Energy Fuels 2009, 23, 5821–5829 Published on Web 10/26/2009
: DOI:10.1021/ef9006609
Soy-Biodiesel Impact on NOx Emissions and Fuel Economy for Diffusion-Dominated Combustion in a Turbo-Diesel Engine Incorporating Exhaust Gas Recirculation and Common Rail Fuel Injection Gayatri Adi,*,† Carrie Hall,† David Snyder,† Michael Bunce,† Christopher Satkoski,† Shankar Kumar,‡ Phanindra Garimella,‡ Donald Stanton,‡ and Gregory Shaver† †
Energy Center, Herrick Laboratories, School of Mechanical Engineering, Purdue University, West Lafayette, Indiana, and ‡ Cummins Inc., Columbus, Indiana Received June 29, 2009. Revised Manuscript Received September 15, 2009
Alternative fuels are gaining importance as a means of reducing petroleum dependence and green house gas emissions. Biodiesel is an attractive renewable fuel; however, it typically results in increased emissions of nitrogen oxides (NOx) relative to petroleum diesel. In order to develop hypotheses for the cause of increased NOx emissions during diffusion-dominated combustion in a modern diesel engine, an effort incorporating both experimental and modeling tasks was conducted. Experiments using a 2007 Cummins diesel engine showed NOx and fuel consumption increases of up to 38% and 13%, respectively, and torque decreases up to 12% for soy-biodiesel. Fuel properties and ignition delay characteristics were implemented in a previously validated engine model to reflect soy-biodiesel. Model predictions are within 3.5%, 7%, and 9.5%, respectively, of experimental engine gas exchange (airflow, charge flow, and exhaust gas recirculation (EGR) fraction), performance (work output, torque, and fuel consumption), and NOx emission measurements. The experimental and model results for the diffusion combustion-dominated operating conditions considered here suggest that higher biodiesel distillation temperatures and fuel-bound oxygen lead to near stoichiometric equivalence ratios in the rich, premixed portion of the flame as well as higher combustible oxygen mass fractions in the diffusion flame front which together result in increased biodiesel combustion temperatures and NOx formation rates.
(PM) emissions.5 Reduction in net carbon dioxide (CO2) is credited to biodiesel produced from crops which consume CO2 from the atmosphere during their growth.6,7 Furthermore, biodiesel is an oxygenated fuel containing approximately 11% oxygen by weight,2 which is believed to yield more complete combustion resulting in lower CO, UHC, and PM emissions.8,5,9 Biodiesel also has some disadvantages including lower energy density and generally higher emissions of nitrogen oxides (NOx) than conventional diesel for many operating conditions.5,2,4 The calorific value for biodiesel is about 12% lower than that for diesel, which means that more biodiesel fuel is required to achieve the same amount of torque or power as compared to diesel fuel.2 The “biodiesel NOx effect” has been an important research subject, and several different theories have been proposed regarding the cause for this
1. Introduction Total global energy consumption is projected to grow by 50% between 2005 and 2030.1 Much of the energy demand is from the transportation sector which is predominantly accommodated with petroleum-based fuels. Since it is produced from renewable sources that are domestically available, biodiesel is an attractive alternative to diesel. However, differences in combustion performance and emissions are observed as a result of fuel property differences (including molecular composition, cetane number, heating value, heat of vaporization, and bulk modulus, among others2-4), as shown in Table 1. More specifically, there are several advantages to using biodiesel as an alternative fuel in diesel engines. It mixes well with diesel and typically reduces carbon monoxide (CO), unburned hydrocarbon (UHC), and particulate matter
(6) Sheehan, J.; Camobreco, V.; Duffield, J.; Graboski, M.; Shapouri, H. Life Cycle Inventory of Biodiesel and Petroleum Diesel for Use in an Urban Bus; National Renewable Energy Laboratory: Golden, CO, 1998. (7) Hill, J.; Nelson, E.; Tilman, D.; Polasky, S.; Tiffany, D. Environmental, economic, and energetic costs and benefits of biodiesel and ethanol biofuels. Proc. Natl. Acad. Sci. U.S.A. 2006, 103 (30), 11206– 11210. (8) McCormick, R.; Tennant, C.; Hayes, R.; Black, S.; Ireland, J.; McDaniel, T.; Williams, A.; Frailey, M.; Sharp, C. Regulated emissions from biodiesel tested in heavy-duty engines meeting 2004 emission standards; SAE 2005-01-2200, Society of Automotive Engineers: Warrendale, PA, 2005. (9) Wang, W.; Lyons, D.; Clark, N.; Gautam, M.; Norton, P. Emissions from nine heavy trucks fuel by diesel and biodiesel blend without modification. Environ. Sci. Technol. 2000, 34, 933–939.
