion Characteristics and NOx Emissions of a Dimethyl-Ether

Sep 27, 2008 - DeVelopment DiVision, Hyundai-KIA Motors, Jangduk-dong, Hwaseong, Gyeonggi-do 445-706, Korea. ReceiVed March 29, 2008. ReVised ...
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Energy & Fuels 2008, 22, 4206–4212

Combustion Characteristics and NOx Emissions of a Dimethyl-Ether-Fueled Premixed Charge Compression Ignition Engine Myung Yoon Kim,† Je Hyung Lee,‡ and Chang Sik Lee*,† Department of Mechanical Engineering, Hanyang UniVersity, 17 Haengdang-dong, Sungdong-gu, Seoul 133-791, Korea, and Powertrain Research and DeVelopment Center, Corporate Research and DeVelopment DiVision, Hyundai-KIA Motors, Jangduk-dong, Hwaseong, Gyeonggi-do 445-706, Korea ReceiVed March 29, 2008. ReVised Manuscript ReceiVed August 9, 2008

This work investigated premixed charge combustion ignition (PCCI) and partial premixed charge compression ignition (PPCCI) combustions for dimethyl ether (DME) fuel to obtain a dramatic reduction in NOx emissions. Negligible NOx emissions were attainable with advanced injection timing under low engine load conditions using PCCI combustion. At higher engine load conditions, a dual injection strategy consisting of an initial injection at 100° BTDC and a second injection later than top dead center (TDC) offered extremely low NOx emissions while maintaining moderate engine output. The second injection timing, which was later than 10° after top dead center (ATDC), showed a reduction in NOx emissions by over 90% compared to the singleinjection case, in which the lowest NOx emission levels could be achieved. The indicated mean effective pressure (IMEP) of PPCCI combustion was less sensitive to the timing of the second injection than compared to that of conventional combustion using single injection near TDC. PCCI combustion with the first injection effectively ignited the fuel for the second injection. This explains the lower sensitivity of IMEP to the timing of the second injection with PPCCI combustion.

1. Introduction Recent events such as the rising costs of petroleum and advent of stringent emission regulations have made the use of clean and renewable fuel more desirable. Moreover, extensive research has been conducted on various approaches to reduce NOx and particulate matter (PM), resulting from the compression ignition (CI) engine, to meet stringent emission regulations. In this regard, dimethyl ether (DME) appears to have great potential and should be considered the fuel of choice for reducing dependence upon petroleum. DME has been considered an ideal diesel fuel substitute for compression ignition (CI) engines because it has excellent ignition properties. Moreover, the absence of soot from DME, chemically CH3-O-CH3, has drawn attention to its possibilities.1,2 DME exhibits starkly different properties from those of other fuels, such as gasoline and diesel fuel, for internal combustion engines. Because of its low boiling point and higher vapor pressure, DME, much like propane and butane, must be kept under pressure to remain liquid. Its high cetane number and soot-free combustion in the engine renders it particularly suitable * To whom correspondence should be addressed: Department of Mechanical Engineering, Hanyang University, 17 Haengdang-dong, Sungdongdu, Seoul 133-791, Korea. Telephone: +82-2-2220-0427. Fax: +82-2-22815286. E-mail: [email protected]. † Hanyang University. ‡ Hyundai-KIA Motors. (1) Choi, C. Y.; Reitz, R. D. An experimental study on the effects of oxygenated fuel blends and multiple injection strategies on DI diesel engine emissions. Fuel 1999, 78 (11), 1303–1317. (2) Choi, C. Y.; Reitz, R. D. Modeling the effects of oxygenated fuels and split injections on DI diesel engine performance and emissions. Combust. Sci. Technol. 2000, 159, 169–198.

