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An experimental study on quenching crevice widths in the combustion chamber of a spark-ignition engine
Twenty-Sixth Symposium (International) on Combustion/The Combustion Institute, 1996/pp. 2605–2611
AN EXPERIMENTAL STUDY ON QUENCHING CREVICE WIDTHS IN THE COMBUSTION CHAMBER OF A SPARK-IGNITION ENGINE SHIZUO ISHIZAWA Department of Mechanical Engineering Gunma College of Technology Maebashi, Japan
Hydrocarbon (HC) emissions from spark-ignition engines are now a major environmental problem in many cities of the world. It is difficult to reduce HC emissions during engine warm-up because the catalysts do not work well at low temperatures. The sources of unburned HCs from spark-ignition engines seem to be crevices in the combustion chamber, oil layer, deposits, and quench layer on the cylinder wall surfaces. Single-surface and two-surface flame quenching (crevice) play a large role in generating unburned HCs. Two-surface flame quenching distances (quenching crevice width) in the combustion chamber of a sparkignition engine were investigated using an ion probe capable of detecting flame arrival at a narrow width. The crevice width could be controlled precisely. Because engine combustion has cycle-by-cycle fluctuations, quenching crevice widths were estimated by the statistical analysis. It was defined as the width when the ion detector could detect flame arrival in 50 of 100 cycles. The effects of the mixture equivalence ratio, exhaust gas recycle (EGR) rate, ignition timing, charging efficiency, and combustion chamber wall temperature on the quenching crevice width were investigated. HC emissions in the exhaust port, cycle-bycycle combustion fluctuation, and temperature of a quenching plate of the ion probe in the combustion chamber were also estimated in the experiments. The quenching crevice width was relatively uniform at the surface of the combustion chamber except in the area close to the ignition spark plug and end gas. The quenching crevice width increased with leaner mixture ratio, larger EGR rate, lower charging efficiency, greater ignition timing, and lower wall temperature.
Introduction Since unburned hydrocarbons (HCs) exhausted from spark-ignition engines are recognized as one of the main causes of photochemical air pollution, a great deal of experimental and theoretical research has been done. The quenching distance in the combustion chamber of a spark-ignition engine was first estimated in photographic studies done by Daniel [1]. Subsequently, HCs from small crevices in the combustion chamber, especially the piston top land crevice, have been investigated extensively [2–5]. Later, it was found in constant-volume combustion chamber experiments that the lubricant layer on the combustion chamber surface contributed significantly to the amount of unburned HCs [6–7]. Several studies examined the interaction between the mixture and the oil layer in the combustion chamber and the cylinder of the engine [8–10]. Recently, the main sources of HCs are thought to be quenching crevices and the oil layer on the combustion chamber walls under normal steady-state operating conditions [11]. However, single-surface flame quenching effects on unburned HCs will be larger with an increase of EGR gas or a decrease of equivalence ratio. Quenching distances have been measured with
various experimental methods. Recently, the effect of wall temperature on single-surface quenching distance was investigated intensively [12]. In most cases, however, the quenching distances have been estimated in the experiments involving a burner flame, a constant-volume combustion chamber, and a CFR engine [13–14]. In this study, two-surface flame quenching distance (the quenching crevice width) in the cylinder of a spark-ignition engine that had a conventional hemispherical combustion chamber was measured with an ion current detector that was fitted to the top of a specially designed probe. It was assumed that the singe-surface quenching distance is proportional to that of two-surface distance. The effect of engine operating parameters on the quenching crevice and the relationship between the quenching crevice width and HCs were investigated. An experimental equation for quenching crevice width was formulated as a function of the maximum cylinder pressure and wall temperature. Experimental Method and Apparatus Experimental Engine and Operating Conditions A single-cylinder four-stroke spark-ignition engine was used in the experiments. The engine specifications are given in Table 1. The combustion
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SPARK IGNITION ENGINES
Measurement of Quenching Crevice Width, HC Concentration, and Cylinder Pressure
TABLE 1 Engine specifications No. of cylinders Valve train Bore and stroke Combustion chamber configuration Compression ratio Valve timing
Intake valve Exhaust valve
1 OHV 85 2 78 mm Hemispherical 8.5 128 BTDC—488 ATDC 608 BBDC—108 ATDC
Fig. 1. Schematic of ion detector.
chamber had a hemispherical shape and fuel was supplied to the engine with a magnetic-electronic– controlled fuel injector installed in the intake pipe. Since emissions from vehicles are a major source of urban air pollution, the engine was operated under conditions equivalent to the load of city driving, that is, a vehicle speed of 40 km/h (approximately 1400 rpm) and engine load of 0.31 MPa indicated mean effective pressure. To study the influence of the mixture equivalence ratio, EGR rate, ignition timing, charging efficiency, and combustion chamber wall temperature on the quenching crevice width, these engine operating parameters were varied in the experiments. Gasoline obtainable in the Japanese domestic market was used as the fuel.
