Research and Publications - Alabama Transportation Institute

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Inframarginal Travelers and Transportation Policy
(Wiley, 2024-08-06) Hall, Jonathan D.
Structural models of traffic congestion, such as the bottleneck model, are used to answer important, policy-relevant questions. However, existing models typically assume that no travelers are inframarginal regarding when to travel; that is, given equilibrium travel times, no travelers strictly prefer their ex-ante departure time to all others. In this paper, I address this shortcoming by incorporating inframarginal travelers into these models. This change significantly improves these models’ ability to fit the data and changes policy prescriptions. In the case of congestion pricing, it typically changes the optimal toll by at least 25% and significantly worsens the distributional impacts.
The Effects of Thick Thermal Barrier Coatings on Low-Temperature Combustion
(2020) Yan, Ziming; Gainey, Brian; Gohn, James; Hariharan, Deivanayagam; Saputo, John; Schmidt, Carl; Caliari, Felipe; Sampath, Sanjay; Lawler, Benjamin; University of Alabama Tuscaloosa
An experimental study was conducted on a Ricardo Hydra single-cylinder light-duty diesel research engine. Start of Injection (SOI) timing sweeps from -350 deg aTDC to -210 deg aTDC were performed on a total number of five pistons including two baseline metal pistons and three coated pistons to investigate the effects of thick thermal barrier coatings (TBCs) on the efficiency and emissions of low-temperature combustion (LTC). A fuel with a high latent heat of vaporization, wet ethanol, was chosen to eliminate the undesired effects of thick TBCs on volumetric efficiency. Additionally, the higher surface temperatures of the TBCs can be used to help vaporize the high heat of vaporization fuel and avoid excessive wall wetting. A specialized injector with a 60° included angle was used to target the fuel spray at the surface of the coated piston. Throughout the experiments, the equivalence ratio, ϕ, was maintained constant at 0.4; the combustion phasing was consistently matched at 6.8 ± 0.4 deg aTDC. It can be concluded that the thick TBC cases achieved 1 to 2 percentage points improvement in combustion efficiency, and generally, a ~2 percentage points increase in indicated engine efficiency. It is also noticed that applying a dense top sealing layer to the TBC further improves the UHC emissions compared to the TBC coated piston with an unsealed surface. From the heat release analysis, it can be concluded that the TBCs have no significant impact on the heat release process and knock intensity while matching the combustion phasing; however, it reduces the intake temperature requirement by up to 20 K. The exhaust gas temperatures were expected to increase for the TBC cases, but the expected increase in exhaust temperature was not conclusive from the results observed in this study.
Investigation Into Reactivity Separation Between Direct Injected and Premixed Fuels in RCCI Combustion Mode
(2019) Yan, Ziming; Gainey, Brian; Hariharan, Deivanayagam; Lawler, Benjamin; University of Alabama Tuscaloosa
This experimental study focuses on the effects of the reactivity separation between the port injected fuel and the direct injection fuel, the amount of external-cooled exhaust gas recirculation (EGR), and the direct injection timing of the high reactivity fuel on Reactivity Controlled Compression Ignition (RCCI) combustion. The experiments were conducted on a light-duty, single-cylinder diesel engine with a production GM/Isuzu engine head and piston and a retrofitted port fuel injection system. The global charge-mass equivalence ratio, ϕ′, was fixed at 0.32 throughout all of the experiments. To investigate the effects of the fuel reactivity separation, different Primary Reference Fuels (PRF) were port injected, with the PRF number varying from 50 to 90. To investigate the effects of EGR, an EGR range of 0 to 55% was used. To investigate the effects of the injection timing, an injection timing window of −65 to −45 degrees ATDC was chosen.The results indicate that there are several tradeoffs. First, decreasing the port injected fuel reactivity (increasing the PRF number) delays combustion phasing, decreases the combustion efficiency by up to 9%, increases the gross indicated thermal efficiency up to 22%, enhances the combustion sensitivity to the direct injection timing, and slightly increases the UHC, CO, and NOx emissions. Second, increasing the EGR percentage delays combustion phasing, lowers the peak heat release rate, and lowers the NOx emissions. The combustion efficiency first increases and then decreases with EGR percentage for high reactivity fuels (low PRF number), but only decreases for low reactivity fuels. Finally, delaying the injection timing advances combustion phasing and increases the combustion efficiency, but decreases the gross indicated thermal efficiency and increases the NOx emissions. Across all of the experiments, delays in CA50 increase the gross indicated thermal efficiency and decrease the combustion efficiency, which represents an inherent tradeoff for RCCI combustion on a light-duty engine.