for CI engines.3,4 Moreover, because it requires less air to produce the amount of heat produced by diesel fuel, a DMEfueled engine can provide higher power performance than a diesel fuel engine.5,6 However, DME has greater compressibility than diesel fuel, leading to more compression work for highpressure injection.7 Because the poor lubricity of DME may result in the abrasion of elements of the fuel injection system, such as the injection pump and injector, the addition of a lubrication improver is necessary to ensure long-term use of the engine. DME is a clean and promising alternative fuel, which, because of the oxygen in its molecular structure, is capable of significantly reducing particulate matter. However, a growing number of studies on NOx emissions have illustrated the importance of strategies to reduce NOx emissions when using DME.6,8 From this standpoint, a method to reduce NOx emissions while maintaining high engine performance is extremely important. In the conventional combustion in the CI engine, the fuel-air mixture combusts during the injection event and it originates a (3) Kajitani, S.; Zhili, C.; Konno, M.; Rhee, K. T. Engine performance and exhaust characteristics of direct-injection diesel engine operated with DME. SAE Paper 972973, 1997. (4) Teng, H.; McCandless, J. C. Can heavy-duty diesel engines fueled with DME meet US 2007/2010 emissions standard using a simplified aftertreatment system? SAE Paper 2006-01-0053, 2006. (5) Teng, H.; McCandless, J. C. Comparative study of characteristics of diesel-fuel and dimethyl-ether sprays in the engine. SAE Paper 2005. (6) Kim, M. Y.; Bang, S. H.; Lee, C. S. Experimental investigation of spray and combustion characteristics of dimethyl ether in a common-rail diesel engine. Energy Fuels 2007, 21 (2), 793–800. (7) Sorenson, S. C.; Glensvig, M.; Abatav, D. L. Dimethyl ether in diesel fuel injection systems. SAE Paper 981159, 1998. (8) Kim, M. Y.; Yoon, S. H.; Park, K. H.; Lee, C. S. Effect of multiple injection strategies on the emission characteristics of dimethyl ether (DME)fueled compression ignition engine. Energy Fuels 2007, 21 (5), 2673–2681.

10.1021/ef800221g CCC: $40.75  2008 American Chemical Society Published on Web 09/27/2008

DME-Fueled PCCI Engine

spatial heterogeneity, which is the main cause of NOx and soot emissions because of the locally high temperature and rich mixture in the combustion chamber. When an injection event is sufficiently advanced in a CI engine, combustion occurs in the lean, premixed mode that can produce extremely low NOx, although it typically increases hydrocarbon (HC) emissions and fuel consumption.9-12 Under this condition, the fuel-air mixture is highly diluted than that of conventional diesel combustion.9 In the present work, this type of combustion is called premixed charge compression ignition (PCCI). The ignition delay period should be extended to achieve PCCI combustion; to this end, early injection with or without exhaust gas recirculation (EGR) is generally implemented. Under low engine load conditions, early injection is helpful for achieving excellent mixing of fuel with air before ignition; as a result, near zero NOx emissions with good fuel economy is achievable. However, when fuel was injected at an early timing under medium- or high-load conditions, the fuel-air mixture may produce knocking during compression stroke along with a high level of NOx, noise, and fuel consumption because the mixture is too rich to be burned with low-temperature premixed combustion. Therefore, an alternative injection strategy, partial premixed charge compression ignition (PPCCI), was introduced.12-14 PPCCI combustion is a combination of PCCI and conventional combustion. With this concept, a portion of the fuel is premixed using early pilot injection, while the remainder of the fuel is injected after top dead center (TDC). The premixed fuel burned at a low temperature and under lean conditions demonstrates very low NOx emission levels. A secondary injection can control combustion and offers more engine output by burning of conventional combustion with a short ignition delay. In addition, the dual injection strategy is also known to effectively minimize HC and CO emissions.12 In this study, half of the fuel was premixed and the remainder was injected near TDC for PPCCI combustion. When the fuel is injected at an earlier time than with the conventional diesel engine, some problems may occur if the fuel spray and piston geometry were not modified from a conventional engine. Under this condition, the deposition of fuel may occur on the wall of the combustion chamber because of inordinate spray penetration as a result of low in-cylinder pressure and temperature during the injection event. This can cause lower thermal efficiency and increase the amount of incomplete combustion products, such as HC and CO. In addition, the fuel spray injected with early timing can lead to an advanced combustion phase during the compression stroke, and this reduces fuel economy. Therefore, several modifications (9) Lee, S.-S.; Reitz, R. D. Spray targeting to minimize soot and CO formation in premixed charge compression ignition (PCCI) combustion with a HSDI diesel engine. SAE Paper 2006-01-0918, 2006. (10) Lee, S.-S.; Reitz, R. D. Investigation of two low emission strategies for diesel engine: Premixed charge compression ignition (PCCI) and stoichiometric combustion. Ph.D. Thesis, Departement of Mechanical Engineering, University of WisconsinsMadison, Madison, WI, 2006. (11) Neely, G. D.; Sasaki, S.; Leet, J. A. Experimental investigation of PCCI-DI combustion on emissions in a light-duty diesel engine. SAE Paper 2004-01-0121, 2004. (12) Kim, M. Y.; Lee, C. S. 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 (17-18), 2871–2880. (13) Mueller, C. J.; Martin, G. C.; Briggs, T. E.; Duffy, K. P. An experimental investigation of in-cylinder processes under dual-injection conditions in a DI diesel engine. SAE Paper 2004-01-1843, 2004. (14) Lechner, G. A.; Jacobs, T.; Chryssakis, C.; Assanis, D. N.; Siewert, R. M. Evaluation of a narrow spray cone angle, advanced injection timing strategy to achieve partially premixed compression ignition combustion in a diesel engine. SAE Paper 2005-01-0167, 2005.

