Table of Contents

    Topic review

    Early Injection for Low Emissions

    View times: 177
    Contributors: Liang Xingyu , Yuesen Wang
    Submitted by: Liang Xingyu

    Definition

    Low-emission and high-efficiency have always been the targets for Internal Combustion Engine development. For diesel engines, homogeneous charge (aka. HCCI) and premixed charge (aka. PCCI) combustion modes provide both low-emission and high-efficiency simultaneously. To achieve these advanced combustion modes, early injection is needed as a relatively longer air-fuel mixing time is guaranteed. Several key parameters, such as the injection timing, pressure, angle, directly determine the final combustion process and thus the emission and efficiency performance. The pros and cons of these key parameters are discussed in detail here to provide a good review of the early-injection strategy.

    1. Advanced Combustion Modes: HCCI and PCCI

    The conventional diesel combustion process can be classified into four major phases: ignition delay, premixed combustion, mixing controlled combustion, and the late burning phase[1]. The conventional combustion phase regime encompasses both NOx and soot islands, as shown in Figure 1. This is not preferable while considering the more and more stringent emission regulations. Therefore, advanced combustion modes that could eliminate or avoid the fuel-rich and high-temperature environment are needed.

    Applsci 09 03737 g001 550

    Figure 1. φ-T diagram of conventional combustion, homogeneous charge compression ignition (HCCI) combustion, and premixed charge compression ignition (PCCI) combustion[2]. φ, equivalence ratio; T, temperature.

    HCCI combustion was first proposed by Onishi et al.[3] and Noguchi et al.[4]. The main characteristic of HCCI is a (more or less homogeneous) premixed air-fuel mixture that undergoes auto-ignition as a result of compression. A major difficulty in HCCI is to get a homogeneous admixture of air and fuel. Besides, the high cetane number of conventional diesel fuel results in large rates of pressure rise and difficulties in combustion phasing control[5][6][7][8][9].

    PCCI combustion has been described as a middle path between conventional and HCCI combustion modes[10][11][12][13]. For PCCI combustion, only part of the fuel undergoes the HCCI type of clean combustion, while the remainder undergoes conventional combustion. Table 1shows that both HCCI and PCCI provide clear advantages out of the conventional diesel combustion mode regarding the soot and NOx emissions.

    Table 1. Comparison of key characteristics of conventional diesel, HCCI, and PCCI combustion.

    2. Early Injection Strategy Definition

    In order to allow enough time for fuel to mix with the air before combustion, the early injection strategy, by which the fuel is injected in an early stage of the compression stroke, has been applied widely in HCCI and PCCI diesel engines. The start of early injection is typically 20–200 before the top dead center(BTDC). Based on the characteristic of HCCI and PCCI combustion, the early injection strategy can be classified as a single injection and two-stage injection, as seen in Figure 2. For a two-stage injection, the first injection is also called the pilot injection, and the second injection is also called the main injection. Based on the injection timing, the early injection strategy can be divided into three patterns, as seen in Figure 3: The injection closest to TDC is defined as late; that farthest from TDC is defined as early, and the one in between is defined as middle[14]. The demarcation points of these three patterns in this paper are defined as 60, 40, and 20 BTDC, respectively.

    Applsci 09 03737 g002 550

    Figure 2. Single and two-stage early injection strategy.

    Applsci 09 03737 g003 550

    Figure 3. Early injection strategy divided by injection timing.

    Using the early injection strategy will cause a wall-wetting problem and leads to (1) low combustion eciency, (2) excessive soot/carbon monoxide (CO)/hydrocarbon (HC) emissions, and (3) (local) oil dilution[15][16]. Many methods, including limiting the injection angle, have been proposed to limit or reduce wall-wetting.

    Overall, there are several key parameters in the early-injection process that define the final combustion and emission performances. The effects of fuel injection pressure, injection timing, and injection angle on engine performance and emissions are discussed in detail separately in the following sections.

    3. Effects of Injection Pressure

    Injection pressure could change the combustion and the emissions significantly as it directly determines the fuel spray, injection duration, and therefore the time to mix air and fuel into a homogenous mixture. However, it is not a straightforward approach to have lower emissions by simply increasing the injection pressure. Table 2 below gives details about the emissions along with changing the injection pressure.

