Article pubs.acs.org/EF
Impact of Physical and Chemical Properties of Alternative Fuels on Combustion, Gaseous Emissions, and Particulate Matter during Steady and Transient Engine Operation Ashwin A. Salvi,* Dennis Assanis,† and Zoran Filipi‡ Department of Mechanical Engineering, Walter E. Lay Automotive Laboratory, University of Michigan, Ann Arbor, Michigan 48109, United States ABSTRACT: Properties of alternative fuels, such as cetane number, aromatic content, bulk modulus, etc., can vary significantly; thus, a detailed investigation is conducted to assess the impacts on engine combustion, efficiency, and emissions during steady-state and transient engine conditions. Insights can be used to maximize the benefits of alternative fuels or to avoid potential problems that stem from differing ignition and combustion characteristics of particular fuels. In this work, biodiesel (20%, 50%, and 100% biodiesel blends), jet fuel JP8, and synthetic jet fuel S8 are compared to diesel #2 (ULSD). Comparisons are made on ignition, combustion, particulate matter, particulate size spectra, and NOX emission at steady state under harmonized operating conditions, that is, with equalized fuel energy input. All experiments are conducted using the same baseline calibration originally developed for diesel fuel. Ignition delay and combustion phasing trends suggest the need for possible adaptation of ECU calibration. The impact of alternative fuels on particulate emission depended strongly on fuel aromatic content and oxygenation. NOX emission was correlated to multiple physical and chemical fuel properties. While the significance of different properties on NOX emission varied with engine condition, the CA50 location was strongly correlated at all conditions. This paper also includes detailed insight into particulate spectra obtained for all six fuels. Transient engine operation over a driving schedule was characterized with the engine-in-the-loop setup. The use of alternative fuels caused more aggressive cyber-driver behavior as a reaction to their lower energy densities. Different fueling histories were recorded with the alternative fuels and led to marked changes in instantaneous emissions traces. In particular, spikes of particulate emission in the exhaust that typically occur at accelerator tipin were reduced with biodiesel, while extended high load operation was observed in order to follow the demanded vehicle velocity trace and led to higher NO. Cumulative results over the complete FTP 75 driving schedule indicate that transient engine operation reduces the particulate matter benefits of the alternative fuels. JP8 and S8 show slight NO emission benefits with biodiesel showing slightly worse NO emission when compared to steady state. The more aggressive driver behavior led to worse fuel economy.
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INTRODUCTION Petroleum consumption in the United States was slightly over 39 quadrillion kJ in 2008, representing nearly 38% of all energy consumed.1 Of those 39 quadrillion kJ, nearly 72% is consumed by the transportation sector.2 Since diesel fuel accounts for approximately 22% of the total transportation fuel consumption in the United States,2 the use of alternative and renewable fuels would help displace a fraction of petroleum fuel, promote energy security, and help mitigate concerns of greenhouse gas emissions on climate impact. However, alternative fuel combustion still generates pollutants, and criteria emissions are still as much a concern as they are with conventional fuel. This study examines four different types of fuels (diesel, biodiesel, jet fuel, and synthetic fuel) to determine the impact of a broad range of fuel properties, such as bulk modulus, cetane number, aromatic content, and oxygen content, on the performance of a medium-duty diesel engine. Soy methyl ester biodiesel was selected due to its partially renewable nature and large availability in North America, jet fuel for its relevance in special applications such as military use, and synthetic jet fuel, produced using the Fischer−Tropsch gas to liquids (GTL) process, due to its significance for energy security. Advanced instrumentation is utilized to correlate the physical and chemical properties of the fuel to variations in combustion and emissions. © 2012 American Chemical Society
Previous diesel engine research has developed some trends in physical fuel properties to engine out emissions. The bulk moduli of fuels can have an impact on the fuel injection timing,3−5 possibly affecting fuel/air mixing, ignition delay, and combustion phasing. Advancements in combustion phasing have been seen to increase NOX emission,6,7 with some research showing negligible NOX increase when combustion phasing is normalized with a change in injection timing or injection pressure.8 Certain trends in chemical fuel properties can also be correlated to engine out emissions. It is generally accepted that an increase in the aromatic content of a fuel also increases particulate emission, while an increase in fuel oxygenation, as in the case of biodiesel, decreases particle emission.5,9−16 In addition, Schönborn et al. associated an increase in the number of double bonds in the fuel chain to an increase in soot emission.17 Benjumea et al. also saw an increase in soot emission, although only slightly, with an increase in the number of double bonds in the fuel,18 while Feng et al. reported a strong correlation.19 Benjumea et al. cited that although an increase in Received: March 28, 2012 Revised: June 8, 2012 Published: June 11, 2012 4231
dx.doi.org/10.1021/ef300531r | Energy Fuels 2012, 26, 4231−4241
Energy & Fuels
Article
