Improving thermal efficiency by reducing cooling losses in hydrogen combustion engines

Hydrogen can be readily used in spark ignition engines as a clean alternative to fossil fuels. However, the higher burning velocity and shorter quenching distance of hydrogen compared with hydrocarbons cause a larger heat transfer from the burning gas to the combustion chamber walls. Because of this cooling loss, the thermal efficiency of hydrogen-fueled engines is sometimes lower than that of conventionally fueled engines. Therefore, reducing the cooling loss is a crucial element in improving the thermal efficiency of hydrogen combustion engines. Previous research by the author and others has proposed the direct injection stratified charge as a technique for reducing the cooling loss in hydrogen combustion and shown its effect in reducing cooling loss through experiments in a constant volume combustion vessel. However, it is known that a reduction in cooling loss does not always improve thermal efficiency due to a simultaneous increase in the exhaust heat loss. This paper explains the relation between cooling loss reduction and thermal efficiency improvements by the direct injection stratified charge in hydrogen combustion engines.     

Exergy analysis of micro-organic Rankine power cycles for a small scale solar driven reverse osmosis desalination system

Exergy analysis of micro-organic Rankine heat engines is performed to identify the most suitable engine for driving a small scale reverse osmosis desalination system. Three modified engines derived from simple Rankine engine using regeneration (incorporation of regenerator or feedliquid heaters) are analyzed through a novel approach, called exergy-topological method based on the combination of exergy flow graphs, exergy loss graphs, and thermoeconomic graphs. For the investigations, three working fluids are considered: R134a, R245fa and R600. The incorporated devices produce different results with different fluids. Exergy destruction throughout the systems operating with R134a was quantified and illustrated using exergy diagrams. The sites with greater exergy destruction include turbine, evaporator and feedliquid heaters. The most critical components include evaporator, turbine and mixing units. A regenerative heat exchanger has positive effects only when the engine operates with dry fluids; feedliquid heaters improve the degree of thermodynamic perfection of the system but lead to loss in exergetic efficiency. Although, different modifications produce better energy conversion and less exergy destroyed, the improvements are not significant enough and subsequent modifications of the simple Rankine engine cannot be considered as economically profitable for heat source temperature below 100 °C. As illustration, a regenerator increases the system’s energy efficiency by 7%, the degree of thermodynamic perfection by 3.5% while the exergetic efficiency is unchanged in comparison with the simple Rankine cycle, with R600 as working fluid. The impacts of heat source temperature and pinch point temperature difference on engine’s performance are also examined. Finally, results demonstrate that energy analysis combined with the mathematical graph theory is a powerful tool in performance assessments of Rankine based power systems and permits meaningful comparison of different regenerative effects based on their contribution to systems improvements.     

柴油发电机组回能型电子负载系统技术研究与应用

针对公司现有柴油机及柴油发电机组试验负载为测功器和水负载,试验产生的机械能及电能消耗在水阻中导致能源浪费问题,进行柴油发电机组回能型电子负载系统技术研究。根据船用柴油发电机组以及往复式内燃机驱动交流发电机组试验特性要求,完成相应技术方案制定,系统参数计算及设备选型。通过某型发电机组回能型电子负载测试试验,验证了所开发的回能型电子负载系统的各性能指标满足发电机组调速特性和负载特性的测试要求;达到了节约能源,降低成本,提高柴油发电机组测试效率的目的。     

Thermodynamic analysis of thermal efficiency and power of Minto engine

Minto engine is a kind of liquid piston heat engine that operates on a small temperature gradient. But there is no power formula for it yet. And its thermal efficiency is low and formula sometimes is misused. In this paper, deriving the power formula and simplifying the thermal efficiency formula of Minto engine based on energy distribution analysis will be discussed. To improve the original Minto engine, a new design of improved Minto engine is proposed and thermal efficiency formula and power formula are also given. A computer program was developed to analyze thermal efficiency and power of original and improved Minto engines operating between low and high-temperature heat sources. The simulation results show that thermal efficiency of improved Minto engine can reach over 7% between 293.15 K and 353.15 K which is much higher than that of original one; the temperature difference between upper and lower containers is lower than half of that between low and high temperature of heat sources when the original Minto engines output the maximum power; on the contrary, it is higher in the improved Minto engines.     