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Figure 1. Shapes of piston bowls and sprays. Table 1. Common Specifications of Test Engines engine type bore (mm) stroke displacement volume (cm3) fuel injection system number of nozzle holes IVO IVC EVO EVC

single-cylinder DI diesel 75 84.5 373.3 Bosch common rail 6 8° BTDC 52° ABDC 8° BBDC 38° ATDC

in terms of spray, including angle, piston geometry, and compression ratio, are implemented for PCCI combustion.12,15-17 This work investigates PCCI and PPCCI combustions with DME fuel to achieve a dramatic reduction in NOx emissions from a compression ignition engine. To achieve this goal, PCCI and PPCCI combustion are investigated along with modified piston geometries and two spray types, including conventional and narrow cone spray angles. The effects of various parameters on NOx and exhaust emissions of a compression ignition engine are measured in terms of ultra-low NOx emissions. In addition, PCCI and PPCCI combustion with a dual injection strategy were implemented to extend operating conditions that produce low NOx emissions. 2. Experimental Apparatus and Procedure 2.1. Experimental Apparatus. A narrowed spray angle injector, modified piston geometry, and a reduced compression ratio were used to create PCCI and PPCCI combustion (Figure 1). Two types of injection nozzles with different spray cone angles (156° and 60°) were used to examine the potential of PCCI and PPCCI combustion of DME fuel. The compression ratio of the test engine was reduced from 17.8 to 15 to retard the combustion phase of the mixture formed by early injection. Common specifications of the test engines are listed in Table 1, and the differences in specifications for the engines are listed in Table 2. Cylinder pressure was measured with a piezo-electric pressure sensor (6052B2, Kistler), and the measured signal was amplified and converted into a voltage with a charge amplifier (5011, Kistler). The pressure histories were recorded during the tests using a data acquisition board (PCI-MIO-16E-1, National Instruments). The sampling, processing, and saving of the pressure data were achieved through a program made by LabVIEW. The pressure traces were ensemble-averaged over 300 cycles to compensate for cycle-bycycle variations. (15) Albrecht, A.; Chauvin, J.; Lafossas, F.-A.; Potteau, S.; Corde, G. Development of highly premixed combustion diesel model: From simulation to cotrol design. SAE Paper 2006-01-1072, 2006. (16) Walter, B.; Gatellier, B. Near zero NOx emissions and high fuel efficiency diesel engine: The NADI concept using dual mode combustion. Oil Gas Sci. Technol. 2003, 58 (1), 101–114. (17) Kim, M. Y.; Kim, J. W.; Lee, C. S.; Lee, J. H. Effect of compression ratio and spray injection angle on HCCI combustion in a small DI diesel engine. Energy Fuels 2006, 20 (1), 69–76.