    Table 2. Variation of performance and emissions after increasing the injection pressure. BSFC, brake-specific fuel consumption. 

    Author

    Injection Pressure (bar)

    Fuel

    BSFC

    NOx

    HC

    CO

    Soot

    Jeong et al.[17]

    500–900

    Diesel

    na

    na

    na

    Fang et al.[18] 

    600/1000

    Diesel

    na

    na

    na

    Shimazaki et al.[19]

    300–1200

    Diesel

    ↓↑

    Kiplimo et al.[20]

    800/1400

    Diesel

    Liu et al.[21]

    600–1400

    Diesel

    na

    na

    na

    na

    Chen et al.[22]

    1000–1400

    Diesel

    na

    na

    Siewert[23]

    800–1600

    Diesel

    na

    Park et al.[24]

    400/1200

    Bioethanol blends

    Arun et al.[25]

    200–240

    Carbon black–water–diesel

    na

    ↓↑

    ↓↑

    ↓↑

    ↓↑

    Nanthagopal et al.[26]

    200–240

    Biodiesel

    4. Effects of Injection Timing

    On one hand, injecting the fuel at an earlier time prolongs the ignition delay and helps to create a more homogeneous mixture. The formed lean mixture is then burned at a low temperature, resulting in low NOx emissions. On the other hand, the cylinder pressure and temperature are low under earlier injection timing, which leads to poor fuel evaporation and the wall-wetting problem.

    4.1 Single Early Injection Timing Effects

    Table 3 shows a summary of the variation of performance and emissions of the HCCI engine after advancing the early injection timing. In general, advancing the injection timing results in better NOx emissions but worse HC and CO emissions. However, the final soot emission depends on the opposite effects mentioned above. Engine performance deteriorates with advanced injection timing due to the increased negative work and incomplete combustion.

    Table 3. Variation of performance and emissions after advancing the early injection timing (single). 

    Author

    Injection Timing (° BTDC)

    Fuel

    BSFC

    NOx

    HC

    CO

    Soot

    Benajes et al.[27]

    33–24

    Diesel

    na

    na

    Kiplimo et al.[20]

    40–20

    Diesel

    Kim and Lee[28]

    70–20

    Diesel

    na

    na

    na

    Fang et al.[30]

    80–40

    Diesel

    na

    na

    na

    Kim et al.[31]

    180–20

    Diesel

    na

    na

    na

    Kim et al.[32]

    180–20

    Diesel

    na

    na

    Miyamoto et al.[33]

    180–20

    Diesel

    na

    na

    Kook et al.[29]

    200–50

    Diesel

    Park et al.[24]

    40–20

    Bioethanol blends

    Yoon et al.[34]

    40–20

    DME

    Kim et al.[35]

    40–20

    Gasoline

    na

    Wamankar and Murugan[36]

    26–20

    Diesel

    4.2 Two-stage Early Injection

    In PCCI combustion, a two-stage early injection strategy is utilized. In general, advancing the first injection timing will decrease NOx and soot emissions and increase HC and CO emissions. Engine performance deteriorates with advanced injection timing due to the increased negative work and incomplete combustion. The second injection is considered to act as the ignition controller and promoter of PCCI combustion. The second injection timing mainly influences the second stage of the combustion process, which is mainly diffusive combustion. With retarded second injection timing, the major combustion event was delayed. The variation of BSFC of different second injection timings mainly depended on whether the combustion event shifted to near TDC. In addition, NOx emissions decreased when the second injection timing was retarded because of the low charge temperature caused by the late combustion. Soot emissions generally increased as the second injection was retarded. This was because of the increased portion of diffusion combustion and low charge temperature. HC and CO emissions also increased with retarded second injection timing.

    5. Effects of Injection Angle

    Wall-wetting caused by the early injection strategy directly influenced the performance and emissions of the HCCI diesel engine. Limiting the injection angle has been proved to be a useful approach to reduce the wall-wetting phenomenon. The magnitude and direction of the spray rotation in the bowl were directly affected by the injection angle, as shown in Figure 4. This difference further impacted the fuel-air mixing in the piston bowl and finally impacted combustion and emissions. As mentioned above, the impingement target is an important factor influencing emissions and is commonly determined by the injection timing, injection angle, and piston structure.

