Energy Fuels 2009, 23, 4908–4916 Published on Web 09/16/2009
: DOI:10.1021/ef900387c
Effect of the Atkinson Cycle Combined with Calibration Factors on a Two-Stage Injection-Type Premixed Charge Compression Ignition Engine Hyungmin Kim, Jaehyeon Lee, Kibum Kim, and Kihyung Lee* Department of Mechanical Engineering, Hanyang University, 1271 Sa1-dong, Sangrok-gu, Gyeonggi-do 426-791, Korea Received May 4, 2009. Revised Manuscript Received July 24, 2009
The Atkinson cycle has been studied as one strategy for premixed charge compression ignition (PCCI) combustion that can reduce NOx and soot emissions. A high exhaust gas recirculation (EGR) is employed to reduce the detrimental engine-out emissions from automotive engines, but it deteriorates the fuel economy of vehicles. Not only can the Atkinson cycle achieve simultaneous reduction in particulate matter (PM) and NOx emissions without increasing fuel consumption, but it can also expand the applicable operational range of the PCCI strategy. Late intake valve closing (LIVC) was used to produce the Atkinson cycle in this study, and the emissions and combustion characteristics of a PCCI diesel engine were investigated using a 30° LIVC in combination with various operating parameters, such as EGR rate, injection timing, swirl ratio, and intake pressure. A LIVC strategy with advancing second injection timing leads to a low compression ratio such that it can reduce NOx emission while the PM emission level and break-specific fuel consumption (BSFC) remain constant.
combustion efficiency by reducing the oxygen concentration in the chamber, resulting from introducing a high exhaust gas recirculation (EGR), and it consequently reduces NOx emission. A significant reduction in NOx emission can also be achieved using a multiple injection strategy, but it results in a low combustion temperature and early ignition, occurring prior to top dead center (TDC), causing higher PM emissions and fuel consumption. To minimize these drawbacks, it is necessary to decrease the compression ratio (CR) of the diesel engine.5 The Atkinson cycle is a well-known strategy for this purpose and requires the advancement or retardation of the intake valve closing.6-8 A low CR decreases in-cylinder pressure and temperature conducive to increasing ignition delay, as shown by Murata et al.7 Benajes et al. investigated the combustion and emission characteristics of a single-cylinder direct diesel engine using the advancing intake valve closing (AIVC) strategy.6 A substantial reduction in NOx was found in this study, but soot emission was not decreased. Late intake valve closing (LIVC) has been used in more studies than AIVC. The LIVC shows much more promising results in terms of the simultaneous reduction of NOx and PM.7-9 In the present study, the LIVC strategy was applied to a multiple-injection-type PCCI diesel engine and the effects of
1. Introduction Upcoming fossil energy depletion and stringent emission regulations on automobiles have motivated substantial developments in diesel engine technologies that can both improve engine performance and reduce emissions. The developments can be largely categorized into the more efficient combustion strategies [e.g., homogeneous charge compression ignition (HCCI) or low-temperature combustion (LTC)] and aftertreatment systems. A big challenge of the after-treatment system is not cost-effective, and a post-injection strategy employed in some after-treatment systems, such as lean NOx trap and swirl control valve (SCR), has the disadvantage of increasing fuel consumption.1 Therefore, it would be better to reduce heavy dependency on the after-treatment systems by improving the combustion performance of the engine itself. The HCCI combustion technique has shown promising results, producing near-zero NOx and particulate matter (PM) emissions. A particular variation of the HCCI combustion technique, premixed charge compression ignition (PCCI) combustion, has recently drawn substantial attention.2-4 The concept behind PCCI combustion is to enhance the process of air and fuel mixing such that premixed combustion occurs simultaneously across the combustion chamber, eliminating flame diffusion. PCCI combustion also decreases
(5) Araki, M.; Umino, T.; Obokata, T.; Ishima, T.; Shiga, S.; Nakamura, H.; Long, W.; Murakami, A. Effects of compression ratio on characteristics of PCCI diesel combustion with a hollow cone spray. SAE Tech. Pap. 200501-2130, 2005. (6) Benajes, J.; Serrano, J. R.; Molina, S.; Novella, R. Potential of Atkinson cycle combined with EGR for pollutant control in a HD diesel engine. Energy Convers. Manage. 2009, 50, 174–183. (7) Murata, Y.; Kusaka, J.; Daisho, Y.; Kawano, D.; Suzuki, H.; Ishii, H.; Goto, Y. Miller-PCCI combustion in an HSDI diesel engine with VVT. SAE Tech. Pap. 2008-01-0644, 2008. (8) He, X.; Durrett, R. P.; Sun, Z. Late intake valve closing as an emissions control strategy at tier 2 bin 5 engine-out NOx level. SAE Tech. Pap. 2008-01-0637, 2008. (9) Murata, Y.; Kusaka, J.; Odaka, M.; Daisho, Y.; Kawano, D.; Suzuki, H.; Ishii, H.; Goto, Y. Achievement of medium engine speed and load premixed diesel combustion with variable valve timing. SAE Tech. Pap. 2006-01-0203, 2006.
