Investigation of a Drive Mechanism Modification to Increase Thermodynamic Power of a Low Temperature Differential Gamma Type Stirling Engine

  • Author / Creator
    Nicol-Seto, Michael E
  • This work presents the findings of an investigation into the modification of the drive mechanism of a low temperature differential gamma type Stirling engine with the aim of improving the thermodynamic performance and power production. A drive mechanism was conceptualized that could modify the displacer and piston motion profiles to be more discontinuous by dwelling the pistons at top dead center and bottom dead center during an engine cycle. The discontinuous motion better replicated the ideal Stirling thermodynamic cycle. Motion modification was achieved using interchangeable non-circular gear sets to vary the displacer and piston crankshaft speeds throughout an engine cycle. Preliminary thermodynamic analysis using a simple isothermal model validated the design, and so an engine was retrofit with the novel drive mechanism. Three sets of oval elliptical non-circular gears were tested: a set of round gears of eccentricity e= 0 used to replicate a conventional unmodified drive mechanism, and two oval gear sets of eccentricity e= 1/5 and e= 1/3 that incrementally increased the dwell of the displacer and piston.
    A series of steady state experiments were conducted on the modified engine that tested the performance of the motion modifications: displacer dwelling, piston dwelling, and combined dwelling. A supplemental set of trials were run that dwelled the displacer mid-stroke to reduce displacer velocity combined with piston dwelling. All experiments were conducted with a thermal source at 90 °C and a thermal sink at 5 °C.
    Results of displacer dwelling experiments simultaneously validated, and went against, the anticipated improvement in performance. Preliminary modeling and reports in the literature anticipated increased indicated cycle work and power. The findings of this investigation indicated that dwelling the displacer did improve the indicated thermodynamic work of the cycle, but was also detrimental to maximum engine power. The reduction in power was caused by a reduction in engine running speed, which was attributed to the increased maximum displacer speed of the dwelled motion. The trial using the e= 1/3 gears reduced maximum power by 27.4%, despite improving improving shaft work by 9.5%. The supplemental trials with reduced maximum displacer speed resulted in increases to engine speed and slight increases to maximum power.
    Experiments where the piston was dwelled had neutral or positive outcomes. The indicated work of the cycle was increased as anticipated, but the shaft work was not improved proportionally. The reduced mechanism effectiveness was attributed to additional mechanism friction. Piston dwelling did not reduce engine velocity. The most substantial gain in engine power was observed during the trial with reduced displacer velocity and the e= 1/5 gear set dwelling the piston, which improved the maximum power by 4.0%.
    The empirical results highlighted a shortcoming of the initially positive thermodynamic modeling results. The model cannot predict engine speed, and so could not anticipate the reduction to engine cyclic rate, despite improvements to the indicated thermodynamic work. The findings of this investigation suggest that improvements to engine running speed and mechanism effectiveness are necessary to realize the thermodynamic gains achieved by the dwelling of the displacer and piston at the tested engine operating conditions.

  • Subjects / Keywords
  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.