Energy & Fuels 2003, 17, 755-761
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Effect of Residual Gas Fraction on the Combustion Characteristics of Butane-Air Mixtures in the Constant-Volume Chamber Myung Yoon Kim, Dae Sik Kim, and Chang Sik Lee* Department of Mechanical Engineering, Hanyang University, 17 Haengdang-dong, Sungdong-ku, Seoul 133-791, Korea Received March 28, 2002
An experimental study was made to investigate the effect of residual gas on the combustion characteristics and flame propagation of butane-air mixtures in a constant-volume combustion chamber. The combustion process and flame propagation are studied under different ratios of residual gas and various equivalence ratios in the combustion chamber. The effects of the residual gas ratio on the combustion pressure, heat release rate, burned fraction, and flame propagation phenomena were studied in detail. The experimental apparatus consists of a constant-volume combustion chamber, a premixture chamber, a data acquisition system, and a laser Schlieren system with a high-speed camera. With an increase of the residual gas ratio in the combustion chamber, the combustion pressure and the rate of heat release decrease and the burning period of the fuel-air mixture is reduced by the increase of residual gas. The effects of residual gas on the combustion characteristics and flame propagation speed are dependent on the amount of residual gas. In the case of a higher residual gas ratio (more than 10%) in the chamber, the combustion pressure and heat release are steeply decreased. It is confirmed that residual gas in the combustion chamber lowered the rate of heat release as a result of the decrease of combustion temperature. The flame propagation speed decreases with the increase of residual gas in the combustion chamber.
1. Introduction The nitrogen oxide (NOx) concentration of the exhaust gas from the automotive engine is primarily a function of combustion temperature. So, the most effective way of reducing NOx emission is to keep the combustion temperature down. Reduction of NOx formation by diluting the incoming air-fuel mixture with a small amount of inert gas is the simplest practical method. Air is available for a diluent gas, but it is not a nonreacting mixture like the exhaust gas. Tabata et al. investigated the effect of EGR (exhaust gas recirculation) under stoichiometric and lean mixture conditions and compared it with the effect of lean operation through the exhaust gas recirculation.1 Arcoumanis et al. analyzed the effect of various levels of EGR on the combustion characteristics in the four-cylinder direct-injection optical diesel engine.2 Their study revealed that the increase of EGR rate showed higher cyclic pressure variations during the warm-up period and reduced flame core temperatures. In the spark ignition engine, the most practical approach for the reduction of exhaust emission and improvement of engine stability is to control the combustion period by enhanced mixture flow in the cylinder. It is well-known that EGR * Corresponding Author. Phone: +82-2-2290-0427. Fax: +82-22281-5286. E-mail: cslee@ hanyang.ac.kr. (1) Tabata, M.; Yamamoto, T.; Fukube, T. Soc. Automot. Eng. 1995; No. 950684. (2) Arcoumanis, C.; Bae, C.; Nagwaney, A.; Whitelaw, J. H. Soc. Automot. Eng. 1995; No. 950850.
is effective in reducing NOx emissions,3 but the problem with exhaust gas recycling is an increase of the particulate matter. Shiozaki et al. measured the flame temperature under EGR conditions with a two-color imaging CCD camera.4 Also, Mitchell et al. measured the relationship between exhaust gas recirculation and intake air dilution on combustion through the optic access in a diesel engine.5 They indicated that flame temperature had a major influence on nitrogen oxide and that carbon monoxide emissions were influenced mainly by the O2 fraction in the intake air. Fundamental studies on exhaust gas recirculation have been carried out by many researchers, both theoretically and experimentally.6-8 Most of the previous researchers conducted engine tests to investigate the overall effect of exhaust gas recycling on emission control. But, these studies of engine combustion in the case of EGR have many uncertainties as a result of the difficulties of conducting experiments in the actual engine. From this point of (3) Baert, R. S. G.; Beckman, D. E.; Verbeek, R. P. Soc. Automot. Eng. 1996; No. 960848. (4) Shiozaki, T.; Nakajima, H.; Kudo, Y.; Miyashita, A.; Aoyagi, Y. Soc. Automot. Eng. 1996; No. 960323. (5) Mitchell, D.; Pinson, J. A.; Lizinger, T. A. Soc. Automot. Eng. 1996; No. 932798. (6) Durnholz, M.; Eifler, G.; Endres, H. Soc. Automot. Eng. 1996; No. 920725. (7) Mouqallid, M.; Lecodier, B.; Trinite, M. Soc. Automot. Eng. 1994; No. 941990. (8) Ropke, S.; Schweimer, G. W.; Strauss, T. S. Soc. Automot. Eng. 1995; No. 950213.
10.1021/ef0200774 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/29/2003
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Figure 2. Constant-volume combustion chamber.