*To whom correspondence should be addressed. Telephone: þ82-31400-5251. Fax: þ82-31-406-5550. E-mail:
[email protected]. (1) Bression, G.; Soleri, D.; Savy, S.; Dehoux, S.; Azoulay, D.; Hamouda, H.; Doradoux, L.; Guerrassi, N.; Lawrence, N. A study of methods to lower HC and CO emissions in diesel HCCI. SAE Tech. Pap. 2008-01-0034, 2008. (2) 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. 790840, 1979. (3) Onishi, S.; Jo, S. H.; Shoda, K.; Jo, P. D.; Katao, S. Active thermoatmosphere combustion (ATAC);A new combustion process for internal combustion engines. SAE Tech. Pap. 790501, 1979. (4) Najt, P. M.; Foster, D. E. Compression-ignited homogeneous charge combustion. SAE Tech. Pap. 830264, 1983. r 2009 American Chemical Society
4908
pubs.acs.org/EF
Energy Fuels 2009, 23, 4908–4916
: DOI:10.1021/ef900387c
Kim et al.
Figure 1. Schematic of a common-rail-injection-type PCCI multi-cylinder engine.
LIVC combined with various operating parameters (e.g., EGR rate, intake pressure, swirl ratio, and injection timing) on the combustion and emissions characteristics in a fourcylinder high-pressure direct-injection (DI)-type PCCI engine system were investigated.
Table 1. Specifications of Engine and Valve Timings Used in This Study
2. Experimental Apparatus and Method A schematic diagram of the 2000 cc, four-cylinder commonrail-injection-type PCCI diesel engine employed in this study is shown in Figure 1. The system consists of an engine with an 83 mm bore and a 93 mm stroke, seven-hole injectors with a nozzle diameter of 0.141 mm, a high-pressure pump, and a commonrail-injection system. The detailed specifications of the engine along with the common-rail-injection system are summarized in Table 1. The engine was also equipped with a SCV, an EGR valve, and a turbocharger, which enabled the execution of various parametric studies. An EC dynamometer (220 kW Meiden) was used to maintain the same engine speed and to measure the engine torque. For emission analysis and fuel supply control, an angle sensor (3600-pulse encoder) and a TDC sensor were attached at the crank and cam shafts, respectively. An injection controller (TDA 8000, TEMS Co.) was employed to control the common-rail pressure, the fuel injection timing, and the injected fuel quantity necessary for achieving a highpressure DI PCCI engine. The required signals for the control of injection timing and fuel quantity were received from the TDC and crank angle sensors. The controller was able to control the rail pressure up to 1600 bar, and it contained a PWM controller that could control the variable geometry turbocharger (VGT), EGR, and SCV. The engine coolant temperature was maintained at 82 ( 2 °C using a water temperature control (WTC); consequently, the temperature of the fuel supplied to the engine was constrained to the range 40 ( 0.5 °C. A system acquiring temperatures, pressures, and engine torque was installed in the dynamometer controller, and a fuel flow rate was measured using a mass flow meter (CFM 010, Micro motion). NOx, total hydrocarbon (THC), CO, CO2, and O2
description
specification
engine type number of cylinders bore stroke (mm) displacement volume (cc) swirl ratio compression ratio fuel injection system injection pressure (bar) hole diameter (mm) spray angle (deg) hole number standard valve train (ATDC)
