Environ. Sci. Technol. 2008, 42, 8865–8870
Reducing NOx Emissions from a Biodiesel-Fueled Engine by Use of Low-Temperature Combustion T I E G A N G F A N G , † Y U A N - C H U N G L I N , * ,‡ TIEN MUN FOONG,§ AND CHIA-FON LEE§ Department of Mechanical & Aerospace Engineering, North Carolina State University, Raleigh, North Carolina 27695, Institute of Environmental Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan, and Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
Received January 16, 2008. Revised manuscript received July 2, 2008. Accepted September 20, 2008.
Biodiesel is popularly discussed in many countries due to increased environmental awareness and the limited supply of petroleum. One of the main factors impacting general replacement of diesel by biodiesel is NOx (nitrogen oxides) emissions. Previous studies have shown higher NOx emissions relative to petroleum diesel in traditional direct-injection (DI) diesel engines. In this study, effects of injection timing and different biodiesel blends are studied for low load [2 bar IMEP (indicated mean effective pressure)] conditions. The results show that maximum heat release rate can be reduced by retarding fuel injection. Ignition and peak heat release rate are both delayed for fuels containing more biodiesel. Retarding the injection to postTDC (top dead center) lowers the peak heat release and flattens the heat release curve. It is observed that low-temperature combustion effectively reduces NOx emissions because less thermal NOx is formed. Although biodiesel combustion produces more NOx for both conventional and late-injection strategies, with the latter leading to a low-temperature combustion mode, the levels of NOx of B20 (20 vol % soy biodiesel and 80 vol % European low-sulfur diesel), B50, and B100 all with postTDC injection are 68.1%, 66.7%, and 64.4%, respectively, lower than pure European low-sulfur diesel in the conventional injection scenario.
1. Introduction Research has invested in searching for new energy sources due to the rapid depletion of fossil fuel supplies. Biodiesel has possibilities over other alternative fuels in its efforts to complement petroleum-based fuels because of its renewability. Biodiesel-fueled diesel engines could reduce emissions of carbon monoxide (CO), carbon dioxide (CO2), total hydrocarbons (THC), sulfur dioxide (SO2), and polycyclic aromatic hydrocarbons (PAHs) but slightly increase brakespecific fuel consumption (BSFC) and NOx emissions (1-12). There are mixed results for particulate matter (PM) reduction in the literature (13); however, most results (>90%) show PM reduction when biodiesel is used. If the biodiesel fuel is * Corresponding author phone: +886-7-5252000 exit 4412; fax: +886-7-5254449; e-mail:
[email protected]. † North Carolina State University. ‡ National Sun Yat-Sen University. § University of Illinois at Urbana-Champaign. 10.1021/es8001635 CCC: $40.75
Published on Web 10/28/2008
2008 American Chemical Society
produced at substantially lower carbon use, net CO2 is generally reduced due to the fact that the biodiesel feed stocks, such as oil plants, absorb CO2 during their life cycle. Emissions of NOx are one of the main concerns when biodiesel is introduced into the energy market. Previous studies and measurements of NOx emissions from biodiesel showed an increase in NOx emissions. Brake-specific NOx emissions could be 15% higher for biodiesel compared to diesel fuels (14-22). However, this increase in NOx emissions is strongly dependent on both speed and load. Tat and Van Gerpen (22) reported that an earlier start of combustion, which causes higher combustion temperature, leads to increased NOx emissions. According to the Zel’dovich mechanism, formation of NOx is prominent at high temperature. By using the two-color method in a rapid compression machine, Senda et al. (23) found that the high flame temperature of biodiesel existed in its whole flame but was only observed in the central part of the diesel flame. This also suggested that higher combustion temperature is likely to increase NOx emissions. Due to the well-known tradeoff between PM and NOx emissions, current diesel engines will have difficulty meeting the ever-stricter emission regulations with simultaneous reductions of NOx and PM. Low-temperature combustion (LTC) is a promising technique to meet these requirements. LTC includes homogeneous-charge compression ignition (HCCI) and several other newer combustion concepts. HCCI combustion is essentially a two-stage autoignition process: an initial cold flame chemical reaction occurring at lower combustion temperature, followed by thermal flame combustion (24). Although there were many studies focused on HCCI combustion in diesel engines (25-36), only a few such as the modulated kinetics (MK) combustion (28, 29, 33, 34) and uniform bulky combustion system mode (25, 26, 30, 32) were implemented in real engines. Simultaneous reduction of NOx and PM was achieved without a penalty on fuel economy. Low-temperature MK combustion was accomplished by using heavy EGR (exhaust gas recirculation) rate, retarding injection timing, enhancing air motion, and increasing injection pressure. However, both NOx and PM emissions remain problematic at high load and high speed operations, even though new concepts were proposed by combining the HCCI mode with conventional diesel combustion (28, 29, 31, 32). The main concern is to expand HCCI mode or improve the quality of high-load diesel combustion with reduced emissions. Choi et al. (34) stated that it was difficult to obtain a real homogeneous mixture in diesel engines with late injection. Partial heterogeneity exists even for very early injection (27). Previous studies have concluded that emission reduction in HCCI combustion is mainly because of the low-temperature reaction rather than homogeneity itself. Therefore, low-temperature combustion is the main reason for emission reduction, and HCCI combustion is a subset of this technique. Recently some works about clean diesel combustion mode such as high-efficiency clean combustion (HECC) (37) and premixed charge compression ignition (PCCI) combustion for biodiesel blends were published, showing emission improvement by use of clean diesel combustion for pure biodiesel or biodiesel blends. This study examines the possibility of controlling or reducing NOx emissions from biodiesel with different combustion modes by varying the injection strategies and fuel blends.