Figure 1. Schematic diagram of experimental apparatus.
view, the effect of residual gas on combustion in a constant-volume combustion chamber is important to understand as the influencing factor for the reduction of combustion temperature and formation of emissions. The purpose of this study is to investigate the influence of residual gas ratio on the combustion and flame propagation characteristics in the constant-volume combustion chamber. The information obtained from the experimental results provides detailed combustion characteristics such as combustion pressure, the rate of heat release, mass fraction burned, and the visualization of the flame propagation under the various equivalence ratios and residual gas ratios. 2. Experimental Section 2.1. Experimental Apparatus. In a spark ignition engine, the combustion happens ideally near the top dead center at the end of the compression stroke, and this period of the stroke cycle is regarded as constant-volume combustion. In this study, the constant-volume combustion chamber was made to investigate the influence of the residual gas ratio on combustion phenomena. Figure 1 shows the schematic diagram of experimental apparatus. The test rig consists of the constant-volume combustion chamber, the fuel supply system, the ignition system, and the data acquisition system. A signal generator was used for synchronization of the ignition system, pressure data acquisition system, and high-speed camera. Butane and air were premixed in a premixture chamber where their equivalence ratio was determined on the basis of their partial pressures measured by a diaphragm pressure transducer. The total volume of the premixture chamber was measured to be 3290 cm3, and the fan at the bottom of the premixture chamber was operated by electronic control. A heater of 1 kW was installed to avoid fuel liquefaction under high pressure in the chamber. The constant-volume combustion chamber consists of a cylindrical chamber, an intake and exhaust valve, and an ignition system, as shown in Figure 2. The combustion chamber is a cylindrical shape with a diameter of 100 mm and a depth of 40 mm. It has extensive optical windowssa pair of 130 mm diameter and 30 mm thickness quartz windows mounted on both sides of the combustion chamber, which has a volume of 341 cm3. Two plate electrical heaters (300 W) were installed on the outside walls of the combustion chamber and coupled to a K-type thermocouple. This system allows the temperature of the chamber to be kept constant at 363 K in order to prevent water condensation on the windows after combustion.
Figure 3. Schematic diagram of the optical system. Ignition was achieved by a transistorized coil ignition system (TCI) with electronic ignition using a 12 V power supply. To ignite the mixture in the center of the combustion chamber, the spark plug was elongated to the center of the combustion chamber. 2.2. Experimental Procedures. The instantaneous pressure variation in the combustion chamber was monitored by a piezoelectric pressure transducer (Kistler, 6061B) connected to a charge amplifier. The output of the amplifier was input to the data acquisition system (Keithley, DAS-58), which digitized and stored the voltage signal with a sampling rate of 1 kHz. Figure 3 shows a schematic diagram of the optical system for combustion visualization. The system of combustion visualization was achieved by a high-speed Schlieren system, which consists of a light source, concave mirror (300 mm), and a high-speed camera (maximum 3000 fps, Phantom). The highspeed Schlieren system and optical system were used to visualize the flame propagation in the combustion chamber. A He-Ne laser was used as a light source, with maximum output of 10 mW and light wavelength of 632.8 nm. Flame propagation speed was calculated from the Schlieren images (512 × 512 pixel, 1000 fps). The experiments were performed for 5 kinds of residual gas ratio that included the range of 0.8 to 1.2 of equivalence ratio and initial mixture pressure from 1 to 5 bar. The fuel and air were mixed in a premixing chamber, where their equivalence ratio was determined on the basis of the partial pressures of component gases such as air and fuel as measured by a pressure transducer. A constant-volume combustion chamber is filled with a butane-air mixture at each equivalence ratio, and the mixture is allowed enough time to decay completely in the chamber to make a quiescent flow condition. When the burned gas temperature decreased to the same temperature as that of the combustion chamber, the burned gas was discharged and then as much as the amount of burned gas that corresponds to the residual gas ratio. At the same initial pressure condition, the fixed amount of fresh mixture is supplied to the combustion chamber. The fuel used in this experiment is butane.
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Figure 4. Time history of combustion pressure at different equivalent ratios.
Figure 5. Effect of residual gas on the combustion pressure at stoichiometric equivalence ratios.