four-stroke DI 4 83 92 1991 1.5 17.3 common-rail DI ∼1600 0.141 148 7 EVO: 232° IVO: 352° EVC: 368° IVC: 502°
emissions were evaluated with an emission analyzer (7100 DEGR, Horiba Co.), and the smoke density was measured with a smoke meter (415S, AVL Co.). While the engine ran under the steady-state conditions, all measurements were averaged over 10 s. The emission data were measured in ppm/FSN, and all data were converted into g kW-1 h-1 to correct sample-sample variation. The cylinder pressure was measured with a glow plug-type pressure sensor attached to the first cylinder. The rate of heat release (ROHR) was calculated, and the indicated mean effective pressure (IMEP) was measured using a combustion analyzer (MTS Co.) and an encoder for the combustion analysis, respectively. In this study, the effects of various calibration factors and LIVC on combustion and emission performance were investigated using a high-pressure DI PCCI engine and the effects were evaluated by a comparison to a conventional diesel engine. The engine-operating conditions included an engine speed of 1500 rpm and a brake mean effective pressure (BMEP) of 0.4 MPa. An injection strategy used in this study was a two-stage injection that consists of an early injection set at the middle of the compression 4909
Energy Fuels 2009, 23, 4908–4916
: DOI:10.1021/ef900387c
Kim et al.
Table 2. Experimental Conditions Including Engine-Operating Conditions, Injection Strategy, and Calibration Factors description
specification
engine speed (rpm) BMEP (MPa) cam profile injection pressure (MPa) injection timing
1500 0.4 standard, LIVO þ30° 85-130 early ATDC -60° late ATDC 0 and 5° split 3:7 EGE rate (%) 20-35 swirl ratio 1.7-2.7 pressure (bar) 0.08-0.16
injection method rate of injection quantity intake conditions
stroke for sufficient premixing and the main injection set near the TDC.10,11 This injection strategy has the advantage of reducing emissions, but the first ignition takes place prior to TDC in case the CR is high, resulting in a low combustion efficiency. The CR can be reduced with the LIVC strategy, and an additional investigation was performed on the effects of the LIVC combined with other prevalent calibration parameters affecting PCCI combustion (e.g., EGR rate, swirl ratio, injection pressure, and intake pressure) on combustion and emission characteristics. The detailed injection and operating conditions are summarized in Table 2.
Figure 2. Effect of the IVC timing on the cylinder pressure.
3. Results and Discussion To assess the effect of the intake valve closing on in-cylinder pressure, a simulation study was carried out using onedimensional software (WAVE).12 Figure 2 shows the incylinder pressure as a function of intake valve closing (IVC). The in-cylinder pressure decreases with increasing retardation of the IVC because the intake air flows out of the cylinder in the early compression stroke. Using Figure 2, CRs were determined and plotted as a function of IVC in Figure 3. As the IVC retards, the effective CR becomes smaller, and the CR becomes approximately 16 when the IVC is retarded by 30°. These data were used for a retarded cam design. In addition, the data was fitted using a polynomial linear line fit, and the following correlation was developed: CR ¼ -6:8e-6 IVC3 þ 11e-2 IVC2 -6:21IVC þ 1169
Figure 3. Relation between the CR and IVC timing.
