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Laser-ignited internal combustion engine
COMBUSTION A N D F L A M E 30, 319- 320 (1977)
319
BRIEF COMMUNICATIONS Laser-Ignited Internal Combustion Engine J. D. DALE, P. R. SMY and D. WAY-NEE University of Alberta, Edmonton, Alberta, Canada T6G 2G7
and R. M. CLEMENTS University of Victoria, British Columbia, Canada VSW 2Y2
We have been investigating the effect of different methods of achieving ignition in internal combustion engines. In one series of measurements, where a pulsed plasma jet was used as the ignition device [1], we found significant increases in output power, fuel economy and the lean operation limit and substantial changes (both positive and negative) in pollution levels. Since the plasma jet effectively contains far more energy than a conventional igniting spark and because it also achieves good penetration of the combustible mixture, we extended our investigation by carrying out measurements on an internal combustion engine which is ignited by the "spark" formed by a pulsed high power laser beam [2]. It is these measurements that are discussed in this communication. Laser ignition has been investigated before but only with single igniting pulses of visible and near infrared radiation focused into combustion bombs. Reasons for carrying out these earlier measurements involved the relative freedom offered in choosing where the spark was generated, and in choosing more or less precisely (within limits) the amount of energy dissipated in the spark (no electrode losses). These earlier measurements [3-5] gave evidence for more rapid flame speed with minimum ignition energies which were dictated by the minimum energy to create a laser spark. The laser used in the measurements reported here was a pulsed repetitive TEA (transverse
excited atmospheric) CO2 laser, operating at 10.6 /zm wavelength at pulsed powers of up to 1 MW and with a duration of ~1 /as. The advantages of this laser are that it is far more efficient than those operating in the visible and near infrared regions, and that breakdown can be achieved with lower pulse energies. (In fact, the shift to longer wavelengths was seen as desirable by Hickling and Smith [3] in similar experiments.) The engine usea was an ASTM-CFR (6:1 CR) coupled directly to an electric dynanometer running on locally obtained commercial leaded gasoline (RON 92). The carburetor was an updraft Ensign Dual Fuel type. The laser beam was focused to form a spark in the combustion chamber using a 15 cm focal length lens and a 1 cm-thick zinc selenide window set in the wall of the cylinder head. The measurements taken show strong similarities to those made with a plasma jet ignitor. When using lean mixtures the output power is increased by a substantial factor (---3 X at an air-fuel ratio of 19.5:1) and the air fuel lean limit is extended from ----20-23. Pressure time traces substantiate earlier combustion bomb findings [3] in that they reveal a substantial decrease in the time to peak pressure (an increase in the flame speed). Successive pressure traces are found to show much less cycle to cycle variation than was experienced with the same engine operating with conventional spark ignition. Quantitative analysis of the pressure traces was not done. Averaged pressure-time traces Copyright © 1977 by The Combustion Institute Published by Elsevier North-Holland, Inc.
320
P.R. SMY ET AL.
2000 Y "& looo
o
,b ;2
,'82o
(ms)
Fig. 1. Averaged pressure-time traces for (a) laser, and (b) conventionally ignited engine starting from time of spark discharge. Engine speed 1000 rpm, 6:1 CR, Timing 10° BTDC, water jacket temperature 98°C, air intake temperature 25°C, oil temperature 65°C, pressure transducer Norwood model 111. (starting from the time of spark firing) for the engine operated with laser and conventional coil ignition (6 V system, ~20 mJ spark energy) are shown in Fig. 1. Clearly, the laser ignition does initiate a more rapid rate of pressure rise which results in more engine power and improved efficiency (lower specific fuel consumption). Measurements of pollutants showed essentially no change in CO concentrations over the A / F range. The HC concentrations were the same up to an A / F ~ 17.5. Above this A / F ratio both ignition systems showed rising HC's, but the laser ignition produced lower concentrations (~30% lower at A / F ~ 20/1) due to the lack of misfiring. The NO gas concentrations were the same up to an A / F 15, above which the laser ignition produced significant increases in NO concentrations. At an A / F ~ 16, the levels were 4100 ppm and 2600 ppm for the laser and conventional ignition systems respectively, dropping to 350 and 100 ppm respectively at A / F ~ 20. The measured NO levels for the laser were below estimated equilibrium levels for isooctane-air mixtures. Finally, the overall behaviour of the engine showed little variation with position of the laser sparks or with the energy of the laser beam, providing it was sufficient to generate a spark. The laser beam required an energy burst of 90 mJ to break down in room air and about 100 mJ to ignite an A / F ~ 15 at the compression pressure of 720 kPa (105 psi). For performance tests the energy was raised to about 300 mJ.