2. Experimental Section 2.1. Test Fuels and Test Engine. Four test fuels were used: European low-sulfur diesel (B0), B20 (20 vol % soy biodiesel VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Specifications of Single-Cylinder DIATA Research Enginea bore stroke displacement/cylinder compression ratio swirl ratio valves/cylinder intake valve diameter exhaust valve lift maximum valve opening intake valve opening intake valve closing exhaust valve opening exhaust valve closing
70 mm 78 mm 300 cc 19.5:1 2.5 4 24 mm 21 mm 7.30/7.67 mm (intake/exhaust) 13 CAD ATDC (at 1 mm valve lift) 20 CAD ABDC (at 1 mm valve lift) 33 CAD BBDC (at 1 mm valve lift) 18 CAD BTDC (at 1 mm valve lift)
a
CAD, crank angle degree; ATDC, after top dead center; ABDC, after bottom dead center; BBDC, before bottom dead center; BTDC, before top dead center.
and 80 vol % B0), B50, and B100. The engine used for experimentation was modified from a single-cylinder DIATA (direct injection aluminum through-bolt assembly) research engine, supplied by Ford Motor Co. The characteristics of the engine include an all-aluminum structure, a small displacement volume of 300 mL/cylinder, a four-valve combustion system, and a common-rail injection system. The specifications of the DIATA engine are given in Table 1. The drop-liner design employed at Sandia National Laboratories in Livermore, CA, is adopted (38). Optical access to the combustion chamber is attained from the side, through a window, or from below, through a fused silica piston top attached to a Bowditch-type piston extension. The optical engine design maintains the geometry of the ports and combustion chamber of the original engine. A Bosch common-rail electronic injection system with injection pressures up to 1350 bar is installed. A valve-covered orifice injector with six 0.124 mm diameter nozzle holes is used. A needle lift sensor is fitted in the injector for monitoring needle operation during injection. Detailed information on this optical engine can be found in previous publications (39, 40). The engine speed remained constant at 1500 rpm, which is a typical engine speed for a passenger car diesel engine when driving on highways. The injection pressure was 600 bar for all the tests. Low load conditions with 2 bar IMEP were matched by adjusting the fuel quantity per cycle. The testing conditions are summarized in Table 2. In comparison with conventional injection timing (case 2), it is found that the indicated specific fuel consumption (ISFC) values are decreased except B100 when late injection timing is used (case 3). The reduction fractions are 8.37% for B0, 11.1% for B20, and 10.5% for B50. 2.2. Data Acquisition. National Instruments LabView version 6.0 was used as the data acquisition and timing software. An optical shaft encoder with 0.25 crank angle resolution was installed to provide the time basis. The engine temperatures and pressures were monitored with a multifunction data acquisition board. The necessary timing was controlled by 16 up/down 32-bit counter/timers. A 12-bit unintensified high-speed video camera (Phantom v7.0 from Vision Research, Inc.) was used to obtain the combustion videos. The camera was operated at 12 000 frames/s. This frame rate corresponds to an interval of 0.75 crank angle degree (CAD) between two sequential images at 1500 rpm. The same lens f-stop was used for all the conditions; therefore, perceived intensities are directly comparable. 2.3. NOx Analyzer. An important benefit of low-temperature combustion is the low NOxemission. Because of thermal loading, it is risky to run the optical engine in a continuous firing mode. Instead, it was operated in a pattern of three 8866
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injection cycles followed by 10 motoring cycles. A MEXA-720 NOx analyzer from Horiba was used to measure the diluted NOx concentration from the exhaust pipe. A NOx sensor was installed in the exhaust pipe and connected to the signal analyzer for NOx concentration readings. This nonsampling NOx analyzer provided a faster response. The sensor had a response time of about 0.7 s. The analyzer was calibrated according to the operation manual before emission measurement. The engine was operated in the skip-fire mode for a certain time to reach a steady state with a stable NOx reading. The NOx concentration was recorded. Then the engine was shut off and cleaned. Five data points were collected for each condition. The final NOx emission values were corrected on the basis of the duty cycle of the operation pattern.
