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Research Papers: Gas Turbines: Cycle Innovations

The Benefits of an Inverted Brayton Bottoming Cycle as an Alternative to Turbocompounding

[+] Author and Article Information
Colin D. Copeland

Department of Mechanical Engineering,
University of Bath,
Bath BA2 7AY, UK
e-mail: C.D.Copeland@bath.ac.uk

Zhihang Chen

Department of Mechanical Engineering,
University of Bath,
Bath BA2 7AY, UK
e-mail: z.chen3@bath.ac.uk

Contributed by the Cycle Innovations Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 23, 2015; final manuscript received October 12, 2015; published online December 4, 2015. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(7), 071701 (Dec 04, 2015) (11 pages) Paper No: GTP-15-1374; doi: 10.1115/1.4031790 History: Received July 23, 2015; Revised October 12, 2015

The exhaust gas from an internal combustion engine contains approximately 30% of the thermal energy of combustion. Waste heat-recovery systems aim to reclaim a proportion of this energy in a bottoming thermodynamic cycle to raise the overall system thermal efficiency. The inverted Brayton cycle (IBC) considered as a potential exhaust-gas heat-recovery system is a little-studied approach, especially when applied to small automotive power plants. Hence, a model of an air-standard, irreversible Otto cycle and the IBC using finite-time thermodynamics (FTT) is presented to study heat recovery applied to an automotive internal combustion engine. The other two alternatives power cycles, the pressurized Brayton cycle and the turbocompounding system (TS), are compared with the IBC to specify the strengths and weaknesses of three alternative cycles. In the current paper, an irreversible Otto-cycle model with an array of losses is used as a base for the bottoming cycle. The deviation of the turbomachinery from the idealized behavior is described by the isentropic component efficiencies. The performance of the system as defined as the specific power output and thermal efficiency is considered using parametric studies. The results show that the performance of the IBC can be positively affected by five critical parameters—the number of compression stages, the cycle inlet temperature and pressure, the isentropic efficiency of the turbomachinery, and the effectiveness of the heat exchanger. There exists an optimum pressure ratio across the IBC turbine that delivers the maximum specific power. In view of the specific power, installing a single-stage of the IBC appears to be the best balance between performance and complexity. Three alternative cycles are compared in terms of the thermal efficiency. The results indicate that the pressurized and IBCs can improve the performance of the turbocharged engine (TCE) only when the turbomachinery efficiencies are higher than a value which changes with the operating condition. High performance of the IBC turbomachinery is required to ensure that the TCE with the IBC is superior to that with TS.

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References

Figures

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Fig. 1

(Top left) Turbocharging and turbocompounding; (top right) pressurized Brayton; and (bottom) inverted Brayton

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Fig. 2

Schematic of the TCE with the IBC

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Fig. 3

Temperature and entropy diagram of a TCE with three stages compression of IBC

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Fig. 4

P–V diagram of the air-standard Otto cycle

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Fig. 5

The system-specific work output as a function of subatmospheric pressure and number of compression stages in the IBC

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Fig. 6

Effect of compression stages and subatmospheric pressure on the thermal efficiency

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Fig. 7

The specific work output variation depending on the bottoming turbine inlet temperature and subatmospheric pressure

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Fig. 8

The thermal efficiency variation depending on the bottoming turbine inlet temperature and subatmospheric pressure

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Fig. 9

IBC performance versus subatmospheric pressure plotted for different values of turbomachinery efficiency (turbine and compressor—ηit  and ηic: (a) 0.9 and 0.85; (b) 0.85 and 0.8; (c) 0.8 and 0.75; (d) 0.75 and 0.7; and (e) 0.7 and 0.65)

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Fig. 10

PIBC* –  ηIBC  curves varying with p6  and rt

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Fig. 11

PIBC* –  ηIBC  curves varying with plow  and ηex 

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Fig. 12

Schematic of the TCE with the pressurized Brayton cycle

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Fig. 13

Comparison of three alternative power cycles at different values of the turbomachinery efficiency, heat exchanger effectiveness, and turbine pressure ratio

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Fig. 14

The efficiency of TCE with various stages IBC versus IBC turbomachinery efficiency

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Fig. 15

The efficiency of TCE with various stages IBC versus pressure drop in the heat exchanger

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