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

Fundamental Explorations of Spring-Varied, Free Piston Linear Engine Devices

[+] Author and Article Information
Matthew C. Robinson

Mechanical and Aerospace Engineering,
West Virginia University,
Morgantown, WV 26505
e-mail: mrobin16@mix.wvu.edu

Nigel N. Clark

Mechanical and Aerospace Engineering,
West Virginia University,
Morgantown, WV 26505
e-mail: nigel.clark@mail.wvu.edu

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received February 28, 2015; final manuscript received March 6, 2015; published online March 31, 2015. Editor: David Wisler.

J. Eng. Gas Turbines Power 137(10), 101502 (Oct 01, 2015) (8 pages) Paper No: GTP-15-1066; doi: 10.1115/1.4030094 History: Received February 28, 2015; Revised March 06, 2015; Online March 31, 2015

The conventional crank-based internal combustion engine faces many challenges to remain a viable option for electric power generation. Limitations in mechanical, thermal, and combustion efficiencies must be overcome by innovations in existing technologies and progress toward new ones. The free piston linear engine (FPLE) has the potential to meet these challenges. Friction losses are reduced by avoiding rotational motion and linkages. Instead, electrical power is generated by the oscillation of the translator through a stator. Naturally, variable compression ratio provides a unique platform to employ advanced combustion regimes. However, possibly high variations in stroke length result in unknown dead center piston positions and greater difficulties in compression control as compared to conventional engines. Without control, adverse occurrences such as misfire, stall, over-fueling, and rapid load changes pose greater complications for stable system operation. Based on previous research, it is believed that incorporating springs will advance former designs by both increasing system frequency and providing a restoring force to improve cycle-to-cycle stability. Despite growing interest in the FPLE, current literature does not address the use of springs within a dual, opposed piston design. This investigation is an extension of recent efforts in the fundamental analysis of such a device. Previous work by the authors combined the dynamics of a damped, spring mass system with in-cylinder thermodynamic expressions to produce a closed-form nondimensional model. Simulations of this model were used to describe ideal Otto cycle as the equilibrium operating point. The present work demonstrates more realistic modeling of the device in three distinct areas. In the previous model, the work term was a constant coefficient over the length of the stroke, instantaneous heat addition (representing combustion) was only seen at top dead center (TDC) positions, and the use of the Otto cycle included no mechanism for heat transfer except at dead center positions. Instead, a position based sinusoid is employed for the work coefficient causing changes to the velocity and acceleration profiles. Instantaneous heat addition prior to TDC is allowed causing the compression ratio to decrease toward stable, Otto operation, and a simple heat transfer scheme is used to permit cylinder gas heat exchange throughout the stroke resulting in deviation from Otto operation. Regardless, simulations show that natural system stability arises under the right conditions. Highest efficiencies are achieved at a high compression ratio with minimal heat transfer and near-TDC combustion.

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References

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Figures

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Fig. 1

Illustration of simple engine system

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Fig. 2

Position/velocity profile for basic equilibrium system

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Fig. 3

Velocity/acceleration profile, heat addition in one cylinder occurs simultaneously with heat rejection in the other, continued from Fig. 2

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Fig. 4

In-cylinder pressure, continued from Fig. 2

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Fig. 5

Work profiles from previous (constant) and current (sinusoidal) analyses

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Fig. 6

Comparison of velocity profiles for different work profiles

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Fig. 7

Comparison of acceleration profiles for different work profiles, continued from Fig. 6

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Fig. 8

Position/velocity changes with combustion pressure at 20% of initial pressure

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Fig. 9

Compression ratio changes, continued from Fig. 8

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Fig. 10

Pressure changes, continued from Fig. 8

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Fig. 11

Pressure curve comparison with and without heat transfer

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Fig. 12

Expanded adverse work area, expanded from Fig. 11

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Fig. 13

Excess heat loss and work resulting in misfire and stall

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Fig. 14

Low compression ratio equilibrium example, position/velocity profile

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Fig. 15

Fall of compression ratio to achieve equilibrium, continued from Fig. 14

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Fig. 16

Cylinder pressure profiles for equilibrium operation, continued from Fig. 14

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Fig. 17

Energy consumption distribution, continued from Fig. 14

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Fig. 18

High compression ratio equilibrium example, position/velocity profile

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Fig. 19

Change of compression ratio to achieve equilibrium, continued from Fig. 18

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Fig. 20

Velocity/acceleration profile, continued from Fig. 18

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Fig. 21

Continued from Fig. 18, energy consumption distribution

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