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Table 2. Specifications of Two Types of Engines Engine Type A (Conventional Diesel Engine) spray included angle (deg) 156 compression ratio 17.8 bowl diameter (mm) 44 Engine Type B (Modified for PCCI Combustion) spray included angle (deg) 60 compression ratio 15 bowl diameter (mm) 35.6

Table 3. Experimental Conditions engine speed (revolutions/min) injection pressure (MPa) coolant temperature (°C) oil temperature (°C) injection timing single injection for PCCI combustion dual injection for PPCCI combustion intake air temperature (°C) bulk equivalence ratio (Φ) EGR rate (%)

1500 50 70 70 200-2° BTDC τ1st ) 100° BTDC τ2nd ) 10° BTDC-20° ATDC 38-46 0.1-0.4 0

2.2. Experimental Conditions. The experiment was conducted at a fixed engine speed of 1500 rpm, and the experimental conditions used in this study are summarized in Table 3. In this work, the injection timing was defined as the time when the injection signal is conducted to the injector. Because virtually no soot emission is produced by a DME engine, relatively low injection pressure compared to a standard diesel engine is generally selected for a DME engine; however, an injection pressure of 50 MPa was chosen for this work to extend the PCCI combustion region by shortening the injection period. To find optimized injection timing for PCCI combustion, the injection timing was swept from 2° to 200° BTDC under various equivalence ratio conditions with no EGR. From this step, the optimized injection timing for PCCI combustion using single early injection, which can achieve low NOx emissions and moderate engine output, was determined. Then, to extend the operating condition for the engine with low NOx emissions, PPCCI combustion using dual injection was implemented and optimized operating conditions were investigated. The possible engine loads were limited at Φ ) 0.4 by the engine durability. Three injection cases, a traditional single injection near TDC (to act as a baseline for comparison), an early single injection case for PCCI combustion, and dual injections consisting of a single early injection and followed by a late injection after TDC for PPCCI combustion, were considered in the present work. Parts a and b of Figure 2 show the injection strategies and combustion characteristics, which correspond to the injection strategies. For the three kinds of injection strategies, the total mass of injection fuel was fixed to maintain the bulk equivalence ratio of the mixture at 0.3, and that condition corresponds to medium engine load. As shown in Figure 2b, a single intense reaction, which originated in diffusive combustion with a partially premixed mixture, was indicated for the single injection case. In this case, NOx emissions can be easily reduced by delaying injection without consideration of increasing soot emission because of the characteristics of DME; however, the delaying is subject to restriction by misfire or increasing cycle-tocycle variation. Consequently, the optimal injection timing may be determined considering the NOx emissions and the engine output or the combustion instability with retarded injection timing. PCCI combustion shows two peaks that are similar to those reported in the literature for HCCI operation.18-22 The first peak is attributed (18) Kim, D. S. Combustion and emission characteristics of homogeneous charge compression ignition diesel engine. Ph.D. Thesis, Department of Mechanical Engineering, Hanyang University, Seoul, Korea, 2004. (19) Kim, D. S.; Kim, M. Y.; Lee, C. S. Effect of premixed gasoline fuel on the combustion characteristics of compression ignition engine. Energy Fuels 2004, 18 (4), 1213–1219. (20) Kim, D. S.; Kim, M. Y.; Lee, C. S. Combustion and emission characteristics of partial homogeneous charge compression ignition engine. Combust. Sci. Technol. 2005, 177 (1), 107–125.

Figure 2. Injection strategies and combustion characteristics that correspond to the injection strategies (engine type B).

to low-temperature combustion, after which there is a short ignition delay because of the negative temperature coefficient behavior of the fuel. The second larger peak is due to a premixed hightemperature reaction. The heat release of the combustion of the first injection shows a similar pattern of homogeneous charge compression ignition (HCCI) combustion. The ignition of the lowtemperature reaction was advanced by 2°, relative to the PCCI combustion using single injection. This is presumably because the second injection at a later time point causes an increase in exhaust gas temperature and engine operating temperature, and the increase in charge temperature and wall temperatures leads to an advance in the low-temperature reaction. Previous research showed that the start of the low-temperature reaction is primarily determined by the temperature of the charge regardless of injection timing and (21) Kim, D. S.; Kim, M. Y.; Lee, C. S. Reduction of nitric oxides and soot by premixed fuel in partial HCCI engine. J. Eng. Gas Turbines Power 2006, 128 (3), 497–505. (22) Kim, D. S.; Kim, M. Y.; Lee, C. S. Combustion and emission characteristics of a partial homogeneous charge compression ignition engine when using two-stage injection. Combust. Sci. Technol. 2007, 179 (3), 531– 551.