*Corresponding author. E-mail:
[email protected]. (1) International Energy Outlook 2008 DOE/EIA; Energy Information and Administration: Washington, D.C., 2008. (2) Yuan, W. Computational Modeling of NOx Emissions from Biodiesel Combustion Based on Accurate Fuel Properties. Ph.D. thesis, Iowa State University, 2003. (3) Tat, M. E.; Van Gerpen, J. H. Fuel property effects on biodiesel, Paper 036024, ASAE Meeting, Las Vegas, Nevada, July 2003. (4) Szybist, J.; Song, J.; Alam, M.; Boehman, A. Biodiesel combustion, emissions, and emission control. Fuel Process. Technol. 2007, 88, 679–691. (5) A comprehensive analysis of biodiesel impacts on exhaust emissions; United States Environmental Protection Agency: Washington, D.C., 2002. r 2009 American Chemical Society
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: DOI:10.1021/ef9006609
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controlled common rail fuel injection systems because such systems are not significantly impacted by bulk modulus. Differences in biodiesel adiabatic flame temperatures may also play a role. Some theoretical analysis of fatty acid methyl esters (FAME) which constitute biodiesel has shown that higher hydrocarbon chain length and unsaturation level lead to lower adiabatic flame temperatures.16 However, other studies have reported higher flame temperatures with biodiesel combustion,11,14 which could lead to higher NOx formation rates. The change in fuel consumption and emissions for biodiesel are also a result of engine control settings. Diesel engine control modules (ECMs) are typically programmed to make decisions, including desired values for air-fuel ratio (AFR), exhaust gas recirculation (EGR), and injection timing and pressure, with reference tables based on engine speed and demanded torque to optimize the fuel economy while meeting emissions regulations. Torque is usually based on fuel flow estimates. Since biodiesel has a lower specific energy content, the use of fuel flow rate leads to overestimates of torque. This leads to ECM decisions corresponding to a higher torque than that at which the engine is actually operating. As a result, the ECM decisions will fail to correctly optimize fuel consumption and emissions.14
Table 1. Fuel Properties property
units
diesel
biodiesel
molecular composition density heat of vaporization T10 T50 T75 lower heating value cetane number
kg/m3 KJ/kg K K K MJ/kg
C13.5H23.6 830 250 490 542 562 43.25 49
C18.8H34.5O2 877.25 357 617 635.5 638 37.5 55
increase, but a consensus for the exact cause has yet to be reached.10-16 One hypothesis focuses on autoignition characteristics. Biodiesel and other fuels with low aromatic content have higher cetane numbers and thus ignite more readily. Shorter ignition delays result in reduced mixing which may cause higher NOx. Also, shorter ignition delays may lead to early combustion in a smaller cylinder volume characterized by temperature increase-induced elevations in NOx. Another theory relates increases in NOx to the reductions in PM emissions typically observed with biodiesel combustion. The presence of diesel particulate matter in a flame contributes to radiant heat losses from flames.11,12 Less soot in biodiesel combustion may result in less heat being carried away from the flame front by soot radiation, elevating local temperatures, and in turn increasing NOx production rates. A third theory relates NOx increases to the difference in bulk modulus between diesel and biodiesel.15 Biodiesel has lower compressibility due to higher isentropic bulk modulus and higher speed of sound than regular diesel. For engines using mechanical fuel injectors in a pump-line-nozzle style fuel injection system, the fuel pump produces pressure pulses which travel down individual fuel lines to each injector. The time at which the pressure wave reaches the fuel injectors directly influences the fuel injection time. For biodiesel, the pressure wave travels down the fuel lines more rapidly due to higher speed of sound and bulk modulus, thereby advancing the injection timing, in some cases by several crank angle degrees.3 As a result of earlier injection, residence times are longer for burned gases and more heat release occurs closer to top dead center (TDC) when the cylinder volume is reduced. The cylinder temperatures are therefore increased, leading to higher NOx emissions.13 This effect is not likely to be significant in modern diesel engines using electronically
2. Study Objectives This study aims to leverage a combination of experimental and modeling results to demonstrate, and develop hypotheses for, biodiesel-induced increases of fuel consumption and NOx emissions at three different diffusion combustion-dominated operating conditions in a modern diesel engine. For both the experimental tests and model simulations performed in this work, ECM decisions were dictated with values that are consistent with the decisions that the ECM would make for conventional diesel fuel. The purpose of this was to examine only the influence of fuel differences on the combustion process, not on ECM decision making. 3. Diesel and Biodiesel Experiments 3.1. Experimental Setup. Experiments were run at the Purdue University Herrick Laboratories on a test cell utilizing an inline six cylinder 2007 Cummins ISB 6.7L turbo-diesel production engine. The engine is equipped with an externally cooled exhaust gas recirculation (EGR) circuit and a variable geometry turbocharger (VGT). An eddy current dynamometer provides engine loading, and external sensors are used to measure various quantities including temperatures, pressures, torques, air and fuel flows, and exhaust emissions. A dSPACE data acquisition system logs data from all the external and factory-installed engine sensors. Exhaust emissions analyzers are used to measure CO2, NOx, and PM. The CO2 analyzer is a fast response nondispersive infrared (NDIR) Cambustion analyzer which measures CO2 and carbon monoxide (CO) concentrations with a sampling time of 7 ms. Two sample heads are used in the setup, one in the intake manifold and another in the engine exhaust, downstream of the turbocharger. This arrangement enables EGR fraction calculation using the CO2 concentration in the intake and exhaust. The NOx concentration in the exhaust is measured using a fNOx400 Cambustion analyzer with a 4 ms sampling time. An AVL Microsoot sensor (AVL 483) is used for PM concentration measurement in the engine exhaust. 3.2. Experimental Procedure. Tests were conducted using conventional petroleum diesel fuel and soy-biodiesel at three very different speed-torque operating points as shown in
(10) McCormick, R.; Graboski, M.; Alleman, T.; Herring, A.; Tyson, K. Impact of biodiesel source material and chemical structure on emissions of criteria pollutants from a heavy-duty engine. Environ. Sci. Technol. 2001, 35, 1742–1747. (11) Ban-Weiss, G.; Chen, J.; Buchholtz, B.; Dibble, R. A numerical investigation into the anomalous slight NOx increase when burning biodiesel; a new (old) theory. Fuel Process. Technol. 2007, 88, 659–667. (12) Muaculus, M. Measurements of the influence of soot radiation on in-cylinder temperatures and exhaust nox in a heavy-duty di diesel engine; SAE 2005-01-0925, Society of Automotive Engineers: Warrendale, PA, 2005. (13) Szybist, J.; Kirby, S.; Boehman, A. NOx emissions of alternative diesel fuels: A comparative analysis of biodiesel and FT diesel. Energy Fuels 2005, 19, 1484–1492. (14) Eckerle, W.; Lyford-Pike, E.; Stanton, D.; LaPointe, L.; Whitacre, S.; Wall, J. Effects of methyl ester biodiesel blends on NOx emissions; SAE 2008-01-0078, Society of Automotive Engineers: Warrendale, PA, 2008. (15) Tat, M.; Van Gerpen, J. Measurement of biodiesel speed of sound and its impact on injection timing; SR-510-31462; National Renewable Energy Laboratory: Golden, CO, 2003. (16) Jha, S.; Fernando, S.; Filip To, S. Flame temperature analysis of biodiesel blends and components. Fuel 2007, 87, 1982–1988.