包含多变过程的内可逆Otto循环有限时间热力学分析

应用有限时间热力学理论分析了包含多变过程的内可逆Otto循环,由数值计算给出了考虑传热损失时循环输出功与压缩比、效率与压缩比以及输出功与效率的特性关系,分析了多变指数和传热损失对循环性能的影响,通过分析可知多变指数和传热对Otto循环性能有较大影响。计算所得的结果对实际Otto热机的设计和改进有一定的指导意义。     

On efficient utilization of fuels

Various methods are developed to conserve fuels by increasing the efficiencies of heat engines. These efforts are limited by fundamental constraints on efficiency, as specified by the Carnot engine.We consider a cyclic process during which heat is converted into work while no other body, except the atmosphere, acts as both the source and sink, and show that the possibility of constructing heat engines with 100% efficiency need not be totally ruled out. If the cycle of a heat engine partially coincides with a natural cycle, its efficiency can reach the value of 100%.Although there exist many cycles in nature, the most convenient cycle for this purpose is the cycle of rain formation. Cycles of engines converting CDE or concentration difference energy (i.e., the potential energy stored in two similar and separated solutions of unequal concentrations) into work can always be so designed that they partially coincide with the cycle of rain formation. For a practical CDE engine, the evaporation of water from dilute solution may have to be carried out rapidly by burning fuels instead of using heat from the atmosphere. While this procedure will have an adverse effect on the overall engine efficiency, the efficiency of fuel utilization may still be very high.A possible arrangement of a CDE engine utilizing a semipermeable membrane is discussed. Calculations indicate that the efficiency of fuel utilization of such an engine would be independent of temperature and would increase (theoretically indefinitely) with the solubility of the solute.     

Performance analysis of an irreversible Miller heat engine and its optimum criteria

An irreversible cycle model of the Miller heat engine is established, in which the multi-irreversibilities coming from the adiabatic compression and expansion processes, finite time processes and heat leak loss through the cylinder wall are taken into account. The power output and efficiency of the cycle are optimized with respect to the pressure ratio of the working substance. The optimum criteria of some important parameters such as the power output, efficiency and pressure ratio are given. The influence of some relevant design parameters is discussed. Moreover, it is expounded that the Otto and the Atkinson heat engines may be taken as two special cases of the Miller heat engine and that the optimal performance of the two heat engines may be directly derived from that of the Miller heat engine.     

新型热管技术开发及应用

介绍了制冷热管(RHP)、热管发电引擎(TSR)等新型热管技术的原理和近年来的研究发展情况。新型热管技术,特别是热管和透平相结合的TSR技术,提出了一种能源利用和回收的新思路,具有广泛而特殊的工程应用背景。对TSR技术的改进和深入研究将有助于提高该技术能源的转化效率。     

Comparison of the theoretical performance potential of fuel cells and heat engines

Fuel cells have decided advantages including compatibility with renewable fuels such as hydrogen, methanol and methane. It is often claimed that they have greater potential for efficient operation than heat engines because they are not restricted by the Carnot limitation. However, in this paper a generalized (exergy analysis) approach is utilized to clarify the comparison of the theoretical performance potential of heat engines and fuel cells, in particular, to show that fuel cell conversion is restricted by the second law of thermodynamics in the same way as heat engines. The Carnot efficiency is simply a manifestation of the second law for the heat engine excluding the combustion process. It is shown that the maximum work obtainable from the conversion device is related to the change in flow exergy between reactants and products, that is in general, not equivalent to the change in Gibbs free energy. For equivalent reactant and product temperatures, the difference between the change in Gibbs free energy and the change in flow exergy is equal to the exergy flux of heat transfer that must be rejected by the device due to absorption of entropy from the reactant-product flow. The importance of exergetic (second-law) efficiencies for evaluating performance is demonstrated. Also, exergy analysis is utilized to resolve a number of efficiency related issues for endothermic reactions.     