DME-Fueled PCCI Engine

Figure 3. Effect of engine configurations on the combustion characteristics of DME for PCCI combustion (τinj ) 60° BTDC, Φ ) 0.2).

Figure 4. Effect of the equivalence ratio on the characteristics of conventional combustion (engine type B, τinj ) 6° BTDC).

the equivalence ratio.17 Combustion of the second volume of injected fuel began just after the injection without a premixed spike. The ignition delay of the second injection is noticeably shortened relative to the PCCI case.

3. Results and Discussion 3.1. Effect of Engine Configuration on the Combustion of DME. Figure 3 shows the effect of engine configuration on the combustion characteristics of DME fuel in the engine. In this case, the fuel is injected at 60° BTDC to achieve low NOx emissions using early single-injection PCCI combustion. When the PCCI combustion was implemented, a modified configuration (engine type B) offered an increase in IMEP because the combustion phase was delayed to TDC during the compression stroke. In addition, engine type B showed brisk heat release with respect to the high-temperature reaction (HTR). This induced better heat release as a result of optimized mixture formation, because the narrowed spray cone angle injector avoided out-of-bowl injection and wall wetting issues, which generally occurs at advanced injection timing.14 Figure 4 shows the effect of the equivalence ratio on the characteristics of conventional combustion in engine type B with injection timing at 6° BTDC. The combustion pressure increased, and the ignition delay was shortened as the equivalence ratio increased. As illustrated in the cylinder pressure-crank angle diagram of PCCI combustion, stable combustion could

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Figure 5. Effect of the equivalence ratio on the characteristics of PCCI combustion (τinj ) 60° BTDC).

be achieved in the range of equivalence ratios from Φ ) 0.2 to 0.4 and combustion duration increased as the equivalence ratio increased. 3.2. Effect of a Single Early Injection on PCCI Combustion (Engine Type B). The effect of the equivalence ratio on the characteristics of PCCI combustion with engine type B is shown in Figure 5. In this instance, injection timing was advanced to 60° BTDC to extend the ignition delay to form an adequately diluted lean fuel-air mixture before ignition. Increasing the equivalence ratio led to a simultaneous advance in LTR and HTR. In addition, the peak of HTR was significantly influenced by the equivalence ratio. The advance of ignition timing and rapid heat release during the compression stroke appeared to significantly increase the cylinder pressure. Moreover, the increase in the heat transferred through the wall of the combustion chamber originating from the advance of the combustion phase may severely deteriorate the thermal efficiency of the engine.11,17,20 A fluctuation in the combustion pressure was found at an equivalence ratio of 0.3, and this phenomenon will be discussed in Figure 6. Parts a and b of Figure 6 show the effect of the equivalence ratio on the instantaneous cylinder pressure history measured from a single engine cycle and the cycle-to-cycle variations of peak combustion pressure in the case of PCCI combustion, respectively. A stable and mild combustion could be achieved at an equivalence ratio of 0.2. In addition, negligible NOx emissions were recorded under this condition. However, a heavy knocking induced by sharp pressure rise was encountered when the equivalence ratio was increased to 0.3. As a result of knocking and an abnormally rapid increase in the cylinder pressure, NOx emissions increased to 888 ppm, which was significantly higher than the 5 ppm recorded when the equivalence ratio was 0.2. At the same time, the advance in the combustion phase brought about the combustion fluctuation at an equivalence ratio of 0.3 as shown in Figure 6a. In addition, the IMEP at an equivalence ratio of 0.3 was slightly higher than the results achieved at Φ ) 0.2. This shows inferior fuel economy at the equivalence ratio of 0.3. These findings would seem to indicate that the limit equivalence ratio for engine operation with PCCI combustion with low NOx emissions is somewhere between 0.2 and 0.3. Lowering NOx emissions requires that several strategies, such as EGR or PPCCI combustion obtained by dividing the injection event, should be applied when operating the engine at higher load conditions. Figure 7 shows the effect of the equivalence ratio on NOx emissions in the case of PCCI combustion when the injection timing was fixed at 60° BTDC. Although negligible NOx emissions were recorded at equivalence ratios between 0.1 and

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Figure 8. Effect of injection timing on the IMEP and NOx emissions (single injection, Φ ) 0.2).

