Analysis of a Combined Regenerative and Reheat Gas Turbine Cycle using MATLAB 1Mohammed 1 Jasim Mohammed 2Dr. Mohammad Tariq Research Scholar, Department of Mechanical Engineering, SSET, SHIATS-DU, Naini, Allahabad, India 1 Assistant Professor, Department of Mechanical Engineering, SSET, SHIATS-DU, Naini, Allahabad, India ABSTRACT In recent development of gas turbine cycles, new software for calculating thermal efficiency and power output of a reheat and regenerative cycles are applied. There are one compressor and one turbine used in regeneration gas turbine cycle and one compressor and two turbines are used in reheat cycle. In the combined cycle, one compressor and two turbines (high pressure and low pressure respectively) is using in the present work. The temperature after reheating assumes to be reaching at the same temperature of HPT inlet. In the present work, a more sophisticated method is developed from the earlier work and is used to calculate the various parameters of a combined gas turbine cycle. The combination of the reheat and regenerative cycle has been taken in the present work and the thermodynamic analysis has been performed using MATLAB 10 software. The parameters taken in the well defined range for overall pressure ratio, turbine inlet temperatures and ambient temperature. The cycle performs the analysis for various regenerative effectiveness and it is found that the thermal efficiency has been increases on increasing the regenerative effectiveness. Also found that the heat required in the burner decreases for higher regenerative effectiveness. Keywords: Gas Turbine, Regenerative Effectiveness, Reheat cycle, Thermal Efficiency. 1. Introduction gas turbine, you need to be schooled in 1.1 Gas turbines theory various topics. To define the compressor and Gas turbines can be divided into important the turbine, you need to use aerodynamics. categories. There are industrial gas turbines To get an efficient combustion, knowledge and there are jet engine gas turbines. Both on thermodynamics is required. Finally, to types of gas turbines have a short but make sure the engine survives the big interesting background. When designing a temperature differences and high forces, you must be familiar with material sciences. A Then the effects of oxygen utilization, fuel gas turbine cycle is modeled to investigate utilization, the efficiencies of the gas turbine components effects of parameters important and on the system performance of the RHE cycle temperature (CIT), turbine inlet temperature and the EGR cycle were discussed in detail. (TIT) and pressure ratio (PR) on the overall The effect of turbine efficiency on the cycle performance and CO2 emissions [5]. system efficiency was more obvious than the The reheat process is replaced by processes effect of the compressor and recuperator of heating the expanded gases while passing efficiencies through different turbine stator blades. Small components. It was also indicated that amount of combusted gases is utilized to improving the flow inside such blades for heating and efficiencies for the RHE cycle increased mixing with the expanded gases [2]. A system efficiency higher than that for the computational the EGR cycle [17]. The majority of the performance of different gas turbine plants Western world’s naval fleets already use is presented in this paper. The work includes gas-turbine engines for propulsion and the effect of relative humidity, ambient inlet electric power generation. The General air temperature and types of fuels on gas Electric LM2500 gas turbines used to power turbine plants. Investigation also covers ships have a simple-cycle thermal efficiency economic analysis and effect of fuels on of 37 percent. The General Electric WR-21 emissions [9]. The performance of actual gas turbines equipped with intercooling and cycles being used in gas turbines is regeneration have a thermal efficiency of 43 discussed. A general overview of combined- percent and produce21.6 MW (29040 hp). cycle plants is provided [13]. The regeneration also reduces the exhaust Two cycle schemes of recuperative heat temperature from 600°C to350°C. Air is exchanger gas compressed to 3 atm before it enters the described intercooler. Compared to steam-turbine and according to the air reheating method. The diesel propulsion systems, the gas turbine