Theory and performance analysis of a new heat engine for concentrating solar power

The objectives of this paper are to introduce a new heat engine and evaluate its performance. The new heat engine uses a gas, such as air, nitrogen, or argon, as the working fluid and extracts thermal energy from a heat source as the energy input. The new heat engine may find extensive applications in renewable energy industries, such as concentrating solar power (CSP). Additionally, the heat engine may be employed to recover energy from exhaust streams of internal combustion engines, gas turbine engines, and various industrial processes. It may also work as a thermal‐to‐mechanical conversion system in a nuclear power plant and function as an external combustion engine in which the heat source is the combustion gas from an external combustion chamber. The heat engine is to mimic the performance of an air‐standard Otto cycle. This is achieved by drastically increasing the time duration of heat acquisition from the heat source in conjunction with the timing of the heat acquisition and a large heat transfer surface area. Performance simulations show that the new heat engine can potentially attain a thermal efficiency above 50% and a power output above 100 kW under open‐cycle operation. Additionally, the heat engine could significantly reduce CSP costs and operate in open cycles, effectively removing the difficulties of dry cooling requirement for CSP applications. Copyright © 2014 John Wiley & Sons, Ltd.     

System-level energy efficiency is the greatest barrier to development of the hydrogen economy

Current energy research investment policy in New Zealand is based on assumed benefits of transitioning to hydrogen as a transport fuel and as storage for electricity from renewable resources. The hydrogen economy concept, as set out in recent commissioned research investment policy advice documents, includes a range of hydrogen energy supply and consumption chains for transport and residential energy services. The benefits of research and development investments in these advice documents were not fully analyzed by cost or improvements in energy efficiency or green house gas emissions reduction. This paper sets out a straightforward method to quantify the system-level efficiency of these energy chains. The method was applied to transportation and stationary heat and power, with hydrogen generated from wind energy, natural gas and coal. The system-level efficiencies for the hydrogen chains were compared to direct use of conventionally generated electricity, and with internal combustion engines operating on gas- or coal-derived fuel. The hydrogen energy chains were shown to provide little or no system-level efficiency improvement over conventional technology. The current research investment policy is aimed at enabling a hydrogen economy without considering the dramatic loss of efficiency that would result from using this energy carrier.     

Development of an experimental test rig for cogeneration based on a Stirling engine and a biofuel burner

A system consisting of a last-generation Stirling engine (SE) and a fuel burner for distributed power generation has been developed and experimentally investigated. The heat generated by the combustion of two liquid fuels, a standard Diesel fuel and a rapeseed oil, is used as a heat source for the SE, that converts part of the thermal energy into mechanical and then electric energy. The hot head of the SE is kept in direct contact with the flame generated by the burner. The burner operating parameters, designed for Diesel fuel, were changed to make it possible to burn vegetable oils, not suitable for internal combustion engines. The possibility of adopting different configurations of the combustion chamber was taken into account to increase the system efficiency. The preliminary configurations adopted allowed to operate this integrated system, obtaining an electric power up to 4.4 kW_el with a net efficiency of 11.6%.     

Evaluation of exergy and energy efficiencies of photothermal solar radiation conversion