2.3. Residual Gas Ratio and Heat Release. In this experiment, the residual gas ratio rRG in the combustion chamber is defined as
rRG )
mRG × 100 (%) mmix + mRG
(1)
where mRG and mmix are the mass of residual gas and mass of butane-air mixture (without residual gas), respectively. The butane-air mixture with residual gas composition was obtained as follows. A constant-volume combustion chamber is filled with a butane-air mixture at each equivalence ratio, and it is ignited by an electric spark plug. After the burned gas temperature was decreased to that of the test condition, the burned gas was discharged with the state of test residual gas ratio. And the burned gas and fresh fuel-air were mixed in a combustion chamber where the residual gas ratio was determined on the basis of their partial pressure. Also, initial pressure in the combustion chamber increases with the increase of residual gas fraction as follows:
Pi ) Pmix + PRG
(2)
where Pi is the initial pressure in the combustion chamber (with residual gas), Pmix is the pressure of the fuel-air mixture gas, and PRG is the pressure of the residual gas. The experiments were carried out for each condition, which included five residual gas ratios (0, 5, 10, 15, and 20%) at various equivalence ratios from 0.8 to 1.2 and various fuel-air mixture pressures. Also, the residual gas ratio was controlled from 0 to 20% with 5% intervals at each experimental condition. The combustion characteristics were obtained from the pressure data in the combustion chamber, and the flame propagation characteristics were analyzed by the optical system and high-speed digital camera system. The mass fraction burned Mb(τ), was calculated from the measured pressure trace on the basis of the assumption that combustion pressure corresponds to the mass fraction, and it can be expressed as follows:
Mb(τ) )
P(τ) - Pi Pmax - Pi
(3)
where Mb(τ) is the mass fraction burned at time τ, P(τ) is the instant pressure in the combustion chamber, Pi is the initial pressure in the combustion chamber before combustion, and Pmax is the maximum combustion pressure in the combustion chamber.
Figure 6. Effect of residual gas on the combustion pressure at Φ ) 0.8 and 1.2. In this paper, the combustion duration was defined as elapsed time required to reach the maximum pressure from spark ignition in a constant-volume combustion chamber. Also, in order to determine the heat release rate, if we ignore the heat transfer to the combustion chamber wall, the first law equation can be written as follows:
dV dQ dT ) mcv +P dτ dτ dτ
(4)
where dQ is the gross heat energy released as a result of combustion, m is the mass of the mixture, cv is the specific heat at constant volume, dT is the gas temperature change in the combustion chamber, P is the pressure in the combustion chamber, and dV is the change in the cylinder volume. Applying the ideal gas equation to the gas mixture in the combustion chamber and differentiating about time
PV ) mRT
(5)
dV dP dT P +V ) mR dτ dτ dτ
(6)
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Figure 7. Effect of initial pressure on combustion duration.
Figure 9. Effect of residual gas ratio and equivalent ratio on the combustion duration (Pmix ) 3 bar).
Figure 8. Effect of residual gas ratio on the maximum combustion pressure at Pmix ) 3 bar. Substitution of eq 6 into eq 4 and rearrangement of the terms give the usual form of the first law of heat release equation:
1 dP κ dV dQ ) V + P dτ κ - 1 dτ κ - 1 dτ
(7)
where κ is the ratio of specific heats.
3. Experimental Results and Discussion 3.1. Effect of Residual Gas on Combustion Characteristics. Figure 4 shows the effect of the equivalence ratio on the combustion pressure history in the combustion chamber at an initial mixture pressure of 3 bar (without residual gas). The maximum pressure appears in the equivalence ratio Φ ) 1.2. Figure 5 shows the effect of residual gas on the combustion pressure in the chamber at the mixture pressure of 3 bar and equivalence ratio of 1.0. As shown in Figure 5, the peak value of combustion pressure decreases with an increase of the residual gas fraction in the chamber because of the inert effect of the residual gas. Also, the length of time required to reach the maximum value of the combustion pressure is retarded in accordance with the increase of residual gas ratio. The effects of residual gas on the combustion pressure in the chamber at different equivalence ratios are illustrated in Figure 6. This figure
Figure 10. Effect of residual gas on the mass fraction burned.
shows that the combustion pressure decreases with an increase of residual gas ratio. This is a result of the increase of the inert gas fraction that the residual mass left over from the previous combustion in the chamber. As a result of the residual fraction, the combustion duration is increased with the increase of residual gas ratio and equivalence ratio. As indicated in Figure 6, there are very small variations of maximum pressure with the increase of the residual gas ratio at low equivalence ratio of Φ ) 0.8. In the range of lean mixture, it cannot be fired at over 15% of the residual gas ratio. Also, the timing at which the maximum pressure appears is retarded in proportion to the increase of the residual gas fraction in the mass of mixture. The effects of initial pressure on the total combustion duration under various residual gas conditions are illustrated in Figure 7. The total combustion period was proportional to the increase of initial pressure.
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Figure 11. Effect of residual gas ratio on the rate of heat release at different equivalence ratios.
Figure 8 shows the effect of equivalence ratio on the maximum pressure with different residual gas ratios. The residual gas ratios are slightly affected by the maximum pressure at the range within 10% of the residual fraction. However, in the case of higher residual gas ratios, the maximum combustion pressures are steeply decreased. Figure 9 shows the influence of the residual gas ratio on the combustion duration for different equivalent ratios. As the residual gas fraction in the chamber is increased, the combustion duration is longer than that of the lower ratio of residual gas. As illustrated in the figures, the difference of the combustion durations between 0% and 20% of residual gas ratio is very large since a high residual gas ratio brings about the decrease of combustion temperature and an increase of heat loss.
