A Study of the Theoretical Potential of Thermochemical Exhaust Heat Recuperation for Internal Combustion Engines PDF Download

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Languages : en
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We present a detailed thermodynamic analysis of thermochemical recuperation (TCR) applied to an idealized internal combustion engine with single-stage work extraction. Results for several different fuels are included. For a stoichiometric mixture of methanol and air, TCR can increase the estimated ideal engine Second Law efficiency by about 3% for constant pressure reforming and over 5% for constant volume reforming. For ethanol and isooctane the estimated Second Law efficiency increases for constant volume reforming are 9% and 11%, respectively. The Second Law efficiency improvements from TCR result primarily from the higher intrinsic exergy of the reformed fuel and pressure boost associated with gas mole increase. Reduced combustion irreversibility may also yield benefits for future implementations of combined cycle work extraction.

Author: David R. Vernon
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ISBN: 9781124509464
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Languages : en
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The thermochemical recuperation process uses endothermic reformation reactions to upgrade a portion of an engine's primary fuel into a hydrogen-rich gas, thereby converting part of the exhaust heat from an internal combustion engine into chemical potential energy. Enriching the primary fuel air mixture of the internal combustion engine with this hydrogen-rich gas potentially enables combustion with very lean or dilute mixtures, resulting in higher efficiency and lower emissions as compared to standard combustion regimes. It may be possible to simplify thermochemical recuperation system architecture by directly mixing exhaust gases with the fuel in the reformation process to supply a significant portion of the heat and water required. To evaluate the effect of direct exhaust gas mixing on ethanol autothermal reformation, this work experimentally and theoretically investigated dilution with a mixture of nitrogen and carbon dioxide to simulate an exhaust composition, in combination with a range of inlet temperatures, to simulate exhaust gas temperatures, at a constant steam to carbon ratio. Parameters such as the chemical coefficient of performance, chemical energy output divided by chemical energy input, are introduced to better enable quantification of thermochemical recuperation. Trends in yield and performance metrics for ethanol autothermal reformation were observed under operating conditions across a range of oxygen to carbon ratio, a range of dilution amount, and a range of inlet temperature. For high inlet temperature cases, dilution increases hydrogen yield and chemical coefficient of performance suggesting that direct exhaust mixing would be beneficial. However, for low inlet temperatures, dilution decreased hydrogen yield and other performance metrics suggesting that direct exhaust mixing would not be beneficial. Dilution decreased methane production for many conditions. High inlet temperature conditions were found to cause homogeneous oxidation and homogenous conversion of ethanol upstream of the catalyst leading to high conversions of ethanol and high methane yields before reaching the catalyst. Coke formation rates varied over two orders of magnitude, with high coke formation rates for the high inlet temperature cases and low coke formation rates for the low inlet temperature cases. Dilution decreased the rate of coke formation. Models of intrinsic rate phenomenon were constructed in this study. The models predict that mass transport rates will be faster than the rate of chemical reaction kinetics over the range of ethanol concentrations and temperatures measured in the catalyst monolith both with and without dilution. Bounding cases for heat generation and transfer rates indicate that these phenomena could be the rate limiting mechanism or could be faster than both chemical kinetics and mass transport rates depending upon the distribution of oxidation heat between the catalyst and gas stream. Based on these results direct exhaust gas mixing is expected to be a practical method for supplying heat and water vapor for ethanol autothermal reformation in thermochemical recuperation systems when exhaust temperatures are above a certain threshold. For low exhaust temperatures direct exhaust gas mixing can supply water vapor but reduces other performance metrics.

Author: Flavio Dal Forno Chuahy
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Languages : en
Pages : 0

