Just google claresnyder electric fiat 128 and you will get quite a few
hits, including this one from 1996 -----
by Clarence Snyd » Thu, 05 Dec 1996 04:00:00
>I'd like to calculate the hp requirements for a car at a
>given speed. What is the formula for that. Assume I already
>know the Cd. Also what is the typical hp loss through the
>drivetrain (manual transmission) and what is the typical
>rolling resistance. I imagine this last one goes up
>exponentially with speed too, but would like a more definite
>answer.
>Thanks for any info,
>Andreas
THe question of horsepower requirement is not a simple one. Tire
rolling resistance itself is not a cut and dried calculation, and it
is one of the simpler ones. The rolling resistance drops dramaticly
with warm-up, and increases with speed and load. The amount of tread
left on the tire makes a difference, as does the tire construction.
Steel radials have the lowest RR, and increase the least with
speed.Up
to about 40MPH, the RR coefficient stays about consta/nt with radial
and bias/belted tires. The radial runs about .012, with the bias
belted about .014. A bias tire starts at about .013, but starts to
rise immediately. By 70MPH, the Bias tire has reached almost .02. The
Bias Belted is almost .016, and the radial is almost .014
The effect of warmup is also extensive, with a cold tire taking about
1.4 times as much as a tire having run 20 miles under load (reached
100F operating temp and increased pressure of 5PSI)
Load increases the rolling resistance in a linear fashion On a new
bias tire, the Crr of .012 at a light load .ncreases to .014 at 20%
loa/d, .016 at 40%, .020 at 60%, .022 at 80%, and .024 at 100%. This
is at full rated pressure. With worn tread, this increase is less
than
half.
Brake drag also comes into the picture, Disk brakes and self adjusting
drum brakes increase the drag. With the GM X car, they reduced disk
brake drag from 2.0
ft.lb to 0.5 ft lb drag, according to GM
engineers, but real world testing showed drag of about 1.5 ft lb.
Factored with the radius of the wheel, etc, this amounts to 1.7 lbs
of
rolling resistance per brake, or an increase in CRR of .002 to .004
for vehicles with disk brakes. Self adjusting drums are about equal,
and manually adjusted brakes can be less than.half, or .001 to .002
CRR increase.
Alignment adds to the resistance as well, with a well set up vehicle
adding a CRR of roughly .001
Aero drag has fewer variables. If you know the CD, and the frontal
area, Aero Drag = CD X A (in sq ft) X (mph)^2/391
The CD of a ford pinto was about .5, with 19.4 sq ft frontal area.
A chevette was .502 and 18.99
The Rabbit was .459 and 19.59.
The beetle was .458, and less than 19 sq ft.,
Aero drag for winds has to be added to this. iThe factor Kw is
calculated as (0.98(W/V)^2 + 0.63 (W/V)) CD(M)/CD -0.40(W/V)
W= average wind speed,V= vehicle speed
Cd(m) is the maximum CD for any yaw angle relative to the wind, This
is typically about 1.4 for a sedan, 1.2 for wagons and hatchbacks,
and
about 1.6 for more slender vehicles.
Climbing and accelleration requirements are dependent almost totally
on weight. For acceleration,, the formula is Faccel=Kr x gvw x
a/21.94 = gvw x Ka where a=accel in mph/sec, and Kr accounts for
inertia of rotating masses. Kr=1 + (0.04 + 0.0025 i^2) for an
internal combustion engined car. The .04 reflects the inertia of
wheels and tires,, whilke the .0025 reflects the rotating engine and
driveline parts that rotate at i times the speed of the wheels (rear
drive ratio) This usually works out to about 1.06 to 1.2
Climbing power , or Fclimb= gvw X Kc. The Kc is roughly equal to
percent grade, for all intents and purposes.
These calculations tell you what TRACTIVE FORCE is required.
This still has to be converted to horsepower.
This information was extrapalated from the book EV Engineering
Guidebook: EV conversions for the 80's by Paul R Shipps, of 3E
Vehicles, Box 19409, San Diego Ca 92119.
There is a lot more in the book. See if it is still available if you
are interested.
I got involved in this when I electrified a '75 Fiat 128L coupe a
number of years back. 8 HP was adequate around town. (electric motor
-
high torque at low RPM)
Clare Snyder, Class "A" Licenced Auto Mechanic
Recycled as a Computer Solution Provider
Waterloo, Ontario, Canada