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Figure 1. Testing point locations in an engine speed-torque map.
Figure 2. Measured torque, BSFC, PM, and NOx emissions comparison for diesel and biodiesel. The error bars correspond to one standard deviation.
Table 2. Engine Parameters parameter
value
rating displacement volume no. of cylinders valves per cylinder compression ratio intake valve opening/closing exhaust valve opening/closing intake and exhaust valve diam
325 hp at 2500 rpm 6.7 6 4 17.3 20 BTDC/200 ATDC 220 BTDC/20 ATDC 29.27 and 29.4
units hp L
CAD CAD mm
Figure 1. The shaded portion is referred to as the not-to-exceed (NTE) region, specified by the US EPA for engine emissions assessment.17 At these operating conditions, the fuel and air mixture burns almost entirely in a diffusion flame. So, unlike operating points at lower speeds and torques, these three points exhibit minimal premixed burning. At these, and other operating conditions, the engine control module (ECM) uses reference maps to make several important decisions, including the fuel injection profile and quantity, rail pressure, air handling parameters, etc. The aim of the decision making is to keep the emission levels within the EPA specified limits while simultaneously attempting to maximize fuel efficiency. The ECM decisions were dictated via Cummins proprietary software to ensure that all the inputs to the engine except for the fuel properties remained constant across different fuels. The goal was to examine the difference in combustion behavior of the two fuels due to fuel property differences. The ECM parameters that were held constant for a given speed/demanded torque operating condition include pilot, post, and total fueling (mg/stroke), timing for the start of the main pulse (deg before TDC (BTDC)), pilot to main and main to post separation timings (μs), rail pressure (bar), EGR fraction, and charge flow (kg/min). After allowing the engine to attain steady state by running for 5 min, data was collected for conventional diesel and biodiesel at each operating point using the on-engine sensors as well as the external sensors. The in-cylinder pressure was logged at 50 kHz in order to achieve good resolution for accurately calculating the heat release rate, indicated mean effective pressure (IMEP), centroid of heat release, and peak in-cylinder pressure. Other data collected with the dSPACE system were logged at 0.1 Hz while PM data was logged at 1 Hz using an AVL software interface for the analyzer. The experimental data was used to calculate various quantities including gross indicated mean
Figure 3. Measured (normalized) heat release rate comparison between diesel and biodiesel combustion.
effective pressure (IMEP), apparent heat release rate,18,19 and brake specific NOx and PM emissions. 3.3. Anticipated and Observed Results. Soy-biodiesel and diesel have different physical and chemical properties, as summarized in Table 1. Biodiesel contains approximately 11% oxygen by weight, resulting in a lower energy content than diesel fuel. Due to the reduction in energy content, the brake specific fuel consumption (BSFC) is expected to increase with the use of biodiesel. In other words, more fuel is generally required to get the same amount of work done with biodiesel since it is less energy dense. The experimental observations were consistent with these expected trends, as shown in Figure 2. All the data presented here have been normalized due to confidentiality using the corresponding values obtained for the 2000 rpm, 350 lb-ft operating point with diesel fuel. The reduction in torque was 10.7%, 12.3%, and 10.3% at the three points with biodiesel. The percentage increase observed for BSFC was 11.3%, 10.3%, and 10.8% for the three operating conditions. Interestingly, the heat release rates for the two fuels are very similar as shown in Figure 3, indicating that there are only very minor differences in the rate of combustion, even though there are considerable differences in the fuel properties (namely molecular composition, evaporation, and heating value, as shown in Table 1). According to a 2002 US EPA report on emission impacts of biodiesel for 1997 and older heavy-duty highway engines, the emissions of PM, CO and unburned hydrocarbons (UHC) typically reduce dramatically with increases in biodiesel
(17) Control of Emissions of Air Pollution From New Motor Vehicles: In- Use Testing for Heavy-Duty Diesel Engines and Vehicles; United States Environmental Protection Agency: Washington, D.C., 2005. (18) Stone, R. Introduction to Internal Combustion Engines, 3rd ed.; Society of Automotive Engineers: Warrendale, PA, 1999. (19) Heywood, J. Internal Combustion Engine Fundamentals; McGraw-Hill: New York, 1998.
