Mensagem: 24
Data: Fri, 9 Nov 2001 18:33:21 -0200
De: "Sukys" <rsukys@yahoo.com>
Assunto: Dinamômetro Virtual
Pessoal,
Para quem gosta de numeros, segue abaixo o texto que recebi sobre uma forma aproximada de determinar
a potência de um carro.
Evidente que os valores encontrados não são absolutos mas pode ser uma boa forma de determinar ganhos
(ou perdas) com modificações no motor, câmbio e pneus. Para tal basta fazer as medições antes e depois das
modificações e observar a variação relativa na medição.
Sukys
rsukys@yahoo.com
Speaking of dynos, here is the coast-down method of measuring HP.
You really only need a stopwatch and an accurate speedometer but I use a
radar gun for more precision in the speed measurement.
To measure the power available at a given speed, you first time how long
it takes to accelerate from a bit below to a bit above that speed.
Then you time how long it takes to decelerate with the car in neutral
through the same speed interval. The first measurement measures
how much power over and above drag and rolling resistance is available
for acceleration. The second measurement measures the total of all
parasitic losses including drag, rolling resistance, transmission
losses, etc. The sum of these two is the power the engine is making.
If you're interested in the peak power of the engine, select the range
of speeds to just bracket the RPM where the peak power is made.
It may take some experimenting to find out where this is. Needless
to say, unless you compensate for it, this test must be done
on flat, level, dry road. A run in either direction over the same
course and the results averaged is the most accurate.
The first thing to do is to weigh the car. Many trukstops have
scales and will weigh your car free with a fill-up. I use a full
tank of gas because that is easy to replicate. Include your weight,
of course.
The procedure is simply. Get your stopwatch ready. Accelerate
at wide open throttle and when you hit your lower speed, start the
stopwatch. When you hit the upper speed, stop it. Write down the
starting speed, ending speed and time. Accelerate a bit more, reset the
stopwatch, put the car in neutral, coast down. When you hit the upper
speed, start the watch and stop it when you hit the lower speed.
Record the time.
Here are the computations.
Sal = Speed, accelerating, lower
Sau = Speed, accel, upper
Ta = Time, accel
Sdl = Speed, decel, lower
Sdu = Speed, decel, upper
Td = time, decel
W = weight of the car
net_ft-lbsA = net ft-lbs during acceleration
net_ft-lbsD = net ft-lbs during deceleration
HPa = HP during acceleration
HPd = HP during deceleration
Constants:
0.0336 = Magic number to unify the measurement units.
550 = ft-lbs per second in one HP
Step 1: Compute net ft-lbs, accelerating
(0.0336 * W * (Sau)^2 ) - (0.0336 * W * (Sal)^2) = net_ft-lbsA
Step 2: Compute net ft-lbs decelerating.
(0.0336 * W * (Sdu)^2 ) - (0.0336 * W * (Sdl)^2) = net_ft-lbsD
Step 3: Compute HP, accelerating
HPa = (net_ft-lbsA / Ta) / 550
Step 4: Compute HP, decelerating
HPd = (net_ft-lbsD / Td) / 550
Step 5: Compute total HP
HPd + HPa = total HP.
Example:
A car weighing 3200 lbs. It accelerates from 60 mph to 80 mph in 5
seconds and decelerates through the same range in 20 seconds.
That works out to 68.4 hp accelerating and 17.1 hp decelerating,
85.5 hp total.
This is a pretty sensitive test and can show, for example, the
added rolling resistance from driving in the rain with water standing
on the pavement. Just remember that drag is a cube-law functio of
speed so don't try to "average" over too wide a speed range.
ABRAÇOS
Théo
Fpolis