With less soot to act as a heat sink, the in-cylinder temperatures can increase allowing for the thermally dependent Zeldovich reactions to produce more NOX. Oxygen in the fuel can also act to increase the stoichiometric zone of the diffusion flame, thus increasing the localized and overall flame temperature and NOX production. Adi et al. suggest that biodiesel fuel increases NOX emission by operating closer to stoichiometric ratios during premixed combustion and supply higher combustible oxygen mass fractions in the diffusion flame. In addition, higher concentrations of O2 can be found in the exhaust gases with biodiesel combustion, which could increase the flame temperature and NOX emission in engines with exhaust gas recirculation.27 Benjumea et al. correlated an increase in NOX emission to increased premixed combustion due to an increase in double bonds.18 The increase in double bonds can also increase hydrocarbon radicals and contribute to prompt NOX,28 while Ban-Weiss et al. advanced a theory that increased double bonds cause higher flame temperatures.3 However, Mueller et al. state that while the adiabatic flame temperature does contribute to NOX emission, it is not a primary driver.7 Mueller et al. summarize that NOX formation with biodiesel is a result of multiple coupled parameters affecting incylinder and flame temperatures: advanced combustion phasing, lower radiated heat loss, and larger stoichiometric zones in the diffusion flame causing higher flame temperatures. With sometimes drastic variations in fuel properties such as cetane number, aromatics, oxygenates, and energy content, impacts on combustion and emissions can be significant. It is important to distinguish between variations in engine performance and emissions that can be linked directly to variations in ignition and combustion due to physical properties (e.g., bulk modulus, density) and those attributable to chemical composition (e.g., aromatics, presence of oxygen in the molecule) or both (e.g., cetane number), since variations in ignition can be compensated for with modifications to the engine calibration. However, combustion effects linked to the chemical properties of the fuel will be unavoidable. Thus, the objective of this research is to develop insights into the physical and chemical properties of alternative fuels and determine their impacts on engine combustion, emissions, and performance on a medium-duty diesel engine. This paper is organized as follows: a brief description of the experimental setup is introduced first, followed by the fuel properties and approach. Results and discussions of the steady-state and transient experiments are next, and finally, we state our conclusions.
the number of double bonds should lower smoke emission through an increase in premixed combustion, soot precursors actually increased and led to higher smoke. Variations of fuel cetane number will have a direct impact on ignition delay and the ratio between premixed and diffusion burning but the impact on emissions is a bit unclear. Schönborn et al. found NOX emission was greatly increased with an increase in premixed combustion, a characteristic associated with fuel cetane number and in-cylinder temperature.17 Szybist et al. concluded that NOX emission was not as sensitive to the maximum cylinder temperature and rate of heat release but was more sensitive to the crank angle timing where these maxima occurred.6 A literature review20 conducted by the Environmental Protection Agency (EPA) reported that NOX emission generally increased with increasing fraction of biodiesel in a fuel. However, considerable scatter is seen in the data, with some conditions reporting lower NOX emission, as shown in Figure 1.
Figure 1. NOX values reported in literature with biodiesel combustion.20
This discrepancy could be due to many factors: different engine operating conditions, heavy duty vs medium duty vs light duty engines, fuel injection system differences (timing, pressure, unit injectors, high pressure common rail), outdated engine technology, combustion phasing, calibration differences, and the basis for comparison, such as matched fueling rate, start of injection, start of combustion, crank angle 50% location, and engine system responses, as highlighted by Song et al.21 A scientific consensus regarding the observed NOX increase has yet to be established. NOX increases have been recorded by multiple other researchers, with some attributing it to biodiesel oxygenation, contributing to the Zeldovich mechanism directly or through soot radiative heat transfer.3,4,22−25 The presence of oxygen in biodiesel fuel can result in an increase in the NO chemical kinetics rate, eq 1. Simplified NO Formation from the Extended Zeldovich Mechanism
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EXPERIMENTAL SETUP
The engine used in this study is a 2004 turbocharged direct-injection 6.0 L International V-8 diesel. This engine is equipped with a hydraulic electric unit injector (HEUI) fuel injection system that is capable of a single main injection event at pressures upward of 2000 bar. The turbocharger is equipped with a variable geometry turbine (VGT), enabling performance benefits, as well as adding a level of control over intake and backpressures. This engine is also equipped with an exhaust gas recirculation (EGR) system that provides cooled exhaust gases to the intake manifold in an effort to reduce NOX emission. In addition, the engine is designed with a retarded fuel injection strategy that introduces fuel into the cylinder after top dead center (TDC) for in-cylinder NOX mitigation. Engine parameters are monitored and controlled with a link to the engine’s power train control module. This link is established with the use of ETAS INCA software and allows the control of fuel injection timing, fuel injection pressure, VGT vane position, and EGR valve position.