stroke. A better method for determining the effective CR is to apply a linear line fit to the compression process on a P-v diagram in logarithmic scale and to define the corresponding volume at the intersection of the linear line and the intake pressure as shown in Figure 4. The effective CR can be determined using the following equation:
ð1Þ
As mentioned above, the late IVC affects the actual incylinder compression process that is generally well-represented by a pressure that is determined using the effective CR, taking the amount of escaped intake air through the intake valve as a result of the delay of the intake valve closing into account for calculation. Therefore, the effect of the late IVC can be described in terms of the effective CR, commonly defined as the ratio of cylinder volume at IVC to the TDC volume.4 However, it is not suitable way for the Atkinson cycle to represent the actual in-cylinder compression process because of a high air flow drag that usually occurs until the intake valve is completely closed during the compression
CR ¼ ðVc þVl Þ=Vc
ð2Þ
where Vc is the combustion chamber volume and Vl is the displacement volume. The results are tabulated in Table 3 along with geometric CR, which is a typical ratio and volumetric CR representing the ratio at the moment of IVC. An experiment was carried out to compare LIVC with a standard cam under the same experimental conditions (e.g., injection timing, injection pressure, injection quantity, and various operating conditions). The optimum injection timing and injection rate were determined in the previous study.13 Figure 5 shows that LIVC 30° affects the cylinder pressure as a function of the crank angle and volume. The LIVC decreases
(10) Chung, J. W.; Kang, J. H.; Kim, N. H.; Kang, W.; Kim, B. S. Effects of the fuel injection ratio on the emission and combustion performances of the partially premixed charge compression ignition combustion engine applied with the split injection method. Int. J. Automot. Technol. 2008, 9 (1), 1–8. (11) Kim, H. M.; Kim, K. B.; Lee, K. H.; Ikeda, Y. A study on the optimization of operating conditions for simultaneous reduction in NOx and PM in a 4-cylinder premixed diesel engine. SAE Tech. Pap. 2009-010926, 2009. (12) Ricardo Co. WAVE;Engine Reference Manual, Version 5.3, 2003.
(13) Kim, H. M.; Lee, J. H.; Kim, K. B.; Lee, K. H. A study on the injection strategy to expand the operating region of premixed charge compression ignition engine. The 7th Thermal and Fluid Engineering Conference, 2008.
4910
Energy Fuels 2009, 23, 4908–4916
: DOI:10.1021/ef900387c
Kim et al.
Figure 4. Definition of the effective CR. Table 3. CRs of the 30° Retarded Cam contents
value
geometric CR effective CR volumetric CR
17.5 15.8 13.8
the effective CR, resulting in a low cylinder pressure and ignition delay. The location of the maximum cylinder pressure is also shown to be delayed in Figure 5a. It is a great advantage that the LIVC can compensate for a short ignition delay, which is a common issue in two-stage injection-type PCCI engines. In other words, the fuel in the first injection is typically ignited before TDC, but LIVC lengthens the ignition delay by decreasing the compression pressure. Moreover, LIVC can extend the ignition delay for secondarily injected fuel. Figure 5b shows logarithmic P-v diagrams of engines with LIVC 30° and standard cam. In comparison to standard cam data, the LIVC 30° data cause a decrease in work and IMEP. One can compensate for the reduction in engine work by advancing the second injection timing. Pumping loss can also be decreased using the LIVC 30°. In addition, it was investigated how a variety of operating conditions combined with the Atkinson cycle affects breakspecific fuel consumption (BSFC) and the emission characteristics of the engine. To evaluate this, the operating conditions were fixed at 1500 rpm and BMEP of 4 bar by varying the injection quantity. 3.1. Effect of the EGR Rate. Figure 6 represents the effects of LIVC and standard closing with various EGR rates on BSFC and engine-out emissions with fixed injection timings at ATDC -60° and 0°. The LIVC increases BSFC for all EGR rates, because the CR of the LIVC is lower than that of the standard cam. Turbo- or supercharging can prevent the increase in BSFC in LIVC application. As shown in Figure 7, the LIVC decreases the in-cylinder pressure during the compression stroke, leading to a lower combustion temperature. As a result, NOx emission generally decreases, while CO, THC, and PM emission increase. The CO emissions are inconsistent with the results, well-known knowledge for EGR rates below 30%. The low combustion temperature decreases combustion efficiency, and a longer ignition delay allows for excessive dispersion of the fuel-air mixture before the ignition. This causes the formation of an excessively lean mixture, leading to the increase in CO and HC emissions. As shown in Figure 7, ROHR is becoming retarded with an
Figure 5. Effect of the IVC timing on combustion: (a) in-cylinder pressure and (b) pressure-volume diagram.
Figure 6. Effect of LIVC with various EGR rates on BSFC and emissions.
increasing EGR rate and using LIVC. It indicates that the ignition delay period is extended, helping sufficient mixing for the air and fuel in the cylinder. It is believed that a 4911
Energy Fuels 2009, 23, 4908–4916
: DOI:10.1021/ef900387c
Kim et al.
