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Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

Fuel Efficiency Optimization for a Divided Exhaust Period Regulated Two-Stage Downsized Spark Ignition Engine

[+] Author and Article Information
Bo Hu

Department of Mechanical Engineering,
University of Bath,
Claverton Down,
Bath BA2 7AY, UK
e-mail: B.Hu@bath.ac.uk

Sam Akehurst

Department of Mechanical Engineering,
University of Bath,
Claverton Down,
Bath BA2 7AY, UK
e-mail: S.Akehurst@bath.ac.uk

Chris Brace

Department of Mechanical Engineering,
University of Bath,
Claverton Down,
Bath BA2 7AY, UK
e-mail: C.J.Brace@bath.ac.uk

Pengfei Lu

Department of Mechanical Engineering,
University of Bath,
Claverton Down,
Bath BA2 7AY, UK
e-mail: P.Lu@bath.ac.uk

Colin D. Copeland

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

J. W. G. Turner

Department of Mechanical Engineering,
University of Bath,
Claverton Down,
Bath BA2 7AY, UK
e-mail: J.Turner@bath.ac.uk

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 28, 2015; final manuscript received September 11, 2015; published online November 11, 2015. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(5), 051507 (Nov 11, 2015) (9 pages) Paper No: GTP-15-1377; doi: 10.1115/1.4031624 History: Received July 28, 2015; Revised September 11, 2015

In our previous paper, a new gas exchange concept termed divided exhaust period regulated two-stage (DEP R2S) system has been proposed. In this system, two exhaust valves in each cylinder are separately functioned with one valve feeding the exhaust mass flow into the high-pressure (HP) manifold, while the other valve evacuating the remaining mass flow directly into the low-pressure (LP) manifold. By adjusting the timing of the exhaust valves, the target boost can be controllable while improving the engine's pumping work and scavenging is attainable which results in better fuel efficiency from the gas exchange perspective. This paper will continue this study by adding an appropriate knock model to examine the benefits this concept could bring to the combustion phasing. The results at full load showed that under knock limited spark advance (KLSA) and fully optimized exhaust valve timing condition, the DEP R2S system benefited from lower pumping loss and better scavenging due to the reduced backpressure and improved pulsation interference despite suffering from reduced expansion ratio and expansion work. The combustion phasing was advanced across the engine speed which is mainly attributed to the reduced residual and the reduced requirement of gross indicated mean effective pressure (IMEP). The net brake-specific fuel consumption (BSFC) was observed to improve by up to 3% depending on the engine operating points. At part load, the DEP R2S system could be used as a mechanism to extend the “duration” of the exhaust valve. This will reduce the recompression effect of the exhaust residuals during the beginning and the end of the exhaust stroke compared to the original R2S model with late exhaust valve opening and early exhaust valve opening. In addition, increased internal exhaust gas recirculation (EGR) due to the increased overlap between the LP and the intake valve is also beneficial for the improved pumping mean effective pressure (PMEP) as the throttle can be further opened to reduce the corresponding throttling loss. The average net BSFC improvement is expected to be approximately 6–7%.

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References

Figures

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Fig. 1

Four-cylinder engine gas exchange diagram [1]

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Fig. 2

Schematic of a DEP R2S system

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Fig. 3

Schematic of an original R2S system

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Fig. 11

Pumping loops in a standard P–V diagram at 1000 rpm 4.99 bar BMEP

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Fig. 10

Mass flow rate across the valves for the original system at 1500 rpm

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Fig. 9

Mass flow rate across the valves for the DEP R2S system at 1500 rpm

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Fig. 8

Pumping loops in a standard P–V diagram at 1500 rpm

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Fig. 7

Mass flow rate across the valves for the original system at 3000 rpm

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Fig. 6

Mass flow rate across the valves for the DEP R2S system at 3000 rpm

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Fig. 5

Pumping loops in a standard P–V diagram at 3000 rpm

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Fig. 4

Genetic algorithm procedure

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Fig. 12

Valve profile and mass flow rate across each valve

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Fig. 13

Pumping loops in a standard P–V diagram at 1000 rpm 4.99 bar BMEP

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