system performance with operating pressure, offers greater power for a given size and turbine inlet temperature and fuel cell load weight, high reliability, long life, and more were studied based on the simulation results. convenient operation. The engine start-up study (RHE) (EGR) compressor temperature inlet recirculated like operating operating to and assess exhaust were among gas the gas turbine turbine component time has been reduced from 4 h required for gas turbine, the dual pressure reformer can a typical steam propulsion system to less give a further benefit, as far as an accurate than 2 min for a gas turbine. Many modern optimization of the steam management is marine propulsion systems use gas turbines performed [4]. A regenerative gas turbine together with diesel engines because of the engine, with isothermal heat addition, high fuel consumption of simple-cycle gas- working under the frame of a Brayton cycle turbine engines. In combined diesel and gas- has been analyzed. With the purpose of turbine systems, diesel is used to provide for having a more efficient small-sized gas efficient low-power and cruise operation, turbine engine, the optimization has been and gas turbine is used when high speeds are carried out numerically using the maximum needed. In gas-turbine power plants, the power (MP) and maximum power density ratio of the compressor work to the turbine (MPD) method [18]. work, called the back work ratio, is very high. Usually more than one-half of the 1.2 The Regenerative Cycle turbine work output is used to drive the The compressor. The situation is even worse prominent in these days of tight fuel when the isentropic efficiencies of the reserves and high fuel costs. The amount of compressor and the turbine are low. This is fuel needed can be reduced by the use of a quite in contrast to steam power plants, regenerator in which the hot turbine exhaust where the back work ratio is only a few gas is used to preheat the air between the percent. This is not surprising, however, compressor and the combustion chamber. since a liquid is compressed in steam power The regenerator increases the temperature of plants instead of a gas, and the steady-flow the air entering the burner, thus reducing the work is proportional to the specific volume fuel-to-air ratio and increasing the thermal of the working fluid. Two solutions are efficiency. proposed for the heat recovery scheme: a 1.3 Reheat Cycle first reformer has a single pressure level In the reheat gas turbine cycle, there are two while the second has two in order to match turbines used namely; high pressure turbine the different pressures of the combustors. (HPT) and low pressure turbine (LPT). The While the single pressure scheme gives good gases first expand in the high pressure performance with respect to the stand alone turbine and then in low pressure turbine. The regenerative cycle is becoming temperature after expansion in the HPT becomes low and then further head added in the second combustor to increase the temperature before expansion in the LPT. Generally the temperature after expansion in the HPT has to be increases up to the same temperature of HPT inlet temperature. The pressure of expansion in the turbine has been taken as the root mean square value of the overall pressure for optimum results. Figure 2 T-s representation of combined regenerative and reheat cycle 1.4 Reheat and Regenerative Cycle There are one compressor and one turbine 2. Materials and Methodology used in regeneration gas turbine cycle. In the 2.1Analysis of the Ideal Cycle reheat or recuperation cycle, one compressor The Brayton cycle in its ideal form consists and two turbine is using in the present work. of two isobaric processes and two isentropic Fig 1 and fig 2 represents the combined processes. The two isobaric processes effect of reheating and regeneration of a gas consist of the combustor system of the gas turbine cycle. turbine and the gas side of the HRSG. The two isentropic processes represent the compression (Compressor) and the expansion (Turbine Expander) processes in the gas turbine. A simplified application of the first law of thermodynamics to the airstandard Figure 1 Schematic of combined regenerative and reheat cycle Brayton cycle (assuming no changes in kinetic and potential energy) has the following relationships: Work of compressor WC = ṁa (h2 − h1 ) (1) Work of high pressure turbine Whpt = (ṁa + ṁf )(h3 − h4 ) Work of low pressure turbine (2) W Wlpt = (ṁg + ṁf2 )(h5 − h6 ) act ηc = (LHV)η b Total work output As shown in fig 1, air enters the compressor Wcyc = Whpt + Wlpt − Wc (3) at a state defined by T1 and P1. The Net turbine work is given by compressor exit pressure, P2, is given by: Wnet = Whpt + Wlpt p2 = rp × p1 Heat added to combustor 1 and combustor 2 given as follows; Q2,3 = ṁf × LHVfuel = (ṁa + ṁf )(h3 ) − ṁa (h2 ) (4) Q4,5 = ṁf × LHVfuel = (ṁa + ṁf )(h5 ) − ṁf (h4 ) rp is the compressor pressure ratio. The ideal compressor discharge temperature, T2s is given by the isentropic relation T2′ = T1 × rp [k] The compressor isentropic efficiency, defined as the ratio of the compressor Total heat input is given by; isentropic work to the actual compressor Qtot = Q2,3 + Q4,5 work with both starting at the same initial Thus, the overall thermal cycle efficiency is ηth = Wnet /Qtot (5) state and ending at the same pressure level, may be written as: Assuming the compressor efficiency is (ηc ) and the turbine efficiency is (ηt ), then the ɳc = isentropic work actual work = h2 −h1 h2′ −h1 = T2 −T1 T2′ −T1 actual compressor work and the actual Here the steady-flow energy equation has turbine work is given by: been applied to obtain expressions for the Wca = ṁa (h2 −h1 ) work (6) ηc (7) Thus, the actual total output work is (8) temperature from 2 to 3 is h −h b irreversible adiabatic isentropic compressor in the numerator. Actual compressor discharge temperature is given as The actual fuel required to raise the 3a 2a ṁf = (LHV)η an compressor in the denominator and for an Wta = (ṁa + ṁf )(h3 − h4 )ηt Wact = Wta − Wca for (9) Thus, the overall adiabatic thermal cycle T2′ = T1 + T2 −T1 ηc [k] The work needed by the compressor, Wc: WC = Cp (T2′ − T1 ) = Cp (T2 −T1 ) ηc kJ ⁄kg efficiency can be calculated from the After leaving the compressor at an elevated following equation: pressure and temperature, the air then enters the combustion chamber, where it completely oxidizes a liquid or a gaseous m°f,b FAR = m°a fuel injected under pressure. Pressure and temperature at exhaust of γa−1 pe γa 2.3 Turbine T2 = T1 ( p ) i andηc is In this equation Te is the turbine inlet combustion efficiency. i Pi .LCVf −Cpg . Te temperature of HP compressor, ηb is the p pe = pi ( pe ) Pe b temperature, Ti is stagnation or total exit compressor by relation; where Cpg .Te −Cpa . Ti =η Turbine produces power to drive engine is the pressure ratio of compressor compressor due to expansion of gas stream. polytrophic The efficiency of gas turbine is taken care efficiency of the compressor. by considering polytropic efficiency of Wc = h2 − h1 turbine. In actual compressor process the work T3 required by the compressor for unite mass T4 P3 (ɤ−1) ɤ = (P ) 4 flow rate is given by the following equation Wt = ηt Cpg (T3 − T4 ) Wc = Cpa (T2′ − T1 ) The turbine power output is then ma [kJ/kg] (1+FAR) Wt, where, as seen earlier, ma(1+ FAR) is the mass flow rate of combustion 2.2 Combustion chamber The combustion process raises the gas flowing through the turbine. combustion gas temperature to the turbine The cycle thermal efficiency is the ratio of inlet temperature T3. One of the goals of the network to the heat supplied to the combustion chamber design is to minimize heater: the pressure loss from the compressor to the ηt = Q net = W add Cpg (T3 −T′ 4 ) − Cpa (T2′ −T1 ) Cpg (TIT)−Cpa (T2′ ) turbine. Ideally, then, P3= P2, as assumed by 2.4 Regeneration Effect the Air Standard analysis. The rate of heat In a simple gas turbine cycle the turbine exit released by the combustion process may temperature is nearly always appreciably then be expressed as: higher than the temperature of the air ° Qadd = m a (1 + FAR)Cpg (T3 − T2 ) leaving the compressor. Obviously, the fuel Where FAR is the mass fuel-air ratio and requirement can be reduced by the use of a calculated by equation: regenerator in which the hot turbine exhaust gas preheats the air between the compressor and the combustion chamber. In an ideal 3600 AFR . Wnet SFC = case the flow through the regenerator is at constant pressure. The regenerator effectiveness is given by the following relationship: ɳreg = T3 − T2 T5 − T2 Fuel to air ratio is given by 1 AFR Thermal efficiency is