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not made, given the significant challenges reported in the literature. 4.1. Base Model Validation Using Cummins-Provided Diesel Data. In the prior work done at Purdue University and the Cummins Technical Center,23 the simulation model was calibrated for an engine from the same family as the experimental engine used for this study, using experimental data obtained from tests performed at Cummins Inc. with conventional diesel fuel. The basis of the model includes 1D compressible flow equations for gas flow through the intake and exhaust manifolds coupled with a quasi-dimensional, multizone predictive combustion model to simulate the diesel spray penetration, evaporation, entrainment, mixing, ignition, and combustion. Quasi-dimensional models25,28,29,30 build on the zero-dimensional modeling approach by incorporating multiple zones in the cylinder. Using this approach, it is possible to simulate fuel spray evolution (including transport, evaporation, and air entrainment) and spatial variations in mixture composition and temperature, both prior to, during, and after combustion. The result is a fairly accurate, computationally inexpensive model capable of estimating diesel engine combustion performance and NOx emissions. This approach however, is generally speaking not as accurate as more complex multidimensional combustion models which completely couple the chemical kinetics and fluid mechanics in three dimensions.31-34 The quasi-dimensional model does not represent the in-cylinder temperature and composition inhomogeneities as accurately as the multidimensional models and, therefore, is not as accurate in predictions of soot emissions.23 The model used in this study is a quasi-dimensional model incorporating 1000 zones in the cylinder. It effectively captures the interaction between the EGR loop and variable geometry turbocharger (VGT) and incorporates complex multipulse fuel injection. Fuel injection mass flow rate profiles and total fuel quantities were dictated to the model and the target EGR fraction and air flow were achieved by EGR valve position and VGT rack position modulation in a manner consistent with EGR/air-path control in modern diesel engines.23 The predictive capability of the model was demonstrated at 22 engine operating conditions in the NTE region.
percentage. As shown in Figure 2, dramatic reductions in particulate matter of 90%, 92.8%, and 93.3% were observed in this study.5 NOx emissions on the other hand generally show an increase of approximately 10% from diesel to biodiesel,5 though the biodiesel NOx observations reported in the literature show some variation. Some studies have reported no change9 or even reductions20 in NOx emissions, while most studies have reported increases in NOx emissions.8,21,4,22 According to studies by McCormick et al.8 and Eckerle et al.,14 the NOx increase as a result of biodiesel combustion may be more significant in modern engines. In agreement with those studies, the Purdue experimental results show significant increases in NOx of 18.9%, 16.7%, and 37.9%, respectively, at the three points considered, as seen in Figure 2. In sum, the experimental results at three different diffusion combustion-dominated operating points in a modern six cylinder diesel engine show anticipated increases in fuel consumption and NOx and decreases in PM and torque. It should be stated that the increases and decreases in NOx and PM, respectively, are generally much higher than reported elsewhere.
4. Simulation Modeling In an effort to determine the cause of, and ultimately develop mitigation strategies for, increased biodiesel fuel consumption and NOx emissions as well as describe other differences between diesel and biodiesel combustion outputs, a simulation model was developed and validated. This work extends the use of a previously created engine model23 to capture biodiesel combustion effects that were experimentally observed;mainly, increases in BSFC and NOx and reductions in torque. The model described in ref 23 was calibrated for a similar Cummins diesel engine with data provided by Cummins at 22 operating conditions over a wide operating range. The same model with minor changes in calibration has been used for this study. The model was calibrated for diesel combustion in an engine at Purdue University, and biodiesel combustion was reflected in the model by changing the fuel properties as will be presented in section 5. The base engine model was developed in GT-Power24,25 to incorporate modern diesel engine components including the intake and exhaust manifolds, valves, multipulse injector, cylinders, turbocharger, and cooled EGR loop. The model incorporates a multizone, quasi-dimensional combustion submodel (“DI-Jet”) which has been shown to be capable of predicting in-cylinder heat release, pressure, and NOx.25-27,23 An attempt to simulate particulate matter formation was
(28) Yoshizaki, T.; Nishida, K.; Hiroyasu, H. Reduction of heavy duty diesel engine emission and fuel economy with multi-objective genetic algorithm and phenomenological model; SAE 2004-01-0531, Society of Automotive Engineers: Warrendale, PA, 2004. (29) Jung, D.; Assanis, D. Multi-zone DI diesel spray combustion model for cycle simulation studies of engine performance and emissions; SAE 2001-01-1246, Society of Automotive Engineers: Warrendale, PA, 2001. (30) Pitsch, H.; Barths, H.; Peters, N. Three-dimensional modeling of NOx and soot formation in DI-diesel engines using detailed chemistry based on the interactive flamelet approach; SAE 962057, Society of Automotive Engineers: Warrendale, PA, 1996. (31) Gopalakrishnan, V.; Abraham, J. Computed NO and soot distribution in turbulent transient jets under diesel conditions. Combust. Sci. Technol. 2004, 176, 603–641. (32) Amsden, A.; Butler, T.; O’Rourke, P.; Ramshaw, J. KIVA-A comprehensive model for 2-D and 3-D engine simulations; SAE 850554, Society of Automotive Engineers: Warrendale, PA, 1985. (33) Agarwal, A.; Assanis, D. Multi-dimensional modeling of natural gas ignition under compression ignition conditions using detailed chemistry; SAE 980136, Society of Automotive Engineers: Warrendale, PA, 1998. (34) Xin, J.; Montgomery, D.; Han, Z.; Reitz, R. D. Multidimensional modeling of combustion for a six-mode emissions test cycle on a DI diesel engine. J. Eng. Gas Turbines Power 1997, 119 (3), 683–691.
