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Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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Catalytic Exhaust Gas Recirculation-Loop Reforming for High Efficiency in a Stoichiometric Spark-Ignited Engine through Thermochemical Recuperation and Dilution Limit Extension, Part 2: Engine Performance Yan Chang, James P. Szybist,* Josh A. Pihl, and D. William Brookshear Fuels, Engines, and Emissions Research Center, Oak Ridge National Laboratory, NTRC Building, 2360 Cherahala Blvd., Knoxville, Tennessee 37932, United States ABSTRACT: This is the second part of a two-part investigation of on-board catalytic fuel reforming to increase the brake efficiency of a multicylinder, stoichiometric spark-ignited (SI) engine. In Part 1 of the investigation, we analytically and experimentally characterized the energetics and kinetics of a candidate reforming catalyst over a range of reforming equivalence ratios and oxygen concentration conditions to identify the best conditions for efficient reforming. In the present part of our investigation, we studied an engine strategy that combined exhaust gas recirculation (EGR)−loop reforming with dilution limit extension of the combustion. In our experiments, we found that, under an engine operating condition of 2000 rpm and brake mean effective pressure (4 bar), catalytic EGR reforming made it possible to sustain stable combustion with a volumetric equivalent of 45%−55% EGR. Under this same operating condition with stoichiometric engine exhaust (and no reforming), we were only able to sustain stable combustion with EGR under 25%. These results indicate that multicylinder gasoline engine efficiency can be increased substantially with catalytic reforming combined with and higher EGR operation, resulting in a decrease of more than 8% in fuel consumption, compared to baseline operation.
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INTRODUCTION Efficiency improvements in spark-ignited (SI) engines are desired as a way to meet increasingly challenging fuel economy and CO2 emission regulations.1 External cooled exhaust gas recirculation (EGR) provides known thermodynamic benefits while maintaining compatibility with conventional three-way catalysts for emissions control.2 Caton3 summarized the thermodynamic benefits of EGR, which include reduced pumping work at partial-load conditions, decreased heat transfer due to lower cylinder temperature, and increased ratio of specific heats. External EGR is also a proven way to decrease the knocking propensity for a given fuel, which can be used as the basis for additional increases in efficiency through more advanced combustion phasing or higher compression ratio.4 Lastly, EGR reduces NOx emissions over a broad range of speed and load conditions.5 The amount of EGR dilution that can be used is limited due to cycle-to-cycle combustion instability, thereby limiting the potential efficiency benefit of EGR.6−8 The root cause of the cyclic instability with EGR has been linked to a decrease in flame speed, which elongates the initial flame kernel development process, making it more susceptible to stochastic turbulence variation.9−11 While several technologies unrelated to fuel reforming are being developed to extend the EGR dilution limit, including high-energy long-duration ignition systems12 and incorporation of different higher turbulence combustion chamber flows,13 the focus of this work is to extend the EGR dilution limit by using the reformed products of the fuel to increase the EGR dilution tolerance. The addition of the major products of fuel reforming, H2 in particular, increases the combustion rate and can increase © XXXX American Chemical Society
engine efficiency because of the shorter combustion duration.14−20 Alger et al.21 reported that H2 can also be used to extend the EGR dilution limit, with the addition of 1 vol % H2 extending the EGR limit from 25% to more than 50% for gasoline and from 20% to 28% for compressed natural gas. Fennell et al.22 showed that simulated H2-rich reformate could extend EGR dilution in an SI engine from 21% to 27% at the same combustion stability. Ivanič et al.23 added reformate at levels of 15% and 30% gasoline energy equivalent in a single cylinder engine and found that, under partial-load conditions, lean dilution can improve engine efficiency by as much as 12% while EGR dilution delivers 8% improvement. Although there are many approaches to generating reformate on board, they can generally be classified into two broad categories. The first category is where fuel is reformed in one or more cylinders in an engine using noncatalytic processes. This category includes the Dedicated EGR (D-EGR) strategy developed by Southwest Research Institute, which is the most developed strategy in this category.24 D-EGR uses fuel-rich combustion in one cylinder and recirculates its exhaust to the intake system, generating brake thermal efficiency as high as 42.5%25 and demonstrating a vehicle-level fuel consumption decrease of more than 10%.26 This category also includes injecting fuel during the negative valve overlap period for a homogeneous charge compression ignition engine to manipulate the fuel−air mixture autoignition propensity.27−32 The second category of fuel reforming, and the category of the Received: August 30, 2017 Revised: December 15, 2017 Published: December 19, 2017 A