d[NO] 6 × 1016 (−69090/ T ) e [O2 ]1/2 = e [N2]e (1) dt T1/2 However, Lapuerta et al. refute the influence of fuel bound oxygen on NOX emission, citing that a balance on oxygen availability shows the oxygen/fuel mass ratio is lower than conventional fuel.14 In previous studies,9,26 optical diagnostics verified that biodiesel produces less soot during combustion than diesel. 4232
dx.doi.org/10.1021/ef300531r | Energy Fuels 2012, 26, 4231−4241
Energy & Fuels
Article
The engine power is absorbed by a 330 kW AVL ELIN series 100 APA AC dynamometer. This dynamometer is a versatile machine that lends itself to transient engine operation due to a 5 ms response time and ultra low rotational inertia. During steady-state experiments, gaseous emissions (CO, CO2, EGR, THC, NO/NOX, O2) are investigated using an AVL CEB II emission analysis system while a Cambustion CLD500 measured the transient NO emission. Particulate emissions were measured by a Cambustion differential mobility spectrometer 500 (DMS500) and an AVL 415S variable smoke meter. The DMS500 differential mobility spectrometer measures the particulate number concentration according to their respective diameters with a range from 5 to 1000 nm. Particles in a sample of exhaust flow have a charge imparted on them by a corona discharger and are deflected into electrometer rings where the exhaust particles deposit their charge according to their charge to drag ratio. A correlation is then applied to determine the particle diameter and number of particles at that diameter. A dilution ratio of 4:1 was used with a charger sheath temperature of 40 °C and a cyclone temperature of 55 °C. An example output from the instrument is shown in Figure 2, where the x-axis is the log particle diameter and the y-axis is the spectral density.
the fuels. However, injection timing, injection pressure, EGR set point, and turbine vane position are fixed according to the baseline diesel condition so as to replicate the conditions in a standard engine and to provide insight that could subsequently be used for development of multifuel strategies. Biodiesel blends were volumetric mixtures of neat biodiesel with stock diesel to produce 20% and 50% blends of biodiesel. Fuel properties are shown in Table 1. The alternative fuels will be compared on the basis of combustion characteristics, gaseous emissions, and particulate matter. Using pressure-based combustion diagnostics in combination with an instrumented fuel injector, combustion characteristics such as ignition delay, rates of heat release, duration of combustion, and combustion phasing can be determined. The in-depth combustion analysis and emissions diagnostics offer insights into mechanisms responsible for variations in emissions observed with different fuels. Transient engine-in-the-loop with virtual vehicle testing is conducted to gain insights into real-world driving scenarios. Using standard engine calibration, the impacts of alternative fuels on driver behavior, simulated in-vehicle engine performance, and transient particulate and NO emissions over the FTP 75 driving schedule are investigated. The driver behavior brings an interesting new dimension, since the natural tendency of a human or cyber driver will be to compensate for variations of fuel energy content and do so in a very dynamic manner.
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RESULTS AND DISCUSSION Impact of Physical Fuel Properties. The impact of the physical properties of the fuel can be addressed by utilizing the instrumented injector to measure fuel injector needle lift and injection pressure. Select injection pressure traces from the high load case will be discussed although similar trends apply for other engine conditions. Figure 3 shows the upper and lower bounds (B100 and S8, respectively) of peak injection pressures with diesel as reference. The rate of pressure increase in the 2−9 CA degree window is different for each fuel. This phenomenon is attributed to the bulk modulus of the liquid fuel and the method of fuel injection. Among the four fuels, the literature has shown the bulk modulus of S8 to be the lowest, followed by JP8, diesel, and biodiesel.31−34 B100s higher bulk modulus leads to a higher rate of fuel injection, a higher peak of injection pressure, and an advanced location of peak injection pressure as seen in Figure 3. The opposite is true for S8, which exhibits the lowest bulk modulus of the test fuels and results in a lower fuel injection rate, lower peak injection pressure, and delayed location of peak injection pressure. This results in approximately a 130 bar injection pressure difference between B100 and S8 with B100s peak pressure occurring 0.8 CA degrees earlier.
Figure 2. DMS500 spectral density example. The area of this curve results in the number of particles per cc of exhaust flow. The conversion from spectral density to mass is performed via Hagena et al.29
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APPROACH To quantify the impact of alternative fuels on engine performance and emissions, 750 rpm 1.5 bar BMEP, 1200 rpm 7 bar BMEP, and 1800 rpm 11 bar BMEP conditions were considered and referred to as low, mid, and high load conditions, respectively. The Janssen et al.30 group performed experiments with harmonized energy input conditions by adjusting the volumetric fuel flow rate while changing the start of injection to match the CA50 location of the test fuels to the baseline diesel. The equalized energy input into the engine provides matched power output and forms a good foundation for the comparison of fuels on a combustion and emissions basis. The results presented in this paper are obtained with harmonized energy input between Table 1. Fuel Properties
density energy content cetane no. H/C ratio O/C ratio % aromatics
3
kg/m MJ/kg GJ/m3
diesel (D2)
JP8
S8
B20 (20% Bio, 80% D2)
B50 (50% Bio, 50% D2)
B100 (100% Bio)
847.7 42.7 36.1 51.4 1.792