(20) Proc, K.; Barnitt, R.; McCormick, R.; Technical Report NREL RTD Biodiesel (B20) Transit Bus Evaluation: Interim Review Summary; National Renewable Energy Laboratory: Golden, CO, 2005. (21) Sharp, C.; Ryan, T.; Knothe, G. Heavy-duty diesel engine emissions tests using special biodiesel fuels; SAE 2005-01-3671, Society of Automotive Engineers: Warrendale, PA, 2005. (22) Tat, M. Investigation of Oxides of Nitrogen Emissions from Biodiesel-Fueled Engines. Ph.D. thesis, University of Illinois at UrbanaChampaign, 2003. (23) Shaver, G.M.; Frazier, T. R.; Kulkarni, A.; Popuri, S.; Stanton, D. W. Validation of a computationally efficient wholeengine model of a Cummins 2007 turbocharged diesel engine. J. Gas Turbines Power, in press. (24) GT-Power User’s Manual; version 6.2, Gamma Technologies, 2006. (25) Morel, T.; Wahiduzzaman, S. Modeling of diesel combustion and emissions. XXVI FISITA Congress, Prague, June 16-23, 1996. (26) Ciesla, C.; Keribar, R.; Morel, T. Engine/powertrain/vehicle modeling tool applicable to all stages of the design process; SAE 200001-0934, Society of Automotive Engineers: Warrendale, PA, 2000. (27) Morel, T.; Keribar, R.; Silvestri, J.; Wahiduzzaman, S. Integrated engine/vehicle simulation and control; SAE 1999-01-0907, Society of Automotive Engineers: Warrendale, PA, 1999.
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Table 3. Average Absolute Error Across All 22 Operating Points (From a Previous Study Utilizing Data Supplied by Cummins, Inc.)23 a
Table 5. Average Absolute Error between Purdue Engine Data and Simulation for Three Operating Points Considereda Average absolute error %
Average Absolute Error airflow
charge flow
A/F
Tur-in-p
3.93
3.86
3.75
5.99
EGR
NOx
IMT
PCP
1.67
11.00
4.99
4.30
torque
BSFC
IMEP
IMP
3.4
3.72
4.34
4.13
airflow
charge flow
A/F
Tur-in-p
3.4
3.2
3.9
2.1
EGR
NOx
IMT
2.8
9.2
2.1
3.9
torque
BSFC
IMEP
IMP
4.17
4.75
4.19
5.09
PCP
a
Where A/F is the air-to-fuel ratio, Tur-in-p is the turbine inlet pressure, IMT is the intake manifold temperature, PCP is peak cylinder pressure, and IMP is the intake manifold pressure. Fuel: petroleum diesel.
a Where A/F is the air-to-fuel ratio, Tur-in-p is the turbine inlet pressure, IMT is the intake manifold temperature, PCP is peak cylinder pressure, and IMP is the intake manifold pressure. Fuel: petroleum diesel.
Table 4. Model Calibration Parameter Settings cal parameters
ref cal
new cal
entrainment mult before combustion entrainment mult. after combustion entrainment mult. after impingement combustion rate mult droplet drag mult droplet evaporation mult droplet breakup length mult Sauter mean diam mult activation temp overall combustion delay mult delay exponent for pressure overall NOx mult compressor mass mult compressor efficiency mult convection mult
0.8 0.5 1.2 0.6 0.9 1.0 1.0 1.0 3000 K 1.0 -1.25 1.5 1.0 0.97 0.9
0.8 0.5 1.2 0.6 0.9 1.0 1.0 1.0 3000 K 1.0 -1.25 1.5 1.0 0.9 0.7
As summarized in Table 3, the average absolute error of the model for 12 important engine outputs at the 22 operating conditions is less than 6% for predictions of variables related to the gas exchange process, less than 5% for engine performance parameters and 11% for NOx emissions. 4.2. Model Calibration for Purdue Engine Using Diesel Fuel. Model calibration for the engine at Purdue was also completed, in this case using diesel data at the three different points tested in the NTE region as described in section 3. The engine hardware configuration at Purdue is the same as the engine used to generate the data leveraged in the prior work.23 Table 4 shows that the majority of the calibration parameter values are the same as those for the previous calibration23 which used data from a different engine at Cummins. The same model calibration (with only minor changes) was therefore appropriate for a different engine setup indicating the robustness of the modeling and data collection approaches. Table 5 and Figure 5 demonstrate the effectiveness of the model at capturing the performance, NOx emissions, and the gas exchange process including the air and charge flows, the air-to-fuel ratio, and EGR fraction. The two solid lines in the figures are drawn at (10% values of the experimental data. The data presented here has been normalized for confidentiality purposes. Figure 4 shows the apparent heat release rate comparisons between the model predictions and the calculated experimental heat release rates based on in-cylinder pressure measurements. The experimental curves for heat release rates are from pressure data collected from different cylinders during tests on different days.
Figure 4. Normalized apparent heat release rate comparison. Fuel: diesel.
Figure 5. Model predictions for conventional diesel combustion. The error bars correspond to one standard deviation.
