Building a Prony Brake

[Triangle.Stirling]

hello everybody,

To test engines, i want to make a so called prony brake. I searched the internet, but there is not that much info that is helpful.
I looked at this diy prony brake, and i think i understand the working of it now.

To test engine power, you have to attach a clamp or similar to the axis of the engine. That axis then presses on a scale, where you can read how much pressure is applied.
The clamp on the axis must then be slipping, slowing down the engine. You then have the momentum, and with the engine-speed you can calculate power output.

wikipedia learns me this: The constant 5252 is the rounded value of (33,000 ft·lbf/min)/(2π rad/rev).

If i convert that to the units we use in the Netherlands i come up with the following:
P(Hp) = T(N/m) * f(rpm) / 7121
and
P(Watt) = T(N/m) * f(rpm) / 9,682

Is the above correct? I hope there is somebody that can confirm the formula's above, we don't work with feets en pounds here in the Netherlands, we use meters and kilos.
Also, is there anybody that has built a prony brake? If you did, i'd love to see the photo's of it.

Thanks in advance!

[vamoose]

Hey T.S.

Check these out. There are some very helpful calculations and explanations in them by the authors..
[hr]
Design criteria for Stirling cycle engine - Step 6: Power calculations and performance


[hr]
Mousepower Dyno Testing


[hr]
vamoose

[Ian S C]

The brake that I made has a 6" arm, the brake runs on a spool 3/4" dia., and the brake material is a thin strip of PTFE. The arm is marked in inches, and has notches every 1/4" along the top, a weight hanger fits on here, it weighs .5oz, and extra weights *(washers) can be put on. I now have a set of digital scales similar to Vamoose's, but I'm yet to try them. Ian S C

[Triangle.Stirling]

Thanks for the replies!
When the engine is ready, i'll be building a prony brake, or just making a little setup to test the power.
I also got a nice explanation from instructables.com
I post it below, maybe someone can use it.

ABBC3_SPOILER_SHOW

P=Power, W
T=Torque, Nm
N=Engine rotational speed, RPM

Mechanical energy = force * distance.
For example, if you push a block along a floor with a force of 5N for a distance of 3m, you have expended = 5N * 3m = 15 Nm = 15J of energy.

Power = the time rate of making or using energy = J/s.
for example, if you use (or make) 15J of energy in 3s, the power = 15J/3s = 5J/s = 5W.

For a rotating shaft, you use the same concepts, but convert the definition of mechanical energy (energy= linear force * distance) into force exerted around a circle. So if you have a force of 1N acting at a radius of 1m, that 1N force will travel 2* π = 2 π m in one revolution, equal to 2 π Nm energy per recolution. Also, the 1N force acting at 1m radius is a torque of 1 Nm. (Note the SI symbols of energy and torque are the same, Nm, but we are talking about two different things).

For rotational power, then,
Power = the time rate of making or using energy per revolution =
(Torque * 2*π)/ (time for one revolution)
So for 1 RPM, Power = (Torque * 2π*1)/ (1/60) = T * 2π/ 60s = T*0.10472 = Nm/s = Watts
For RPM >1, multiply br RPM (N)
The gereral formula is therefore: P (Watts) = T*N* π /30 = T*N*0.10472, same as P = T*N/9.55

Here is a useful website:
http://www.magtrol.com/support/motorpower_calc.html