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Automotive HorsePower; brake, wheel, or real?

Automotive HorsePower; brake, wheel, or real?

Automotive HorsePower; brake, wheel, or real?

(OP)
I follow a sports car forum. A recent post was someone’s posting of an incoherent article about how to calculate HP using times to speeds during acceleration.

I took interest and tried to write a simplified version using minimal mathematics. Basically calculated power as change in KE over time.
From another poster that had provided a plot taken from those new fangled GPS data loggers. His intent was to show how his +500 bHP engine (documented) now achieved a 0 to 60 mph less than 4 seconds. Appling the KE to power calculations the required HP to do so is about 250 wHP. there is an "Eng Tool Box" calculator that gave similiar results

So my query is why does if appear to take 2X the required at the engine? Is driveline losses that much?

RE: Automotive HorsePower; brake, wheel, or real?

There's a hideous amount of entropy in Bench Racing.

Mike Halloran
Pembroke Pines, FL, USA

RE: Automotive HorsePower; brake, wheel, or real?

0-60 in a manual trans spends most of its time near peak torque rather than peak power. The losses are quite large, 15% in the driveline for a start. Powertrain rotational inertia also upsets your calculations, in first gear one car I worked on the referred inertia was about 40% of the rest mass.

Cheers

Greg Locock


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RE: Automotive HorsePower; brake, wheel, or real?

(OP)
Am I to understand that there will be 15% drive line losses for a power measurement at steady state. But for accelerating not only is the power need to accelerate to car's weight, there will be power needed for increase rotation of the drive line due to its inertia.

that makes sense.

since I sleep on it I had a though, would the descrepancy be also the fact documented engine power is at a peak, where measure performance is over a wider rpm range, thus performance is effective power "under the curve"?

so I think I now see where
the difference in brake to wheel would be the 15% driveline effeciny,
and
the difference from wheel to "real" is the power required due to driveline inertia

Thanks

RE: Automotive HorsePower; brake, wheel, or real?

Greg could have been talking about losses in the dynamic situation. The energy to bring the mass of the driveline from 0 to their top rotational speed is not even present at steady-state (fairly constant RPM).

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

RE: Automotive HorsePower; brake, wheel, or real?

(OP)
My query WAS the comparison of dyno brake power with the calculated power to accelerate a 1500kg mass to 100kph in 3.5 seconds. thus I forgot about the power that is needed to spin the drive line.

a very quick google search on the subject implies, like Greg mention, the "rule of thumb" equivelant mass of the vehicle is 1.4 its actual. there is a LOT more engineering I should study, if I wish to continue understanding this

I think I got my answer! thanks

edit...FYI
I found this article
http://hpwizard.com/rotational-inertia.html#simple

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