3. Results and Discussion 3.1. Fuel Specifications. Four different fuels or blends (B0, B20, B50, and B100) were used. Selected fuel properties are tabulated in Table 3. Biodiesel has higher density than European low-sulfur diesel. Higher density could cause longer liquid penetration for biodiesel. Sulfur content in biodiesel is significantly lower. The boiling point, which is important for air-fuel mixing, is generally higher for biodiesel. Higher boiling point may lead to longer penetration. The viscosity for biodiesel is higher than for diesel fuel. High viscosity may influence spray development and droplet atomization as well as injection dynamics. Another important property is the cetane number, which greatly affects the autoignition characteristics. In this study the soybean biodiesel, Stepan Biodiesel SB-W, was made by Stepan Co. According to their report, the average cetane number is 50 (41). The cetane number of European low-sulfur diesel is about 54.0, which is significantly higher than 42 for the no. 2 diesel fuel in the U.S. market. However, a recent study (42) showed a comparison of B100 with different fuels with designed cetane numbers, and the cetane number of B100 could be matched to a reference fuel with cetane number of 80 based on the heat release rate. 3.2. In-Cylinder Pressure. The in-cylinder pressure curves are shown in Figure 1. The effects of injection timing on the in-cylinder pressure are quite similar for different fuel blends. With early pre-TDC (top dead center) injection, ignition takes place well in advance of TDC. The in-cylinder pressure is higher than other injection timings. Meanwhile, due to early combustion, the hot burnt gas mixture is compressed by the piston, leading to further in-cylinder temperature increase. This high in-cylinder temperature is likely to promote NOx formation. Ignition occurs at approximately TDC for injection timing at -10 CAD ATDC (crank angle degree after top dead center). Although the incylinder pressure peak is still quite high, it is lower than the early injection timing case. With post-TDC injection timing at 3 CAD ATDC, a much longer ignition delay than with the case of -10 CAD ATDC injection timing is observed. The in-cylinder pressure is much lower than the other two injection schemes discussed. For some cases, the combustion pressure peak is even lower than the motoring peak pressure due to late ignition after TDC. The effects of different fuel blends on the in-cylinder pressure are quite apparent. With increasing biodiesel percentage, the ignition delay is lengthened for all injection timings. Biodiesel has a higher boiling point that slows the droplet evaporation rate; therefore, preparation of the ignitable air-fuel mixture is delayed. Moreover, biodiesel has a relatively lower cetane number than European lowsulfur diesel fuel, and thus a longer ignition delay is expected. On the basis of these differences, higher biodiesel content lengthens the ignition delay. This observation, however, contrasts the results reported in previously published biodiesel papers (43-45). In these papers, NOx emissions
TABLE 2. Summary of Engine Operating Conditions fuel type
case
engine speed (rpm)
rail pressure (bar)
main SOIa (CAD ATDC)
fuel quantity (mm3)
fuel mass (mg)
IMEP (bar)
ISFCb [g (kW-h)-1]
B0 B0 B0 B20 B20 B20 B50 B50 B50 B100 B100 B100
1 2 3 1 2 3 1 2 3 1 2 3
1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500
600 600 600 600 600 600 600 600 600 600 600 600
-25 -10 3 -25 -10 3 -25 -10 3 -25 -10 3
8.4 4.9 4.4 8.8 5.2 4.7 9.1 5.6 5.0 9.3 5.6 6.0
7.1 4.1 3.7 7.4 4.4 3.9 7.8 4.8 4.3 8.2 4.9 5.2
2.00 2.06 2.03 2.01 2.02 2.02 2.03 2.01 2.01 2.02 2.00 2.04
426 239 219 442 261 232 461 287 257 487 294 306
a
SOI, start of injection.
b
ISFC, indicated specific fuel consumption.
