Hofmann Dynatest 112
i have a new issue today. We did some pulls on a big turbo Volvo S60R and somehow the power graph is not crossing at 0 hp when going to retardation. Usually the power curve goes down to 0 then continues with dotted line ( WHP). Did you ever see something similar?
today i tried again to set the MOI as per instructions,but still it is not matching. The up and down power and torque is simply not paralell,wherever i set the MOI,the lines are crossing each other. I compared to an inertial only run,the ramp up is precisely matching with the results. I checked the times between 3000-6000 (tried the MOI setup with 3000-6000-3000 profile), in inertia only mode it was 4s and with braked profile 7s up and 7s down. So in my opinion,the MOI and load cell calibration is matching,but still do not understand,why i have much less power during ramp down.
If the engine has a turbo one explanation could be this: The engine is stronger below 3500 RPM or so the way down, because the turbo boost is there already (or some other reason). You see lower number on the way down at higher RPMs, because the MOI is set too high.
Does it makes sense?
Do a steady state test at for example 5000 RPM, and you will see which MOI is correct.
no,this was a 2.0 16v N/A car. I was thinking the same as you mentioned,thats why i tried it again with a naturally aspirated engine.
I made previously the steady state trial and set the moi accordingly. Currently it is 11,3 on the dyno with 318mm rollers (4) and eddy brake.
With these setting the readings seems to be reasonable.
I just see your video today with the new software version. One thing is not considered with the ramp up and down MOI calculation: the load on the tire must be higher when going down,isnt it?
i did a bit more research on the logged data,i made the transfer function from the raw log to get wheel torque reading from the retarder only.Now i can compare the blue retarder tq+inertia tq with retarder only. From the graph i would expect,that the form will be similar going up and down.When going up,the tq has a peak somewhere around 4500 then goes down,but when running the rpm down,retarder torque increases all the way up.
Can someone send me a datalog when MOI was tried to defined,just for comparison? I am interested now to solve this question.
There are some differences when ramping up and down:
When ramping up, the load on the load cell is less, since part of the engine torque is taken by the inertia. On ramp down, the load cell needs to take the torque of the engine PLUS the torque from the inertia. The sum should be the same in both instances (assuming same engine power).
I will send you our MOI test file and user.config file. You can backup your settings and load mine and play around.
today i made another test to determine the inertia. There is an excel around the web helping to design an inertia dyno,so i took this calculation and made measurements on the rollers. Single roller was 2,35kg/m2, the dual roller with the brake was 6,6,so the total system inertia should be 11,3,just what i found best from previous trials 😀
Cool! So that is the inertia of the dyno itself. Using the MOI from this method will give the true wheel power.
You can use this number and add a power correction. Note that the power correction needs to include compensation for the car's drive train MOI which is not included when you do the rope based MOI test. It is not that large for a hub dyno though.
I dont have a hub dyno,so the inertia of the wheels play a role in the equation. I used the dyno with 0% power correction,the engine hp was for me reasonable,but the loss was low. For example 190whp resulted 220 hp. I will ask someone with a stock car to have more and more data,so resulting more accurate settings in the end.