Question for physics/kinetics

[A80Avant]

Im a bit left behind with my highschool/ college physics so i need your help a bit.

I have searched the web a bit and couldnt come up with a vital solution.

Here is the thing:
I use a few formulas for the perfomance box figures so that i can get a power/torque figure in the end.
The raw data i get from the PB is time vs speed OR time vs distance.

With the weight of the car known, and regardless of all losses(they are calculated elsewhere) we are calculating the speed in m/s and then the acceleration difference between 0.1sec intervals as they come from the perfomance box.

Through acceleration we then calculate the KW and so on...

I think there is a flaw in this process, as acceleration becomes less with time due to increase of losses, air drag, however that does not mean that the engine's power is decreacing as well, as it still has to overcome all that to maintain some acceleration.

So in the graphs i end up getting drop in the power curve rather than a decrease in the steapness of power increase.

In dyno sessions, even when the torque drops quickly the power curve still climbs for many more revs before it reaches a plateu to start dropping, while on my graphs is is pretty much following the acceleration pattern.Torque is closed to real values however power is way off even when all losses are added. Losses calculation is different from the PB data and does not follow the acceleration pattern. It seems to be more consistent with reality.


Question is: is there a way that we can get a power OR a torque value from using the perfomance box data of time vs speed?

[Rusty]

Simply, yes you can

You can't ignore losses, you need to add the air resistance and estimate the losses at the wheels, around 35% at 7k according to a dyno! See my 'Cheap Performance Meter' thread for all the calcs

The measured power will drop off so you need to guess the rest to get it back up to the engine power. Look at any dyno wheels plot and peak power ATW will be quite early. Again, I've got pics somewhere showing a comparison for all this, have a search in the gallery.

HTH

[Jonas_R]

Edit:

No need all in Rusty thread

[A80Avant]

no, i didnt say that i ignore losses. I said that i add the losses afterwards through a different formula, but even with that addition, numbers are not correct.

at 7K i have calculated total losses of 107hp (including air drag+tyres+transmittion as 15%)

[Jonas_R]

Do you have some numbers in excel or something?

[A80Avant]

i dont want to use acceleration as a means of finding out the power.
can i use somehow speed (which i have) over time (in 0.1 sec spaces) and calculate the hp or the torque?

if acceleration gets in the equation then i will get the same results as im already getting, as i am also using acceleration to provide me with the horsepower

for example there is an equation that says Energy (joules) = 0.5 * m * v^2. can this be used somehow to avoid using the acceleration method?


here are the numbers and the way i do the calculations. as you can see i use the acceleration, but i dont want to use that.

[Jonas_R]

that is the formula for kinetic energy and I can't see how you can avoid using acceleration. But you could use the acceleration you get from your speed vs time plot.

[A80Avant]

my thinking is (and it may be wrong) that although kinetic energy is lower as revs go up, that doesnt mean that power decreases as well. Sure there are the losses from various areas and at somepoint all the losses become equal to the top speed and thats how fast the car can go, but in the graphs, there shouldnt be that downcurve that i see, and that is because i use acceleration as a means to provide the power.

[Jonas_R]

What are you using for the acceleration measurements?

[Rusty]

Dyno run-down losses are 35% at 7k As I said, it's all in my existing thread.

p = (m.a.v + 1/2.rho.Cd.A) / (1-transmission loss) is the basic equation if memory serves

[A80Avant]

i use speed vs time and make the deduction from each bin on the excel table from the previous value.

[Jonas_R]

I may have found something that is off in your spreadsheet. When you calculate WHP you time kW by 1.36 this is wrong

[A80Avant]

aha, what should it be?

[Jonas_R]

When you calculate WHP you say it is WHP = kW*1,36 and then you add air and drag and that gives HP without drivetrain. However Power [W] is calculated this way:

P = m*a*v + 1/2*rho*Cd*Af*v^3 + m*v*Ct, then you get Hp(without drivetrain) * 1,36.

[A80Avant]

alright i will change it and see if i get different results