    Figure 4. Schematic diagrams of the tested combustion chamber and fuel spray: (a) conventional diesel engine; (b) modified engine configuration for early injection [28].

    Table 4 shows a summary of the variation of performance and emissions of HCCI and PCCI engines after decreasing the injection angle. In general, decreasing the injection angle will limit or reduce the wall-wetting phenomenon, resulting in decreased HC and CO. However, soot emission is directly affected by the placement of spray targeting. Decreasing the injection angle generally is not good for the control of soot emission, but NOx emission can be suppressed by the rich fuel-air mixture and low combustion temperature.

    Table 4. Variation of performance and emissions after decreasing the injection angle (two-stage).

    Author

    Injection Angle (°)

    Fuel

    BSFC

    NOx

    HC

    CO

    Soot

    Kim and Lee[28]

    60/156

    Diesel

    na

    na

    na

    Fang et al.[18]

    70/150

    Diesel

    na

    na

    na

    Kim et al.[31]

    70–150

    Diesel

    na

    na

    na

    Mobasheri and Peng

    90–145

    Diesel

    ↓↑

    na

    na

    ↓↑

    Vanegas et al.[37]

    100–148

    Diesel

    na

    na

    na

    Kook and Bae

    100/150

    Diesel

    na

    Siewert[23]

    100–158

    Diesel

    na

    na

    Park et al.[38]

    70/156

    Bioethanol blended

    na

    na

    Yoon et al.[34]

    60/70/156

    DME

     6. Combination of Early-Injection and Alternative Fuels

    Changing the fuel properties and using alternative fuel are also promising ways to improve the combustion and emissions of HCCI and PCCI engines[39][40][41]. Biodiesel fuel, as one alternative diesel fuel, is currently of great interest and an important research subject. Biodiesel fuels contain oxygen and thus provide an effective way to eliminate the over-rich regions and enhance the combustion process, resulting in low soot, HC, and CO emissions[42][43][44][45]. Dimethyl ether (DME) is another alternative fuel. Its good ignition capability and high latent heat lead to decreased cylinder temperature in the combustion phase[46][47]. Besides, the oxygenated molecular structure and good atomization properties help in the formation of a leaner and more homogeneous mixture. The alternative fuels bioethanol and n-butanol are also widely used due to their high oxygen concentration[48][49][50][51][52].

    As HCCI combustion is mainly controlled by chemical kinetics, the combustion process and burning rate are dependent on fuel properties. Studies have shown that optimal physicochemical properties are needed under different operating conditions; e.g., fuel with a high cetane number is required for light loads and high-octane fuel for heavy loads[53][54][55][56]. Gasoline/diesel dual-fuel combustion was proved to be a useful approach to control the combustion phasing and heat release rate of HCCI by adjusting the blending ratio according to different operating conditions[57][58].

    7. Summary and Conclusions

    Several key parameters in early injection strategy were covered and discussed here mainly focus on engine combustion and emission performances. Both experimental and numerical works had been conducted widely, and the advantages and disadvantages, in terms of the engine emissions, of early injection strategy are listed in Table 5.

    Table 5. Advantages and disadvantages of early injection parameters.