Our tentative explanation of these findings is that the more efficient and rapid combustion afforded by laser ignition produces higher combustion chamber temperatures and provides a longer time for NO formation in the critical postflame period than is the case with conventional coil ignition [6]. It remains to be seen whether variation in ignition timing, etc. will allow some of the benefits of this type of ignition to be retained without the sacrifice of increased pollutants. One possibility is that the property of enhanced flame speed may be utilized in an engine operating with exhaust gas recirculation. At this stage it seems very doubtful that laser ignition will ever be more than a research tool for internal combustion engines. However, in view of its similarities to plasma jet ignition and its flexibility in terms of "spark" location and energy, the ability to operate in a repetitive ignition situation may be very useful. It is well known that there often exists a large difference between the results obtained for simple geometry, static gas, combustion bombs and those obtained for "real life" engines. The complex geometry and flow patterns of the latter case raises serious questions concerning the possibility of direct transference of results from bomb measurements to engines. However, we have shown that it is feasible to utilize laser ignition in an internal combustion engine and hope that this has shed light on the effects of flow patterns, etc. on this ignition.
REFERENCES 1. Topham, D. R., Smy, P. R., and Clements, R. M., Combust. Flame, 25,187 (1975). 2. Minck,R.W.,J. Appl. Phys., 35,252 (1964).
3. Hiclding, R., and Smith, W. R., S.A.E. Technical Paper No. 740114 (1974). 4. Weinberg, F. J. and Wilson, J. R. Proc. Roy Soc. (London) A321 41 (1971). 5. Bach, G. G., Knystautas, R., and Lee, J. H., Twelfth Symposium (International) on Combustion, 853 (1969). 6. Heywood, J. B., Fifteenth Symposium (Inter. national) on Combustion, 1191 (1974).
Received 20 October 1976; revised 28 April 1977
319
BRIEF COMMUNICATIONS Laser-Ignited Internal Combustion Engine J. D. DALE, P. R. SMY and D. WAY-NEE University of Alberta, Edmonton, Alberta, Canada T6G 2G7
and R. M. CLEMENTS University of Victoria, British Columbia, Canada VSW 2Y2
We have been investigating the effect of different methods of achieving ignition in internal combustion engines. In one series of measurements, where a pulsed plasma jet was used as the ignition device [1], we found significant increases in output power, fuel economy and the lean operation limit and substantial changes (both positive and negative) in pollution levels. Since the plasma jet effectively contains far more energy than a conventional igniting spark and because it also achieves good penetration of the combustible mixture, we extended our investigation by carrying out measurements on an internal combustion engine which is ignited by the "spark" formed by a pulsed high power laser beam [2]. It is these measurements that are discussed in this communication. Laser ignition has been investigated before but only with single igniting pulses of visible and near infrared radiation focused into combustion bombs. Reasons for carrying out these earlier measurements involved the relative freedom offered in choosing where the spark was generated, and in choosing more or less precisely (within limits) the amount of energy dissipated in the spark (no electrode losses). These earlier measurements [3-5] gave evidence for more rapid flame speed with minimum ignition energies which were dictated by the minimum energy to create a laser spark. The laser used in the measurements reported here was a pulsed repetitive TEA (transverse
excited atmospheric) CO2 laser, operating at 10.6 /zm wavelength at pulsed powers of up to 1 MW and with a duration of ~1 /as. The advantages of this laser are that it is far more efficient than those operating in the visible and near infrared regions, and that breakdown can be achieved with lower pulse energies. (In fact, the shift to longer wavelengths was seen as desirable by Hickling and Smith [3] in similar experiments.) The engine usea was an ASTM-CFR (6:1 CR) coupled directly to an electric dynanometer running on locally obtained commercial leaded gasoline (RON 92). The carburetor was an updraft Ensign Dual Fuel type. The laser beam was focused to form a spark in the combustion chamber using a 15 cm focal length lens and a 1 cm-thick zinc selenide window set in the wall of the cylinder head. The measurements taken show strong similarities to those made with a plasma jet ignitor. When using lean mixtures the output power is increased by a substantial factor (---3 X at an air-fuel ratio of 19.5:1) and the air fuel lean limit is extended from ----20-23. Pressure time traces substantiate earlier combustion bomb findings [3] in that they reveal a substantial decrease in the time to peak pressure (an increase in the flame speed). Successive pressure traces are found to show much less cycle to cycle variation than was experienced with the same engine operating with conventional spark ignition. Quantitative analysis of the pressure traces was not done. Averaged pressure-time traces Copyright © 1977 by The Combustion Institute Published by Elsevier North-Holland, Inc.