In summary, the model captures the performance, NOx emissions, and gas exchange behavior on two completely different engines using conventional diesel fuel. 5. Biodiesel Combustion Model Predictions Simulation of biodiesel combustion was implemented in the model by changing the fuel molecular composition, density, heat of vaporization, distillation curve, and lower heating 5825
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Adi et al. Table 7. Observed Biodiesel-Induced Changes in Performance Metrics and NOx Emissions for Biodiesel 1600 rpm 650 lb-ft
experimental simulation
Figure 7. Model predictions for biodiesel combustion. The error bars correspond to one standard deviation. Table 6. Average Absolute Error between Purdue Engine Biodiesel Data and Simulation for the Three Operating Points Considered Average absolute error % charge flow
A/F
Tur-in-p
3.2
3.3
2.5
EGR
NOx
IMT
PCP
0.7 torque
9.4 BSFC
2.8 IMEP
7 IMP
3.2
3.4
4.3
3.8
10.3 10 9.9 7.6
% increase in brake specific fuel consumption experimental 11.3 13.3 simulation 16 10.5
10.8 10.6
% increase in NOx 19 16.7 21.5 18.7
38 44.3
combustion at a certain speed and fueling quantity due to the lower energy density of biodiesel, while brake specific fuel consumption is expected to increase. The model predicted reductions in torque of 14%, 10%, and 10% for the three operating points, while the experimental reductions were 10.7%, 12.3%, and 10.3% at the corresponding points. The model predictions for biodiesel IMEP, which is a measure of work output, showed 11.3%, 7.7%, and 7.6% decreases for biodiesel as compared to diesel, and the experimentally observed reductions at the three operating conditions were 10.2%, 11%, and 9.9%, respectively. The model prediction showed increases in BSFC of 16%, 10.5%, and 10.6% as compared to the 11.3%, 13.3%, and 10.8% increases observed experimentally. As discussed previously, biodiesel combustion leads to higher NOx at some operating points. This trait was observed experimentally with NOx increases at the three operating points of 19%, 16.7%, and 38%. The corresponding simulation predictions are consistent with the experimental data with increases in NOx of 21.5%, 18.7%, and 44.3% for biodiesel. In summary, Table 7 demonstrates that the simulation model reasonably captures the torque, IMEP, BSFC, and NOx trends for biodiesel. Good correlations with the experimental biodiesel results, especially the trends seen between diesel and biodiesel, imply that, as desired, only key fuel property changes in the model are required for capturing experimentally observed behavior. The above results indicate the capability of the model to capture the expected and experimentally recorded trends in the performance and NOx emissions for both diesel and biodiesel fuels. This is essential for leveraging the model to develop hypotheses for the cause of, and ultimately develop mitigation strategies for, biodiesel-induced increases in BSFC and NOx.
Figure 6. Normalized apparent heat release rate comparison. Fuel: biodiesel.
3.4
% decrease in torque 10.7 12.3 14 10
2300 rpm 650 lb-ft
% decrease in indicated mean effective pressure experimental 10.2 11 simulation 11.3 7.7
experimental simulation
airflow
2000 rpm 350 lb-ft
6. Model Limitations
value to the values given in Table 1. Minimal changes to ignition delay parameters were made to obtain a good heat release fit with the experimental data as shown in Figure 6. Figure 7 and Table 6 demonstrate that the model accurately captures the biodiesel combustion, performance, NOx emissions, and gas exchange processes including the fresh air flow, charge flow, air-to-fuel ratio, EGR fraction, BSFC, and NOx. In Figure 7, the two solid lines in the figures are drawn at (10% values of the experimental data. Table 7 shows the effect of biodiesel on these quantities as observed experimentally and as predicted by the model. Torque and IMEP were both expected to drop with biodiesel
Before using the model results to generate hypotheses for biodiesel-induced increases in NOx at certain operating conditions, it is important to identify certain limitations in the model’s ability to predict emissions as mentioned in section 4.2. The quasi-dimensional model is not as accurate as more multidimensional combustion models especially in soot emission predictions. The inaccuracy in soot emissions predictions limits the model’s capability to capture any soot radiationinduced effect on NOx that might exist, as described in section 1. However as shown in sections 4 and 5, and in refs 25-27 and 23, the modeling approach has been shown to be 5826
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Figure 9. Hypothesis for observed increases in biodiesel NOx.
consistent with the model predicted NOx increases depicted in Figure 7 and Table 7. Here, the combustible oxygen mass fraction (COMF) is defined as follows: YO2 , charge mcharge þ YO, fuel mfuel ð1Þ COMF ¼ mcharge þ mfuel where Y represents mass fraction and m represents mass. COMF represents the fraction of total oxygen in the cylinder that is available for combustion. It includes the oxygen from the charge and the fuel but does not include the oxygen atoms associated with the following recirculated combustion products: NOx, CO2, H2O etc. Results similar to Figure 8 are observed at the other two operating points. Conventional mixing-controlled diesel engine combustion generally consists of two modes;premixed and diffusion combustion. Premixed combustion is more prevalent early in the process, while the majority of the fuel is consumed later in a diffusion flame.19,35 For both modes of combustion, increased biodiesel flame temperatures are hypothesized, in the discussion that follows, to be the primary factor causing the increased NOx concentrations for biodiesel. In the premixed combustion mode, it appears to be equivalence ratio differences which are the cause of the increased biodiesel flame temperatures. In the diffusion combustion mode, it is hypothesized that higher oxygen fractions within the diffusion flame are the cause of the increased biodiesel flame temperatures. These hypotheses for the increased biodiesel NOx for the three diffusion-dominated operating points considered in this study are summarized in Figure 9 and will be presented in detail in the following sections. 7.1. NOx Increases During Premixed Combustion. As shown in Figure 8a, the near-flame fuel-to-charge ratios predicted by the model for the 1600 rpm, 650 lb-ft operating point are different at the initial stages of combustion as a consequence of the higher biodiesel distillation temperatures (Table 1). Because of the higher distillation temperatures, biodiesel does not vaporize as readily as diesel; therefore, not as much vaporized fuel is mixed with the air in the early stages of combustion. The impact of the distillation temperatures on the fuel-to-charge ratio predictions was confirmed by running a “modified” biodiesel fuel in the model with the same distillation temperatures as diesel,
Figure 8. Model predictions for diesel and biodiesel at the 1600 rpm, 650 lb-ft operating condition.
