Air-standard Cycle Research Articles

Thermodynamic simulation of performance of a dual cycle with stroke length and volumetric efficiency

This article presents finite-time thermodynamics analysis of an irreversible air standard dual cycle. An irreversible dual cycle model which is more close to practice is established. In this model, the effects of stroke length and volume efficiency by considering the nonlinear relation between the specific heats of working fluid and its temperature, the frictional loss, the internal irreversibility, and heat transfer loss are analyzed. The results show that if compression ratio is less than certain value, the power output increases with increasing stroke length, while if compression ratio exceeds certain value, the power output first increases and then starts to decrease with increasing stroke length. With further increase in compression ratio, the increase of stroke length results in decreasing the power output. The results also show that, throughout the compression ratio range, the power output increases with the increasing volumetric efficiency. The results obtained in this study are of importance to provide good guidance for performance evaluation and improvement of practical internal combustion engines.

Quantitative Reasons Why Ideal Air Standard Engine Cycles are Deficient

The ideal air standard engine (IASE) cycles provide reasonable trend predictions of items such as thermal efficiency with respect to compression ratio, but over-predict the absolute values of the efficiency, as well as cylinder pressures and temperatures, and possess other such deficiencies. This study is aimed at quantifying the reasons for these deficiencies, by comparing the results from a thermodynamic cycle simulation (i.e. a representation of actual engines) and the IASE cycles. One of the major deficiencies of IASE cycles is the assumption of no heat transfer. For one case (compression ratio of 8), the gross indicated thermal efficiency for the cycle simulation increased from 35.9% to 41.9% by eliminating all heat transfer. The 41.9% thermal efficiency was in good agreement with the IASE cycle efficiency (41.8%) for a ratio of specific heats of 1.26. Other deficiencies were the assumed instantaneous combustion and lack of true property evaluations. The maximum cylinder pressures and temperatures were always over-predicted by the IASE cycles.

Performance analysis of irreversible Atkinson cycle with considerations of stroke length and volumetric efficiency

This paper presents finite time thermodynamics analysis of an irreversible air standard Atkinson cycle. An irreversible Atkinson cycle model, which is closer to practice, is established. In this model, the effects of stroke length and volume efficiency by considering the non‐linear relation between the specific heats of working fluid and its temperature, the frictional loss, the internal irreversibility and heat transfer loss are analysed. The results show that if compression ratio is less than certain value, the power output increases with increasing stroke length, while if compression ratio exceeds certain value, the power output first increases and then starts to decrease with increasing stroke length. With further increasing compression ratio, the increase in stroke length results in a decrease in the power output. The results also show that, throughout the compression ratio range, the power output increases with increasing volumetric efficiency. The results obtained in the present study are of importance to provide good guidance for performance evaluation and improvement of practical internal combustion engines.

Effects of mean piston speed, equivalence ratio and cylinder wall temperature on performance of an Atkinson engine

The performance of an air standard Atkinson cycle is analyzed using finite-time thermodynamics. In the model, the linear relation between the specific heat ratio of the working fluid and its temperature, the friction loss computed according to the mean velocity of the piston, the internal irreversibility described by using the compression and expansion efficiencies and the heat transfer loss are considered. The relations between the power output and the compression ratio and between the power output and the thermal efficiency are derived by detailed numerical examples. The results show that if the compression ratio is less than a certain value, the power output decreases with increasing mean piston speed, while if the compression ratio exceeds a certain value, the power output first increases and then starts to decrease with increasing mean piston speed. With further increase in the compression ratio, the increase of mean piston speed results in decreasing the power output. Throughout the compression ratio range, the power output increases with increasing cylinder wall temperature while it first increases and then starts to decrease with the increase of equivalence ratio. The conclusions of this investigation are of importance when considering the designs of actual Atkinson engines.