given by AFR = η Wnet Qadd th = Thus, the overall efficiency for this system's cycle can be written as ɳRcyc = 2.5 Analysis gas turbine cycle with (T4 −T5 )−(T2 −T1 ) (T4 −T3 ) reheating … Increasing the effectiveness of a regenerator calls for more heat transfer surface area, which increases the cost, the pressure drop, The compressor efficiency ( ηc ), the turbine ( ηt ), and effectiveness of regenerator (heat exchanger) are considered in this study. These parameters in terms of temperature are defined as: T ′ 2 − T1 ηC = T2 − T1 T3 − T4 ηt = T3 − T ′ 4 ′ εrc = T 2 − T1 T2 − T1 basic cycle is to increase the specific work output and cycle efficiency. The specific work output from the cycle can be increased and the space requirements of the unit. efficiency The very purpose of any modification in the by including reheater and carrying out the expansion in two stages. The work output of the reheat (reheating is done to its original temperature T5= T3). The turbine work increases as the vertical distance between a pair of pressure line increases with as increase in entropy as: (T5 − T6 ) > (T3 − T4 ) Pi P1 = P2 Pi The pressure ratio of compression is given by P2 Pi P2 = × = R pc1 × R pc2 P1 P1 Pi Power output is given by R pc = P = m′a × Wnet Where the R pc1 and R pc2 are the pressure Air to fuel ratio is given by ratio of expansion-I and expansion-II. AFR = LCVf Qadd Specific fuel consumption The net specific work output of the cycle is given by: Wnet = Whpt + Wlpt − Wc inlet temperatures (TIT) for various ambient Whpt = Cp (T3 − T4 ) temperatures. Heat sully to the combustor Wlpt = Cp (T5 − T6 ) increases on increasing the turbine inlet WC = Cp (T2 − T1 ) temperatures. Fig 4 shows the variation of Net work with TIT for various overall Wnet = Cp [(T3 − T4 ) + (T5 − T6 ) − (T2 pressure ratios. Net work increases on − T1 )] The best pressure Pi, where reheating is increasing the TIT for a given OPR. carried out for maximum specific work output can be obtained by equation: 600 550 Pi = √P1 P2 OPR=10 OPR=20 OPR=30 OPR=40 500 3. Results and Discussion This paper presents the results of the Net Work 450 Tamb=300K REGEFF=0.85 400 350 300 250 combined cycle in the form of graphs for 200 150 various parameters. The software developed 1000 1100 in MATLAB 10 for the thermodynamic 1200 1300 1400 1500 Turbin Inlet Temperature calculations of the cycle and then graphs Fig 4 Variation of Net work vs TIT have been plotted in menu driven software ‘Origin 6.1’. Tamb=280K Tamb=290K Tamb=300K Tamb=310K Tamb=320K Tamb=330K 1000 Total Heat Input 900 Thermal Efficiency 1100 OPR=20 REGEF=0.85 800 700 0.50 0.48 0.46 0.44 0.42 0.40 0.38 0.36 0.34 0.32 0.30 0.28 0.26 0.24 0.22 0.20 0.18 Tamb=300K REGEFF=0.85 OPR=10 OPR=20 OPR=30 OPR=40 1000 600 1100 1200 1300 1400 1500 Turbine Inlet Temperature 500 1000 1100 1200 1300 1400 1500 Turbine Inlet Temperature Fig 5 Variation of Thermal efficiency vs TIT Fig 3 Variation of Head input vs TIT Fig 5 shows the variation of Thermal Figure 3 represents the variations of heat efficiency with TIT for different OPR. It supply to the combustor at different turbine seems from the figure that the thermal efficiency increases on increasing the TIT while the network output increases for a given value of OPR. Fig 6 shows the continuously. Fig 13 shows the variation of variation of Work ratio with TIT. Work ratio mass of fuel required in the combustor one decreases ion increasing the TIT and it is and two with TIT. It has been observed that minimum for minimum OPR. Fig 7 shows the fuel required in the first combustor is the variation of Air rate with TIT and it is more as compared to the second combustor. observed that the air rate decreases on This is due to fact that the temperature is increasing the TIT for a given value of OPR. well above after expansion in HPT. Fig 8 represents the variation of specific fuel 4.0 decreases on increasing the TIT. The rate of 3.5 decrease is less at higher values of TIT. Fig 9 shows the variation of Net power with Work Ratio consumption (SFC) with TIT. The SFC OPR=10 OPR=20 OPR=30 OPR=40 Tamb=300K REGEFF=0.85 3.0 2.5 TIT. The net power increases on increasing 2.0 the TIT. Fig 10 represents the variation of 1.5 heat input with TIT. The heat addition to the 1000 1100 1200 1300 1400 1500 Turbine Inlet Temperature burner is decreases on increasing