Figure 6. Effect of the equivalence ratio on the instantaneous cylinder pressure history measured from a single engine cycle and the peak of cylinder pressure in the case of PCCI combustion using a single early injection (τinj ) 60° BTDC).

Figure 7. Effect of the equivalence ratio on the NOx emissions of PCCI combustion (τinj ) 60° BTDC) and conventional combustion (τinj ) 6° BTDC).

0.2, a rapid increase in NOx emissions appeared at an equivalence ratio of 0.3. The effect of injection timing on IMEP and NOx emissions with engine type B at an equivalence ratio of 0.2 is shown in Figure 8. An advance of injection timing beyond 60° BTDC reduced NOx emissions to a negligible level. The IMEP for PCCI combustion was varied as changing injection timing, and the local IMEP peak was found at an injection timing of 100° BTDC, which also had low NOx emissions. At this injection

Figure 9. Effect of second injection timing on the NOx and IMEP for single injection and PPCCI (Φoverall ) 0.3).

timing, ultra-low NOx emissions can be achieved at the cost of a slight reduction in IMEP compared to the conventional combustion region. 3.3. Effect of PPCCI Combustion Using Dual Injection. From the previous discussions of engine durability and NOx emissions for varied equivalence ratio of PCCI combustion, the equivalence ratio of first injection of PPCCI combustion was maintained at Φ ) 0.2 and the overall equivalence ratio was varied at Φ ) 0.3-0.4. Therefore, the two split portions of PPCCI combustion were chosen for investigation. In addition, from the results of Figures 7 and 8, it can be concluded that

DME-Fueled PCCI Engine

Figure 10. Effect of second injection timing on the NOx and IMEP for single injection and PPCCI (Φoverall ) 0.4).

Figure 11. Effect of second injection timing on the characteristics of PPCCI combustion using dual injection (Φoverall ) 0.4).

PCCI combustion by early injection with low NOx emissions could be achieved with an equivalence ratio of 0.2 because the optimum injection timing for PCCI combustion at Φ ) 0.2 was

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100° BTDC. Accordingly, PPCCI combustion with a split injection strategy consists of an initial injection at τfirst ) 100° BTDC, and a second injection later than TDC was conducted to increase the engine output. In this investigation, early injected DME produces negligible NOx emissions because the DME burns with PCCI combustion. A second injection at a later timing produces extra power with low NOx. As a result, the achievable operating range with low NOx emissions can be extended in comparison to that of PCCI combustion and a single early injection. Parts a and b of Figure 9 compare the NOx emissions and IMEP for conventional combustion using a single injection and PPCCI combustion using two split injections. The total equivalence ratio was kept at Φ ) 0.3 for both injection strategies. For PPCCI combustion, the first injection (Φ ) 0.2) at 100° BTDC and the second injection (Φ ) 0.1) at near TDC (from 10° BTDC to 20° ATDC) were applied to produce low NOx emissions under a higher engine load condition. The results of conventional combustion with fuel injection timing between 20° and 2° BTDC were compared to evaluate the effect of injection timing on different combustion strategies. In the case of single injection, the NOx emissions were linearly reduced as retarded injection timing. However, the delay of injection timing in a single injection beyond 2° BTDC was limited because the engine power was rapidly decreased because of the misfire of DME fuel. For the PPCCI case, an over 90% reduction of NOx could be achieved at the timing of the second injection after 10° ATDC compared to the case of single injection and an injection timing of 2° BTDC, where the lowest NOx could be achieved. The IMEP of PPCCI combustion was less sensitive to the second injection timing as shown in Figure 10b, while a slight variation in IMEP was indicated. The NOx emissions and IMEP for PPCCI combustion with an equivalence ratio of 0.4 are shown in Figure 10. In this case, the halves of total fuel injection were injected via the first injection and the second injection. Similar to the results of Figure 9, remarkably low NOx emissions could be achieved by using the delayed second injection timing; however, the IMEP had more advantages at this condition, and a comparable IMEP was indicated in the single injection case at the second injection timings. Figure 11 compares the combustion characteristics of PPCCI combustion because of dual injection with a fixed first injection at τfirst ) 100° BTDC. The second injection timing was swept from TDC to 20° ATDC in crank angle steps of 10° for PPCCI cases. The combustion characteristics of first injection fuel showed nearly HCCI combustion consisted of two stage combustions (LTR and HTR) because of the long period of ignition delay of the first injection fuel. This first combustion of PCCI corresponds to much leaner combustion and generates less NOx emissions. The second injection fuel showed a remarkably short ignition delay and burned with a low peak heat release rate. Although the second injection timing was delayed to 20° ATDC, the heat release rate was not significantly changed because the combustion from the first injection effectively ignited the fuel of the second injection. This explains the reduced sensitivity of IMEP to the second injection timing as shown in Figures 9b and 10b. 4. Conclusions The present study has evaluated the potential for PCCI and PPCCI combustion using a narrow spray cone angle injector and modified piston geometry to avoid out-of-bowl injection and cylinder wall wetting issues for advanced injection timing