In this paper, a new thermodynamic model for photothermal solar radiation conversion into mechanical through a heat engines is proposed. The developed equations allow for the energy and exergy contents of solar radiation to be found, as well as the energy and exergy efficiencies corresponding to concentration type solar-thermal heat engines operating under a range of conditions. The calculation method remains accurate to other published models when their assumed conditions are imposed to the newly developed model. The heat flux absorbed by the receiver (which is assumed to be a grey body and is placed in the focal point of the solar concentrator) depends on the hemispherical absorptivity and emissivity, concentration ratio and receiver temperature. The model is used to conduct a parametric study regarding the energy and exergy efficiencies of the system for assessing its performance. The use of a selective grey body receiver (having a reduced emissivity and a high absorptivity) for enhancing the conversion efficiency is also studied. If the absorptivity approaches one and the emissivity is low enough the photothermal conversion efficiency becomes superior to the known black body receiver limit of 0.853. It is found that in the limit of receiver emissivity tending to zero and absorptivity lending to one, the present model gives the exergy content of solar radiation because the work generated reaches its maximum. In this situation the energy efficiency approaches the exergy efficiency at 1-ITTIN0/TINS where T_S and T₀ are the sun and ambient temperatures, respectively. The influence of the ambient temperature on the exergy and energy efficiencies becomes apparent, with effects of up to 15%, particularly for high absorptivity and low emissivity. The heat transfer conductances at sink and source of the heat engine have a considerable impact on the efficiency of solar energy conversion. The present model is developed in line with actual power system operations for better practical acceptance. In addition, some irreversibility parameters (absorptivity, emissivity, heat transfer conductivity, etc.) are studied and discussed to evaluate the possible photothermal solar radiation conversion systems and assess their energy and exergy efficiencies.     

Oxy-hydrogen gas as an alternative fuel for heat and power generation applications - A review

One of the important goals in today's world is sustainable power generation by using low or zero polluting fuels and energy conversion devices. In this context, utilization of gaseous fuels in internal combustion (IC) engines is focused more due to their better fuel mixing ability with air, higher combustion efficiency, easier transportation, and lower pollutant formation. Liquefied petroleum gas (LPG), compressed natural gas (CNG), hydrogen, and biogas are considered as commonly available alternative gaseous fuels for IC engines. Yet, a search for other possible alternative gaseous fuels is continuing in the world. In recent years, Oxy-hydrogen (HHO) also known as Brown's gas has been explored by many researchers in the world, for the possibility of using it for heat and power applications. Because of this, a comprehensive review of the production of HHO using different generators and its utilization in heat and power applications has been carried out, and the discussions are presented in this paper.     

Thermal efficiency improvement in high output diesel engines a comparison of a Rankine cycle with turbo-compounding

Thermal management, in particular, heat recovery and utilisation in internal combustion engines result in improved fuel economy, reduced emissions, fast warm up and optimized cylinder head temperatures. turbo-compounding is a heat recovery technique that has been successfully used in medium and large scale engines. Heat recovery to a secondary fluid and expansion is used in large scale engines, such as in power plants in the form of heat recovery steam generators (HRSG) [1]. The present paper presents a thermodynamic analysis of turbo-compounding and heat recovery and utilisation through a fluid power cycle, a technique that is also applicable to medium and small scale engines. In a fluid power cycle, the working fluid is stored in a reservoir and expanded subsequently. The reservoir acts as an energy buffer that improves the overall efficiency, significantly. This paper highlights the relative advantage of exhaust heat secondary power cycles over turbo-compounding with the aid of MATLAB based QSS Toolbox [2] simulation results. Steam has been selected as the working fluid in this work for its superior heat capacity over organic fluids and gases.     

Influence of heat loss on the performance of an air-standard Atkinson cycle

This study is aimed at investigating the effects of heat loss, as characterized by a percentage of fuel’s energy, friction and variable specific heats of the working fluid, on the performance of an air-standard Atkinson cycle under the restriction of the maximum cycle-temperature. A more realistic and precise relationship between the fuel’s chemical-energy and the heat leakage is derived through the resulting temperature. The variations in power output and thermal efficiency with compression ratio, and the relations between the power output and the thermal efficiency of the cycle are presented. The results show that the power output as well as the efficiency, for which the maximum power-output occurs, will rise with the increase of maximum cycle-temperature. The temperature-dependent specific heats of the working fluid have a significant influence on the performance. The power output and the working range of the cycle increase while the efficiency decreases with the rise of specific heats of working fluid. The friction loss has a negative effect on the performance. Therefore, the power output and efficiency of the Atkinson cycle decrease with increasing friction loss. It is noteworthy that the results obtained in the present study are of significance for providing guidance with respect to the performance evaluation and improvement of practical Atkinson-cycle engines.     

