A Study of the Characteristics of Mixture Formation and Combustion in

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Energy & Fuels 2008, 22, 1542–1548

A Study of the Characteristics of Mixture Formation and Combustion in a PCCI Engine Using an Early Multiple Injection Strategy Hyung-min Kim,† Yung-jin Kim,† and Ki-hyung Lee*,‡ Graduate School of Hanyang UniVersity, Department of Mechanical Engineering, Hanyang UniVersity, 1271 Sa 1-dong, Sangrok-gu, Gyeonggi-do, 426-791, Korea, Department of Mechanical Engineering, Hanyang UniVersity, 1271 Sa 1-dong, Sangrok-gu, Gyeonggi-do, 426-791, Korea ReceiVed September 21, 2007. ReVised Manuscript ReceiVed January 7, 2008

As world attention has focused on global warming and air pollution, high-efficiency diesel engines with low CO2 emissions have become more attractive. Premixed diesel engines in particular have the potential to achieve a more homogeneous mixture in the cylinder which results in lower NOx and soot emission. It is well-known that the injection strategies such as the injection timing and the spray included angle are important to create the optimal mixture formation for a PCCI (premixed charge compression ignition) engine. In this research, we investigated the effect of injection angle, injection timing, and frequency on the combustion and mixture formation in a direct injection type PCCI engine using an early multiple injection strategy. The experimental results showed that the mixture formation, IMEP, and emission characteristics in the PCCI engine were dominantly affected by the fuel injection timing and spray included angle. In particular, the injection timing of 65° BTDC with the spray included angle of 100° effectively reduced the smoke emission in the early single injection case. In other words, the smoke number was less than 1 FSN when the IMEP exceeded 3.5 bar. The multiple injection method also resulted in more homogeneous mixture formation due to decrease in spray penetration and increase in the total amount of fuel evaporation in the combustion bowl. In addition, a simulation was conducted in order to estimate the mixture distribution within the cylinder according to injection conditions such as spray included angle and injection timing. The simulation result is very effective to clarify the air fuel distribution of a PCCI combustion.

1. Introduction As the environmental problems caused by vehicles become more severe, many countries have proposed stringent regulations for reducing the exhaust emissions from automotive engines. The diesel engine, which generally has a high thermal efficiency, is an especially attractive candidate for future transportation needs. However, the diesel engine generates a large amount of NOx and soot because of locally excessive richness and overall lean combustion. Thus, it will not be easy to reach these low emission levels for diesel engines. Recently, the premixed charge compression ignition (PCCI) engine has been receiving attention as a new low emission engine concept to cope with the regulations.1–3 It is well-known that high-pressure common rail injection systems mainly used in diesel engines achieve poor PCCI combustion performance because of their direct fuel impinge* Author to whom correspondence should be addressed. E-mail: [email protected]. Telephone: +82-31-400-5251. Fax: +82-31-4065550. † Graduate School of Hanyang University, Department of Mechanical Engineering. ‡ Department of Mechanical Engineering. (1) Noguchi, M.; Tanaka, Y.; Tanaka, T.; Takeuchi, Y. A Study on Gasoline Engine Combustion by Observation of Intermediate Reactive Products during Combustion. SAE Tech. Pap. Ser. 1979, 790840. (2) Onishi, S.; Jo, S. H.; Shoda, K.; Jo, P. D.; Kato, S. Active Thermoatmosphere Combustion (ATAC)-A New Combustion Process for Internal Combustion Engines. SAE Tech. Pap. Ser. 1979, 790501. (3) Najt, P. M.; Foster, D. E. Compression-Ignited Homogeneous Charge Combustion. SAE Tech. Pap. Ser. 1983, 830264.

ment on the combustion chamber surfaces. This behavior is inherent when a standard diesel-type injector injected fuel early into a relatively low gas charge density. Therefore, it was thought that a different fuel injection strategy is needed for direct injection type PCCI combustion that provides lower fuel penetration without sacrificing atomization quality. Thus, the spray characteristics of a multiple injection strategy used in PCCI engine tests was investigated to clarify the effect of injection conditions on the mixture formation and combustion characteristics. This paper describes spray behavior, mixture formation, and emission characteristics of a high-pressure diesel injector, which is intended for use in PCCI engines. High-speed photography was applied to investigate the spray characteristics, and the effect of spray geometry on air–fuel distribution was analyzed by using the spray simulation. In addition, the single-cylinder engine was used to measure the combustion and emission characteristics of PCCI engines. The results show that mixture formation and emission depend on the spray included angle and injection strategies including injection timing and injection frequencies. The experimental and simulation data can be used to provide more detailed information about the spray structure and air–fuel distribution which are needed to optimize PCCI engine combustion. 2. Experimental Apparatus and Procedures 2.1. Experimental Apparatus. Figure 1 shows a spray visualization system for measurement of radial penetration length which