320
P.R. SMY ET AL.
2000 Y "& looo
o
,b ;2
,'82o
(ms)
Fig. 1. Averaged pressure-time traces for (a) laser, and (b) conventionally ignited engine starting from time of spark discharge. Engine speed 1000 rpm, 6:1 CR, Timing 10° BTDC, water jacket temperature 98°C, air intake temperature 25°C, oil temperature 65°C, pressure transducer Norwood model 111. (starting from the time of spark firing) for the engine operated with laser and conventional coil ignition (6 V system, ~20 mJ spark energy) are shown in Fig. 1. Clearly, the laser ignition does initiate a more rapid rate of pressure rise which results in more engine power and improved efficiency (lower specific fuel consumption). Measurements of pollutants showed essentially no change in CO concentrations over the A / F range. The HC concentrations were the same up to an A / F ~ 17.5. Above this A / F ratio both ignition systems showed rising HC's, but the laser ignition produced lower concentrations (~30% lower at A / F ~ 20/1) due to the lack of misfiring. The NO gas concentrations were the same up to an A / F 15, above which the laser ignition produced significant increases in NO concentrations. At an A / F ~ 16, the levels were 4100 ppm and 2600 ppm for the laser and conventional ignition systems respectively, dropping to 350 and 100 ppm respectively at A / F ~ 20. The measured NO levels for the laser were below estimated equilibrium levels for isooctane-air mixtures. Finally, the overall behaviour of the engine showed little variation with position of the laser sparks or with the energy of the laser beam, providing it was sufficient to generate a spark. The laser beam required an energy burst of 90 mJ to break down in room air and about 100 mJ to ignite an A / F ~ 15 at the compression pressure of 720 kPa (105 psi). For performance tests the energy was raised to about 300 mJ.
Our tentative explanation of these findings is that the more efficient and rapid combustion afforded by laser ignition produces higher combustion chamber temperatures and provides a longer time for NO formation in the critical postflame period than is the case with conventional coil ignition [6]. It remains to be seen whether variation in ignition timing, etc. will allow some of the benefits of this type of ignition to be retained without the sacrifice of increased pollutants. One possibility is that the property of enhanced flame speed may be utilized in an engine operating with exhaust gas recirculation. At this stage it seems very doubtful that laser ignition will ever be more than a research tool for internal combustion engines. However, in view of its similarities to plasma jet ignition and its flexibility in terms of "spark" location and energy, the ability to operate in a repetitive ignition situation may be very useful. It is well known that there often exists a large difference between the results obtained for simple geometry, static gas, combustion bombs and those obtained for "real life" engines. The complex geometry and flow patterns of the latter case raises serious questions concerning the possibility of direct transference of results from bomb measurements to engines. However, we have shown that it is feasible to utilize laser ignition in an internal combustion engine and hope that this has shed light on the effects of flow patterns, etc. on this ignition.
REFERENCES 1. Topham, D. R., Smy, P. R., and Clements, R. M., Combust. Flame, 25,187 (1975). 2. Minck,R.W.,J. Appl. Phys., 35,252 (1964).
3. Hiclding, R., and Smith, W. R., S.A.E. Technical Paper No. 740114 (1974). 4. Weinberg, F. J. and Wilson, J. R. Proc. Roy Soc. (London) A321 41 (1971). 5. Bach, G. G., Knystautas, R., and Lee, J. H., Twelfth Symposium (International) on Combustion, 853 (1969). 6. Heywood, J. B., Fifteenth Symposium (Inter. national) on Combustion, 1191 (1974).
Received 20 October 1976; revised 28 April 1977