Exploring the Effects of Piston Bowl Geometry and Injector Included Angle on Dual-Fuel and Single-Fuel RCCI
(ASME, 2021) Hariharan, Deivanayagam; Boldaji, Mozhgan Rahimi; Yan, Ziming; Gainey, Brian; Lawler, Benjamin; University of Alabama Tuscaloosa; Clemson University
Reactivity control compression ignition (RCCI) is a low-temperature combustion technique that has been proposed to meet the current demand for high thermal efficiency and low engine-out emissions. However, its requirement of two separate fuel systems (i.e., a low-reactivity fuel system and a high-reactivity fuel system) has been one of its major challenges in the last decade. This leads to the single-fuel RCCI concept, where the secondary fuel (reformates of diesel) is generated from the primary fuel (diesel) through catalytic partial oxidation reformation. Following the in-depth analysis of the reformate fuel (reformates of diesel) and its benefit as the low-reactivity fuel with diesel, the effects of the start of injection (SOI) timing of diesel and the energy-based blend ratio were also studied in detail. In this study, the effects of piston profile and the injector included angles were experimentally examined using both conventional fuel pairs (gasoline-diesel and natural gas-diesel) and reformate RCCI. A validated computational fluid dynamics (CFD) model was also used for a better understanding of the experimental trends. Comparing a reentrant bowl piston with a shallow bowl piston at a constant compression ratio and SOI, the latter showed better thermal efficiency, regardless of the fuel combination, due to its 10% lower surface area for the heat transfer. Comparing the 150-degree included angle and 60-degree included angle on the shallow bowl piston, the latter showed better combustion efficiency, regardless of the fuel combination, due to its earlier combustion phasing (at constant SOI timing). The effect was particularly prominent on reformate RCCI because of its incredibly high diluent concentration, which retards the combustion further for the 150-deg injector. Later, using convergecfd, seven different injector included angles were studied at a constant SOI. With the change in injector included angle, the region of the cylinder targeted by the fuel spray varies significantly, and it was found to have a significant impact on the combustion efficiency and the engine-out emissions. As the injector included angle changed from 60-deg to 150-deg, the combustion efficiency increased by 15% and the CO, NOx, and HC emissions decreased by 96%, 70%, and 86%, respectively.
Experimental Study of the Effect of Start of Injection and Blend Ratio on Single Fuel Reformate RCCI
(ASME, 2020) Hariharan, Deivanayagam; Gainey, Brian; Yan, Ziming; Mamalis, Sotirios; Lawler, Benjamin; University of Alabama Tuscaloosa; Clemson University; State University of New York (SUNY) System; State University of New York (SUNY) Stony Brook
A new concept of single fuel reactivity-controlled compression ignition (RCCI) has been proposed through the catalytic partial oxidation (CPOX) reformation of diesel fuel. The reformed fuel mixture is then used as the low reactivity fuel and diesel itself is used as the high reactivity fuel. In this paper, two reformate mixtures from the reformation of diesel were selected for further analysis. Each reformate fuel mixture contained a significant fraction of inert gases (89% and 81%). The effects of the difference in the molar concentrations of the reformate mixtures were studied by experimenting with diesel as the direct injected fuel in RCCI over a varying start of injection timings and different blend ratios (i.e., the fraction of low and high reactivity fuels). The reformate mixture with the lower inert gas concentration had earlier combustion phasing and shorter combustion duration at any given diesel start of injection timing. The higher reactivity separation between reformate mixture and diesel, compared with gasoline and diesel, causes the combustion phasing of reformate-diesel RCCI to be more sensitive to the start of injection timing. The maximum combustion efficiency was found at a CA50 before top dead center (TDC), whereas the maximum thermal efficiency occurs at a CA50 after TDC. The range of energy-based blend ratios in which reformate-diesel RCCI is possible is between 25% and 45%, limited by ringing intensity (RI) at the low limit of blend ratios, and coefficient of variance (COV) of net indicated mean effective pressure (IMEPn) and combustion efficiency at the high limit. Intake boosting becomes necessary due to the oxygen deficiency caused by the low energy density of the reformate mixtures as it displaces intake air.
