An irreversible heat engine model including three typical thermodynamic cycles and their optimum performance analysis

An irreversible Dual heat engine model, which can include the Otto and Diesel cycles, is established and used to investigate 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. The power output and efficiency of the cycle are derived and optimized with respect to the pressure ratio of the working substance. The maximum power output and efficiency are calculated. The influence of the various design parameters 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 given. Several special interesting cases are discussed. The results obtained are general, so that the optimal performance of irreversible Otto and Diesel cycles are included in two special cases of the Dual cycle and may be directly derived from that of the Dual heat engine. Moreover, the performance characteristic curves of the three heat engines are presented by using numerical examples.     

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.     

热漏对热机功率效率特性的影响分析

在以往文献的基础上,对定常态流不可逆卡诺热机的热漏模型进行了改进,将热机热漏分为外热漏和内热漏两种方式,分析两种热漏方式对不可逆热机最优性能的影响。分析得出内热漏对热机功率效率特性的影响不同于外热漏,也与摩擦,涡流和非平衡等各种不可逆效应不同,内热漏应单独作为影响热机特性的一种因素。并导出存在热阻和两种热漏损失的热机存在最佳功率和最佳效率两种工作状态,两种工作状态分别对应不同的面积比。所得结果对热机设计具有一定指导意义。     

线性唯象传热定律下广义不可逆卡诺热机的频率特性

以广义不可逆卡诺热机模型为研究对象,考虑工质与热源间传热服从线性唯象定律,研究热机性能与循环频率的关系.得到了不同于内可逆情况下的输出功率、效率以及可利用温差与循环频率和吸、放热时间比的关系式,通过数值计算,分析了热漏、内不可逆性的影响特点.结果表明,在任一循环吸、放热时间比下,存在一个最佳循环频率,使循环输出功率达到最大;存在热漏时,任一循环吸、放热时间比下,存在一个最佳循环频率,使循环效率达到最大.     

Effect of variable heat capacities on performance of an irreversible Miller heat engine

Based on the variable heat capacities of the working fluid, the irreversibility coming from the compression and expansion processes, and the heat leak losses through the cylinder wall, an irreversible cycle model of the Miller heat engine was established, from which expressions for the efficiency and work output of the cycle were derived. The performance characteristic curves of the Miller heat engine were generated through numerical calculation, from which the optimal regions of some main parameters such as the work output, efficiency and pressure ratio were determined. Moreover, the influence of the compression and expansion efficiencies, the variable heat capacities and the heat leak losses on the performance of the cycle was discussed in detail, and consequently, some significant results were obtained.     

Finite-time optimization of a solar-driven heat engine

Maximum power and efficiency at the maximum power point of an internally and externally irreversible finite-size solar thermal power plant heat engine are treated. It was found that the thermal efficiency depends on the internal irreversibility resulting from the working fluid for a given value of reservoir temperatures ratio. It was also concluded that the heat-exchangers optimum size ratio must be less than one for maximum power output.     

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.     

Parametric optimization of a solar-driven Braysson heat engine with variable heat capacity of the working fluid and radiation–convection heat losses

An irreversible solar-driven Braysson heat engine system is presented, in which the temperature-dependent heat capacity of the working fluid, the radiation–convection heat losses of the solar collector and the irreversibilities resulting from heat transfer and non-isentropic compression and expansion processes are taken into account. Based on the thermodynamic analysis method and the optimal control theory, the mathematical expression of the overall efficiency of the system is derived and the maximum overall efficiency is calculated, and the operating temperatures of the solar collector and the cyclic working fluid and the ratio of heat-transfer areas of the heat engine are optimized. By using numerical optimization technology, the influences of the variable heat capacity of the working fluid, the radiation–convection heat losses of the solar collector and the multi-irreversibilities on the performance characteristics of the solar-driven heat engine system are investigated and evaluated in detail. Moreover, it is expounded that the optimal performance and important parametric bounds of the irreversible solar-driven Braysson heat engine with the constant heat capacity of the working fluid and the irreversible solar-driven Carnot heat engine can be deduced from the conclusions in the present paper.     

Power,power density and efficiency optimization of an endoreversible Braysson cycle

The performance optimization of an endoreversible Braysson cycle with heat resistance losses in the hot- and cold-side heat exchangers is performed by using finite-time thermodynamics. The relations between the power output and the working fluid temperature ratio, between the power density and the working fluid temperature ratio, as well as between the efficiency and the working fluid temperature ratio of the cycle coupled to constant-temperature heat reservoirs are derived. Moreover, the optimum heat conductance distributions corresponding to the optimum dimensionless power output, the optimum dimensionless power density and the optimum thermal efficiency of the cycle, and the optimum working fluid temperature ratios corresponding to the optimum dimensionless power output and the optimum dimensionless power density are provided. The effects of various design parameters on those optimum values are studied by detailed numerical examples.     

