A blog on what's new, notable, and next in turbomachinery

A New Application for Air Brake Dynamometers?

By Francis A. Di Bella, P.E.
Jan 12, 2018

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My gym has been crowded with a lot of new people these past couple of weeks. It happens every year in January as people try to stick to their New Year’s Resolution to exercise more. I don’t mind the crowds; they are usually gone by February. One newcomer that did catch my engineering eye, was the new exercycle. It is a bicycle seat with a single, large and bulky looking wheel. The large wheel seems out of place next to some of the other machines that tend to have a much more streamlined appearance. The wheel is spun by the rider much like a typical bicycle and serves to absorb the power from the rider. The exercise cycle is called an Airdyne.

The new gym equipment uses the same principle to absorb the power generated by the gym cyclist as does Concepts NREC’s VAROC® Air Dynamometer; only the power level is WAY different. A human can generate only about ¼ hp on a continuous basis, far lower than the turboshaft engines the VAROC® tests. Still, I was intrigued. A close examination of the new air-based exercycle revealed that the rotor is about 3 ft. in diameter and composed of just simple flat baffles that are acting like inefficient blades on a compressor. The air-drag on the rotor increases, in proportion to the cube of the speed (rpm) of the rotor. Paradoxically, the more inefficient the blades on the rotor, the more power required by the user to keep the rotational speed at a respectable level, making a more effective the exercise system. This is an engineer’s dream come true: the more inefficient the rotor air foil design is, the better the design for the exercise machine! 

Concepts NREC manufactures a patented high technology, high-speed energy absorber, that is called an air brake, engine dynamometer in the engineering world. Our air dynamometer is used to absorb power from high speed (15,000 to 24,000 rpm) gas turbine engines. The VAROC system can also modulate the power by exposing more or less of the air foils on the perimeter of the spinning rotor to the ambient air. The novelty in this product is the ability to absorb as much as 18,000 hp by compressing air. The air brake dynamometer is different from the typical dynamometer that might be seen in any engine test laboratory. The typical dynamometer is usually permanently installed in a laboratory to measure engine power and torque at a controlled speed. The power generated by the engine is absorb by generating electric power by inducing a current in a set of windings that must then be immediately cooled with a continuous flow rate of cool water to avoid the windings from overheating. These water brakes are notorious for failing, expensive to operate and generate "dirty water" (not the good Boston kind) that must be treated. Concepts NREC’s dynamometer can be used in any climate or location where coolant water is not readily available. It uses ambient air to absorb the engine’s power and then releases this energy into the atmosphere.

As I sat there exercising, I decided to exercise my mind by trying to conceptualize a possible new product for our company. The new product would be derived from the same basic principle of using air as the compressible fluid in a power absorbing machine. Ours would have two key innovations to the existing “air brake” exercise bikes: 1. The ability to articulate the air foils on the rotor or open and close the rotor and thus restrict air flow to the rotor, and 2. The exercise bike would have two bikes installed in tandem, but facing each other, to provide an interesting gym experience for friends and potential foes alike: the competitive exercycle machine.

This same competition theme can be extended to a larger group, much like the size of the group that you see in the spinner bike classes that have become so popular in the gym world. For this enhanced competition, I can see many “air brake” exercise bikes arranged in a circle with the bikes lined up, like spokes on a wheel, with the rider facing the center. As the exercise class continues, each rider can adjust the level of power, using throttle-like adjustments on the hand grips of the exercycle to adjust the articulation of the air foil (blades) as well as to increase or decrease the rpm or cadence of the rotor. The competition would be rated based on the power generated by each rider which is constantly monitored by a common computer system. The rider with the largest power generation is identified by a light installed on each bike. This may also be an interesting psychological experiment to see if the riders next to the “leader” tend to work more to “catch-up” or is it the rider diametrically opposite the “leader” the one who feels as if they are in a head-to-head competition with the leader, and is spurred onward to performing better.

To be frank, we may never know. Concepts NREC will probably stick with air dynamometers for testing gas turbine engines, rather than helping people stick to their New Year’s resolutions through competitive exercycling.

Tags: Engineering, Dynamometers

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