An air-standard cycle and a thermodynamic perspective on operational limits of Ranque–Hilsh or vortex tubes

A Thermodynamic air-standard cycle was envisaged for Ranque–Hilsh (R–H) or Vortex Tubes to provide relevant Thermodynamic analysis and tools for setting operating limits according to the conservation laws of mass and energy, as well as the constraint of the Second Law of Thermodynamics. The study used an integral or control volume approach and resulted in establishing working equations for evaluating the performance of an R–H tube. The work proved that the coefficient of performance does not depend on the R–H tube operating mode, i.e., the same value is obtained independently if the R–H tube operates either as a heat pump or as a refrigeration device. It was also shown that the isentropic coefficient of performance displays optima values of cold and hot mass fractions for a given operating pressure ratio. Finally, the study was concluded by comparing the present analysis with some experimental data available in the literature for operating pressures ranging 2–11 atm.

Finite-time thermodynamic modeling and analysis for an irreversible Dual cycle

The performance of an air standard Dual cycle is analyzed by using finite-time thermodynamics. An irreversible Dual cycle model which is more close to practice is established. In the model, the nonlinear relation between the specific heats of working fluid and its temperature, the frictional loss computed according to the mean velocity of the piston, the internal irreversibility described by using the compression and expansion efficiencies, and heat transfer loss are considered. The relations between the power output and the compression ratio, between the thermal efficiency and the compression ratio, and the optimal relation between power output and the efficiency of the Dual cycle are derived by detailed numerical examples. Moreover, the effects of internal irreversibility, heat transfer loss, frictional loss and pressure ratio on the cycle performance are analyzed. The power output versus compression ratio and efficiency versus compression ratio curves of the Diesel and Otto cycles are the maximum and minimum envelope lines of the performance of the Dual cycle, respectively. The results obtained herein may provide guidelines for the design of practical internal combustion engines.

Performance of an endoreversible Diesel cycle with variable specific heats working fluid

SYNOPSIS The performance of an air-standard Diesel cycle with heat transfer loss and variable specific heats of working fluid is analysed by using finite-time thermodynamics. The relationships between work output and compression ratio, between thermal efficiency and compression ratio, as well as the optimal relationship between work output and efficiency of the cycle are derived by detailed numerical examples. Moreover, the effects of heat transfer loss and variable specific heats of working fluid on the cycle performance are analysed. The results show that the effects of heat transfer loss and variable specific heats of working fluid on the cycle performance are obvious and should be considered in practical cycle analysis. The results obtained in this paper may provide guidance for the design of practical internal combustion engines.

Performance of Diesel cycle with heat transfer, friction and variable specific heats of working fluid

AbstractThe performance of an air standard Diesel cycle with heat transfer loss, friction-like term loss and variable specific heats of working fluid is analysed by using finite time thermodynamics. The relationships between the power output and the compression ratio, between the thermal efficiency and the compression ratio, as well as the optimal relationship between the power output and the efficiency of the cycle are derived by detailed numerical examples. Moreover, the effects of temperature dependent specific heats of working fluid on the irreversible cycle performance are analysed. The results show that the effects of temperature dependent specific heats of working fluid on the irreversible cycle performance are obvious, and they should be considered in practice cycle analysis. The results obtained in the present paper may provide guidance for the design of practice Diesel engines.

Performance of endoreversible Atkinson cycle

AbstractThe performance of an air standard Atkinson cycle with heat transfer loss and variable specific heats of working fluid is analysed by using finite time thermodynamics. The relationships between the work output and the compression ratio and between the thermal efficiency and the compression ratio, as well as the optimal relationship between the work output and the efficiency of the cycle, are derived from detailed numerical examples. Moreover, the effects of heat transfer loss and variable specific heats of working fluid on the cycle performance are analysed. The results show that these effects are obvious and they should be considered in practice cycle analysis. The results obtained in the present paper may provide guidance for the design of practical internal combustion engines.

Comparison of performances of air standard Atkinson and Otto cycles with heat transfer considerations

In this paper, the effects of heat transfer on the net output work and the indicated thermal efficiency of an air standard Atkinson cycle are analyzed. Comparisons of the performances of air standard Atkinson and Otto cycles with heat transfer considerations are also discussed. We assume that the compression and power processes are adiabatic and reversible and that any convective, conductive or radiative heat transfer to the cylinder wall during the heat rejection process may be ignored. The heat loss through the cylinder wall is assumed to occur only during combustion and is further assumed to be proportional to the average temperature of both the working fluid and cylinder wall. It is found that the net output work versus efficiency characteristics, the maximum net work output and the corresponding efficiency bound are significantly influenced by the magnitude of the heat transfer. An increase in heat transfer to the combustion chamber walls decreases the peak temperature and pressure and, consequently, reduces the work per cycle and efficiency. The effects of other parameters, in conjunction with heat transfer, including combustion constants, compression ratio and intake air temperature are also reported. An Atkinson cycle has a greater work output and a higher thermal efficiency than the Otto cycle at the same operating condition. The compression ratios that maximize the work of the Otto cycle are always found to be higher than those for the Atkinson cycle at the same operating conditions. The results are of importance to provide good guidance for performance evaluation and improvement of practical Atkinson engines.