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Dual fuel reactivity controlled compression ignition (RCCI) combustion is a promising method to achieve high efficiency with near zero NOx and soot emissions; however, the requirement to carry two fuels on-board has limited practical applications. Advancements in catalytic reforming have demonstrated the ability to generate syngas (a mixture of CO and hydrogen) from a single hydrocarbon stream. The reformed fuel mixture can then be used as a low reactivity fuel stream to enable RCCI out of a single parent fuel. Beyond enabling dual-fuel combustion strategies out of a single parent fuel, fuel reforming can be endothermic and allow recovery of exhaust heat to drive the reforming reactions, potentially improving overall efficiency of the system. Previous works have focused on using reformed fuel to extend the lean limit of spark ignited engines, and enhancing the control of HCCI type combustion. The strategy pairs naturally with advanced dual-fuel combustion strategies, and the use of dual-fuel strategies in the context of on-board reforming and energy recovery has not been explored. Accordingly, the work presented in this dissertation attempts to fill in the gaps in the current literature and provide a pathway to "single" fuel RCCI combustion through a combination of experiments and computational fluid dynamics modeling. Initially, a system level analysis focusing on three common reforming techniques (i.e., partial oxidation, steam reforming and auto-thermal reforming) was conducted to evaluate the potential of reformed fuel. A system layout was proposed for each reforming technique and a detailed thermodynamic analysis using first- and second-law approaches were used to identify the sources of efficiency improvements. The results showed that reformed fuel combustion with a near TDC injection of diesel fuel can increase engine-only efficiency by 4% absolute when compared to a conventional diesel baseline. The efficiency improvements were a result of reduced heat transfer and shorter, more thermodynamically efficient, combustion process. For exothermic reforming processes, losses in the reformer outweigh the improvements to engine efficiency, while for endothermic processes the recovery of exhaust energy was able to allow the system efficiency to retain a large portion of the benefits to the engine combustion. Energy flow analysis showed that the reformer temperature and availability of high grade exhaust heat were the main limiting factors preventing higher efficiencies. RCCI combustion was explored experimentally for its potential to expand on the optimization results and achieve low soot and NOx emissions. The results showed that reformed fuel can be used with diesel to enable RCCI combustion and resulted in low NOx and soot emissions while achieving efficiencies similar to conventional diesel combustion. Experiments showed that the ratio H2/(H2+CO) is an important parameter for optimal engine operation. Under part-load conditions, fractions of H2/(H2+CO) higher than 60% led to pressure oscillations inside the cylinder that substantially increased heat transfer and negated any efficiency benefits. The system analysis approach was applied to the experimental results and showed that chemical equilibrium limited operation of the engine to sub-optimal operating conditions. RCCI combustion was able to achieve "diesel like" system level efficiencies without optimization of either the engine operating conditions or the combustion system. Reformed fuel RCCI was able to provide a pathway to meeting current and future emission targets with a reduction or complete elimination of aftertreatment costs. Particle size distribution experiments showed that addition of reformed fuel had a significant impact on the shape of the particle size distribution. Addition of reformed fuel reduced accumulation-mode particle concentration while increasing nucleation-mode particles. When considering the full range of particle sizes there was a significant increase in total particle concentration. However, when considering currently regulated (Dm>23nm) particles, total concentration was comparable. To address limitations identified in the system analysis of the RCCI experiments a solid oxide fuel cell was combined with the engine into a hybrid electrochemical combustion system. The addition of the fuel cell addresses the limitations by providing sufficient high grade heat to fully drive the reforming reactions. From a system level perspective, the impact of the high frequency oscillations observed in the experiments are reduced, as the system efficiency is less dependent on the engine efficiency. From an engine perspective, the high operating pressures and low reactivity of the anode gas allow reduction of the likelihood of such events. A 0-D system level code was developed and used to find representative conditions for experimental engine validation. The results showed that the system can achieve system electrical efficiencies higher than 70% at 1 MWe power level. Experimental validation showed that the engine was able to operate under both RCCI and HCCI combustion modes and resulted in low emissions and stable combustion. The potential of a hybrid electrochemical combustion system was demonstrated for high efficiency power generation

Author: M. Anusha Wijewardane
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Languages : en
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Today, the investigation of fuel economy improvements in internal combustion engines (ICEs) has become the most significant research interest among the automobile manufacturers and researchers. The scarcity of natural resources, progressively increasing oil prices, carbon dioxide taxation and stringent emission regulations all make fuel economy research relevant and compelling. The enhancement of engine performance solely using incylinder techniques is proving increasingly difficult and as a consequence the concept of exhaust energy recovery has emerged as an area of considerable interest. Three main energy recovery systems have been identified that are at various stages of investigation. Vapour power bottoming cycles and turbo-compounding devices have already been applied in commercially available marine engines and automobiles. Although the fuel economy benefits are substantial, system design implications have limited their adaptation due to the additional components and the complexity of the resulting system. In this context, thermo-electric (TE) generation systems, though still in their infancy for vehicle applications have been identified as attractive, promising and solid state candidates of low complexity. The performance of these devices is limited to the relative infancy of materials investigations and module architectures. There is great potential to be explored. The initial modelling work reported in this study shows that with current materials and construction technology, thermo-electric devices could be produced to displace the alternator of the light duty vehicles, providing the fuel economy benefits of 3.9%-4.7% for passenger cars and 7.4% for passenger buses. More efficient thermo-electric materials could increase the fuel economy significantly resulting in a substantially improved business case. The dynamic behaviour of the thermo-electric generator (TEG) applied in both, main exhaust gas stream and exhaust gas recirculation (EGR) path of light duty and heavy duty engines were studied through a series of experimental and modelling programs. The analyses of the thermo-electric generation systems have highlighted the need for advanced heat exchanger design as well as the improved materials to enhance the performance of these systems. These research requirements led to the need for a systems evaluation technique typified by hardware-in-the-loop (HIL) testing method to evaluate heat exchange and materials options. HIL methods have been used during this study to estimate both the output power and the exhaust back pressure created by the device. The work has established the feasibility of a new approach to heat exchange devices for thermo-electric systems. Based on design projections and the predicted performance of new materials, the potential to match the performance of established heat recovery methods has been demonstrated.

