Datalogging RPM signal

[Eric Fahlgren]

Steve.Flanagan at verizonwireless.com wrote:
> 
> Glen
> 
> Do you have any incite as to the physics being used with that device.

Off the top of my head, the aero force is

   Fa = 0.5 Rho Cd A V^2

where

   Rho = air density, use something like 1.225 kg/m^3
   Cd  = drag coefficient, probably 0.3-0.5, take a guess...
   A   = "frontal area" of the vehicle, take a guess...
   V   = velocity of the vehicle

Good old F = ma is the other part

   m   = mass of vehicle
   a   = acceleration of the vehicle

To get V, measure the delta between subsequent ignition pulses or VSS pulses
or whatever, use the gear ratios and tire diameter to get the units into something
useful.  Differentiate a series of these V numbers to get a. Crudely (and maybe
good enough, although I'd tend to think this would have a lot of noise):

   a  = (Vn+1 - Vn) / dt
   dt = pulse length between the two V measurements

So, crank it all out and you have one big F number, which turns out to be
thrust at the contact patch.  Turn this into T (engine torque delivered to
the drive wheels) by dividing by the tire radius and multiplying by the rear
end and transmission ratios, use the usual T*rpm/5252 to get RWHP.

Note that this last exercise (computing RWT and RWHP) is pretty useless, in
that even if you have a good grasp of Cd and A, you still need to contend
with rolling and frictional losses to get "true" T and HP.  Since rolling
losses are proportional to velocity, you will see "lower" HP numbers as
you shift through the gears; possibly the same thing happens in the gear
box, but other than an automatic locking up, I can't judge which way the
effect might go).

To use such a setup, I'd say you'll need to pay very close attention to
minimizing the change in external factors (road grade, wind speed, gear
ratio and so on).  On the other hand, just plotting the computed acceleration
versus RPM might prove to be a good tuning aid, you can see where changes
in tune are affecting acceleration without introducing a bunch of fudge factors.


-- 
Eric Fahlgren                                           Mechanical Dynamics, Inc
efahl at adams.com                                         Ann Arbor, Michigan, USA

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