Performance optimisation of reciprocating heat engine cycles with internal irreversibility

In the present study, an optimal performance analysis based on maximum power (MP) and maximum efficiency criterion including internal irreversibility for steady state operation has been performed for the air standard Dual cycle. It is shown that the effect of internal irreversibility such as friction on MP may be characterised by the cycle irreversibility parameter representing the ratios of entropy differences for the cycle. The presence of this parameter in the equations for the MP and the corresponding efficiency shows that an engine with internal irreversibility delivers less power and has a lower efficiency than a reversible engine. The effect of the design parameters such as pressure ratio, cut off ratio and extreme temperature ratio of the cycles has also been investigated under the MP and maximum efficiency conditions on the basis of having the internal irreversibility. Also in the present study, performance analysis has been extended to the Otto and Diesel cycles which may be considered as special cases of the Dual cycle.

Performance of an Atkinson cycle with heat transfer, friction and variable specific-heats of the working fluid

The performance of an air standard Atkinson cycle with heat-transfer loss, friction-like term loss and variable specific-heats of the working fluid is analyzed using finite-time thermodynamics. The relations between the power output and the compression ratio, between the thermal efficiency and the compression ratio, as well as the optimal relation between the power output and the efficiency of the cycle are derived by detailed numerical examples. Moreover, the effects of variable specific-heats of the working fluid and the friction-like term loss on the irreversible cycle performance are analyzed. The results show that the effects of variable specific-heats of working fluid and friction-like term loss on the irreversible cycle performance should be considered in cycle analysis. The results obtained in this paper provide guidance for the design of Atkinson engines.

Effects of heat transfer, friction and variable specific heats of working fluid on performance of an irreversible dual cycle

The thermodynamic performance of an air standard dual cycle with heat transfer loss, friction like term loss and variable specific heats of working fluid is analyzed. The relations between the power output and the compression ratio, between the thermal efficiency and the compression ratio, as well as the optimal relation between power output and the efficiency of the cycle, are derived by detailed numerical examples. Moreover, the effects of variable specific heats of the working fluid and the friction like term loss on the irreversible cycle performance are analyzed. The results show that the effects of variable specific heats of working fluid and friction like term loss on the cycle performance are obvious, and they should be considered in practical cycle analysis. The results obtained in this paper may provide guidance for the design of practical internal combustion engines.

THERMODYNAMIC ANALYSIS OF MIRROR GAS TURBINE CYCLE

An interesting configuration of the gas turbine is the mirror gas turbine cycle. There have been several developments in the thermodynamic analysis of various combined or co-generation cycle schemes, with more advanced heat recovery capabilities. One of such novel cycles is the mirror cycle, a conceptual combination of Brayton and inverted Brayton cycles with heat sink by intercooling. In this analysis, an air standard cycle is assumed for the gas turbine. Analysis about the specific net-power and the efficiency of the mirror gas turbine as a function of maximum temperature ratio, compressor pressure ratio, and turbine overall expansion ratio of the cycle are performed. The obtained results provide significant guidance to the evaluation and improvement of the mirror gas turbine.

Heat loss as a percentage of fuel’s energy in air standard Otto and Diesel cycles

Abstract The heat addition process for an air standard Otto (or Diesel) cycle has been widely described in the literature e.g. as subtraction from the fuel’s chemical energy of an arbitrary heat loss parameter times the average temperature of heat the adding period. The heat leakage parameter and the fuel’s energy depend on each other. Their valid ranges given in the literature affect the feasibility of both the air standard Otto and Diesel cycles. If they are selected arbitrarily, they will present unrealistic results and make both the air standard Otto and Diesel cycles unfeasible. For this reason, a more realistic and precise relationship between the fuel’s chemical energy and the heat leakage needs to be derived through the resulting temperature. The mentioned relationship is constituted on a pair of inequalities. The original contribution of this paper is to characterize the amount of heat leakage as a percentage of the fuel’s energy. Hence, the performance analysis of any internal combustion engine can be covered by a more realistic and valid range of the heat loss parameter and the fuel’s energy. The valid ranges are presented in graphs and basic equations.