A schematic of flame ion detector and quenching crevice width measurement technique is shown in Fig. 1. It has a narrow crevice between the quenching plate and ion probe tip of the ion detector. In the preliminary experiment using a Bunsen burner, the function of the ion probe to detect ion current was verified. The crevice width was controlled precisely by a spacer made of aluminum foil. Measurements were made at increments of 0.1 mm. The electrodes were supplied with a voltage of 50 V to detect the ion current. If the flame was extinguished before reaching the electrodes, no ion current was detected. The surface temperature of the quenching plate was measured with a thermocouple (constantan). Although the temperature of the other plate projecting into the combustion chamber was not measured, both wall temperatures were assumed to be equivalent. Quenching crevice width was defined as the width at which the ion detector could detect flame arrival in 50 of 100 cycles (50%). Quenching crevice width depends on the definition of flame quenching criteria. If the criteria of 50% flame arrival is made larger, the quenching crevice width will also increase. However, the dependence on the engine operating parameters of the measured quenching widths was expected to be equivalent. The ion current was observed with an oscilloscope and photographed with a camera. Piezoelectric pressure sensor and a charge amplifier were used to measure the cylinder pressure. Cycle-by-cycle combustion fluctuations were estimated by analyzing the cylinder pressure data of 400 continuous cycles. Since the exhaust gas was sampled at the exit of the exhaust port 20 mm from the exhaust valve, there was little influence of oxidation of HC in the exhaust pipe on HC-emission measurement. The ion detector was replaced with a blind plug when the exhaust gas was sampled, so that the effect of the detector’s quenching crevice on the exhaust HC concentration was removed. The HC-emission concentration was analyzed by a flame ionization detector (FID). Results and Discussion Output of Ion Detector An example of the ion current detected by the ion probe and the cylinder pressure is shown in Fig. 2. Maximum ion current was detected just after the maximum cylinder pressure occurred. It is expected that the flame reaches the ion probe at the end of the combustion process. Ion current outputs for more than 70 continuous cycles are shown in Figs. 3 and 4. In Fig. 3, because the mixture ratio supplied to the cylinder was stoichiometric, the flame was
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Fig. 3. Ion current. Engine conditions: 1400 rpm, f 4 1.0, MBT ignition timing, IMEP 4 0.31 MPa central ignition and ion detector location of D.
Fig. 2. Cylinder pressure and ion current. Engine conditions: 1400 rpm, f 4 1.0, MBT ignition timing, IMEP 4 0.31 MPa central ignition and ion detector location of D.
able to enter the crevice and an ion current was detected by the ion probe in every cycle. The flame reached the electrodes in the crevice of the detector, but each cycle had a different ion current and showed cycle-by-cycle fluctuation. The results in Fig. 4 indicate that cycle-by-cycle fluctuations became larger and some cycles lacked an ion current when a leaner mixture was supplied to the cylinder. Because engine combustion has cycle-by-cycle fluctuations, quenching crevice widths should be estimated by statistical analysis. Effect of Location of Spark Plug and Ion Detector on Quenching Crevice Width The dependence of the quenching crevice width (Wc) on the location of the spark plug and the ion detector in the cylinder head is shown in Fig. 5. Two locations were examined. In one case, the spark plug was located in the center of the combustion chamber (central ignition) and, in the other case, in the neighborhood of the cylinder bore far from the center of the cylinder (side ignition). For central ignition, Wc was measured at four different locations. Widths of about 0.4 mm were measured at three points, but that at the point near the spark plug was 0.28 mm. Because of the flame propagation in the combustion chamber, the flame arrival time depends on the location of the spark plug and the ion detector. The flame reaches the ion probe at the end of the combustion process. The cylinder pressure reaches a maximum and begins to decrease before the end of
Fig. 4. Ion current. Engine conditions: 1400 rpm, f 4 0.87, MBT ignition timing, IMEP 4 0.31 MPa central ignition and ion detector location of D.
combustion. Since the unburned gas temperature in the crevice of the ion detector near the spark plug was higher than those of other locations, the flame could enter the crevice more easily. The lower portion of Fig. 5 shows the Wc results for side ignition. Two points showed about the same Wc results as the values obtained with central ignition, but Wc at the point far from the spark plug was about 1.45 mm. This width was much larger than the other measurements and may have been caused by delayed flame arrival and lower unburned gas and flame temperatures. The measured widths were relatively uniform at the surface of the combustion chamber, excluding the points near the spark plug and far from it. The spark plug was located in the center of the combustion chamber and the ion probe at point D to obtain average quenching crevice width characteristics in the combustion chamber in the following experiment.
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Fig. 7. Effect of EGR rate on quenching crevice width. Engine conditions: 1400 rpm, f 4 1.0, MBT ignition timing, IMEP 4 0.31 MPa.