10.1021/ef700568g CCC: $40.75  2008 American Chemical Society Published on Web 03/18/2008

Mixture Formation and Combustion in a PCCI Engine

Energy & Fuels, Vol. 22, No. 3, 2008 1543

Figure 3. Definition of the spray included angle for the test injectors.

Figure 1. Schematic diagram of spray visualization system.

single-cylinder test engine used in the experiments are given in Table 1. Test injectors, each of them with a different spray included angle (150°, 130°, 100°, and 70°), were used in this study to investigate the effect of the spray included angle on the wall wetting phenomena. The detailed specifications of the test injectors are shown in Figure 3. 2.2. Numerical Analysis of Spray Characteristics. The numerical simulation of the spray injected by the common rail type injector was performed with the VECTIS 3.7 program.4 We assumed a cylinder 200 mm in diameter and 200 mm in height for the domain of calculation. The mesh for calculating the collision of the spray with the wall in the cylinder contained 50 000 points. The injection velocity and the discharge coefficient were calculated using experimental data of the injection rate,5 the initial SMD value (postulated as the nozzle diameter), and an X-square distribution. We also assumed that the breakup of the injected liquid followed the Liu–Mather–Reitz model.6 Using the Kelvin–Helmholtz surface tension wave theory, the wavelength and frequency of the fastest growing waves are expressed as follows: ΛKH )

9.02rd(1 + 0.45(Z)0.5)(1 + 0.4 T0.7)

ΩKH )

(1 + 0.865We1.67)0.6 (0.34 + 0.385We1.5) ((1 + Z)(1 + 1.4T0.6))

σ Fdrd3

(1) (2)

The newly formed droplet size is assumed to be proportional to the Kelvin–Helmholtz wavelength above: Figure 2. Schematic of common rail injection type PCCI single-cylinder engine. Table 1. Specifications of the PCCI Test Engine engine type

single-cylinder direct injection

bore × stroke displacement volume swirl ratio compression ratio

91 mm × 96 mm 624 cm3 2 17.5

[(

rd,stable ) min

rd,stable ) B0ΛKH 3πrd2Ur 2ΩKH

)( 1⁄3

,

3rd2ΛKH 4

)]

(3)

1/3

(ΛKH g rd, once) (4)

The constant B0 is set at 0.6. For Kelvin–Helmholtz breakup, the droplet lifetime is τb )

is an important parameter to affect impingement on the cylinder wall. The ambient density was changed from 2.35 to 26.32 kg/m3 to simulate the cylinder pressure at injection timing using a highpressure chamber. A xenon lamp was used as the light source, and the spray image was acquired by using the high-speed camera. In order to synchronize a spray image capture signal with an injector trigger signal, a pulse generator was used. The spray image was taken from the bottom view. Figure 2 shows a diagram of a common rail injection type PCCI single-cylinder engine. A common rail injector was fitted vertically in the center of the cylinder head as shown in the figure. Coolant, an engine oil supplier, and a 3 kW heater were also installed to control the temperature. The engine was operated at a constant speed using a 30 kW dc dynamometer. In order to measure the position of the piston, an 1800-pulse encoder was fixed to the crank shaft and a TDC sensor was attached to the cam shaft. A PCV (pressure control valve) was used at the common rail to control injection pressure and a counter board was used to control the injection timing and the injected fuel quantity. In addition, in order to identify PCCI combustion, the combustion pressure was measured using a pressure sensor installed in the chamber. The main specifications of the

(ΛKH e rd)

