Braked dyno project

[Amelina]

Hello I`m working on a braked dyno build, I`m using 2 eddy current brakes with load-cells and a NI DAQ.
I`m getting a bit of mess with the software.
Is possible to calculate the power output, knowing the braked torque and the acceleration rate?

Regards

 

[dgallup]

If you are going to calculate transient power you need to know the inertia of all the rotating components in addition to the torque and the acceleration rate.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Amelina]

I forgot to mention this part in the post, I do know the inertia of the brakes, each brake inertia is 2.96 Kg/m2

 

[SomptingGuy]

Why would you need to infer torque from acceleration and inertia if you can measure it directly from load cells? I assume you can measure rotational speed too.

- Steve

 

[Amelina]

I would like to calculate power without setting steady state, so if I have inertia and braked torque and rotational speed or aceleration, can I calculate power correctly?

 

[SomptingGuy]

I guess you're going to need to know the inertia upstream of the load cell if you want a true transient torque (or power) measurement. The inertia of the dyno downstream isn't relevant, except that it'll dictate the actual acceleration. If you knew the total inertia of the whole system and its acceleration, you shouldn't need the load cell. What's wrong with using the brake to control the speed anyway and take a proper curve? If you want a transient measurement, how transient is transient?

- Steve

 

[Amelina]

I`m fighting with pid calibration and think about the possibility to calculate the power/torque from the acceleration rate.
Steady with a constant torque reading is hard to get in a short time so if I want to get a lot of points it take time, I have to figure out a lot yet.
I have to calculate coast down to get a estimated flywheel power, but I like to solve before wheel numbers.

 

[LionelHutz]

The accelerating torque or power can be calculated as follows.

torque = inertia x dN / (308 x dt)

or

P = (inertia x N2^2/2 - inertia x N1^2/2) / dt

Where,
dN is the change is speed
dt is the change in time.


You then have to add this to the eddy brake torque or power.

 

[tbuelna]

With an eddy current brake the load cell is usually connected to the stator housing, so the inertia of the drive shaft and rotor must be considered. If you require a precise measure of dynamic engine operating conditions, then you might want to consider using something like a DC dyno instead of an eddy current brake. The DC dyno can be used to both motor and brake the engine. So it is much easier to characterize the frictions and inertias in the engine. A DC dyno also provides more precise control during transient operating conditions than an eddy current brake dyno does.

 

[Amelina]

Thank you very much Lionelhutz, dN rad/sec and dt is in seconds, right?

On here, P = (inertia x N2^2/2 - inertia x N1^2/2) / dt

What is ^?

Thank you very much tbuelna for the input, where can I see some information on this DC dyno? I already are working on edd current basis but I can test on future projects.

 

[tbuelna]

Here's a good comparison of various dyno systems.

http://www.dynesystems.com/what-is-a-dynamometer.htm

 

[LionelHutz]

The first formula dN is the change in rpm and the other units are imperial. Lbs and ft.

^2 means square the value.

The second formula is using SI units. kgm^2 and rad/sec

It would likely help to understand the second formula. Look up rotational kinetic energy and power (physics) on Wikipedia. The rotational kinetic energy is in J or Watt-seconds. The formula takes the change in rotational kinetic energy (dJ) and divides by the time (dt) taken to effect the change to get the power (W) required for that change.

 

[gruntguru]

Amelina. I don't see any mention of engine inertia in this thread.

If brake torque is being measured from stator reaction, the inertia of ALL rotating and reciprocating components must be considered. The total inertia can be measured by comparing torque at a given rpm using two different acceleration rates (one of which could be zero or negative) and working backwards.

Engineering is the art of creating things you need, from things you can get.

 

[Amelina]

Hello, I had the project a little stopped and now I`d like to put it back to live. Sorry for delay.

Mechanicadesingns, I`m using Labview software.

Gruntguru, can you give a aproach of how you will make the calculation? the power is normally measured at the flywheel so internal inertia should not be in to the maths.

Any recomendation of the best way to measure the mechanical looses?

 

[gruntguru]

The power produced during acceleration is slightly less due to the increasing kinetic energy of the moving parts in the engine. Engines are usually rated for steady-state power beause the reduced power during acceleration will vary depending on the rate of acceleration.

LionelHutz provided a formula above for calculating the difference.

je suis charlie

 

[Overrun]

If you measure the nonaccelerating torque/horsepower and compare it to the results accelerating the engine at a known rate of RPM increase the difference should be a function of the total system inertia -I think.