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Research Papers: Internal Combustion Engines

Analysis and Comparison of the Performance of an Inverted Brayton Cycle and Turbocompounding With Decoupled Turbine and Continuous Variable Transmission Driven Compressor for Small Automotive Engines

[+] Author and Article Information
P. Lu, C. Brace, B. Hu, C. Copeland

Department of Mechanical Engineering,
University of Bath,
Bath BA2 7AY, UK

Contributed by the IC Engine Division of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received October 8, 2016; final manuscript received December 5, 2016; published online February 23, 2017. Editor: David Wisler.

J. Eng. Gas Turbines Power 139(7), 072801 (Feb 23, 2017) (12 pages) Paper No: GTP-16-1492; doi: 10.1115/1.4035600 History: Received October 08, 2016; Revised December 05, 2016

For an internal combustion engine, a large quantity of fuel energy (accounting for approximately 30% of the total combustion energy) is expelled through the exhaust without being converted into useful work. Various technologies including turbocompounding and the pressurized Brayton bottoming cycle have been developed to recover the exhaust heat and thus reduce the fuel consumption and CO2 emission. However, the application of these approaches in small automotive power plants has been relatively less explored because of the inherent difficulties, such as the detrimental backpressure and higher complexity imposed by the additional devices. Therefore, research has been conducted, in which modifications were made to the traditional arrangement aiming to minimize the weaknesses. The turbocharger of the baseline series turbocompounding was eliminated from the system so that the power turbine became the only heat recovery device on the exhaust side of the engine, and operated at a higher expansion ratio. The compressor was separated from the turbine shaft and mechanically connected to the engine via continuous variable transmission (CVT). According to the results, the backpressure of the novel system is significantly reduced comparing with the series turbocompounding model. The power output at lower engine speed was also promoted. For the pressurized Brayton bottoming cycle, rather than transferring the thermal energy from the exhaust to the working fluid, the exhaust gas was directly utilized as the working medium and was simply cooled by ambient coolant before the compressor. This arrangement, which is known as the inverted Brayton cycle (IBC) was simpler to implement. Besides, it allowed the exhaust gasses to be expanded below the ambient pressure. Thereby, the primary cycle was less compromised by the bottoming cycle. The potential of recovering energy from the exhaust was increased as well. This paper analyzed and optimized the parameters (including CVT ratio, turbine and compressor speed and the inlet pressure to the bottoming cycle) that are sensitive to the performance of the small vehicle engine equipped with inverted Brayton cycle and novel turbocompounding system, respectively. The performance evaluation was given in terms of brake power output and specific fuel consumption. Two working conditions, full and partial load (10 and 2 bar brake mean effective pressure (BMEP)) were investigated. Evaluation of the transient performance was also carried out. Simulated results of these two designs were compared with each other as well as the performance from the corresponding baseline models. The system models in this paper were built in GT-Power which is a one dimension (1D) engine simulation code. All the waste heat recovery systems were combined with a 2.0 L gasoline engine.

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Figures

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Fig. 1

The schematic diagram of fuel energy distribution in a medium-sized passenger car

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Fig. 2

Technical objects for strategy optimization to achieve 10% improvement in fuel economy

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Fig. 3

Schematic diagram of (a) CVT supercharged high pressure turbocompounding engine and (b) the inverted Brayton cycle engine

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Fig. 4

Temperature and entropy diagram of a turbocharged engine with single stage inverted Brayton cycle compression

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Fig. 5

Comparison of simulated and measured P–V diagrams

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Fig. 6

Comparison of simulated and measured heat release rates

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Fig. 7

CVT supercharged turbocompounding engine torque and BSFC versus supercharger speed at 3000 rpm

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Fig. 8

CVT supercharged turbocompounding turbine efficiency and power versus supercharger speed at 3000 rpm

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Fig. 9

CVT supercharged turbocompounding volumetric efficiency and power versus supercharger speed at 3000 rpm

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Fig. 10

CVT supercharged turbocompounding engine BSFC versus supercharger speed under low load condition

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Fig. 11

CVT supercharged turbocompounding engine torque and BSFC versus turbine speed at 3000 rpm

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Fig. 12

CVT supercharged turbocompounding engine BSFC and turbine efficiency versus turbine speed under low load condition

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Fig. 13

IBC engine torque and BSFC versus inlet pressure to the bottoming cycle

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Fig. 14

IBC power and PMEP versus inlet pressure to the bottoming cycle

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Fig. 15

IBC power and BSFC versus heat exchanger effectiveness

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Fig. 16

Comparison of the fuel consumption between IBC and CVT supercharged turbocompounding engine under partial load condition

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Fig. 17

IBC and turbocompounding power output and PMEP

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Fig. 18

Full load performance comparison between IBC and CVT supercharged turbocompounding engine

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Fig. 19

Power generation of the waste heat recovery devices and the PMEP of the whole system

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Fig. 20

Transient performance between IBC and CVT supercharged turbocompounding engine at 2000 rpm

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Fig. 21

Transient performance between IBC and CVT supercharged turbocompounding engine at 3000 rpm

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Fig. 22

Transient performance between IBC and CVT supercharged turbocompounding engine at 4000 rpm

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