3.78B1rd ΩKHΛKH

(5)

where the breakup time constant B1 is 10. In order to analyze the spray pattern and evaporation characteristics using the spray model, a spray calculation was conducted in a combustion chamber that simulated the geometry of the real PCCI engine. The engine geometry had a compression ratio of 17.5, a bore of 91 mm, and a stroke of 96 mm. The grid and definition of piston diagram for spray simulation in combustion chamber is shown in Figure 4. We chose 220° as the calculation start time, which corresponded to the closing time of the intake and exhaust valves. TDC was selected as the end time. The engine was run at 1400 rpm, and the swirl ratio produced from the intake port was 2.0. (4) Ricardo Co. VECTIS Theory Manual, 2003. (5) Ryu, J. D.; Kim, H. M.; Lee, K. H.; Cho, H. M. A study on the spray structure and evaporation characteristic of common rail type high pressure injector in homogeneous charge compression ignition engine. The 9th Annual Conference ILASS-Asia, 2004. (6) Liu, A. B.; Mather, D.; Reitz, R. D. Modeling the effects of fuel spray characteristics on diesel engine combustion and emission. SAE Tech. Pap. Ser. 1993, 930072.

1544 Energy & Fuels, Vol. 22, No. 3, 2008

Kim et al.

Figure 4. Grid of piston diagram for spray simulation in combustion chamber. Table 2. Experimental Conditions engine speed injection pressure injection quantity injection timing spray included angle hole diameter intake temperature intake pressure

1400 rpm 100 MPa 12 mm3 ∼ 24 mm3 BTDC 180° ∼ TDC 70°, 100°, 130°, 150° 0.168 mm 300 K natural aspiration

Heat transfer and temperature calculations were added in order to evaluate fuel evaporation. Equations 6 and 7 express the mass and temperature of a droplet. dmd DAB (p - pv,∞) ) -AdSh F ln dt Dd v (p - pv,s)

(6)

dCp,dTd dmd ) -AdNu(Td - T)kmFz + hhg (7) dt dt The spray impingement model proposed by Gosman et al.7 was used to evaluate the spray characteristics. According to the Wein and TLeid, the stick, rebound, spread, and splash conditions occurred after spray impingement. The roughness effect of the combustion chamber was ignored.7,8 2.3. Experimental and Simulation Procedure. In this study, we evaluated the effects of the piston geometry and the collision location of the spray on the mixture formation. We performed a spray simulation as a function of the spray included angle and injection timing. After that, we measured engine performance and emissions accordingly. We also conducted a combustion analysis using the experimental device shown in Figure 1 in order to evaluate the combustion and output performance of the PCCI engine. In addition, emission performance was evaluated by measuring NOX with an emission gas analyzer (Horiba Co.). The smoke density was measured using a smoke meter (AVL Co.). Experimental details are shown in Table 2. The engine speed and the injection pressure were fixed at 1400 rpm and 100 MPa, respectively. The effects of the spray included angle and the injection timing were analyzed. md

3. Experimental Results and Discussion 3.1. Effect of Spray Included Angles and Injection Timing on Spray Characteristics. 3.1.1. Comparison of Spray Penetration Length. Figure 5 shows the radial penetration lengths obtained from the spray images taken at the bottom of (7) Bai, C.; Gosman, A. D. Development of methodology for spray impingement simulation. SAE Tech. Pap. Ser. 1995, 950283. (8) Nandha, K. P.; Abraham, J. Dependence of Fuel-Air Mixing Characteristics on Injection Timing in an Early- Injection Diesel Engine. SAE Tech. Pap. Ser. 2002, 2002–01–0944.

Figure 5. Comparison of radial penetration lengths for various spray included angles.

the injector under high and low ambient density conditions, which corresponds to the cylinder pressure at the time of injection (70°–50° BTDC). In addition, the calculated result of radial spray penetration length illustrated as dotted line in Figure 5 reasonably agrees with the experimental results. As the spray included angle narrowed, the radial spray penetration length decreased. At an ambient density of 13.53 kg/m3, the spray did not collide with the wall of the combustion chamber; it was assumed that the spray penetration must be short in the real combustion chamber (bore size is 91 mm) when the ambient temperature and the flow field were considered. 3.2. Air–Fuel Distribution Characteristics at Various Spray Included Angles and Injection Timings. Figure 6 compares the air–fuel distribution characteristics of the spray at the collision location at the time of ignition (10° BTDC) for the various injection timings and spray included angles. In the case of an early injection, in which spray directly impinged upon the wall, a rich mixture was formed near the wall, and a lean mixture was formed in the center of the combustion chamber and the piston bowl zone. In the case of a 70° spray included angle with an injection time of 80° BTDC and also in the case of a 100° spray included angle and a 60° BTDC injection time, the distribution of the mixture was generally uniform throughout the entire combustion region; the spray collision occurred in