DOI: 10.1021/acs.energyfuels.7b02565 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels WGS:
present work, is where a catalyst is used to reform the fuel outside of the engine cylinders19,22,33−40 Catalytic reforming processes often use the sensible enthalpy in the exhaust to promote reforming reactions over the catalyst to improve the energetics of reforming.18,19,33−36,31−43 The range of possible combustion and reforming reactions that can occur has been presented by Ahmed and Krumpelt44 and by Jamal and Wyszynski,18 and is presented below for completeness using iso-octane as the starting fuel. Equation 1 shows the reaction stoichiometry for the complete combustion of isooctane, while eq 2 defines the equivalence ratio (Φ). For the purposes of this manuscript, Φ refers to the O2/fuel ratio of complete combustion in eq 1, relative to the actual O2/fuel ratio. Equations 3−5 show the primary types of reactions that can occur in a reformer. When O2 is present such that the molar O/C ratio is unity, partial oxidation (POx) occurs where the parent hydrocarbon is converted to CO and H2, resulting in an exothermic process, as shown in eq 3. For an environment devoid of O2 but where H2O is present, the resultant steam reforming reaction, shown in eq 4, is highly endothermic and results in much higher concentrations of H2. Similarly, eq 5 shows a dry reforming reaction where CO2 is consumed to form H2 and CO in a process that is substantially more endothermic than steam reforming, but where the concentrations of H2 are lower. Ideally, the endothermic steam reforming reactions are driven by the sensible enthalpy in the exhaust, allowing for waste heat recovery through thermochemical recuperation (TCR).45 In this work, we studied reforming environments that contain an insufficient amount of O2 to convert all of the fuel to CO and H2 through eq 3 and thus consist of a balance of POx, steam, and dry reforming processes.
CO + H 2O → CO2 + H 2
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(−5050.2 kJ/mol)
METHODOLOGY Experimental Facility. The engine used in this study was a 2.0 L GM Ecotec LNF SI engine equipped with the production side-mounted direct injection fueling system. Engine geometry details are presented in Table 1. The combustion chamber
(1)
equivalence ratio: Φ=
(O2 /fuel)Complete Combustion (O2 /fuel)
(6)
It can be challenging to operate the engine and catalyst systems together, such that good performance of the catalytic reformer and engine are achieved simultaneously. Hwang et al.36 integrated a reforming catalyst containing Rh and Pt into the exhaust manifold of a diesel engine. While they were able to produce high concentrations of H2 (>10%), the reforming system resulted in an overall engine efficiency decrease, because the reformer catalyst equivalence ratio was too low (Φcatalyst as low as 1.5), and the reforming process was too dependent on the exothermic reactions in eq 3. This is consistent with much of the diesel reforming work where high concentrations of H2 can be produced, but because of the dependency on POx reforming, there is a large fuel energy penalty to produce the H2.46,47 In contrast, Ashida et al.33 used a steam reforming catalyst (4 wt % Rh/Al2O3, La additive) in the EGR loop of an SI engine without any additional O2 (Φcatalyst approaching ∞). They initially found high levels of fuel conversion and H2 production, but the steam reforming catalyst began deactivating immediately, with 35 ppm of S fuel resulting in a 90% deactivation within 5 h. In the companion paper (Part 1, “Catalyst Performance”), we reported on the reformer catalyst performance over a range of Φcatalyst conditions in an effort to find a balance between the POx activity required for catalyst durability and the endothermic steam reforming reactions required to minimize energy losses or to achieve TCR during reforming.48 In this paper, we characterize the multicylinder engine performance while maintaining stoichiometric exhaust emissions with the EGR-loop reforming strategy, focusing on whole-engine performance, including dilution tolerance, combustion performance, and brake thermal efficiency.