Characteristics of Quenching Crevice Width
Fig. 5. Effect of location of spark plug and ion detector on quenching crevice width (a) central ignition, (b) side ignition. Engine conditions: 1400 rpm, f 4 1.0, MBT ignition timing, IMEP 4 0.31 MPa.
Fig. 6. Effect of equivalence ratio on quenching crevice width. Engine conditions: 1400 rpm, MBT ignition timing, IMEP 4 0.31 MPa.
The dependence of Wc on the mixture equivalence ratio is shown in Fig. 6 for maximum brake torque (MBT) ignition timing along with the HC concentration to examine the relation between the flame quenching distance and exhaust HCs. The coefficient of variations in indicated mean effective pressure (COVimep) was also shown in Fig. 6 to indicate the cycle-by-cycle combustion fluctuations. The smallest Wc was obtained at an equivalence ratio of 1.13. When the engine was operated on a leaner mixture, Wc increased; when the mixture equivalence ratio was below about 0.8, Wc began to increase sharply. Cycle-by-cycle combustion fluctuation and the number of cycles in which slow burning occurred increased. In this case, there may have been some cycles in which the flame was quenched in the crevice before it reached the electrodes and other cycles in which the flame was extinguished before it reached the detector. It is difficult to distinguish the cycles in which flame quenching occurred in the crevice of the detector from those in which slow and partial burning occurred because of the quenching crevice measuring technique. Wc was affected by slow and partial burn when the engine was operated on a lean mixture. In the case of rich mixture operation, HC emissions increased because of incomplete combustion caused by lack of oxygen. The HC emission concentration was lowest when the engine was operated on a mixture equivalence ratio between 0.8 and 0.9. When a leaner mixture was supplied, HC emissions also increased. In these conditions, the quenching layer is expected to contribute significantly to HC emission. Figure 7 shows the characteristics of Wc, HCemission concentration, and COVimep under engine operation with EGR. The quenching crevice width became larger with an increasing EGR because of a
QUENCHING CREVICE WIDTHS IN SPARK-IGNITION ENGINE
Fig. 8. Effect of ignition timing on quenching crevice width. Engine conditions: 1400 rpm, f 4 1.0, IMEP 4 0.31 MPa.
Fig. 9. Effect of charging efficiency on quenching crevice width. Engine conditions: 1400 rpm, MBT ignition timing.
lower flame temperature. As the EGR rate became larger, cycle-by-cycle fluctuations increased and the number of cycles in which the flame propagated slowly and combustion duration was lengthened also increased. In these cycles, because of lower flame and unburned gas temperatures, the flame was extinguished in the crevices or before entering them. In the partial burning cycles, there was not enough time to complete combustion before the exhaust valve opened and the flame could not reach the detector [15]. The HC concentration and Wc showed good correlation as EGR increased. Since HCs released from sources in the combustion chamber except from the quenching layers have little dependence on EGR rate, the quenching layers are
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expected to contribute significantly to total HC emissions. At EGR levels greater than about 30%, Wc was affected by the slow and partial burning, as well as those in the lean mixture experiments. Figure 8 shows the Wc results, HC-emission concentration, and COVimep when the ignition timing was chosen as the operating parameter. Wc was reduced as the ignition timing was advanced. When the ignition timing was retarded from MBT ignition timing, Wc and COVimep increased, whereas the HC-emission concentration decreased. Compared with MBT ignition timing operation, the combustion duration was longer and the gas temperature in the cylinder higher during the late expansion and early exhaust strokes. Since Wc and the HC concentration show a poor correlation, it is believed that mixing and the gas temperature play an important role in the oxidation of HCs released from the quenching layer after flame propagation in the cylinder during the expansion and exhaust strokes [16–18]. Wc measured at TDC ignition timing would be affected by the slow burning because of the increase of cycleby-cycle fluctuations. Figure 9 shows the Wc results, HC-emission concentration, and COVimep as a function of charging efficiency. As the charging efficiency increased, Wc decreased while the HC-emission concentration increased. The decrease in Wc is due to the increased cylinder pressure when the flame reached the quenching crevice. HC emissions arising from surface quenching would be expected to decrease similarly, implying this mechanism is not dominant under these conditions. HC emissions from the piston top crevice would be expected to be independent of the charging efficiency, since the fraction of the fuel that is trapped is unchanged. The cause of the observed increase in HC-emission concentration with charging efficiency remains unknown. To investigate the dependence of the quenching width on the quenching wall temperature, Wc was estimated at different coolant temperatures. Experiments were performed with two air-fuel mixtures, one stoichiometric and the other a lean mixture. The Wc results and HC-emission concentration are shown in Fig. 10. It is seen that Wc decreased with a rising wall temperature. Since the flammability limit was extended because of reduced heat loss from the flame to the combustion chamber walls, the flame was able to enter the crevices more easily. The HC-emission concentration also showed dependence on the wall temperature and decreased as the wall temperature rose. It was reported recently that the single-surface quenching distance varied approximately linearly with low wall temperature in the steady flame experiments [12]. Dependence of Quenching Crevice Width on Cylinder Pressure and Temperature Friedman and Johnston [13] proposed an equation describing the dependence of quenching distance on pressure and temperature of the form:
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Conclusion
Fig. 10. Effect of cylinder wall temperature on quenching crevice width. Engine conditions: 1400 rpm, MBT ignition timing.
