Liquid-piston dynamometers

A dynamometer is an important tool for studying liquid piston engines (or any kind of engine, for that matter) .  A dynamometer allows us to run the engine under varying external loads and also to measure the power produced by the engine.  All kinds of dynamometers have been developed over the years suited to many different engines and purposes.  While the testing of the solar-powered engine discussed in my last post provided the proof-of-concept, and yielded some useful operational information, the fact that the engine ran completely unloaded (only operating against internal friction) imposed a pretty severe limitation on the amount and usefulness of the information that could be learned.  Essentially, it would be like evaluating a car engine if the car were never taken out of neutral.  Actually, it is even more limiting than that because at least you could rev up a car engine whereas liquid piston engines operate in a resonance, and hence single speed, mode.

A Solar-powered Fluidyne Test Bed

In our laboratory at the University of Colorado at Colorado Springs, we built and instrumented a solar-powered fluidyne (liquid piston heat engine) test bed. [1]

The test bed was intended to serve two purposes.  First, it was used to demonstrate direct solar-powered operation of a fluidyne with sunlight concentrated directly on the fluidyne cylinder rather than on a remote heat exchanger.  In addition, it was used to characterize some aspects of the operational thermodynamic cycle.  The test bed was only intended as a platform to explore feasibility and thermodynamic characteristics, not as a practical, power producing engine.  The test bed was to be powered by the sun using a 50” by 37” Fresnel  lens. The Fresnel lens supplied ample solar power for the fluidyne.  

Measurements of temperature and pressure inside the working space of the engine showed that a temperature gradient existed across the working space at all times.  

This figure shows about 20 seconds of temperature data from a single test.  During this segment, the temperature fluctuations on the hot side were about 4 °C, and the temperature fluctuations on the cold side were approximately 12 °C.  As expected, the temperature fluctuations from the two thermocouples are out of phase with one another.  

This figure shows a pressure-volume plot of the working space.  The enclosed area represents the work of each cycle.  This engine was operated without any external load for this phase of testing.  Therefore, the indicated cycle work is only overcoming internal friction and other losses.   

The instrumented solar-powered fluidyne test bed demonstrated consistent and repeatable operation.  The Fresnel lens used provided ample energy to power the fluidyne.   Indicated work of the engine was collected with pressure and volume measurements.  Since the engine was unloaded, this work only overcame frictional losses.

[1]  J.W. Mason and J.W. Stevens, 2011, “Design and Construction of a of a Solar-powered Fluidyne Test Bed” Proceedings of the ASME 2011 Mechanical Engineering Conference and Exposition IMECE2011, November 11-17, 2011, Denver, Colorado, USA, paper IMECE2011-62194.