Complete Combustion: C8H18 + 12.5O2 → 8CO2 + 9H 2O
(−41 kJ/mol)
(2)
POx reforming:
Table 1. Engine Geometry
C8H18 + 4O2 → 8CO + 9H 2
(−660.4 kJ/mol)
(3)
parameter bore × stroke conrod length wrist pin offset toward expansion stroke compression ratio fuel injection system
steam reforming: C8H18 + 8H 2O → 8CO + 17H 2
(1274.2 kJ/mol) (4)
dry reforming: C8H18 + 8CO2 → 16CO + 9H 2
(1603.8 kJ/mol)
value/remark 86.0 mm × 86.0 mm 145.5 mm 0.8 mm 9.2:1 direct injection, side-mounted, production injector with opposite linear wall-directed six-hole spray pattern
(5) 38
geometry and camshaft profiles were unchanged from the stock configuration. All engine experiments presented in this manuscript were conducted with primary reference fuel isooctane obtained from Haltermann, as shown in Table 2. The companion paper presented a detailed characterization of the catalyst performance and equilibrium energetics,48 while this paper focuses on performance of a multicylinder engine. We used a laboratory fueling system for the engine in this study. A lift pump delivered fuel to the engine at a pressure of 5 bar through a Coriolis effect flow meter. A cam-driven fuel pump was used to maintain a constant rail pressure of 100 bar
The work by Leung et al. showed that, for ethanol, the steam reforming reaction (eq 4) is very active at engine exhaust temperatures, converting almost 100% of the fuel energy at 600 °C. In contrast, dry reforming (eq 5) produces bore). An undersquare geometry increases the mean piston speed and, by extension, the in-cylinder turbulence, which scales with mean piston speed.52 Higher turbulence shortens the duration of the early flame kernel development by transitioning to turbulent combustion sooner, thereby increasing combustion stability, and shortens combustion duration, which increases efficiency. Finally, no attempt was made to optimize the fuel injection timing or targeting, or more broadly, optimize the overall catalyst fueling strategy. This engine used a side-mounted fuel injector, making fuel spray impingement on the cylinder wall during the post-injection probable. A centrally mounted injector and specifically designed fuel injector targeting for these conditions would likely provide better control. Whether the post-injection fueling strategy is preferable to direct fueling to the reforming catalyst in the exhaust system has not been studied and may provide improved performance and/or improved control. Barriers to Implementation. In this engine, as with any combustion strategy using high levels of EGR, the transient performance of the engine poses a difficult control challenge. This is because the reforming and EGR loop represents a substantial volume, making it difficult to rapidly change the intake manifold pressure required when varying load to follow transients in SI engines. The EGR-loop reforming system provides further transient complications because Figures 9−11 showed that achieving a steady-state temperature in the catalyst took several minutes, which is significantly longer than engine transient operation. Intake system optimization to minimize the volume combined with calibration optimization can provide
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SUMMARY AND CONCLUSIONS The engine experiments described here demonstrate the potential benefits of EGR-loop reforming for combustion stability and brake efficiency. Specifically, we found that the catalytic reforming strategy could produce intake manifold H2 concentrations as high as 5% at the 2000 rpm and 4 bar BMEP condition investigated. The reforming strategy also introduced a high level of dilution in the engine, equivalent to 45%−55% EGR. However, despite the high level of dilution, the strategy resulted in good combustion stability. A combined analysis of both the conventional EGR and the reformate-assisted combustion shows that combustion stability problems occur when the spark-to-CA05 duration exceeds 40 °CA. With the high flame speed components in the reformate, namely, H2, this threshold was not exceeded for large parts of the reforming conditions investigated. The brake efficiency for our multicylinder engine was increased substantially with EGR-loop reforming, thereby decreasing fuel consumption by more than 8%. This efficiency improvement was achieved while maintaining stoichiometric engine exhaust, meaning that this technology should be compatible with conventional three-way emissions control technology for gasoline engines. We expect that additional efficiency increases should be possible by combining EGR-loop reforming with other engine hardware and operating strategies that enhance the positive impacts of reformate on the combustion profile.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
James P. Szybist: 0000-0002-3550-4423 Josh A. Pihl: 0000-0002-9798-4012 H
DOI: 10.1021/acs.energyfuels.7b02565 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Notes
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This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC0500OR22725 with the U.S. Department of Energy. The U.S. government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for U.S. government purposes. The U.S. Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
The authors gratefully acknowledge the support of the U.S. Department of Energy Vehicle Technologies Office, particularly program managers Gurpreet Singh and Mike Weismiller. Y.C. was also enrolled as a Ph.D. candidate at the University of Michigan at the time of this publication. She would like to express gratitude for the strong support she received from her coadvisors, Prof. Andre Boehman and Dr. Stani Bohac.
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ABBREVIATIONS ATDC = after TDC BMEP = brake mean effective pressure CA = crank angle CA50 = the crank angle at which 50% of the mass fraction has burned CAD = crank angle degrees COV = coefficient of variation D-EGR = dedicated exhaust gas recirculation EGR = exhaust gas recirculation IMEP = indicated mean effective pressure PMEP = pumping mean effective pressure POx = partial oxidation TCR = thermochemical recuperation TDC = top dead center WGS = water−gas shift Φcatalyst = equivalence ratio at the catalyst inlet Φcombustion = equivalence ratio in the combustion chamber for the main combustion event
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DOI: 10.1021/acs.energyfuels.7b02565 Energy Fuels XXXX, XXX, XXX−XXX
