How to measure your MOI
Here is a rundown of how to use YourDyno to measure the Moment of Inertia of your brake dyno. This MOI (if large enough) can be used to calculate the drive train losses, and it can be used to compensate for the power it takes to accelerate the dyno system during a ramp. I will post a video a bit later that may make it more clear, but it should be possible to understand from this explanation too.
- Turn on Raw data logging. This is done in Options->Noise Filtering. Choose a suitable directory for the logs. This feature is very useful, as it enables you to do the same run many times with different parameters. It was originally made to easily find the optimum noise filtering setup, but can be used for other parameters too.
- Turn off any frictional losses correction factor (under Options->Horse Power Correction).
- Turn off compensation for inertia effects during acceleration (under Options->Basic Dyno Setup->Brake Dyno...).
Now you are ready for the static run:
- Do a run with a fixed RPM. This can be done with automatic brake or manual brake, but make sure the RPM stabilizes. You can choose any RPM in theory, but in practice avoid RPMs where there is a massive change in power from small RPM changes. You may well choose the RPM that gives max power.
- Go to Results vs Time. Note down the torque and horsepower the run stabilized at, and note down the exact RPM.
Next do a ramp:
- Do a normal length ramp run using a brake ramp curve (if automatic brake control) or manual operation.
- Go to the directory you chose for your raw data logs. Find the latest run (the filename with the highest number) and rename it to something like "RampRawData.txt".
Now you have all the data you need to find your MOI. Compare the power of the two runs at the fixed RPM you chose in the first run. You will see that the ramp curve reading is lower. Now comes an iterative process to find the MOI that gives the same reading:
- Turn on compensation for inertia effects.
- Guess a MOI. A wheel is about 1.5-2 kg m^2 as a reference. A heavy roller may be 5-10.
- Goto Run->New Run...
- Choose Load raw log file, and choose the RampRawData.txt
- The exact same run as before is now performed, but with data from the file instead of from the YourDyno box.
- Save the results.
- Compare with the static run, and adjust MOI up or down depending if the reading is too high or too low.
- Go back to 2. with a new MOI and continue until readings are very close.
- Turn on Enable correction for frictional losses from retardation data (under Horse Power Correction).
I tested this with the newest 3.01.47 version and the change of the MOI didnt effect the power/torque readings at all. Only turning on compensation for inertia effects made a small difference, but there might be some problem with this?
BTW, this new version improved automatic brake control works great with my hub dyno 🙂
As you mention, there are two ways the MOI can change the readings for brake dynos.
- The compensation for the inertia during acceleration. This is significant for heavy rollers, but for hub dynos like yours it will be relatively modest difference due to the low MOI.
- The calculation and compensation of drive train losses (turn on under Horsepower Correction option). In order for this to work, you need to record data both going up in RPM and down again, all the way to the starting RPM (clutch in, no brake). If you do this, you will see two curves, one for wheel HP/torque and one for engine HP/torque.
The point 2 above surely works too, I have tested it many times. Did you record data both going up and down in RPM?
I imagine that for your setup with relatively low MOI, your retardation run may be very short. This calculation is better suited for heavy rollers that take a while to retard, and therefore more data can be collected during retardation to get accurate results. If you right-click and choose "Set scale to default" you should see the retardation portion of the run too, as negative hp/torque.
I am interested to know the results with your setup, with your MOI. Do you get a smooth retardation curve?
Glad to hear you like the new automatic brake control!
For the hub dyno the inertia is low for sure. But this car witch i am working with is making 370nm at 4000rpm in static run and in acceleration run it makes 320nm at 4000rpm when boost/other effecting parameters are exactly same. So there is still some inertia effect witch causes the readings be a bit low. This car has been in inertia roller dyno, there it made about 360nm at 4000rpm in acceleration run (cant be compared straight to this hub dyno but this is for example...).
I did exactly same as you explained in the first post, but the problem starts from here:
"7. Compare with the static run, and adjust MOI up or down depending if the reading is too high or too low."
The readings drops to 275nm at 4000rpm when turning on compensation of inertia effects. Adjusting the MOI up or down doesnt effect the reading/curve in any way, it stays same no matter if MOI is 2 or 20.
I did a run where the RPM goes up and down and tested the calculation and compensation of drive train losses. It does work fine, but im not interested in measuring engine hp/tq. The wheel hp/tq is what matters 🙂
Indeed! Sorry, there was a bug there. It read the MOI from the Inertia menu instead of from the Brake menu, and when I was testing this I was changing both places, so I did not notice this bug.
New version released now, 3.01.48! Thanks!!
No worries, its nice to see that the software is developing and gets better and better all the time 🙂
I added a video on how to measure the MOI. It is a better method than the method explained above. See: https://www.youtube.com/watch?v=0RVmxpCk0SE
Hi , this is for a roller dyno and hub dyno setup, with yourdyno software how would one measre MOI in a water brake engine dynamometer absorber ? Run a ramp up power run and then ramp power down ? Average of the two graphs is compensating for the dyno+connection shaft the energy required to turn those components plus the test engine??
I have watched your explanation you posted on your YouTube Channel.
Hi, for a water brake you do a power run up then power down in RPM. Use the log file and modify the MOI until the curves overlap. It is the same operation as for eddy brakes.
I'm using an engine Dyno with a small amount of Integra can I put in a percentage of powertrain loss? I used to be able to add 10% to account for the gearbox I was running through but it all seems to be different now if I have power compensation ticked it affects the Run
@stevedarman, yes just add Power correction factor, it is still there. This is the power related loss percentage. You will then not have a speed loss component.
I am amazed at the effort you put into constantly updating the software.
Very Happy Tuner