Two-surface flame quenching distances (the quenching crevice width) under various engine operating conditions were investigated with a specially equipped ion probe that could control a width precisely and detect the flame arrival at a narrow width. The quenching crevice width was relatively uniform at the surface of the combustion chamber and did not depend sensitively on the location in the combustion chamber, excluding the area close to the spark plug and the end gas area. It was narrower near the spark plug than in other locations and widest in the end gas region. The quenching crevice width increased with a leaner mixture ratio, larger EGR rate, lower charging efficiency, greater ignition timing retard, and lower wall temperature. When the mixture equivalence ratio was below about 0.8 and EGR rate exceeded about 30%, it began to increase sharply. This may be due to the increase of cycle-by-cycle fluctuation in addition to a lower flame temperature. As the unburned mixture becomes lean or diluted with EGR, the flame quenching layers contribute significantly to total HC emissions. An experimental equation for the quenching crevice width that was effective for a near-stoichiometric mixture, without EGR and identical engine speed, was deduced as a function of the quenching plate temperature and maximum cylinder pressure. Acknowledgments The author wishes to thank Mr. M. Hashimoto and Mr. Y. Hashizume in the Nissan Research Center of Nissan Motor Co. for their invaluable assistance in conducting the experiments.
Fig. 11. Experimental dependence of quenching width on cylinder pressure and cylinder wall temperature. Engine conditions: 1400 rpm, f 4 1.0.
Wc 4 K1 Pmax Tw 10.9
10.5
(1)
where K1 is a constant, Pmax is the absolute maximum cylinder pressure (MPa), and Tw is the quenching wall temperature (K). The parameters in the equation will depend on stoichiometry and dilution; thus, for proper comparison with literature results, we present data in this form only without EGR and at a single engine speed of 1400 rpm. Figure 11 shows a good correlation with K1 4 14.8. Friedman and Johnston [13] found K1 4 4.2 for their measurements of critical flashback slit width with propane as the fuel. Goolsby and Haskell obtained K1 4 7.0 from measurements of quenching crevice widths in a CFR engine with iso-octane fuel [14]. The differences are probably due mainly to differences in the fuel, although other design and operating parameters could play a part.
REFERENCES 1. Daniel, W. A., Sixth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1957, pp. 886–894. 2. Wentworth, J. T., SAE Transactions 77:403–416 (1968). 3. Wentworth, J. T., Combust. Sci. Technol. 4:97–100 (1971). 4. Adamczyk, A. A., Kaiser, E. W., and Lavoie, G. A., Combust. Sci. Technol. 33:261–277 (1983). 5. Namazian, M. and Heywood, J. B., SAE Paper No. 820088, 1982. 6. Kaiser, E. W., Adamczyk, A. A., and Lavoie, G. A., Eighteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, pp. 1881–1890. 7. Adamczyk, A. A., Rothschild, W. G., and Kaiser, E. W., Combust. Sci. Technol. 44:113–124 (1985). 8. LoRusso, J. A., Kaiser, E. W., and Lavoie, G. A., Combust. Sci. Technol. 25:121–125 (1981).
QUENCHING CREVICE WIDTHS IN SPARK-IGNITION ENGINE 9. Dent, J. C. and Lakshminarayanan, P. A., SAE Paper No. 83-0652, 1983. 10. Ishizawa, S. and Takagi, Y., JSME Int. J. 30:310–317 (1987). 11. Heywood, J. B., Internal Combustion Engine Fundamentals, McGraw Hill, New York, 1988, pp. 596–619. 12. Cleary, D. J. and Farrel, P. V., SAE Paper No. 94-0683, 1994. 13. Friedman, R. and Johnston, W. C., J. Appl. Phys. 22:791–797 (1950).
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14. Gooisby, A. D. and Haskell, W. W., Combust. Flame 26:105–114 (1976). 15. Kuroda, H., Nakajima, Y., Sugihara, K., Takagi, Y., and Muranaka, S., SAE Paper No. 78-0006, 1978. 16. Drobot, F. H., Cheng, W. K., Trinker, F. H., Kaiser, E. W., Siegl, W. O., Cotton, D. F., and Underwood, J., Combust. Flame 99:422–430 (1994). 17. Min, K., Cheng, W. K., and Heywood, J. B., SAE Paper No. 94-0306, 1994. 18. Hellstrom, T. and Chomiak, J., SAE Paper No. 952539, 1995.