    The entry is from 10.3390/app9183737

    References

    1. Heywood, J.B. Internal Combustion Engine Fundamentals; McGraw Hill: New York, NY, USA, 1988.
    2. Mansoury, M.; Jafarmadar, S.; Talei, M.; Lashkarpour, S.M. Optimization of HCCI (Homogeneous Charge Compression Ignition) engine combustion chamber walls temperature to achieve optimum IMEP using LHS and Nelder Mead algorithm. Energy 2017, 119, 938–949.
    3. Onishi, S.; Jo, S.H.; Shoda, K.; Jo, P.D.; Kato, S. Active Thermo-Atmosphere Combustion (ATAC)—A New Combustion Process for Internal Combustion Engines; SAE International: Warrendale, PA, USA, 1979.
    4. Noguchi, M.; Tanaka, Y.; Tanaka, T.; Takeuchi, Y. A Study on Gasoline Engine Combustion by Observation of Intermediate Reactive Products During Combustion; SAE International: Warrendale, PA, USA, 1979.
    5. Stanglmaier, R.H.; Roberts, C.E. Homogeneous Charge Compression Ignition (HCCI): Benefits, Compromise, and Future Engine Applications; SAE International: Warrendale, PA, USA, 1999.
    6. Yao, M.F.; Zheng, Z.L.; Liu, H.F. Progress and recent trends in homogeneous charge compression ignition (HCCI) engines. Prog. Energy Combust. Sci. 2009, 35, 398–437.
    7. Lu, X.C.; Han, D.; Huang, Z. Fuel design and management for the control of advanced compression-ignition combustion modes. Prog. Energy Combust. Sci. 2011, 37, 741–783.
    8. Swor, T.A.; Kokjohn, S.; Andrie, M.; Reitz, R.D. Improving Diesel Engine Performance Using Low and High Pressure Split Injections for Single Heat Release and Two-Stage Combustion; SAE International: Warrendale, PA, USA, 2010.
    9. Harada, A.; Shimazaki, N.; Sasaki, S.; Miyamoto, T.; Akagawa, H.; Tsujimura, K. The Effects of Mixture Formation on Premixed Lean Diesel Combustion; SAE International: Warrendale, PA, USA, 1998.
    10. Simescu, S.; Fiveland, S.B.; Dodge, L.G. An Experimental Investigation of PCCI-DI Combustion and Emissions in a Heavy-Duty Diesel Engine; SAE International: Warrendale, PA, USA, 2003.
    11. Kanda, T.; Hakozaki, T.; Uchimoto, T.; Hatano, J.; Kitayama, N.; Sono, H. PCCI Operation with Early Injection of Conventional Diesel Fuel; SAE International: Warrendale, PA, USA, 2005.
    12. Simescu, S.; Ryan, T.W.; Neely, G.D.; Matheaus, A.C.; Surampudi, B. Partial Pre-Mixed Combustion with Cooled and Uncooled EGR in a Heavy-Duty Diesel Engine; SAE International: Warrendale, PA, USA, 2002.
    13. Weiskirch, C.; Mueller, E. Advanced in Diesel Engine Combustion: Split Combustion; SAE International: Warrendale, PA, USA, 2007.
    14. Eastwood, P.G.; Morris, T.; Tufail, K.;Winstanley, T.; Hardalupas, Y.; Taylor, A.M.K.P. The Effects of Fuel-InjectionSchedules on Emissions of NOx and Smoke in a Diesel Engine during Partial-Premix Combustion; SAE International: Warrendale, PA, USA, 2007.
    15. Boot, M.; Rijk, E.; Luijten, C.; Somers, B.; Albrecht, B. Spray Impingement in the Early Direct Injection Premixed Charge Compression Ignition Regime; SAE International: Warrendale, PA, USA, 2010.
    16. Han, M.; Assanis, D.N.; Bohac, S.V. Sources of hydrocarbon emissions from low-temperature premixed compression ignition combustion from a common rail direct injection diesel engine. Combust. Sci. Technol. 2009, 181, 496–517.
    17. Jeong, K.; Lee, D.; Park, S.; Lee, C.S. Effect of Two-Stage Fuel Injection Parameters on NOx Reduction Characteristics in a DI Diesel Engine. Energies 2011, 4, 2060.
    18. Fang, T.G.; Coverdill, R.E.; Lee, C.F.F.; White, R.A. Effects of injection angles on combustion processes using multiple injection strategies in an HSDI diesel engine. Fuel 2008, 87, 3239.
    19. Shimazaki, N.; Tsurushima, T.; Nishimura, T. Dual Mode Combustion Concept with Premixed Diesel Combustion by Direct Injection near Top Dead Center; SAE International: Warrendale, PA, USA, 2003.