the regenerative effectiveness. Overall heat Fig 6 Variation of Work ratio vs TIT input increases on increasing the turbine 20 inlet temperature. Fig 11 represents the 18 variation of heat input in the burner at OPR=10 OPR=20 OPR=30 OPR=40 16 observed that the heat input decreases on increasing the ambient temperature. At Air Rate 14 various ambient temperatures. It has been Tamb=300K REGEFF=0.85 12 10 8 6 higher regenerative effectiveness, the heat 4 1000 input is minimum. Fig 12 shows the variation of Net work and thermal efficiency with TIT. The thermal efficiency and net work both increases on increasing the TIT but after reaching an optimum value; the rate of increase is low for thermal efficiency 1100 1200 1300 1400 Turbine Inlet Temperature Fig 7 Variation of Air rate vs TIT 1500 655 0.46 650 0.44 Specific Fuel Consumption 0.38 0.36 640 Tamb=300K REGEFF=0.85 635 Heat input to combustor OPR=10 OPR=20 OPR=30 OPR=40 0.40 REGEFF=0.75 REGEFF=0.80 REGEFF=0.85 REGEFF=0.90 REGEFF=0.95 TIT=1400K OPR=20 645 0.42 0.34 0.32 0.30 0.28 0.26 630 625 620 615 610 605 600 0.24 595 0.22 590 0.20 585 580 0.18 1000 1100 1200 1300 1400 280 1500 290 300 310 320 330 Ambient Temperature (K) Turbine Inlet Temperature Fig 11 Variation of Heat input vs ambient Fig 8 Variation of SFC vs TIT temperature 550 Net Power Output (kW) 450 0.55 Tamb=300K REGEFF=0.85 3 Net Work (x10 ) Thermal Efficiency 0.50 Net Work and Thermal Efficiency OPR=10 OPR=20 OPR=30 OPR=40 500 400 350 300 250 200 0.45 0.40 0.35 Tamb=300K OPR=20 ETAGEN=0.85 0.30 0.25 150 0.20 1000 1100 1200 1300 1400 1500 1000 Turbine Inlet Temperature 1100 1200 1300 1400 1500 Turbine Inlet Temperature (K) Fig 9 Variation of Net power vs TIT Fig 12 Variation of Net work and thermal 700 REGEFF=0.75 REGEFF=0.80 REGEFF=0.85 REGEFF=0.90 REGEFF=0.95 600 0.017 0.016 550 Mass of fuel in Combustor 1 Mass of fuel in Combustor 2 0.015 500 Mass of Fuel (kg/s) Heat input to combustor 650 efficiency vs TIT Tamb=300K OPR=20 450 400 1000 1100 1200 1300 1400 Turbine Inlet Temperature (K) 1500 0.014 0.013 OPR=20 Tamb=300K REGEFF=0.85 0.012 0.011 0.010 0.009 0.008 Fig 10 Variation of Heat input to TIT 1000 1100 1200 1300 1400 Turbine Inlet Temperature (K) Fig 13 Variation of Mass of Fuel vs TIT 1500 from 75% to 95%, the efficiency of the CC1 at OPR=10 CC2 at OPR=10 CC1 at OPR=20 CC2 at OPR=20 CC1 at OPR=30 CC2 at OPR=30 CC1 at OPR=40 CC2 at OPR=40 Mass of Fuel in Combustion Chambrer 1 and 2 0.018 0.016 0.014 regenerative Tamb=300K REGEFF=0.85 cycle is higher than its counterpart in the simple cycle. The work output per kg of air is about the same or 0.012 slightly less than that experienced with the 0.010 simple cycle. Increasing the pressure ratio 0.008 0.006 and the turbine firing temperature increases 0.004 the Brayton cycle efficiency. The increase in 0.002 0.000 1000 1100 1200 1300 1400 1500 the pressure ratio increases the overall Turbine Inlet Temperature (K) efficiency at a given firing temperature; Fig 14 Variation of mass of fuel required in the combustor 1 and 2 with TIT beyond a certain value at any given firing Figure 14 shows the variation of the mass of fuel required in the combustor 1 and 2 at various OPR. Figures however, increasing the pressure ratio show the improvement in cycle efficiency because of heat recovery with respect to a simple opencycle gas turbine. Cycle efficiency drops with an increasing pressure drop in the regenerator. There are two types of heat exchangers Regenerative and Recuperative. The term ``regenerative heat exchanger'' is used for a system in which the heat transfer between two streams is affected by the exposure of a third medium alternately to the two flows. The heat flows successively into and out of the third medium, which undergoes a cyclic temperature. These types of heat exchangers are widely used where compactness is essential. For a regenerator assumed to have an effectiveness varies temperature can actually result in lowering the overall cycle efficiency. It should also be noted that the very high-pressure ratios tend to reduce the operating range of the turbine compressor. This causes the turbine compressor to be much more intolerant to dirt build up in the inlet air filter and on the compressor blades and creates large drops in cycle efficiency and performance. 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