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of DME fuel. The following conclusions can be drawn from the experimental results: (1) A configuration of the engine with a narrow spray cone angle and modified piston geometry showed a significant increase in IMEP when the injection timing was advanced for PCCI combustion, while maintaining negligible NOx emissions. The delayed combustion phase to top dead center and brisk heat release in terms of the high-temperature reaction with PCCI combustion are the main factors, which can be explained by the optimized mixture formation of the engine configuration provided by the narrow cone spray injector and modified piston bowl. (2) In PCCI combustion, an increase in the equivalence ratio led to a simultaneous advance in ignition timing for low- and high-temperature reactions. In addition, the peak of the high-temperature reaction was significantly increased as the equivalence ratio increased. (3) The maximum equivalence ratio for engine operation with PCCI combustion, which produces low NOx emissions, is between 0.2 and 0.3. In addition, optimized injection timing with moderate engine output while maintaining low NOx emissions was indicated at 100° BTDC for PCCI combustion with an equivalence ratio of 0.2. (4) PPCCI combustion using a dual injection strategy consisting of the first injection at 100° BTDC and the second injection later than TDC was effective for increasing the engine load while maintaining low NOx emissions. The second injection timing, which was later than 10° ATDC, led to a reduction in NOx emissions by over 90% compared to the case of single injection, where the lowest NOx could be achieved. (5) The IMEP of PPCCI combustion was less sensitive to the second injection timing compared to that of single injection near TDC for conventional combustion. The second injection fuel showed a remarkably short ignition delay and burned with a low peak heat release rate. Although the second injection timing was delayed to 20° BTDC, the heat release rate was not significantly changed because the combustion during the first injection effectively ignited the fuel of the second injection. This explains

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the reduced sensitivity of IMEP to the second injection timing for PPCCI combustion. Acknowledgment. This work is financially supported by the Ministry of Education and Human Resource Development (MOE), the Ministry of Commerce, Industry, and Energy (MOCIE), and the Ministry of Labor (MOLAB) through the fostering project of the Laboratory of Excellency. Also, this study was supported by the Center for Environmentally Friendly Vehicle (CEFV) of the Eco-STAR project from the Ministry of Environment (MOE), Republic of Korea. This work was also supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (R01-2006-000-10932-0).

Nomenclature ATDC ) after top dead center BTDC ) before top dead center CI ) compression ignition CA ) crank angle DC ) direct current DME ) dimethyl ether EGR ) exhaust gas recirculation HCCI ) homogeneous charge compression ignition HTR ) high-temperature reaction IMEP ) indicated mean effective pressure LTR ) low-temperature reaction PCCI ) premixed charge compression ignition PM ) particulate matter PPCCI ) partial premixed charge compression ignition TDC ) top dead center Greek τ ) injection timing Subscripts inj ) injection 1st ) first injection 2nd ) second injection EF800221G