Quote:
Originally Posted by dcb
how about a CDA or BSFC calculator?
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To calculate Cd you need to know:
ρ (rho), the density of the fluid (constant). In this case air. Check.
v (velocity). VSS frequency, or averaged pulses over time. Check.
A (frontal area). Although inconvenient, this can be measured. Check.
Fd (drag force). I'm not sure how this could be actively measured.
Maybe replace an engine mount with a strain gauge? That would still require calibration. Even then, we would be measuring net drag rather than aerodynamic drag.
If you know the vehicle's mass, I suppose you could do a coast-down on level ground, tracking the vehicle's speed along the way (F=M*a). The rate of deceleration at very low speeds (where aero drag is negligible) should let you gauge mechanical drag. Once the fixed drags are known, they can be subtracted from measurements taken at higher speeds where aero is playing a significant role.
While it would certainly be a useful instrument when working on aero mods, I don't see it having any use during day to day driving. Altering your speed, throttle behavior or drafting won't affect your Cd or Cd*A.
What
would be useful IMO is a bar graph of where energy is going. Typical physics class stuff: kinetic, potential and other (drag) bars. This would require some way to gauge engine power output. BSFC (brake-specific fuel consumption) is basically fuel consumption / power output, so that requires some means of gauging power as well. Kinetic energy is easy - mass and velocity... Mass is fairly fixed, and velocity is easily measurable. Differentiating between drag and potential energy would require either directly measuring drag (via a strain gauge or something), or directly measuring vertical changes. An altimeter would probably be the cheapest but least accurate method, due to pressure changes from wind buffeting and such. GPS is probably the most expensive, but not terrifically accurate. A 6 degree-of-freedom accelerometer/gyroscope chip could be used in conjunction with the vehicle speed to judge when the car is traveling up or down hill... Expensive for a bar graph though.
Maybe a pitot tube could be useful?