Experimental Study of Spark-Ignition Combustion using the Anode Off-Gas from a Solid Oxide Fuel Cell
(2020) Ran, Zhongnan; Assanis, Dimitris; Hariharan, Deivanayagam; Mamalis, Sotirios; University of Alabama Tuscaloosa
Hybridizing Solid Oxide Fuel Cells (SOFCs) with internal combustion engines is an attractive solution for power generation at high electrical conversion efficiency while emitting significantly reduced emissions than conventional fossil fueled plants. The gas that exits the anode of an SOFC operating on natural gas is a mixture of H2, CO, CO2, and H2O vapor, which are the products of the fuel reforming and the electrochemical process in the stack. In this study, experiments were conducted on a single-cylinder, spark-ignited Cooperative Fuel Research Engine using the anode off-gas as the fuel, at compression ratio of 11:1 and 13:1, engine speed of 1200 rev/min and intake pressure of 75 kPa, to investigate the combustion characteristics and emissions formation. A comparison was drawn with combustion with Compressed Natural Gas (CNG) at the same engine operating conditions. The experimental results revealed that the anode off-gas can be used as a potential alternative fuel for spark-ignition combustion, and an engine can be used to provide additional power to a hybrid SOFC-engine system. Combustion with the anode off-gas resulted in similar net indicated efficiency with CNG at CR of 13:1, but with negligible NOx emissions and zero total hydrocarbon emissions. However, combustion with the anode off-gas resulted in lower volumetric efficiency and lower load than CNG as a result of high levels of dilution in the off-gas, which greatly reduces the lower heating value of the fuel. This study demonstrated the feasibility of using the SOFC anode-off gas as a potential fuel for spark-ignition engines with good fuel conversion efficiency and minimal NOx and THC emissions.
Experimental study of lean spark ignition combustion using gasoline, ethanol, natural gas, and syngas
(Elsevier, 2019) Ran, Zhongnan; Hariharan, Deivanayagam; Lawler, Benjamin; Mamalis, Sotirios; State University of New York (SUNY) System; State University of New York (SUNY) Stony Brook; University of Alabama Tuscaloosa
In the development of internal combustion engines, engineers and researchers are facing the challenge of improving engine efficiency while reducing harmful exhaust emissions. Previous research has shown that lean combustion is one of the viable techniques that can improve engine efficiency while effectively reducing exhaust emissions. Lean burn engines operate at low burned gas temperatures and can achieve high thermal efficiency based on favorable mixture thermodynamic properties. However, under high dilution levels, a lean misfire limit is reached where the combustion process becomes unstable and incomplete combustion starts to occur. Instability significantly affects engine efficiency, driveability, and exhaust emissions, which limit the full potential of lean burn engines. The lean misfire limit is not only dependent on engine design but also on fuel properties. Therefore, fuels that are conducive to lean combustion can provide the opportunity for enhanced efficiency and reduced emissions. Spark ignited (SI) combustion with conventional gasoline has shown to have relatively narrow range of fuel-air equivalence ratio; therefore, it is desired to explore the lean limit of SI combustion by using alternative fuels, which can also contribute to the reduction of greenhouse gas emissions from transportation and power generation. Experiments were conducted on a Cooperative Fuel Research (CFR) engine with varying fuel-air equivalence ratio (phi) to assess the engine performance and emissions with three alternative fuels, natural gas, ethanol, and syngas, at compression ratio of 8:1 and engine speed of 1200 rev/min. Equivalence ratio was varied by decreasing the mass of fuel while keeping the mass of air the same. The lean misfire limit was defined as the equivalence ratio where the CoV of IMEP across multiple consecutive engine cycles was greater than 5%. It was found that syngas can maintain stable combustion at extremely lean conditions and has the lowest lean misfire limit. Natural gas combustion achieved a lower lean misfire limit than gasoline and ethanol. Gasoline and ethanol had similar lean misfire limits, but it was found that gasoline helped the engine to achieve higher load and fuel conversion efficiency compared to the three alternative fuels.