AN EXPERIMENTAL STUDY ON QUENCHING CREVICE WIDTHS IN THE COMBUSTION CHAMBER OF A SPARK-IGNITION ENGINE SHIZUO ISHIZAWA Department of Mechanical Engineering Gunma College of Technology Maebashi, Japan
Hydrocarbon (HC) emissions from spark-ignition engines are now a major environmental problem in many cities of the world. It is difficult to reduce HC emissions during engine warm-up because the catalysts do not work well at low temperatures. The sources of unburned HCs from spark-ignition engines seem to be crevices in the combustion chamber, oil layer, deposits, and quench layer on the cylinder wall surfaces. Single-surface and two-surface flame quenching (crevice) play a large role in generating unburned HCs. Two-surface flame quenching distances (quenching crevice width) in the combustion chamber of a sparkignition engine were investigated using an ion probe capable of detecting flame arrival at a narrow width. The crevice width could be controlled precisely. Because engine combustion has cycle-by-cycle fluctuations, quenching crevice widths were estimated by the statistical analysis. It was defined as the width when the ion detector could detect flame arrival in 50 of 100 cycles. The effects of the mixture equivalence ratio, exhaust gas recycle (EGR) rate, ignition timing, charging efficiency, and combustion chamber wall temperature on the quenching crevice width were investigated. HC emissions in the exhaust port, cycle-bycycle combustion fluctuation, and temperature of a quenching plate of the ion probe in the combustion chamber were also estimated in the experiments. The quenching crevice width was relatively uniform at the surface of the combustion chamber except in the area close to the ignition spark plug and end gas. The quenching crevice width increased with leaner mixture ratio, larger EGR rate, lower charging efficiency, greater ignition timing, and lower wall temperature.
Introduction Since unburned hydrocarbons (HCs) exhausted from spark-ignition engines are recognized as one of the main causes of photochemical air pollution, a great deal of experimental and theoretical research has been done. The quenching distance in the combustion chamber of a spark-ignition engine was first estimated in photographic studies done by Daniel [1]. Subsequently, HCs from small crevices in the combustion chamber, especially the piston top land crevice, have been investigated extensively [2–5]. Later, it was found in constant-volume combustion chamber experiments that the lubricant layer on the combustion chamber surface contributed significantly to the amount of unburned HCs [6–7]. Several studies examined the interaction between the mixture and the oil layer in the combustion chamber and the cylinder of the engine [8–10]. Recently, the main sources of HCs are thought to be quenching crevices and the oil layer on the combustion chamber walls under normal steady-state operating conditions [11]. However, single-surface flame quenching effects on unburned HCs will be larger with an increase of EGR gas or a decrease of equivalence ratio. Quenching distances have been measured with
various experimental methods. Recently, the effect of wall temperature on single-surface quenching distance was investigated intensively [12]. In most cases, however, the quenching distances have been estimated in the experiments involving a burner flame, a constant-volume combustion chamber, and a CFR engine [13–14]. In this study, two-surface flame quenching distance (the quenching crevice width) in the cylinder of a spark-ignition engine that had a conventional hemispherical combustion chamber was measured with an ion current detector that was fitted to the top of a specially designed probe. It was assumed that the singe-surface quenching distance is proportional to that of two-surface distance. The effect of engine operating parameters on the quenching crevice and the relationship between the quenching crevice width and HCs were investigated. An experimental equation for quenching crevice width was formulated as a function of the maximum cylinder pressure and wall temperature. Experimental Method and Apparatus Experimental Engine and Operating Conditions A single-cylinder four-stroke spark-ignition engine was used in the experiments. The engine specifications are given in Table 1. The combustion
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SPARK IGNITION ENGINES
Measurement of Quenching Crevice Width, HC Concentration, and Cylinder Pressure
TABLE 1 Engine specifications No. of cylinders Valve train Bore and stroke Combustion chamber configuration Compression ratio Valve timing
Intake valve Exhaust valve
1 OHV 85 2 78 mm Hemispherical 8.5 128 BTDC—488 ATDC 608 BBDC—108 ATDC
Fig. 1. Schematic of ion detector.
chamber had a hemispherical shape and fuel was supplied to the engine with a magnetic-electronic– controlled fuel injector installed in the intake pipe. Since emissions from vehicles are a major source of urban air pollution, the engine was operated under conditions equivalent to the load of city driving, that is, a vehicle speed of 40 km/h (approximately 1400 rpm) and engine load of 0.31 MPa indicated mean effective pressure. To study the influence of the mixture equivalence ratio, EGR rate, ignition timing, charging efficiency, and combustion chamber wall temperature on the quenching crevice width, these engine operating parameters were varied in the experiments. Gasoline obtainable in the Japanese domestic market was used as the fuel.