Figure 7. Effect of (a) standard and (b) late IVC timings with various EGR rates on in-cylinder pressure.
decrease in the exhaust gas flow rate because of LIVC could be a possible reason for low CO emissions. The increase in the EGR rate from 30 to 35% results in a further reduction of both the combustion temperature and the oxygen concentration, and it dramatically reduces NOx emission but causes an undesirable increase in PM, THC, and CO emissions and BSFC. Note that LIVC decreased the dependency on EGR to reduce NOx emission, which is a well-known advantage of the Atkinson cycle. The same propensity was observed for the various intake valves close timings in this study. On the basis of results, it could be concluded that the LIVC strategy combined with an EGR of 30% is the optimum condition in terms of engine-out emissions and BSFC. Figure 7 shows incylinder pressure as a function of crank angles and EGR rates for both standard and LIVC timings. The increasing EGR gas decreases the turbine work of a turbocharger, consequently reducing the compression pressure. To eliminate this issue during this experiment, the intake pressure was controlled by changing the vane angle of the VGT. Therefore, it was confirmed that the decreased in the compression pressure does not result from a decrease in intake pressure, and the in-cylinder pressure was reduced by only increasing EGR deteriorating ignition efficiency of the fuel in the first injection. As discussed above, LIVC decreases the in-cylinder pressure and the increasing EGR rate causes a further drop of the overall in-cylinder pressure and maximum in-cylinder pressures. It is also noted that the ignition timing is retarded in Figure 7. 3.2. Effect of the Injection Pressure. Figure 8 shows how LIVC and standard closing with various injection pressures affects the combustion and emission characteristics of the
Figure 8. Effect of LIVC with injection pressures on BSFC and emissions.
PCCI engine. The injection pressure of a conventional diesel engine is around 750 bar at 1500 rpm and a BMEP of 4 bar, but the injection pressure varied from 850 to 1300 bar in this 4912
Energy Fuels 2009, 23, 4908–4916
: DOI:10.1021/ef900387c
Kim et al.
Figure 9. Effect of (a) standard and (b) late IVC timings with injection pressures on in-cylinder pressure.
study. As the injection pressure increases from 850 to 1300 bar, emissions in CO, THC, and PM decrease slightly because of better atomization of the fuel, resulting in improved combustion efficiency. With respect to BSFC data, the standard closing initially increases until an injection pressure of 1150 bar and then decreases at 1300 bar. The LIVC is shown to increase at 1300 bar because a low incylinder pressure causes wall wetting. For the same reason, CO emission in the Atkinson cycle hardly decreases. This fact can also be illustrated in Figure 9, which plots in-cylinder pressure as a function of crank angles and injection pressures for standard and LIVC timings. Increasing injection pressure with the standard closing not only raises the maximum incylinder pressure but also advances the combustion phasing slightly closer to TDC, which means that the combustion efficiency before TDC is improved. On the other hand, the maximum in-cylinder pressure using LIVC increases up to the injection pressure of 1150 bar, but it finally drops at an injection pressure of 1300 bar. 3.3. Effect of the Swirl Ratio. The effects of LIVC and standard closing timings with various swirl ratios on the combustion and emission characteristics of the PCCI engine are shown in Figure 10. It is difficult to notice any distinct trend in both cases in terms of swirl ratios from Figure 10, but emission results with the standard closing decrease a bit as the swirl ratio increases. It is believed that the swirl contributed to the formation of a better air-fuel mixture. The swirl does not seem to significantly influence the emission characteristics of the PCCI engine with LIVC. This fact can also be illustrated in Figure 11, showing in-cylinder pressure and ROHR affected by swirl ratio for standard
Figure 10. Effect of LIVC with swirl ratios on BSFC and emissions.
closing and LIVC. Increasing swirl strength with the LIVC not only raises the maximum in-cylinder pressure but also advances the combustion phasing slightly closed to TDC, which means that the combustion efficiency before TDC is improved. On the other hand, the maximum in-cylinder 4913
Energy Fuels 2009, 23, 4908–4916
: DOI:10.1021/ef900387c
Kim et al.