Efficiency and Emissions Characteristics of an HCCl Engine Fueled by Primary Reference Fuels
(SAE International, 2018) Yang, Ruinan; Hariharan, Deivanayagam; Zilg, Steven; Mamalis, Sotirios; Lawler, Benjamin; State University of New York (SUNY) System; State University of New York (SUNY) Stony Brook; University of Alabama Tuscaloosa
This article investigates the effects of various primary reference fuel (PRF) blends, compression ratios, and intake temperatures on the thermodynamics and performance of homogeneous charge compression ignition (HCCl) combustion in a Cooperative Fuels Research (CFR) engine. Combustion phasing was kept constant at a CA50 phasing of 5 degrees after top dead center (aTDC) and the equivalence ratio was kept constant at 0.3. Meanwhile, the compression ratio varied from 8:1 to 15:1 as the PRF blends ranged from pure n-heptane to nearly pure isooctane. The intake temperature was used to match CA50 phasing. In addition to the experimental results, a GT-Power model was constructed to simulate the experimental engine and the model was validated against the experimental data. The GT-Power model and simulation results were used to help analyze the energy flows and thermodynamic conditions tested in the experiment. The results indicate that an increase of compression ratio causes higher thermal efficiency and fuel conversion efficiency; however, at the same compression ratio, an increase in PRF number results in lower efficiency due to the required increase in intake temperature and the associated decrease in charge density. While the efficiency does increase with compression ratio, the results show that the effect of increased expansion work is partially offset by higher heat transfer losses and lower ratios of specific heats at higher compression ratios. The results indicate that the maximum pressure rise rate (MPRR) in HCCl significantly increases with compression ratio. Combustion efficiency shows a strong trend with peak temperature regardless of the PRF number or compression ratio, indicating that the CO-to-CO2 conversion is independent of the parent fuel chemistry in the case of the PRFs, whereas the unburned hydrocarbon emissions showed the opposite trend, depending mostly on the parent fuel's autoignition tendency.
Effects of Single versus Two-Stage Heat Release on the Load Limits of HCCI using Primary Reference Fuels
(2019) Hariharan, Deivanayagam; Yang, Ruinan; Mamalis, Sotirios; Lawler, Benjamin; University of Alabama Tuscaloosa
Homogeneous Charge Compression Ignition (HCCI) enables combustion with high efficiency and low emissions. Control over the combustion process and its narrow operating range are still the biggest challenges associated with HCCI. To expand the operable load ranges of HCCI, this paper explores the effects of single versus two-stage ignition fuels by studying the Primary Reference Fuels (PRF) in a variable compression ratio Cooperative Fuel Research (CFR) engine. The PRF fuels, iso-octane and n-heptane, are blended together at various concentrations to create fuel blends with different autoignition characteristics. Experiments were conducted using these PRF blends to explore the extent to which the load range can be extended with two-stage ignition fuels at various compression ratios and intake temperatures. The reactivity of the PRF blends increases with the fraction of n-heptane and so does the amount of low temperature heat release (LTHR). Since the low PRF number fuels have a higher reactivity, they can be autoignited at very low compression ratios while maintaining comparable combustion phasing and equivalence ratios. At the lower compression ratios, the low load limits were found to be extended while maintaining high combustion efficiencies. Additionally, lower peak pressures and pressure rise rates were achieved at low PRF number fuels as a result of its two-stage heat release, which can be used to reach higher loads. In addition, the energy released from the LTHR can be used to delay the CA50 combustion phasing (i.e., the crank angle timing where 50% of the energy has been released) beyond what is possible with a single-stage ignition fuel, which allows further high load extension. However, using lower compression ratios has a negative impact on the thermal efficiency. The effects of the extended load, single- and two-stage heat release, combustion phasing, and equivalence ratios on combustion efficiency, thermal efficiencies, and combustion durations were also explored.