Heat Transfer Processes in Low-Heat-Rejection Diesel Engines

The past five years have seen considerable interest in the possibility of constructing diesel engines in which all or part of the combustion chamber is insulated from the environment. either by ceramic materials or by high-temperature alloys backed with an air gap. It is hoped that such a design will permit engine mass to be reduced by the elimination of water cooling, and engine efficiency to be increased by the use of a secondary cycle to extract heat from the exhaust gases. In assessing the practicality of an uncooled engine, it is essential to establish the degree of insulation achievable by a proposed design. This requires identification of the principal heat loss mechanisms. This paper describes the construction of a three-dimensional time-varying combustion-chamber model, and shows that the cyclic motion of the piston leads to an important heat loss not calculable by static models. Insulation is shown to increase the importance of this loss mechanism. Estimates of the loss are given. By adapting methods established for the analysis of thermal regenerators, an approximate closed-form expression for the heat loss is obtained.     

Efficiencies of heat engines and fuel cells: The methanol fuel cell as a competitor to otto and diesel engines

As the real cost of fuel rises the efficiency of energy conversion devices will become of increasing importance. Efficiency is a variable factor depending inter alia on load factor. Whereas heat engines commonly yield optimum efficiencies at near to maximum power, fuel cells yield optimum efficiencies at zero power. Projections based on realistic developments suggest that fuel cells will operate overall with higher efficiencies than heat engines when load factors are below ～45%. Road transportation generally operates at load factors much lower than this and represents a suitable market for fuel cells.     

Stoichiometric H2ICE with water injection and exhaust and coolant heat recovery through organic Rankine cycles

High power density, stoichiometric, turbocharged, directly injected engines with water injection and a three way catalytic converter after treatment have been proposed as one of the most promising H₂ICE [1]. These throttle controlled engines have top brake efficiencies exceeding 40%, but large penalties in efficiency reducing the load with 1 bar BMEP values approaching 10%. Recovery of the large amount of fuel energy lost in the coolant especially at low loads as well as the fuel energy lost in the exhaust that is significant at high loads and speeds may push not only the top brake efficiencies to exceed the 45% mark, but also to dramatically increase the low load efficiency compromised by the throttling. In this paper, recovery of the waste heat from the exhaust gases and the coolant in a H₂ICE is performed with Organic Rankine Cycles (ORC). The engine without ORC has a maximum efficiency of 42% and an average efficiency over the map points of 32.7%. With the exhaust ORC, neglecting the possible back pressure increase due to the heat exchanger downstream of the catalytic converter the maximum efficiency increases to 45.6%, and the average efficiency rises to 35.3%. With the coolant ORC, neglecting the reduced mechanical efficiency for the coolant back pressure increment, the maximum efficiency increases to 43.4% and the average efficiency increases to 34.6%. Finally, combining the two ORC with same assumptions, the maximum efficiency increases to 46.9% and the average efficiency to 38%.     

Unified cycle model of a class of internal combustion engines and their optimum performance characteristics

The unified cycle model of a class of internal combustion engines is presented, in which the influence of the multi-irreversibilities mainly resulting from the adiabatic processes, finite-time processes and heat leak loss through the cylinder wall on the performance of the cycle are taken into account. Based on the thermodynamic analysis method, the mathematical expressions of the power output and efficiency of the cycle are calculated and some important characteristic curves are given. The influence of the various design parameters such as the high-low pressure ratio, the high-low temperature ratio, the compression and expansion isentropic efficiencies etc. on the performance of the cycle is analyzed. The optimum criteria of some important parameters such as the power output, efficiency and pressure ratio are derived. The results obtained from this unified cycle model are very general and useful, from which the optimal performance of the Atkinson, Otto, Diesel, Dual and Miller heat engines and some new heat engines can be directly derived.