Performance analysis and parametric optimum criteria of an irreversible Atkinson heat-engine

Multi-irreversibilities, mainly resulting from the adiabatic processes, finite-time processes and heat loss through the cylinder wall, are considered in the cycle model of an Atkinson heat engine. The power output and efficiency of the cycle are derived by introducing the pressure ratio and the compression and expansion efficiencies. The performance characteristic curves of the cycle are presented. The bounds of the power output and efficiency are determined. The optimum criteria of some important parameters, such as the power output, efficiency and pressure ratio are given. The influences of the various design parameters on the performance of the cycle are analyzed in detail. The results obtained may provide a theoretical basis for both the optimal design and operation of real Atkinson heat engines.     

Performance analysis and parametric optimum criteria of a quantum Otto heat engine with heat transfer effects

The influence of both the quantum degeneracy and the finite rate heat transfer between the working substance and the cylinder wall on the optimal performance of an Otto engine cycle is investigated. Expressions for several important parameters such as the power output and efficiency are derived. By using numerical solutions, the curves of the power output and efficiency varying with the compression ratio of two isochoric processes are presented. It is found that there are optimal values of the compression ratio at which the power output and efficiency attain their maximum. In particular, the optimal performance of the cycle in strong and weak gas degeneracy and the high temperature limit are discussed in detail. The distinctions and connections between the quantum Otto engine and the classical are revealed. Moreover, the maximum power output and efficiency and the corresponding relevant parameters are calculated, and consequently, the optimization criteria of some important parameters such as the power output, efficiency and compression ratio of the working substance are obtained.     

Thermodynamic Analysis and Comparison of Performances of Air Standard Atkinson,Otto, and Diesel Cycles with Heat Transfer Considerations

This paper focuses on the overall performances of Otto, Atkinson, and Diesel air standard cycles. This study compares performance of these cycles with regard to parameters such as variable specific heat ratio, heat transfer loss, frictional loss, and internal irreversibility based on finite‐time thermodynamics. The relationship between thermal efficiency and compression ratio, and between power output and compression ratio of these cycles are obtained by numerical examples. In this study, it is assumed that during the combustion process, the heat transfer occurs only through the cylinder wall. The heat transfer is affected by the average temperature of both the cylinder wall and the working fluid. The results show that for each cycle, with the increase of the compression ratio in the specific mean piston speed, power output and thermal efficiency first increase and after reaching their maximum value, start to decrease. The results also indicate that maximum power output and maximum thermal efficiency of an Atkinson cycle could be higher than the values of these parameters in Diesel cycle and Otto cycle in the same operating conditions. The maximum power output and the maximum thermal efficiency of the Otto cycle have the lowest value among studied cycles. By increasing the mean piston speed, power output and thermal efficiency of Atkinson, Diesel, and Otto cycles start to decrease. The results of this study provide guidance for the performance analysis and show the improvement areas of practical Otto, Atkinson, and Diesel engines.     

Thermodynamic Analysis and Optimization of an Irreversible Maisotsenko-Diesel Cycle

So far, Maisotsenko cycle has been applied to many fields such as heating ventilation and air-conditioning, power industry, chemical production, and so on. A lot of researches about classical thermodynamic analyses of Maisotsenko cycle have been made. A new cycle model of combined Diesel and Maisotsenko cycles considering heat transfer loss(HTL), piston friction loss(PFL) and internal irreversible loss(IIL) was proposed in this paper. By using the finite time thermodynamic(FTT) theory, the power and efficiency performances of the Maisotsenko-Diesel cycle(MDC) were studied. Effects of mass flow rate(MFR) of water injection in the Maisotsenko air saturator(MAS) and the other parameters related to the design of Diesel engine on the optimal cycle performances were analyzed. Furthermore, it was testified that irreversible MDC was superior than conventional irreversible Diesel cycle in both power output and thermal efficiency. The results can expand the application of Maisotsenko cycle(M-cycle) and provide some theoretical guidelines for the practical devices.     

Carbon Dioxide as Working Fluids in Transcritical Rankine Cycle for Diesel Engine Multiple Waste Heat Recovery in Comparison to Hydrocarbons

In consideration of the high-temperature characteristic of engine's waste heat and stricter environmental regulations, natural substance, including CO_2 and hydrocarbons, have been treated as promising working fluid for diesel engine waste heat recovery due to its environment friendly and excellent physical and chemical properties. This paper presented a comprehensive performance analysis on transcritical Rankine cycles for diesel engine multiple waste heat recovery using hydrocarbons and CO_2 as working fluid. The optimal turbine inlet pressures corresponding to maximum net power output, maximum exergy efficiency and minimum electricity production cost(EPC) were obtained. The effect of working fluid on these optimal pressures has been discussed. For fluids with low critical temperature, the optimal pressure corresponding to maximum net power output is lower than the one for maximum exergy efficiency, while the opposite results can be found for fluid with high critical temperature. Then, the effect of various working fluid properties in transcritical cycle performance is discussed. Comparison results show that CO_2 obtains only more power output than Ethane, Propane and Propene, but CO_2 is capable of absorbing more energy from engine coolant and regeneration heat with comparable total heat transfer areas and has an advantage in turbine size, particularly for hydrocarbons with high critical temperature.     