Catalytic partial oxidation reformation of diesel, gasoline, and natural gas for use in low temperature combustion engines
(Elsevier, 2019) Hariharan, Deivanayagam; Yang, Ruinan; Zhou, Yingcong; Gainey, Brian; Mamalis, Sotirios; Smith, Robyn E.; Lugo-Pimentel, Michael A.; Castaldi, Marco J.; Gill, Rajinder; Davis, Andrew; Modroukas, Dean; Lawler, Benjamin; State University of New York (SUNY) System; State University of New York (SUNY) Stony Brook; City University of New York (CUNY) System; City College of New York (CUNY); University of Alabama Tuscaloosa
Onboard reforming has relevance to both conventional and advanced combustion concepts. Most recently, onboard reforming has been proposed to enable "Single-Fuel RCCI" combustion and therefore, this paper explores catalytic partial oxidation reforming of three potential transportation-relevant fuels: gasoline, diesel, and natural gas. Reformation is performed at two pressure levels (between 15 and 60 psig) for each parent fuel for equivalence ratios ranging from 3.7 to 7.6 and the gaseous reformate mixtures are characterized with gas chromatography. The percentage of diesel oxidized during reformation is similar across all of the equivalence ratios. However, the percentage of gasoline and natural gas oxidized during reformation decreased with increasing equivalence ratio. The energy released during the reformation process is also calculated and presented for each gaseous reformate fuel. The lower heating value of every reformate fuel is lower than 20% of their respective parent fuel, due to the high concentration of inert gases (mostly nitrogen) in the reformate fuel mixtures. Two reformed fuels for each parent fuel were then selected to study their autoignition characteristics using HCCI combustion on a Co-operative Fuel Research (CFR) engine. The equivalence ratio is maintained at 0.31 and the combustion phasing was held constant by varying the intake temperature. Although the equivalence ratio is constant, the input energy from the different reformate fuels is not constant due to the component concentrations in the fuel. The gaseous reformate fuels are then compared to gasoline, natural gas, and the primary reference fuels in HCCI to determine an effective Primary Reference Fuel (PRF) number or effective octane rating for each gaseous reformate fuel. The effective octane rating for the gaseous reformate fuels fell slightly above the PRF number scale at an effective octane number of -110.
Single-fuel reactivity controlled compression ignition through catalytic partial oxidation reformation of diesel fuel
(Elsevier, 2019) Hariharan, Deivanayagam; Boldaji, Mozhgan Rahimi; Yan, Ziming; Mamalis, Sotirios; Lawler, Benjamin; State University of New York (SUNY) System; State University of New York (SUNY) Stony Brook; Clemson University; University of Alabama Tuscaloosa
A single-fuel RCCI concept has been proposed to avoid the need for a secondary fuel system required for conventional RCCI by generating the secondary fuel from the primary fuel through catalytic partial oxidation (CPOX) reformation. In conventional RCCI, gasoline or natural gas can be used as the low-reactivity fuel, and diesel can be used as the high-reactivity fuel. In this study, two reformate mixtures generated by reforming diesel fuel at different operating conditions were used as the low-reactivity fuel, with the parent diesel as the high reactivity fuel. The combustion characteristics of reformate-diesel RCCI were compared with the conventional RCCI. A CFD model was also developed and validated against the experimental results. The model was then used to validate a necessary approximation of the reformate mixture's species concentrations. Compared to conventional RCCI fuel pairs, reformate-diesel RCCI shows marginally better thermal efficiency, approximately 10% better THC emissions, approximately 50% lower NOx emissions, and good controllability. Because the reformate mixture has a high concentration of diluents it displaces a large fraction of intake air and acts similarly to EGR. The combustion behavior of reformate-diesel RCCI is in between that of gasoline-diesel and natural gas-diesel conventional RCCI. From the results, it can be concluded that reformate-diesel RCCI is not overly sensitive to the reformation process itself and the exact species concentrations in the reformate mixture. A small change in the start of injection of diesel, blend ratio, and EGR fraction can be used to compensate for reformate mixture concentration differences.
