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.

In a current project we are developing and evaluating two different dynamometer schemes to use with liquid-piston engines.  In one approach, we impose a small orifice on the output column so that every oscillation has to push and pull air through the restriction.  This will provide a continuous net drag on the engine operation, fluctuating in magnitude proportional to the square of the velocity of the output column.  We’ve tested a variety of orifices and configurations.  Our other approach uses an elastic balloon attached to the end of the output column.  The engine motion inflates the elastic balloon when the output column rises, and recovers some of the energy when the output column sinks.  The net load on the engine is the hysteresis between the inflation and deflation of the balloon, and is not expected to be large.  However, the balloon inflation (and deflation) imposes a different external pressure-phase load on the engine than is present with either atmospheric (constant) pressure or with the orifice drag.

This figure demonstrates the effect of loading, with one of the orifice configurations, and one of the balloon configurations on the resonant frequency.

This figure illustrates the effect of loading with orifices on the phasing of the column oscillations.