Figure 6. Air–fuel distributions at various spray included angles and injection timings.

the upper piston and the border region of the piston bowl.9 We conclude from these results that the distributions of the mixture became more uniform throughout the entire combustion chamber when the spray injected toward to the upper piston and to the border of piston bowl zone. 3.3. Engine Performance and Smoke Characteristics. 3.3.1. Effect of Various Spray Included Angles and Injection Timings on Performance Characteristics. Figure 7 shows the characteristics of the IMEP (indicated mean effective pressure) for premixed combustion as a function of various spray included angles. When the spray collided with the wall of the combustion chamber, the IMEP decreased because the combustion could not be completed due to the wetting of the wall. When the spray collided with the border of the upper piston and the bowl zone, the IMEP increased because the mixture was distributed uniformly throughout the whole combustion chamber. When the spray collided directly with the bowl zone of the piston, the mixture was densely distributed in that region and the IMEP showed the maximum value which tends to result in knocking at this region. The criterion of engine knocking was judged by the increasing rate of combustion pressure, which was defined the less than 5 bar/deg as the criterion in the study. 3.3.2. Effect of Various Spray Included Angles and Injection Timings on Smoke Characteristics. Figure 8 shows the smoke characteristics according to various spray included angles and injection timings. The generation of smoke is proportional to the IMEP, and it is thought that there is a close relationship (9) Walter, B. Gatellier, B. Development of the High Power NADITM Concept Using Dual Mode Diesel Combustion to Achieve Zero NOx and Particulate Emissions. 2002, 2002–01–1744.

Figure 7. Effect of various spray included angles and injection timings on the IMEP.

between the smoke and the collision location of the spray and the formation of the mixture. In particular, when a dense mixture is formed locally in the combustion chamber, the smoke generation increased, i.e., in the cases of the spray included angle of 70° with BTDC 80° injection timing and the spray included angle of 100° with BTDC 60° injection timing. However, when the mixture was distributed uniformly in the bowl zone, the generation of smoke decreased. 3.3.3. Smoke and IMEP Characteristics According to Various Spray Included Angles and Injection Timings. Figure 9 shows the smoke characteristics according to the IMEP. In


Kim et al.

Figure 8. Effect of various spray included angles and injection timings on the characteristics of smoke.

Figure 10. Comparison of spray penetration length at various injection strategies. Figure 9. Effect of the IMEP on smoke characteristics.

general, when the IMEP increased, the amount of smoke tended to increase. This means that a rich mixture is good for engine performance, but it makes the smoke level increase. However, when the spray included angle was 100°, smoke levels were less than 1 FSN even though the IMEP was over 3.5 bar. Therefore, in the case of an early injection, a spray included angle of 100° with an injection timing of 65° BTDC effectively reduces smoke without sacrificing IMEP. 3.4. Effect of Various Injection Strategies on Spray Characteristics. 3.4.1. Comparison of Spray Penetration Length. Figure 10 shows the penetration length at various injection strategies according to ambient density changing from 2.35 kg/ m3 to 26.32 kg/m3 by using a high-pressure chamber. This data was also used in the multiple spray simulation. In the case of the single injection, we repeated the data three times so that we could easily compare the results with the triple injection data. The triple injection data showed that, as the ambient pressure increased, the spray penetration length decreased. This is similar to the single injection data, but the penetration length in triple injection case was shorter than that of single injection case. 3.5. Effect of Injection Strategies and Quantities on Characteristics of Spray and Combustion. 3.5.1. Characteristics of Mixture Formation and EVaporation for Multiple Injections. Figure 11 compares the effects of the injection

Figure 11. Effect of injection times on spray and mixture formation.

strategy on the spray and mixture formation. The injection timing was 65° BTDC for the single injection, 70° and 60° BTDC for the double injection, and 75°, 65°, and 55° BTDC for the triple injection. The collision position of all the main spray was found in the border region of the upper piston and the bowl zone. The results indicate that, as the number of injections increases, the mixture in the combustion chamber becomes enriched. This mixture helps the combustion to enhance. 3.5.2. Fuel EVaporation Characteristics. Figure 12 quantitatively shows the total fraction of evaporated fuel in the combustion chamber. The total fraction of evaporated fuel tends


Figure 12. Effect of injection times on total fraction of fuel evaporation.