Multiple injection strategies for reducing HC and CO emissions in diesel-methane dual-fuel low temperature combustion
(Elsevier, 2021) Hariharan, Deivanayagam; Krishnan, Sundar Rajan; Srinivasan, Kalyan Kumar; Sohail, Aamir; University of Alabama Tuscaloosa; University of Michigan System; University of Michigan
Dual fuel low temperature combustion (LTC), while promising extremely low engine-out emissions of oxides of nitrogen (NOx) and particulate matter (PM), is beset with high unburned hydrocarbon (HC) and carbon monoxide (CO) emissions, especially at low engine loads. In the present work, diesel dual injection is experimentally shown to achieve simultaneous reduction of HC and CO emissions without compromising NOx and PM benefits. The motivation to use a second late diesel injection (typically after top dead center (ATDC)) is to oxidize HC arising from incomplete methane oxidation in dual-fuel combustion initiated by the first early diesel injection (around 310 CAD). Since the second diesel injection occurs during the expansion stroke, the NOx-formation propensity is reduced due to low local temperatures. The experimental matrix consisted of twenty-two distinct operating points on a single cylinder research engine (SCRE) adapted for diesel-ignited methane dual fueling. All the experiments were performed at a constant intake pressure of 1.5 bar and fixed first diesel injection timing at 310 CAD whereas, the second diesel injection timing was varied between 320 and 375 CAD and the injection pressure was varied between 500 and 1500 bar. In addition to reductions in the following emissions: indicated specific hydrocarbons or ISHC (54%, to 13.7 g/kW-hr), ISCO (46%, to 2.7 g/kW-hr), and indicated specific oxides of nitrogen or ISNOx (7%, to 0.31 g/kW-hr) emissions, an 11% increase in indicated fuel conversion efficiency (IFCE), and a 12% increase in combustion efficiency (eta c) were achieved relative to the baseline single injection (at 310 CAD) dual-fuel LTC. Although smoke emissions increased slightly from 0.03 to 0.06 Filter Smoke Number (FSN), they are still considerably lower than for conventional diesel operation.
A comprehensive experimental investigation of low-temperature combustion with thick thermal barrier coatings
(Pergamon, 2021) Yan, Ziming; Gainey, Brian; Gohn, James; Hariharan, Deivanayagam; Saputo, John; Schmidt, Carl; Caliari, Felipe; Sampath, Sanjay; Lawler, Benjamin; Clemson University; University of Alabama Tuscaloosa; State University of New York (SUNY) System; State University of New York (SUNY) Stony Brook
Thick thermal barrier coatings (TBCs) have a significant potential to increase thermal efficiency by reducing heat transfer losses. However, in conventional combustion modes, the drawbacks associated with charge heating and higher propensity to knock have outweighed the efficiency benefits. Since the advanced low-temperature combustion (LTC) concepts are fundamentally different from the conventional combustion modes, these penalties do not exist in most of LTCs. The current experimental study shows the feasibility and benefits of thick TBCs with advanced LTC enabled by two different fuels: conventional gasoline and wet ethanol 80 (WE80, i.e., 80% ethanol and 20% water by mass). A total of five pistons were tested, including two metal baselines and three TBCcoated pistons with different thicknesses or surface finishes. A load sweep was conducted with each fuel on each piston within the same constraints. The thick TBCs extends the low load limit by about 15% for both gasoline and WE80 cases. A deterioration of the high load limit was not observed, which implies that the charge heating penalty does not occur in LTCs. The combustion efficiency increased for the thicker TBC by up to 2% points, and the fuel conversion efficiency was increased by up to 4.3%. The gasoline cases experience the largest benefits at low load, while the wet ethanol experiences the largest benefits at mid-to-high load. The intake temperature requirement is successfully reduced by 10-15 K. It is also observed that the dense sealing layer results in a significant improvement to UHC emissions. All of the coated pistons survived the 10-20 h of engine operation with no visual failure. (c) 2021 Elsevier Ltd. All rights reserved.

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