Optimization criteria for the important parameters of an irreversible Otto heat-engine

An irreversible cycle model of an Otto heat-engine is established, in which the main irreversibilities result from the non-isentropic compression and expansion processes; finite-time processes and heat loss through the cylinder wall are taken into account. The power output and efficiency of the cycle are derived. The curves of the power output and efficiency varying with the compression ratio of two isochoric processes are presented. It is found from the curves that there are optimal values of the compression ratio at which the power output and efficiency attain their maxima. Moreover, the maximum power-output and efficiency and the corresponding relevant parameters are calculated, and consequently, the optimization criteria of some important parameters such as the power output, efficiency, compression ratio, and temperatures of the working substance are obtained.     

Performance of irreversible Meletis–Georgiou vane rotary engine cycle with variable specific heats of working fluid

An irreversible cycle model of Meletis–Georgiou (MG) engine consisting of an isochoric heating branch, isochoric and isobaric cooling branches, two non-isentropic compression and two non-isentropic expansion branches and with heat transfer loss, internal irreversibility and the linear relation between specific heat of the working fluid and its temperature is established by using the theory of finite time thermodynamics. The analytical relations of the work output versus compression ratio, the efficiency versus compression ratio, as well as the work output versus efficiency are obtained by using numerical examples. The results show that the work output versus the efficiency characteristic of the irreversible MG cycle is a loop-shaped curve which is consistent with the general heat engine performance, and the cycle model considering the internal irreversibility and linear relation between specific heats of the working fluid and its temperature is closer to practice than the endo-reversible model with constant specific heats of the working fluid.     

基于遗传算法的汽油机朗肯循环余热回收系统的优化

为了更加高效利用汽油机排气余热,分析了某款汽油机排气余热回收潜力,建立了基于蒸发器和活塞式膨胀机的汽油机-朗肯循环联合余热回收系统模型。利用遗传算法,同时考虑膨胀机输出功、排气利用率、蒸发器效率和膨胀机绝热效率,以膨胀机输出功和系统总效率为优化目标,以蒸发压力和膨胀机转速为优化变量,对汽油机4个工况下朗肯循环系统的最佳运行参数进行了研究。结果表明,在整个发动机转速范围内,排气最大可利用效率均高于46%,转速越高则排气品质越高。在不同工况下存在最优的膨胀机转速和蒸发压力。经过优化,在选取的4个工况下,功率提高率均在6%以上,最高达到7.08%。     

热漏对热机最佳功率与最佳效率特性的影响

对于有限时间热力学,以往文献对于卡诺热机最佳效率与最佳功率间关系的分析大多只考虑了热阻和装置热漏(本文称为外热漏),而没有考虑到工质热漏(本文称为内热漏)的影响。将热漏分为外热漏和内热漏两种方式,经过分析得出了由于内热漏损失使热机存在最佳功率和最佳效率两种不同工作状态,指出了内热漏的影响不同于外热漏,也不可将内热漏简单归结于内不可逆的重要结论。     

A Parametric Study of an Irreversible Closed Intercooled Regenerative Brayton Cycle

Entropy generation minimization technique is used in the analysis of an irreversible closed intercooled regenerative Brayton cycle coupled to variable-temperature heat reservoirs. Mathematical models are developed for dimensionless power and efficiency for a multi-stage Brayton cycle. The dimensionless power and efficiency equations are used to analyze the effects of total pressure ratio, intercooling pressure ratio, thermal capacity rates of the working fluid and heat reservoirs, and the component (regenerator, intercooler, hot- and cold-side heat exchangers) effectiveness. Using detailed numerical examples, the optimal power and efficiency corresponding to variable component effectiveness, compressor and turbine efficiencies, intercooling pressure ratio, total pressure ratio, pressure recovery coefficients, heat reservoir inlet temperature ratio, and the cooling fluid in the intercooler and the cold-side heat reservoir inlet temperature ratio are analyzed.     

Optimal Performance Characteristics of Subcritical Simple Irreversible Organic Rankine Cycle

Based on the theory of finite time thermodynamics, a subcritical simple irreversible organic Rankine cycle (SSIORC) model considering heat transfer loss and internal irreversible losses is established in this paper. The total heat transfer surface area is taken as a constraint, and R245fa is adopted as working fluid of the cycle in the performance optimization. The evaporator heat transfer surface area and mass flow rate of the working fluid are optimized to obtain the maximum power output and thermal efficiency of the SSIORC, respectively. In addition, the influences of the internal irreversibilities on the optimal performances are also investigated. The results show that when the evaporator heat transfer surface area is varied, the relationship between power output and thermal efficiency is a loop-shaped curve, and there exist maximum power output and thermal efficiency points, respectively. However, the two maximum points are very close to each other. When the mass flow rate of the working fluid is varied, the relationship between power output and thermal efficiency is a parabolic-like curve. With the decreases of expander and pump irreversible losses, the performances of the irreversible SSORC are close to those of the endoreversible SSORC with the only loss of heat transfer loss.