Figure 14. Characteristics of smoke as a function of the injection quantity at various injection strategies.

the smoke level decreased, we could not increase the amount of fuel because NOx levels increased significantly. 4. Conclusions

Figure 13. Characteristics of IMEP as impacted by the injection quantity at various injection strategies.

to increase as the number of injections increases. This is because the shorter spray penetration causes a decrease in the amount of wall wetting, and this phenomenon results in a more homogeneous mixture in the combustion chamber. 3.5.3. Emission and IMEP Characteristics at Various Injection Frequencies and Quantities. Figure 13 shows the characteristics of the IMEP as a function of the injected fuel quantity for early injections. The injection timing was fixed at 65° BTDC for the single injection, 70° and 60° BTDC for the double injection, and 75°, 65°, and 55° BTDC for the triple injection. These injection timings result in an optimized mixture distribution in the combustion chamber. As shown in the figure, the IMEP increased as the injection amount increased, and the IMEP range enlarged according to the number of injections increased. It should be noted that we could not continue the single and double injection experiments for over 20 mm3 of injection quantity due to knocking and a high level of NOx. Figure 14 shows the characteristics of smoke according to the injection quantity at the early injection timings. The injection timings were the same as those of the IMEP test. Results show that smoke increased as the injection quantity increased up to 20 mm3. The smoke level of the triple injection decreased when the injection quantity surpassed 20 mm3. However, even though

In this study, we investigated the spray and combustion behavior in a homogeneous charge compression ignition engine as a function of the spray included angle, injection timing, and injection frequency using the early direct injection system. We reached the following conclusions. 1. When the injected fuel impinged on the upper piston and on the border of the piston bowl zone, the distributions of the mixture became more uniform throughout the entire combustion chamber than those of the other cases. 2. In the case of the 100° spray included angle, the mixture is distributed homogeneously throughout the combustion chamber because the spray does not collide with the cylinder liner but collides with the upper piston and the border of the bowl zone. 3. In the case of single injection at 65° BTDC with the 100° spray included angle, the IMEP was about 3.5 bar and the smoke number was less than 1 FSN. However, in case of 50° BTDC injection timing, the smoke level increased rapidly, though the IMEP was increased up to 4.5 bar. It is thought that the reason for this is that the spray does not collide with the cylinder liner and the homogeneous mixture enhances the combustion more actively. 4. When the mixture in the cylinder was homogenized using the early double injection with injection times of 60° and 70° BTDC, the IMEP was about 5 bar. In the case of the triple injection using injection times of 75°, 65°, and 55° BTDC, it was increased up to 6 bar. Therefore, the multiple injection strategy is very effective for the premixed combustion. 5. As the total injection amount increased, the IMEP and the smoke increased simultaneously in most of the cases. In the case of an early double injection, smoke exceeded 2 FSN at an IMEP of over 6 bar. On the other hand, the IMEP exceeded 6 bar in the case of the triple injection, and the smoke was less than 2 FSN. Thus, we found that the IMEP limit in the premixed combustion area was about 6 bar. Acknowledgment. This work was supported by the research fund of Hanyang University (HY-2006-I).


Nomenclature rd ) the droplet radius T ) Taylor number () Z(We)0.5) Z ) Ohnesorge number () (We)0.5 Re-1) We ) Weber number σ ) surface tension Ur ) relative velocitybetween ambient and drop md ) droplet mass Ad ) droplet frontal area Sh ) Sherwood number DAB ) mass diffusivity Fv ) vapor density Fa ) ambient density

Kim et al. Pv,s ) partial pressure at the droplet surface Pv,∞ ) partial pressure at the ambient Cp,d ) specific heat ofliquid Nu ) Nusselt number Km ) mixture thermal conductivity hhg ) latent heat of evaporation Td ) droplet temperature Wein Weber number of the impinging droplet TLeid ) Leidenfrost temperature IMEP ) indicated mean effective pressure FSN ) filter smoke number EF700568G

A Study of the Characteristics of Mixture Formation and Combustion in

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