    20. Kiplimo, R.; Tomita, E.; Kawahara, N.; Yokobe, S. Effects of spray impingement, injection parameters, and EGR on the combustion and emission characteristics of a PCCI diesel engine. Appl. Therm. Eng. 2012, 37, 175.
    21. Liu, H.F.; Ma, S.Y.; Zhang, Z.; Zheng, Z.Q.; Yao, M.F. Study of the control strategies on soot reduction under early-injection conditions on a diesel engine. Fuel 2015, 139, 481.
    22. Chen, L.; Yang, F.Y.; Yang, Y.P.; Yang, X.Q.; Ouyang, M.G. Application of Narrow Cone Angle Injectors to Achieve Advanced Compression Ignition on a Mass-Production Diesel Control Strategy and Engine Performance Evaluation; SAE International: Warrendale, PA, USA, 2009.
    23. Siewert, R.M. Spray Angle and Rail Pressure Study for Low NOx Diesel Combustion; SAE International: Warrendale, PA, USA, 2007.
    24. Park, S.H.; Cha, J.; Kim, H.J.; Lee, C.S. Effect of early injection strategy on spray atomization and emission reduction characteristics in bioethanol blended diesel fueled engine. Energy 2012, 39, 387.
    25. Arun, K.S.; Ashok, K.; Murugan, S. Experimental investigation of the effect of compression ratio, injection timing & pressure in a DI (direct injection) diesel engine running on carbon black-water-diesel emulsion. Energy 2015, 93, 520.
    26. Nanthagopal, K.; Ashok, B.; Raj, R.T.K. Influence of fuel injection pressures on Calophyllum inophyllum methylester fuelled direct injection diesel engine. Energy Convers. Manag. 2016, 116, 173.
    27. Benajes, J.; Garcia-Oliver, J.M.; Novella, R.; Kolodziej, C. Increased particle emissions from early fuel injection timing Diesel low temperature combustion. Fuel 2012, 94, 190.
    28. 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, 2880.
    29. Kook, S.; Park, S.; Bae, C. Influence of Early Fuel Injection Timings on Premixing and Combustion in a Diesel Engine. Energy Fuels 2008, 22, 337.
    30. Fang, T.G.; Coverdill, R.E.; Lee, C.F.; White, R.A. Smokeless Combustion within a Small-Bore HSDI Diesel Engine Using a Narrow Angle Injector; SAE International: Warrendale, PA, USA, 2007.
    31. Kim, H.M.; Kitn, Y.J.; Lee, K.H. A Study of the Characteristics of Mixture Formation and Combustion in a PCCI Engine Using an Early Multiple Injection Strategy. Energy Fuels 2008, 22, 1548.
    32. Kim, H.; Ryu, J.; Lee, K. A Study on the Characteristics of Spray and Combustion in a HCCI Engine according to Various Injection Angles and Timings. J. Mech. Sci. Technol. 2007, 21, 140.
    33. Miyamoto, T.; Tsurushima, T.; Shimazaki, N.; Harada, A.; Sasaki, S.; Hayashi, K. Modeling Ignition and Combustion in Direct Injection Compression Ignition Engines Employing Very Early Injection Timing. JSME Int. J. 2002, 45, 872–880.
    34. Yoon, S.H.; Cha, J.P.; Lee, C.S. An investigation of the effects of spray angle and injection strategy on dimethyl ether (DME) combustion and exhaust emission characteristics in a common-rail diesel engine. Fuel Process. Technol. 2010, 91, 1372.
    35. Kim, K.; Kim, D.; Jung, Y.; Bae, C. Spray and combustion characteristics of gasoline and diesel in a direct injection compression ignition engine. Fuel 2013, 109, 626.
    36. Wamankar, A.K.; Murugan, S. Effect of injection timing on a DI diesel engine fuelled with a synthetic fuel blend. J. Energy Inst. 2015, 88, 413.
    37. Vanegas, A.; Won, H.; Peters, N. Influence of the Nozzle Spray Angle on Pollutant Formation and Combustion efficiency for a PCCI Diesel Engine; SAE International: Warrendale, PA, USA, 2009.
    38. Park, S.H.; Yoon, S.H.; Lee, C.S. HC and CO emissions reduction by early injection strategy in a bioethanol blended diesel-fueled engine with a narrow angle injection system. Appl. Energy 2013, 107, 88.