TABLE 3. Summary of Selected Fuel Properties for European Low-Sulfur Diesel Fuel and Soybean Biodiesel Fuel European low-sulfur diesel fuel 0.837 196 54.7 202 (initial boiling point); boiling point (°C) 270 (50%); 356 (end point) viscosity (cps) 3.2 (at 40 °C) cetane number 54.0
fuel property specific gravity sulfur (ppm) flash point (°C)
a
soybean biodiesel fuel 0.877 ∼0 >93.9 >316 7 (at 25 °C) 50a
Reference 41.
FIGURE 2. Heat release rate in various cases.
FIGURE 1. In-cylinder pressure in various cases (SOI, start of injection). increased with increasing biodiesel fuel, which was attributed to the early injection timing or ignition timing of biodiesel with the same mechanical pump settings. The early ignition timing was observed from the in-cylinder pressure. The differences in the current results can be attributed to different fuels and configurations of the injection systems. First, European low-sulfur diesel has a higher cetane number than the no. 2 diesel in those studies. Second, their fuel injection systems were mechanically controlled injection pumps. The fuel bulk modulus yields significant effects on the pressure wave propagation in the high-pressure fuel line. Biodiesel has higher bulk modulus than no. 2 diesel. Also, the highpressure fuel line for a mechanical injection system is typically longer than for a common-rail system. The longer the high-
pressure fuel line, the larger the difference in the timing of the pressure wave arriving at the injector tip. Therefore, the injection timing is more advanced for biodiesel than for no. 2 diesel for a mechanical system. However, for a commonrail system, the line pressure is established and maintained by the high-pressure pump and pressure regulator in the fuel rail. Thus difference in bulk modulus has only slight effects for a common-rail system (46). This was also observed by Zhang and Boehman (47) in their recent paper. The combined effects of earlier injection timing and higher cetane number compared to the no. 2 diesel fuel in those papers result in earlier ignition compared to previous studies for biodiesel (4345). 3.3. Heat Release Rate. The heat release rates are illustrated in Figure 2. Most of the conditions have premixedcombustion-dominated heat release rate patterns. For a certain type of fuel with different injection timings, an early pre-TDC injection at -25 CAD ATDC results in a higher peak heat release rate followed by the conventional injection timing and then by the post-TDC injection timing. Similar observations are made for the post-TDC injection timing cases by Choi et al. (34). The heat release duration for late post-TDC injection is longer and the rate is lower than the early preTDC injection cases. This heat release pattern is preferable for noise reduction. The ignition delay is longer with higher biodiesel content. For case 1 of all tested fuels, the heat release rate curves are quite similar with approximately equal combustion duration, except for slight differences in peak heat release rate. These peaks decrease when the fraction of biodiesel in the blend increased due to the higher boiling VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Time integration of spatially integrated flame luminosity (SIFL) and NOx emissions of the 12 cases with different injection strategies and fuel blends. point of biodiesel. For the conventional injection timing cases, B0 has a lower heat release rate peak than other fuel blends, with some evidence of diffusion combustion after the dominant premixed heat release event, as observed from the combustion images with liquid spray being injected into premixed combustion flames. However, for other fuel blends, there is little diffusion combustion. This is because of the longer ignition delay for biodiesel blends than B0. For retarded post-TDC injection (case 3), the heat release rate is lower and broader for higher biodiesel content. An interesting observation can be seen upon comparison with the results by Cheng et al. (42), which show that the ignition delay of biodiesel with cetane number 50 can be closely matched by a reference fuel with cetane number 80. This reveals that the standard cetane number measurement is not always indicative of the relationship in actual ignition delay between different fuels under DI combustion. 3.4. Spatially Integrated Flame Luminosity. Spatially integrated flame luminosity (SIFL) was obtained for the 12 cases by summing up the pixel values of the bottom-viewed combustion images. Time-integrated SIFL (TISIFL) and NOx emissions data are plotted in Figure 3. TISIFL is a parameter reflecting soot emission characteristics, while NOx emissions represent the concentration in the exhaust pipe. For case 1, B0 has the highest value of TISIFL and it increases with increasing biodiesel content for B20, B50, and B100. For case 2 with conventional injection timing, TISIFL for B0 is much higher than for B20, B50, and B100. However, B50 has the highest value of TISIFL among B20, B50, and B100 due to combined effects of fuel volatility and oxygen content. Case 2 shows a higher value of TISIFL than cases 1 and 3 for all fuel types. For case 3 with delayed injection, the values observed are lower for B20, B50, and B100 than for B0. On the basis of these results, it can be summarized that biodiesel significantly reduces soot emissions because the oxygen content in the fuel prohibits soot formation and promotes its oxidation in combustion for both conventional and late injection timings. Both early and late injections are helpful in reducing soot compared with the conventional injection timing for all the fuel blends. But more significant reductions are observed for B20, B50, and B100 with post-TDC injection timing. Lower SIFL for B100 was found for typical mixing8868