capable of capturing in-cylinder heat release, NOx, pressure, IMEP, and BSFC. Furthermore, the performance and NOx emissions trends seen experimentally between diesel and biodiesel are effectively captured by the model as demonstrated in Table 7. This model capability provides an opportunity for developing hypotheses for the cause of NOx emissions increases observed at the three operating points considered in this study. 7. Hypotheses for Observed Increases in Biodiesel NOx The model allows the “observation” of in-cylinder phenomena which are difficult or impossible to observe in experimental studies, including (1) fuel transport, air entrainment, and evaporation processes; (2) local temperatures and combustible oxygen mass fractions; and (3) NOx generation rates. As will be presented in this section, these model predictions motivate a hypothesis for the biodiesel-induced increases in NOx at the diffusion combustion-dominated operating conditions considered in this study. Figure 8 shows various model predictions for the 1600 rpm, 650 lb-ft operating point plotted against the fraction of fuel burned, including the (a) local fuel-to-charge ratio which is a ratio of the fuel mass to the total charge (fresh air and EGR) mass, (b) the near flame equivalence ratios, (c) the combustible oxygen mass fractions, (d) postcombustion gas temperatures, (e) NOx formation rates, and (f) the average cylinder NOx concentration. Note that the NOx formation rate and concentration elevations for biodiesel in Figure 8e are
(35) Dec, J. A conceptual model of DI diesel combustion based on lasersheet imaging; SAE 970873, Society of Automotive Engineers: Warrendale, PA, 1997.
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ratios which are closer to stoichiometric in the flame at the flame lift-off length.36,37 7.2. NOx Increases During Diffusion Combustion. The above explanation may hold true for the premixed portion of the combustion process; however, in the diffusion portion of the process, the equivalence ratio at the flame front is essentially always at a stoichiometric value.39-41 Therefore, once the fuel is largely being consumed in a diffusion flame, it is more relevant to consider the fraction of oxygen that is present. It is well-known that higher oxygen fractions yield higher diesel combustion temperatures and NOx formation rates for diffusion flames.42-46 Figure 8c shows that the predicted combustible oxygen mass fraction is consistently higher for biodiesel than for diesel during the near-stoichiometric conditions (also true during rich conditions), resulting in increased NOx formation (Figure 8e and f). The reason for higher combustible oxygen mass fractions for the biodiesel combustion cases considered here is twofold. First, there is additional oxygen in the flame that is contributed directly by the oxygen in the fuel. Second, when EGR is present, the mass fraction of O2 in the charge gas is higher for biodiesel than for diesel. This is because exhaust O2 fractions are higher for biodiesel.47 Since the exhaust contains more O2, the same EGR fraction will result in higher charge O2 fractions for biodiesel. In fact, the principal reason that EGR is effective at reducing NOx in diesel combustion is that it lowers the oxygen fraction.42-46 Reduced oxygen fractions reduce flame temperatures because, while the flame is still stoichiometric, more inert species (CO2, H2O, etc.) are present to absorb the heat of combustion.42,44 In biodiesel combustion, EGR is not as effective at reducing temperature because it is not as effective at reducing the oxygen fraction, due to the oxygen present in the fuel and the additional oxygen in the EGR gas following biodiesel combustion. Or, in other words, more EGR is required to reduce charge oxygen
Figure 10. Calculated constant pressure adiabatic flame temperatures vs fuel-charge ratio and equivalence ratio using HPFLAME equilibrium code.38 AFR and EGR correspond to the 1600 rpm, 650 lb-ft case. Reactant temperature = 865 K. Pressure = 100 bar.
resulting in nearly identical fuel-charge ratios (results not shown). The difference in fuel-to-charge ratios coupled with the difference in the stoichiometric air-to-fuel ratio (AFR) between diesel and biodiesel, affect the near-flame equivalence ratios for the two fuels, as shown in Figure 8b. Equivalence ratio is defined as the ratio of stoichiometric AFR to the actual AFR. More precisely, in the case of oxygenated fuels, the equivalence ratio is defined as the ratio of stoichiometric oxygen-to-fuel ratio to the actual oxygen-to-fuel ratio. The stoichiometric oxygen-to-fuel ratio for biodiesel is lower than that of diesel (2.974 vs 3.309). So, as can been seen from Figure 8b, biodiesel exhibits lower equivalence ratios than diesel throughout the combustion process. Again, this is for two reasons: the lower fuel-to-charge ratios (in the early stages of combustion) and also the lower stoichiometric oxygen-to-fuel ratio. Figure 8b shows that, for a portion of the combustion process, the predicted equivalence ratio near the flame is rich (i.e., >1). Most premixed combustion in conventional diesel engines occurs in fuel-rich regions.35 Under these rich, premixed conditions the near-flame equivalence ratio has a profound effect on the postcombustion temperature, as shown in Figure 10. It can be seen that there is not much difference between the flame temperatures for diesel and biodiesel at any a particular equivalence ratio. However, if the adiabatic flame temperatures for the two fuels are compared with respect to the fuel-to-charge ratio, a significant difference can be observed in the flame temperatures for diesel and biodiesel at the same fuel-to-charge ratio as seen in Figure 10. Specifically, higher temperatures result for mixture conditions just rich of stoichiometric. Because the premixed combustion is rich and equivalence ratios for biodiesel are lower than diesel, biodiesel combustion is closer to stoichiometric, resulting in higher predicted flame temperatures and NOx formation rates (Figure 8d and e). This hypothesis, summarized in the top half of Figure 9, is consistent with studies using an optical access engine which reported that biodiesel combustion exhibits equivalence