    39. Kiplimo, R.; Tomita, E.; Kawahara, N.; Yokobe, S. Effects of spray impingement, injection parameters, and EGR on the combustion and emission characteristics of a PCCI diesel engine. Appl. Therm. Eng. 2012, 37, 175.
    40. Ryan, T.W.; Matheaus, A.C. Fuel Requirements for HCCI Engine Operation; SAE International: Warrendale, PA, USA, 2003.
    41. Aceves, S.M.; Flowers, D.; Martinez-Frias, J.; Espinosa-Loza, F.; Pitz, W.J.; Dibble, R. Fuel and Additive Characterization for HCCI Combustion; SAE International: Warrendale, PA, USA, 2003.
    42. Kawano, D.; Naito, H.; Suzuki, H.; Ishii, H.; Hori, S.; Goto, Y.; Odaka, M. Effects of Fuel Properties on Combustion and Exhaust Emissions of Homogeneous Charge Compression Ignition (HCCI) Engine; SAE International: Warrendale, PA, USA, 2004.
    43. Agarwal, A.K. Biofuels (alcohols and biodiesel) applications as fuels in internal combustion engines. Prog. Energy Combust. Sci. 2007, 32, 233–271.
    44. Demirbas, A. Biodiesel production from vegetable oils via catalytic and non-catalytic supercritical methanol transesterification methods. Prog. Energy Combust. Sci. 2005, 31, 466–487.
    45. Graboski, M.S.; McCormick, R.L. Combustion of fat and vegetable oil derived fuels in diesel engines. Prog. Energy Combust. Sci. 1998, 24, 125–164.
    46. Komninos,N.P.; Rakopoulos, C.D.ModelingHCCI combustion of biofuels: A review. Renew. Sustain. Energy Rev. 2012, 16, 588–610.
    47. Teng, H.; McCandless, J.C.; Schneyer, J.B. Compression Ignition Delay (Physical + Chemical) of Dimethyl Ether—an Alternative Fuel for Compression—Ignition Engines; SAE International: Warrendale, PA, USA, 2003.
    48. Yao, M.F.; Zheng, Z.Q.; Xu, S.D.; Fu, M.L. Experimental Study on the Combustion Process of Dimethyl Ether (DME); SAE International: Warrendale, PA, USA, 2003.
    49. Sahin, Z.; Durgun, O. Prediction of the effects of ethanol–diesel fuel blends on diesel engine performance characteristics, combustion, exhaust emissions, and cost. Energy Fuel 2009, 23, 1707–1717.
    50. Mohammadi, A.; Kee, S.S.; Ishiyama, T.; Kakuta, T.; Matsumoto, T. Implementation of Ethanol Diesel Blend Fuels in PCCI Combustion; SAE International: Warrendale, PA, USA, 2005.
    51. Park, S.H.; Cha, J.; Lee, C.S. Effects of bioethanol-blended diesel fuel on combustion and emission reduction characteristics in a direct-injection diesel engine with exhaust gas recirculation (EGR). Energy Fuels 2010, 24, 3872–3883.
    52. Chen, Z.; Liu, J.P.; Han, Z.Y.; Du, B.; Liu, Y.; Lee, C. Study on performance and emissions of a passenger-car diesel engine fuelled with butanol–diesel blends. Energy 2013, 55, 638–646.
    53. Bessonette, P.W.; Schleyer, C.H.; Duffy, K.P.; Hardy, W.L.; Liechty, M.P. Effects of Fuel Property Changes on Heavy-Duty HCCI Combustion; SAE International: Warrendale, PA, USA, 2007.
    54. Inagaki, K.; Fuyuto, T.; Nishikawa, K.; Nakatita, K.; Sakata, I. Dual-Fuel PCI Combustion Controlled by in-Cylinder Stratification of Ignitability; SAE International: Warrendale, PA, USA, 2006.
    55. Yao, M.F.; Zhang, B.; Zheng, Z.Q.; Chen, Z. Experimental study on homogeneous charge compression ignition combustion with primary reference fuel. Combust. Sci. Technol. 2007, 179, 2539–2559.
    56. Liu, H.F.; Yao, M.F.; Zhang, B.; Zheng, Z.Q. Influence of fuel and operating conditions on combustion characteristics of a homogeneous charge compression ignition engine. Energy Fuels 2009, 23, 1422–1430.
    57. Yang, B.B.; Li, S.J.; Zheng, Z.Q.; Yao, M.F.; Cheng,W. A Comparative Study on Different Dual-Fuel Combustion Modes Fuelled with Gasoline and Diesel; SAE International: Warrendale, PA, USA, 2012.
    58. Leermakers, C.A.J.; Van den Berge, B.; Luijten, C.; Somers, L.M.T.; de Goey, L.P.H.; Albrecht, B.A. Gasoline–Diesel Dual Fuel: Effect of Injection Timing and Fuel Balance; SAE International: Warrendale, PA, USA, 2011.
    More