Author: Ross P. Owen
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Languages : en
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Abstract: A zero-dimensional fluid and thermodynamic model of an engine, cooling system, and exhaust system was developed in order to simulate the operation of an advanced thermal management system. The model was calibrated with experimental data where available. The thermal management system modeled in this work employed waste heat recovery to reduce engine, coolant, and lubricating fluid warm-up times and fuel consumption following a cold-start. The model was used to develop a control strategy for two valves in the exhaust system which control the flow of exhaust through an exhaust-to-coolant heat exchanger. The objective of the controller was to minimize coolant warm-up time without violating any of the system constraints. A model-based open-loop controller was developed that was able to reduce warm-up time by nearly 35% on the FTP city drive cycle while respecting the limitations of the system.

Author: Bhanu Teja Krishnavajjala
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Languages : en
Pages : 176

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134a, R-245fa and R-236fa. The sensitivity of the model to these working fluids at different mass flow rates and the effects of the component sizing was studied and presented in this thesis. The efficiency of the ORC model and power developed were examined. The results showed that using R-245fa as the working fluid for waste heat recovery with ORC resulted in an engine peak brake thermal efficiency close to 45%.

Author: Daniela Gewald
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ISBN: 9783843912877
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Languages : de
Pages : 170

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Author: Miloš Ž Vujić
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Languages : en
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Author: Mark Aaronian Severy
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Category : Biomass
Languages : en
Pages : 296

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Natural gas internal combustion engines release over half of the fuel's energy as waste heat and emit pollution that harms human health and accelerates climate change. Enriching natural gas with hydrogen has been shown to mitigate these impacts by reducing emissions and increasing engine efficiency. Thermal energy in the exhaust gas from natural gas engines can be used to drive chemical reactions to reform a biomass-derived feedstock into a hydrogen-rich gas. This gas can be blended with the primary fuel to enhance combustion and displace some of the natural gas demand. Two types of chemical reformation processes, aqueous-phase reformation (APR) and vapor-phase reformation (VPR), have been identified which can convert a biomass-derived sugar feedstock solution into a hydrogen-rich gas by recovering waste heat from engine exhaust gas. VPR operates at higher temperatures than APR, which limits the amount of heat that can be transferred from the exhaust gas to the reaction temperature. This study used a thermodynamic pinch analysis to compare the performance of these two processes based on their respective process heat demands and the thermal energy available from engine exhaust gas to determine how many moles of feedstock can be reformed. The calculations were performed using specifications for eight natural gas engines with reactor conditions from fourteen APR and ten VPR experiments, using glycerol as a model compound.

Author: Joaquin G. Ruiz
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ISBN:
Category :
Languages : en
Pages : 72

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Less than 30% of the energy in a gallon of gasoline reaches the wheels of a typical car; most of the remaining energy is lost as heat. Since most of the energy consumed by an internal combustion engine is wasted, capturing much of that wasted energy can provide a large increase in energy efficiency. For example, a typical engine producing 100 kilowatts of driveshaft power expels 68 kilowatts of heat energy through the radiator and 136 kilowatts through the exhaust. The possibilities of where and how to capture this lost energy are examined in this paper. The solution of recovering heat energy from the exhaust through the catalytic converter with a Stirling engine was examined due to its practicality. A novel approach for combining a Stirling engine and a catalytic converter that would be effective was designed. The power output and efficiency of the Stirling Engine were analyzed and it was found that the average overall car efficiency could be raised 7% with the new design.