Low Temperature Difference Alpha-Type Stirling Engine for the Experimental Determination of Optimal Parameters to Maximize Shaft Power

• Author / Creator

  Michaud, Jason

• An investigation was performed experimentally to determine the combination of parameters that would produce the maximum shaft power for an alpha-type Stirling engine using a thermal source with a temperature below 100 °C. The primary objective of the study was to design and build an alpha-type Stirling engine, using air as the working fluid with a mean pressure near atmospheric pressure, which would turn a crank shaft on its own accord. Parameters would then be varied to measure the corresponding shaft power output of the engine. If the manufactured Stirling engine could not rotate the crankshaft, the engine would be driven to determine the combination of parameters that would minimize the power input required to rotate the engine.

  Three different prototype opposed piston alpha (OPA) Stirling engines were designed and assembled to develop the engine to be used as the testing system: the OPA MK I, the OPA MK II, and the OPA MK III. The OPA MK III was chosen as the engine to be used for experiments, and utilized rubber bellows as piston seals, compact radiators for heat exchangers, and a triple Scotch yoke mechanism. The mechanism allowed the stroke length to be set to 50.8 mm or 76.2 mm for the expansion and compression pistons, and it allowed the phase angle to be varied from 90° to 180° in 5° increments.
  The engine was instrumented to measure internal pressure, source and sink temperatures, gas temperatures, angular position of the crankshaft, and torque. An experiment plan was developed that manipulated the expansion and compression piston stroke lengths, the phase angle between the pistons, and the engine angular velocity. The phase angle was varied between 120° and 180° in 5° increments, the speed was varied between 30 rpm and 60 rpm in 10 rpm increments, and three engine swept volumes were possible, which resulted in 156 parameter combinations to be tested.

  Manual attempts at running the engine with a thermal source temperature of 95 °C and a thermal sink temperature of 2 °C were performed, where the engine failed to run under all tested configurations. As such, significant effort was put into determining the reason it failed to run. To ensure confidence in the results, in-depth studies into the uncertainty of the bellows volume variation, modelling mechanism energy transfer, instrument calibration and uncertainty reduction, and the time required for engine warm-up and steady state operation were performed. Furthermore, the experiment was split into two 156 configuration sets: a baseline experiment with an inactive thermal source and sink, along with a thermal experiment with an active thermal source and sink, so that any performance improvement and thermal effects could be observed.

  Results included analyzing and comparing measurements between baseline and thermal experiments for pressure fluctuations, gas temperatures, indicator diagrams, and shaft power input. It was found that compressive heating and expansion cooling had a significant effect at higher compression ratios, such that the measured temperature difference decreased from approximately 90 °C to 75 °C as the phase angle decreased, indicating that the heat exchangers could not sufficiently offset the compressive heating and expansion cooling. Furthermore, comparison of the ideal pressure ratio to the measured pressure ratio at 180° was found to be lower, suggesting the gas temperature was not uniform. The mechanism effectiveness was investigated and found to vary between 0.57 and 0.81. By using the measured shaft work, a mechanism effectiveness above 0.9 would be required for net positive shaft work to appear in the range of 170° to 160° with the current heat exchangers. Therefore, it was concluded that the engine failed to run primarily due to insufficient heat transfer and a low mechanism effectiveness.
   
  Optimal parameter combinations were investigated by comparing the shaft power input of the baseline and thermal experiments. The range of phase angles that contained the optimal value was determined to be between 140° and 160°, with phase angles of 150° and 160° being the most promising. The exact optimal swept volume and phase angle combination that reduced the shaft power input could not be concluded due to uncertainty, but an optimal compression ratio was estimated to lie within the range of approximately 1.12 to 1.24 for a similar engine and operating conditions.

• Subjects / Keywords

  • Scotch Yoke Mechanism
  • Maximize Shaft Power
  • Low Temperature Difference
  • Alpha Stirling Engine
  • Stirling Engine

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-m3jj-1646

• License

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