A schematic of flame ion detector and quenching crevice width measurement technique is shown in Fig. 1. It has a narrow crevice between the quenching plate and ion probe tip of the ion detector. In the preliminary experiment using a Bunsen burner, the function of the ion probe to detect ion current was verified. The crevice width was controlled precisely by a spacer made of aluminum foil. Measurements were made at increments of 0.1 mm. The electrodes were supplied with a voltage of 50 V to detect the ion current. If the flame was extinguished before reaching the electrodes, no ion current was detected. The surface temperature of the quenching plate was measured with a thermocouple (constantan). Although the temperature of the other plate projecting into the combustion chamber was not measured, both wall temperatures were assumed to be equivalent. Quenching crevice width was defined as the width at which the ion detector could detect flame arrival in 50 of 100 cycles (50%). Quenching crevice width depends on the definition of flame quenching criteria. If the criteria of 50% flame arrival is made larger, the quenching crevice width will also increase. However, the dependence on the engine operating parameters of the measured quenching widths was expected to be equivalent. The ion current was observed with an oscilloscope and photographed with a camera. Piezoelectric pressure sensor and a charge amplifier were used to measure the cylinder pressure. Cycle-by-cycle combustion fluctuations were estimated by analyzing the cylinder pressure data of 400 continuous cycles. Since the exhaust gas was sampled at the exit of the exhaust port 20 mm from the exhaust valve, there was little influence of oxidation of HC in the exhaust pipe on HC-emission measurement. The ion detector was replaced with a blind plug when the exhaust gas was sampled, so that the effect of the detector’s quenching crevice on the exhaust HC concentration was removed. The HC-emission concentration was analyzed by a flame ionization detector (FID). Results and Discussion Output of Ion Detector An example of the ion current detected by the ion probe and the cylinder pressure is shown in Fig. 2. Maximum ion current was detected just after the maximum cylinder pressure occurred. It is expected that the flame reaches the ion probe at the end of the combustion process. Ion current outputs for more than 70 continuous cycles are shown in Figs. 3 and 4. In Fig. 3, because the mixture ratio supplied to the cylinder was stoichiometric, the flame was
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Fig. 3. Ion current. Engine conditions: 1400 rpm, f 4 1.0, MBT ignition timing, IMEP 4 0.31 MPa central ignition and ion detector location of D.
Fig. 2. Cylinder pressure and ion current. Engine conditions: 1400 rpm, f 4 1.0, MBT ignition timing, IMEP 4 0.31 MPa central ignition and ion detector location of D.
able to enter the crevice and an ion current was detected by the ion probe in every cycle. The flame reached the electrodes in the crevice of the detector, but each cycle had a different ion current and showed cycle-by-cycle fluctuation. The results in Fig. 4 indicate that cycle-by-cycle fluctuations became larger and some cycles lacked an ion current when a leaner mixture was supplied to the cylinder. Because engine combustion has cycle-by-cycle fluctuations, quenching crevice widths should be estimated by statistical analysis. Effect of Location of Spark Plug and Ion Detector on Quenching Crevice Width The dependence of the quenching crevice width (Wc) on the location of the spark plug and the ion detector in the cylinder head is shown in Fig. 5. Two locations were examined. In one case, the spark plug was located in the center of the combustion chamber (central ignition) and, in the other case, in the neighborhood of the cylinder bore far from the center of the cylinder (side ignition). For central ignition, Wc was measured at four different locations. Widths of about 0.4 mm were measured at three points, but that at the point near the spark plug was 0.28 mm. Because of the flame propagation in the combustion chamber, the flame arrival time depends on the location of the spark plug and the ion detector. The flame reaches the ion probe at the end of the combustion process. The cylinder pressure reaches a maximum and begins to decrease before the end of
Fig. 4. Ion current. Engine conditions: 1400 rpm, f 4 0.87, MBT ignition timing, IMEP 4 0.31 MPa central ignition and ion detector location of D.
combustion. Since the unburned gas temperature in the crevice of the ion detector near the spark plug was higher than those of other locations, the flame could enter the crevice more easily. The lower portion of Fig. 5 shows the Wc results for side ignition. Two points showed about the same Wc results as the values obtained with central ignition, but Wc at the point far from the spark plug was about 1.45 mm. This width was much larger than the other measurements and may have been caused by delayed flame arrival and lower unburned gas and flame temperatures. The measured widths were relatively uniform at the surface of the combustion chamber, excluding the points near the spark plug and far from it. The spark plug was located in the center of the combustion chamber and the ion probe at point D to obtain average quenching crevice width characteristics in the combustion chamber in the following experiment.
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Fig. 7. Effect of EGR rate on quenching crevice width. Engine conditions: 1400 rpm, f 4 1.0, MBT ignition timing, IMEP 4 0.31 MPa.
