- #1

madhatter106

- 141

- 0

[tex]I=1/2*m*(r_{1}^{2}-r_{2}^{2})[/tex]

[tex]\alpha=\frac{d\omega }{dt}[/tex]

[tex]\tau =I*\alpha[/tex]

these are some of the equations I'm using, the torque should be in Nm when [tex]MOI=kg/m^2[/tex]

the information I'm using comes from data sets off the ECU. I'll explain a bit, a test development engine has been run on the dyno using a chassis, the numbers are good. what I was asked is if it's possible to back out and use the ECU information to output similar numbers to the dyno, first blush I figure sure it shouldn't be that difficult, famous last words, right?

I have the rpm vs time data in milliseconds, specifically the rpm is counted every 20 milliseconds, I have the dyno numbers for each recorded run so I can compare results, i.e. the data set for one pull has a recorded ECU run for all sensors, rpm,time, temp etc...

I have the gear ratios for all gears, wheel dia etc..I have put together a spread sheet with the input rpm to the gear box, output for each gear in rpm and mph to the drum roller dynomometer, drum dyno rpm and mph, and delta between them. the rpm on the dyno drum is converted to radians per second.

step 1:

engine rpm to drum rpm (calculated via gear ratios)

step 2:

drum rpm to radians [tex]\omega /sec[/tex]

step 3:

[tex]\alpha =(\omega _{1}-\omega _{2})/(t_{1}-t{2})[/tex]

step 4:

[tex]\tau Nm=I*\alpha[/tex]the numbers are wildly high, the curve looks about right, peaks near the dyno's output. however there are massive spikes due to the time count where the rpm is recorded in whole numbers to the millisecond count so the time moves forward but the rpm holds a bit, on avg it's 60~80 milliseconds per revolution count then it jumps a fair amount in the next step. resolution could be better but it's all I've got to work with.

I'm assuming at this point I've calc'd wrong somewhere but can't seem to figure where. I have the drum mass and radius data for a correct MOI. using the gear reduction I have the drum rpm to engine rpm and assume that should be accurate enough neglecting slip and friction loss.

the reason I'm going to this trouble is the inertia drum mass exceeds the chassis weight by a factor of 2.2, the loads are very high for the final working loads the engine will be under. If I can re-calc the dyno run then I should be able to factor the delta in the dyno's inertia to the chassis inertia and approx the acceleration of the chassis in the real world by using this equation:

[tex]\I =[\frac{Dm-Cm}{Cm}*Dm]+Dm[/tex]

Dm=drum mass

Cm=Chassis mass

Hopefully someone can point out my error, Thanks..