Applying Thermodynamics in Search of Superior Engine Efficiency

Historically, a succession of thermodynamic processes has been used to idealize the operating cycles of internal combustion engines. In this study, the 256 possible combinations of four reversible processes—isentropic, isothermal, isochoric, and isobaric—are surveyed in search of cycles promising superior thermal efficiency. Regenerative cycles are excluded. The established concept of the air-standard cycle, which mimics the internal combustion engine as a closed-cycle heat engine, is used to narrow the field systematically. The approach relies primarily on graphical interpretation of approximate temperature-entropy diagrams and is qualitative only. In addition to identifying the cycles offering the greatest efficiency potential, the compromise between thermal efficiency and mean effective pressure is addressed.

Heat transfer effects on the performance of an air standard Dual cycle

There are heat losses during the cycle of a real engine that are neglected in ideal air standard analysis. In this paper, the effects of heat transfer on the net work output and the indicated thermal efficiency of an air standard Dual cycle are analyzed. Heat transfer from the unburned mixture to the cylinder walls has a negligible effect on the performance for the compression process. Additionally, the heat transfer rates to the cylinder walls during combustion are the highest and extremely important. Therefore, we assume that the compression and power processes proceed instantaneously so that they are reversible adiabatics, and the heat losses during the heat rejection process can be neglected. The heat loss through the cylinder wall is assumed to occur only during combustion and is further assumed to be proportional to the average temperature of both the working fluid and the cylinder wall. The results show that the net work output versus efficiency characteristics and the maximum net work output and the corresponding efficiency bounds are strongly influenced by the magnitude of the heat transfer. Higher heat transfer to the combustion chamber walls lowers the peak temperature and pressure and reduces the work per cycle and the efficiency. The effects of other parameters, in conjunction with the heat transfer, including combustion constants, cut-off ratio and intake air temperature, are also reported. The results are of importance to provide good guidance for the performance evaluation and improvement of practical Diesel engines.

Performance optimization of a new combined power cycle based on power density analysis of the dual cycle

In this paper, maximum power density (MPD) analysis of an air standard internal combustion Dual cycle has been performed. Based on the obtained results for MPD analysis of the Dual cycle, a new combined power cycle model (Dual+Joule-Brayton) has been introduced and optimized. Optimal performance and design parameters are obtained analytically under the MPD conditions of the Dual cycle. The obtained results are discussed in terms of thermal efficiency, power and engine sizes. It is shown that for the combined cycle, a design based on the MPD conditions is more advantageous from the point of view of engine sizes and thermal efficiency.

The effect of friction on the performance of an air standard dual cycle

An irreversible air standard Dual cycle model is proposed in this paper. The finite-time evolution of the cycle's compression and power stroke is taken into account and its global losses lumped in a friction-like term is also considered. The analytical formulas of power output versus compression ratio and efficiency versus compression ratio of the cycle are derived. They are generalized formulas for internal combustion engines because they include the performance characteristic of special cases of Diesel and Otto engines. The effects of various design parameters on the performance of the cycle are demonstrated by one numerical example. The model leads to loop-shaped power-versus-efficiency curve as is common to almost all real heat engines.

Solar Chimney Cycle Analysis With System Loss and Solar Collector Performance

An ideal air standard cycle analysis of the solar chimney power plant gives the limiting performance, ideal efficiencies and relationships between main variables. The present paper includes chimney friction, system, turbine and exit kinetic energy losses in the analysis. A simple model of the solar collector is used to include the coupling of the mass flow and temperature rise in the solar collector. The method is used to predict the performance and operating range of a large-scale plant. The solar chimney model is verified by comparing the simulation of a small-scale plant with experimental data. [S0199-6231(00)00503-7]