Characteristics of Quenching Crevice Width
Fig. 5. Effect of location of spark plug and ion detector on quenching crevice width (a) central ignition, (b) side ignition. Engine conditions: 1400 rpm, f 4 1.0, MBT ignition timing, IMEP 4 0.31 MPa.
Fig. 6. Effect of equivalence ratio on quenching crevice width. Engine conditions: 1400 rpm, MBT ignition timing, IMEP 4 0.31 MPa.
The dependence of Wc on the mixture equivalence ratio is shown in Fig. 6 for maximum brake torque (MBT) ignition timing along with the HC concentration to examine the relation between the flame quenching distance and exhaust HCs. The coefficient of variations in indicated mean effective pressure (COVimep) was also shown in Fig. 6 to indicate the cycle-by-cycle combustion fluctuations. The smallest Wc was obtained at an equivalence ratio of 1.13. When the engine was operated on a leaner mixture, Wc increased; when the mixture equivalence ratio was below about 0.8, Wc began to increase sharply. Cycle-by-cycle combustion fluctuation and the number of cycles in which slow burning occurred increased. In this case, there may have been some cycles in which the flame was quenched in the crevice before it reached the electrodes and other cycles in which the flame was extinguished before it reached the detector. It is difficult to distinguish the cycles in which flame quenching occurred in the crevice of the detector from those in which slow and partial burning occurred because of the quenching crevice measuring technique. Wc was affected by slow and partial burn when the engine was operated on a lean mixture. In the case of rich mixture operation, HC emissions increased because of incomplete combustion caused by lack of oxygen. The HC emission concentration was lowest when the engine was operated on a mixture equivalence ratio between 0.8 and 0.9. When a leaner mixture was supplied, HC emissions also increased. In these conditions, the quenching layer is expected to contribute significantly to HC emission. Figure 7 shows the characteristics of Wc, HCemission concentration, and COVimep under engine operation with EGR. The quenching crevice width became larger with an increasing EGR because of a
QUENCHING CREVICE WIDTHS IN SPARK-IGNITION ENGINE
Fig. 8. Effect of ignition timing on quenching crevice width. Engine conditions: 1400 rpm, f 4 1.0, IMEP 4 0.31 MPa.
Fig. 9. Effect of charging efficiency on quenching crevice width. Engine conditions: 1400 rpm, MBT ignition timing.
lower flame temperature. As the EGR rate became larger, cycle-by-cycle fluctuations increased and the number of cycles in which the flame propagated slowly and combustion duration was lengthened also increased. In these cycles, because of lower flame and unburned gas temperatures, the flame was extinguished in the crevices or before entering them. In the partial burning cycles, there was not enough time to complete combustion before the exhaust valve opened and the flame could not reach the detector [15]. The HC concentration and Wc showed good correlation as EGR increased. Since HCs released from sources in the combustion chamber except from the quenching layers have little dependence on EGR rate, the quenching layers are
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expected to contribute significantly to total HC emissions. At EGR levels greater than about 30%, Wc was affected by the slow and partial burning, as well as those in the lean mixture experiments. Figure 8 shows the Wc results, HC-emission concentration, and COVimep when the ignition timing was chosen as the operating parameter. Wc was reduced as the ignition timing was advanced. When the ignition timing was retarded from MBT ignition timing, Wc and COVimep increased, whereas the HC-emission concentration decreased. Compared with MBT ignition timing operation, the combustion duration was longer and the gas temperature in the cylinder higher during the late expansion and early exhaust strokes. Since Wc and the HC concentration show a poor correlation, it is believed that mixing and the gas temperature play an important role in the oxidation of HCs released from the quenching layer after flame propagation in the cylinder during the expansion and exhaust strokes [16–18]. Wc measured at TDC ignition timing would be affected by the slow burning because of the increase of cycleby-cycle fluctuations. Figure 9 shows the Wc results, HC-emission concentration, and COVimep as a function of charging efficiency. As the charging efficiency increased, Wc decreased while the HC-emission concentration increased. The decrease in Wc is due to the increased cylinder pressure when the flame reached the quenching crevice. HC emissions arising from surface quenching would be expected to decrease similarly, implying this mechanism is not dominant under these conditions. HC emissions from the piston top crevice would be expected to be independent of the charging efficiency, since the fraction of the fuel that is trapped is unchanged. The cause of the observed increase in HC-emission concentration with charging efficiency remains unknown. To investigate the dependence of the quenching width on the quenching wall temperature, Wc was estimated at different coolant temperatures. Experiments were performed with two air-fuel mixtures, one stoichiometric and the other a lean mixture. The Wc results and HC-emission concentration are shown in Fig. 10. It is seen that Wc decreased with a rising wall temperature. Since the flammability limit was extended because of reduced heat loss from the flame to the combustion chamber walls, the flame was able to enter the crevices more easily. The HC-emission concentration also showed dependence on the wall temperature and decreased as the wall temperature rose. It was reported recently that the single-surface quenching distance varied approximately linearly with low wall temperature in the steady flame experiments [12]. Dependence of Quenching Crevice Width on Cylinder Pressure and Temperature Friedman and Johnston [13] proposed an equation describing the dependence of quenching distance on pressure and temperature of the form:
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SPARK IGNITION ENGINES
Conclusion
Fig. 10. Effect of cylinder wall temperature on quenching crevice width. Engine conditions: 1400 rpm, MBT ignition timing.