(37) Cheng, A.; Upatnieks, A.; Mueller, C. Investigation of the impact of biodiesel fuelling on NOx emissions using an optical direct injection diesel engine. Int. J. Engine Res. 2006, 7, 297–318. (38) Olikara, C.; Borman, G. A computer program for calculating properties of equilibrium combustion products with some applications to I. C. engines; SAE 750468, Society of Automotive Engineers: Warrendale, PA, 1975. (39) Turns, S. An Introduction to Combustion, 2nd ed.; McGraw-Hill: New York, 2000. (40) Glassman, I. Combustion, 3rd ed.; Academic Press: San Diego, CA, 1996. (41) Lapuerta, M.; Armas, O.; Rondriguez-Fernandez, J. Effect of biodiesel fuels on diesel emissions. Prog. Energy Combust. Sci. 2007, 34, 198–223. (42) Ladommatos, N.; Abdelhalim, M.; Zhao, H.; Hu, Z. The dilution, chemical, and thermal effects of exhaust gas recirculation on diesel engine emissions - part 1: Effect of reducing inlet charge oxygen; SAE 961165, Society of Automotive Engineers: Warrendale, PA, 1996. (43) Ropke, S.; Schweimer, G.; Strauss, T. NOx formation in diesel engines for various fuels and intake gases. SAE 950213, Society of Automotive Engineers: Warrendale, PA, 1995. (44) Nakayama, S.; Fukuma, T.; Matsunaga, A.; Miyake, T.; Wakimoto, T. A new dynamic combustion control method based on charge oxygen concentration for diesel engines; SAE 2003-01-3181, Society of Automotive Engineers: Warrendale, PA, 2003. (45) Ogawa, H.; Miyamoto, N.; Shimizu, H.; Kido, S. Characteristics of diesel combustion in low oxygen mixtures with ultra-high EGR; SAE 2006-01-1147, Society of Automotive Engineers: Warrendale, PA, 2006. (46) Mitchell, D.; Pinson, J.; Litzinger, T. The effects of simulated EGR via intake air dilution on combustion in an optically accessible DI diesel engine; SAE 932798, Society of Automotive Engineers: Warrendale, PA, 1993. (47) Snyder, D.; Adi, G.; Bunce, M.; Satkoski, C.; Shaver, G. Steadystate biodiesel blend estimation via a wideband oxygen sensor. J. Dynam. Syst. Measur. Control 2009, 131(4), 1-9.
(36) Mueller, C.; Boehman, A.; Martin, G. An experimental investigation of the origin of the increased NOx emissions when fueling a heavy-duty comression-ignition engine with soy biodiesel; SAE 2009-01-1792, Society of Automotive Engineers: Warrendale, PA, 2009.
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the other engine parameters constant at each point. The main conclusions of this work can be listed as follows: 1. The comparison of diesel and biodiesel data showed reductions in PM of about 90%, increases in NOx as high as 38% at one operating point, and increases in BSFC up to 13%. 2. The diesel data was used to calibrate a model which incorporates a multizone, quasi-dimensional combustion submodel that predicts engine performance and NOx emissions. Model predictions for heat release rate, torque, IMEP, BSFC, and NOx emissions are in satisfactory agreement with data from both diesel- and biodiesel-fueled engine experiments, indicating the model’s usefulness for investigating the cause of the observed increases in biodiesel combustion fuel consumption and NOx emissions. 3. Hypotheses for biodiesel NOx increases (at the operating conditions considered here) link the near stoichiometric equivalence ratios in the rich, premixed portion of the flame as well as higher combustible oxygen mass fraction in the diffusion flame front to higher temperatures and NOx formation rates for biodiesel combustion.
Figure 11. Calculated constant pressure stoichiometric adiabatic flame temperatures vs EGR fraction using HPFLAME equilibrium code.38 AFR corresponds to the 1600 rpm, 650 lb-ft case. Reactant temperature = 865 K. Pressure = 100 bar.
fractions for biodiesel. This is evident from the calculated constant pressure stoichiometric adiabatic flame temperatures shown in Figure 11. Note that there is very little difference between stoichiometric adiabatic flame temperature when there is no EGR. However, as the EGR fraction is increased, the biodiesel flame temperatures becomes higher relative to diesel temperatures. This is because EGR is less effective at reducing the oxygen mass fraction for biodiesel since the EGR gas contains more oxygen following biodiesel combustion. This hypothesis may help to explain why the biodiesel NOx effect has been observed to be significantly higher in modern diesel engines with EGR.8,14
Acknowledgment. Funding for this work was provided by Cummins Inc., the Office of Naval Research, the Energy Center at Purdue Discovery Park, and the Purdue Research Foundation. The authors wish to thank the following researchers at the Cummins Technical Center in Columbus, Indiana: Tim Frazier, Wayne Eckerle, and Sriram Popuri. Also, special thanks to Cummins Inc., for providing the engine and technical support. The authors are also grateful to the Ray W. Herrick Laboratories Technical Services staff of Fritz Peacock, Bob Brown, Gil Gordon, and Frank Lee. We would also like to thank British Petroleum for providing the fuel and fuel analysis.
8. Conclusions The main purpose of this study was to develop an enhanced understanding of the impact of biodiesel combustion on engine performance characteristics and NOx emissions. Experimental tests were conducted on a 6.7L six cylinder Cummins 2007 diesel engine with conventional diesel and biodiesel fuel at three diffusion combustion-dominated operating conditions to study the effect of biodiesel combustion on the engine outputs. The only change made during the tests was the fuel, keeping all
Note Added after ASAP Publication. There were changes made to the Acknowledgement section in the version of this paper published ASAP October 26, 2009; the corrected version published ASAP November 3, 2009.
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