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controlled combustion mode compared with a reference fuel, and it should be noted that smoke emission from biodiesel or diesel fuel is also strongly dependent on the engine load, as shown by Cheng et al. (42). 3.5. NOx Emissions. Interesting trends are observed in NOx emissions (Figure 3). For cases 2 and 3, NOx emissions increase with biodiesel, which is consistent with previously published results. For case 2, NOx increases for B20, B50 and B100 were 5.19%, 22.2% and 37.8% higher, respectively, compared with B0. It is found that the increase of NOx emission in case 2 is relatively higher than that obtained by U.S. EPA (48). This may be due to the fact that the combustion here is mainly dominated by premixed combustion at low load conditions, which is quite different from a diffusionflame-dominated conventional combustion mode. For case 3, NOx increases 3.61%, 8.43%, and 15.7% for B20, B50, and B100, respectively. However, for early pre-TDC injection timings, NOx emissions decrease with higher biodiesel content. Then, after the biodiesel content passes a critical value, NOx emissions increase again if more biodiesel is used. This implies a tradeoff between different parameters. For case 1, the reductions of NOx are 6.62% for B20, 17.1% for B50, and 13.1% for B100, compared with B0. Low NOx is observed for a retarded post-TDC injection strategy due to HCCI-like low-temperature combustion. Averaged reductions of NOx with late injection are 69.3% for B0, 69.7% for B20, 72.7% for B50, and 74.2% for B100 compared with conventional diesel combustion within the engine. These results showed that late injection (low-temperature combustion) could effectively reduce NOx emissions. In other words, retarding injection timing benefited NOx emissions because of less thermal NOx formation during low-temperature combustion. Although NOx emission increases with higher percentage of biodiesel for both conventional and late injection strategies, on average, NOx reduces 68.1% for B20, 66.7% for B50, and 64.4% for B100 by use of late injection strategy. As for PM emission during low-temperature combustion, it will also be reduced due to lower soot formation rate as reported by many papers (28, 49). The emission of CO, however, will generally increase. But CO is relatively easier to remove than PM and NOx in the exhaust.
Simultaneous reduction of NOx and soot is possible by use of low-temperature HCCI combustion modes. Recalling that the ignition delay lengthens as biodiesel fuel content increases, it is expected that NOx emissions positively correlate with biodiesel content irrespective of the ignition delay, as explained in previous results (43-45). For conventional or late injection strategies, in which ignition occurs near or after TDC, a longer ignition delay means a lower in-cylinder global temperature. Thus, a longer ignition delay helps reduce NOx emissions if ignition occurs after TDC. Although the ignition delay is longer for biodiesel in this study, higher NOx emissions are observed. This could be attributed to the oxygen content in biodiesel. In addition, higher oxygen content results in relatively lean combustion compared with regular diesel fuel. Therefore, increasing the amount of biodiesel would cause more NOx in the exhaust. The results indicate, although indirectly, that the oxygen content actually has more pronounced influence than ignition delay on NOx emissions, for both conventional and late injection strategies. But for early pre-TDC injection timing, there is a tradeoff between the ignition delay and the oxygen content. For ignition occurring much earlier than TDC, advancing the ignition time would result in higher incylinder temperature. Due to piston compression, the incylinder temperature continues to rise. Consequently, there is a balancing effect of high in-cylinder temperature resulting from early ignition and the high oxygen content in the biodiesel. The tradeoff of ignition delay and oxygen content in biodiesel for different fuel blends leads to the observed trend of NOx emissions for case 1. When case 1 is compared with cases 2 and 3, NOx emissions are significantly higher. Retarding fuel injection until after TDC is an effective method for simultaneous reduction of soot and NOx emissions. Meanwhile, problems associated with biodiesel NOx emissions can be solved by low-temperature combustion. Therefore, low-temperature HCCI or PCCI combustion modes can be promising solutions for low-emission biodiesel engines.
Acknowledgments This research was supported in part by the Department of Energy under Grant DE-FC26-05NT42634, by Department of Energy GATE Centers of Excellence under Grant DE-FG2605NT42622, and by the Ford Motor Company under University Research Program. We also thank Paul Miles of Sandia National Laboratories and Evangelos Karvounis and Werner Willems of Ford for their assistance on the design of the optical engine and on the setup of the experiments.
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