Figure 11. Effect of (a) standard and (b) late IVC timings with swirl ratios on in-cylinder pressure.
timings with various intake pressures on the combustion and emission characteristics of the PCCI engine. Elevated intake pressure leads to a slight increase in NOx emission and BSFC. The high intake pressure was considered to promote the first combustion efficiency, which reduced ignition delay for the second combustion, as shown in Figure 13. Thus, not only did locally fuel-rich combustion occur in the combustion chamber, but the combustion pressure also increased significantly and, thus, so did NOx emissions. Furthermore, the first ignition occurred prior to TDC, which might be the reason for the high BSFC. LIVC reduces the effective CR inside the cylinder such that the shortcomings of high NOx emission are overcome without a large increase in BSFC. 3.5. Effect of the Advanced Injection Timing. Finally, the effects of LIVC with advancing injection timing on the combustion and emission characteristics of the PCCI engine are shown in Figure 14. The optimal injection timings in a two-stage injection PCCI engine were determined ATDC -60° and 5° in the previous study.13 The reader is expected to refer to further information on the injection timings. In comparison to conventional diesel combustion, PCCI combustion applied with multiple injections at ATDC -60° and 5° shows a simultaneous reduction in PM and NOx emissions. However, an increase in the BSFC of PCCI is inevitable because of decreased combustion efficiency. The BSFC increased by approximately 13%. The increase in BSFC can be compensated by advancing the second injection timing. When the second injection timing is advanced 5°, an increase in combustion temperature is accompanied with a decrease in BSFC and PM emission. High NOx emission occurs in this case, but LIVC leads to a low CR and, thus, can reduce NOx
Figure 12. Effect of LIVC with intake pressures on BSFC and emissions.
pressure and combustion phasing in the standard closing are not affected by the swirl ratio. 3.4. Effect of the Intake Pressure. In the same manner, Figure 12 shows the effects of LIVC and standard closing 4914
Energy Fuels 2009, 23, 4908–4916
: DOI:10.1021/ef900387c
Kim et al.
Figure 13. Effect of (a) standard and (b) late IVC timings with intake pressures on in-cylinder pressure.
emissions while the PM emission level and BSFC remain constant. 4. Conclusions In this study, the effects on the combustion and emission characteristics of the PCCI engine of LIVC and standard closing timings with various operating conditions were investigated. The principle conclusions can be summarized as follows: (1) The CR was reduced by approximately 1.5 with a 30° LIVC compared to the standard cam. (2) Regardless of a type of cam, an increase in the EGR rate decreases the combustion temperature and the air/fuel ratio. As a result, NOx emissions decrease rapidly but PM emissions and BSFC increase. (3) An increase in injection pressure with a standard cam results in a slight decrease in PM, THC, and CO emissions but a slight increase in NOx emissions. In addition, the first ignition prior to TDC is a direct cause of an increase in BSFC. However, the high injection pressure of 1300 bar along with the LIVC results in wall wetting, which causes a high BSFC. (4) In comparison to the standard cam, the LIVC increases BSFC but decreases NOx emissions substantially while maintaining PM emissions. This is attributed to a low CR, resulting in a low combustion temperature and a longer ignition delay. A typical way to reduce NOx emission is to use EGR, but a promising result was obtained with the LIVC capable of reducing NOx emission from the engine. Therefore, LIVC is expected to decrease the dependency on EGR to reduce NOx emission. Acknowledgment. The authors acknowledge the financial support provided for this research by the Korea Automotive
Figure 14. Effect of operating conditions on NOx, PM, and BSFC.
4915
Energy Fuels 2009, 23, 4908–4916
: DOI:10.1021/ef900387c
Kim et al.
IMEP = indicated mean effective pressure IVC = intake valve closing IVO = intake valve opening LIVC = late intake valve closing LTC = low-temperature combustion PCCI = premixed charge compression ignition ROHR = rate of heat release SCV = swirl control valve TDC = top dead center THC = total hydrocarbon VGT = variable geometry turbocharger WTC = water temperature control
Technology Institute. This study was performed as a part of a project to optimize injection conditions in HCCI engines.
Nomenclature AIVC = advancing intake valve closing ATDC = after top dead center BMEP = brake mean effective pressure CR = compression ratio EGR = exhaust gas recirculation EVC = exhaust valve closing EVO = exhaust valve opening HCCI = homogeneous charge compression ignition
4916