Two-surface flame quenching distances (the quenching crevice width) under various engine operating conditions were investigated with a specially equipped ion probe that could control a width precisely and detect the flame arrival at a narrow width. The quenching crevice width was relatively uniform at the surface of the combustion chamber and did not depend sensitively on the location in the combustion chamber, excluding the area close to the spark plug and the end gas area. It was narrower near the spark plug than in other locations and widest in the end gas region. The quenching crevice width increased with a leaner mixture ratio, larger EGR rate, lower charging efficiency, greater ignition timing retard, and lower wall temperature. When the mixture equivalence ratio was below about 0.8 and EGR rate exceeded about 30%, it began to increase sharply. This may be due to the increase of cycle-by-cycle fluctuation in addition to a lower flame temperature. As the unburned mixture becomes lean or diluted with EGR, the flame quenching layers contribute significantly to total HC emissions. An experimental equation for the quenching crevice width that was effective for a near-stoichiometric mixture, without EGR and identical engine speed, was deduced as a function of the quenching plate temperature and maximum cylinder pressure. Acknowledgments The author wishes to thank Mr. M. Hashimoto and Mr. Y. Hashizume in the Nissan Research Center of Nissan Motor Co. for their invaluable assistance in conducting the experiments.
Fig. 11. Experimental dependence of quenching width on cylinder pressure and cylinder wall temperature. Engine conditions: 1400 rpm, f 4 1.0.
Wc 4 K1 Pmax Tw 10.9
10.5
(1)
where K1 is a constant, Pmax is the absolute maximum cylinder pressure (MPa), and Tw is the quenching wall temperature (K). The parameters in the equation will depend on stoichiometry and dilution; thus, for proper comparison with literature results, we present data in this form only without EGR and at a single engine speed of 1400 rpm. Figure 11 shows a good correlation with K1 4 14.8. Friedman and Johnston [13] found K1 4 4.2 for their measurements of critical flashback slit width with propane as the fuel. Goolsby and Haskell obtained K1 4 7.0 from measurements of quenching crevice widths in a CFR engine with iso-octane fuel [14]. The differences are probably due mainly to differences in the fuel, although other design and operating parameters could play a part.
REFERENCES 1. Daniel, W. A., Sixth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1957, pp. 886–894. 2. Wentworth, J. T., SAE Transactions 77:403–416 (1968). 3. Wentworth, J. T., Combust. Sci. Technol. 4:97–100 (1971). 4. Adamczyk, A. A., Kaiser, E. W., and Lavoie, G. A., Combust. Sci. Technol. 33:261–277 (1983). 5. Namazian, M. and Heywood, J. B., SAE Paper No. 820088, 1982. 6. Kaiser, E. W., Adamczyk, A. A., and Lavoie, G. A., Eighteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, pp. 1881–1890. 7. Adamczyk, A. A., Rothschild, W. G., and Kaiser, E. W., Combust. Sci. Technol. 44:113–124 (1985). 8. LoRusso, J. A., Kaiser, E. W., and Lavoie, G. A., Combust. Sci. Technol. 25:121–125 (1981).
QUENCHING CREVICE WIDTHS IN SPARK-IGNITION ENGINE 9. Dent, J. C. and Lakshminarayanan, P. A., SAE Paper No. 83-0652, 1983. 10. Ishizawa, S. and Takagi, Y., JSME Int. J. 30:310–317 (1987). 11. Heywood, J. B., Internal Combustion Engine Fundamentals, McGraw Hill, New York, 1988, pp. 596–619. 12. Cleary, D. J. and Farrel, P. V., SAE Paper No. 94-0683, 1994. 13. Friedman, R. and Johnston, W. C., J. Appl. Phys. 22:791–797 (1950).
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14. Gooisby, A. D. and Haskell, W. W., Combust. Flame 26:105–114 (1976). 15. Kuroda, H., Nakajima, Y., Sugihara, K., Takagi, Y., and Muranaka, S., SAE Paper No. 78-0006, 1978. 16. Drobot, F. H., Cheng, W. K., Trinker, F. H., Kaiser, E. W., Siegl, W. O., Cotton, D. F., and Underwood, J., Combust. Flame 99:422–430 (1994). 17. Min, K., Cheng, W. K., and Heywood, J. B., SAE Paper No. 94-0306, 1994. 18. Hellstrom, T. and Chomiak, J., SAE Paper No. 952539, 1995.