3.2. Mass Fraction Burned and Heat-Release Rate. Thermodynamic analysis of measured cylinder pressure data is a very powerful tool used for quantifying combustion parameters.9,10 There are two main approaches, which are often referred to as “burn-rate analysis” and “heat-release analysis”. Burn-rate analysis is used mainly to obtain the mass fraction burned, which is a normalized quantity with a scale of 0 to 1. Heat-release analysis is used to produce absolute energy. The rate of heat release is a very important parameter because this has a very significant influence on pressure-rise rate and NOx emissions. Figure 10 shows the mass fraction burned at the residual gas ratios rRG ) 0% and rRG ) 10%. With an (9) Kodah, Z. H.; Soliman, H. S.; Abu Qudais, M.; Jahmany, Z. A. Appl. Energy 2000, 66, 237-250. (10) Krieger, R. B.; Borman, G. L. ASME 1966; 66-WA/DGP-4.
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Figure 12. Schlieren pictures of flame propagation in various residual gas ratios at Pmix ) 3 bar.
increase of residual gas ratio, the total combustion duration increases, as shown in the comparison of mass fraction burned. Moreover, the combustion durations were considerably affected by residual gas in the rich equivalence ratios. Figure 11 shows the effect of residual gas ratio on the heat-release rate. As indicated in the figures, the peak value of the heat-release rate was rapidly decreased and its timing was retarded at high residual gas ratios. From these results of pressure and heat-release rate, it can be inferred that the increase in residual fraction in the mass of the mixture plays an important role in the decrease of maximum temperature in the chamber. 3.3. Effect of Residual Gas on Flame Propagation. The speed of flame propagation is dependent on the residual gas fraction and equivalence ratio. The effects of residual gas fraction on the flame propagation are shown in Figure 12. The flame behaviors of a mixture are obtained from a continuous recording of a single spark event. In the case of rRG ) 20%, flame core shape is an ellipsoidal trace because of interaction between spark energy and heat transfer with the buoyancy. With an increase in the residual gas ratio in
Figure 13. Effect of residual gas ratio on the flame propagation speed.
the chamber, the flame propagation speed decreases, as shown in the pictures. Figure 13 shows the measured flame speed obtained by a Schlieren picture at three equivalence ratios and variable residual gas ratio. The flame speed was calculated from digitized video images consisting of a 512 × 512 pixel array. Also, image processing software was used to calculate the diameters of the flame. Under the effect of buoyancy, the center of the flame was raised and the flame shape was distorted with a high residual gas ratio. Therefore, the diameters of the flame were calculated from the left and right end of the flame surface and flame speed was calculated from the diameters. As illustrated in the flame pictures, flame speed is fastest when the flame passes through the middle region of the combustion chamber. In the case of higher residual gas ratios, the flame speed is very low compared to the same condition without residual gas. The speed of early flame propagation is lower than the intermediate stage because of higher heat loss at the early stage of combustion. Also, influenced by the rise
Combustion Characteristics of Butane-Air Mixtures
of unburned gas pressure, flame propagation speed is slightly lower in the final stage than in the intermediate stage. 4. Conclusions An experimental study was carried out to investigate the influence of the residual gas on the combustion characteristics and flame propagation in a constant-volume chamber. The effect of residual gas on the combustion characteristics and flame propagation speed are analyzed by using the constant-volume chamber with an optical arrangement and a high-speed Schlieren system. The main results of this work are summarized as follows: (1) The combustion pressure and heat release rate of the butane-air mixture were decreased in accordance with an increase of residual gas ratio. It is confirmed that the residual gas effect shows a lowered rate of heat release as a result of the decrease of combustion temperature. (2) From the result of mass fraction burned, the increase of residual gas fraction in the constant-volume
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chamber influences the burning period. In the case of a higher residual gas ratio in the chamber, the combustion characteristics such as combustion pressure and heat release are steeply decreased. (3) The residual gas ratios are slightly affected at the maximum pressure at the range within 10% of the residual gas fraction. In the case of a higher residual gas ratio, the maximum pressure of gas in the combustion chamber is steeply decreased, compared to the case of a lower residual gas ratio. (4) The flame propagation speed is dependent on the residual gas ratio and equivalence ratio. With an increase of the residual gas portion in the chamber, the flame propagation speed decreases. Acknowledgment. This work is supported by the fund of National Center for Cleaner Production of Korea Institute of Industrial Technology (Project No.: 99-1K-34). EF0200774
