BlackBird05

The question: when you calculate HP based on a change in speed, should you add in the power it takes to move the vehicle at that speed, and when you do, is this the flywheel HP?

This question arose from a project on how to determine the power required to move the car at cruise speeds. The basic premise is to use a data logger to track time and velocity while coasting (engine in neutral), then run the change in velocity and time through a formula for power.
(For the curious, here's the formula: start with P=F*V, then put F=m*a into it yielding P=m*a*V, then from kinematics a=(Vf-Vi)/(tf-ti) which gives the final of P=(Vf-Vi)/(tf-ti)*m*Vf)(after that you get to convert the units appropriately)

So when you're accelerating, you can track the excess HP to accelerate the vehicle, but as you speed up, the power required to move the vehicle also increases eventually leading to a constant speed in the mid 100's due to air and component drag. So the question again, should the power required to push the car through the air and turn all the components be added to the change in velocity HP to yield the E-shaft HP?

RIWWP

iTrader: (2)

If I understand that correctly (which I'm not sure I do), then yes.

Seems like a somewhat odd way to approach the problem though, as gearing can make a difference since it changes how much power is actually being put to the ground at a given point in the rpm band, before you even get into non-flat power curves where shifting the rpm point at aero-max could be changing to a higher or lower power point in the power curve.

If the project includes practical experiments, the way I would approach it is to get a dyno chart of that particular engine (max power pull as well as cruising charts), overlayed with the MAF data of airflow so you can see the relationship between the MAF data and the resulting power output, with no aerodynamic component involved. Then, record the MAF data with the car at cruise at various RPM points, and you should be able to subtract the difference in the MAF to get the aerodynamic cost portion of that, then the prior maf to hp calculations to determine the power being used.

BlackBird05

Oh my, I did forget about the power multiplication that occurs via the transmission and differential. Fortunately those numbers can be found and I'll start trying to integrate them into the formula.
But the original goal of determining the power it takes to move the car at cruise speeds can still be done w/o modifications.

RIWWP

iTrader: (2)

Don't forget the "gear ratio" that is the wheel radius.

BlackBird05

a wheel radius that'll change with velocity...

BlackBird05

Ok, so to deal with the power multiplication occurring due to the tranny, diff, and tires, one would calculate HP via the data logger, add in the HP to move the vehicle at that each speed, then divide by 3 values: tranny ratio, differential ratio, and radius of tire in ft.

Please show my where my logic is incorrect (besides me using a formula for acceleration that assumes it is constant)

j9fd3s

yes, isn't it flywheel hp - friction/aero losses = acceleration?

monchie

Yeah...i understand completely. lol

staticlag

This sounds like a complicated way to approach a basic premise. Basically if I understand correctly you would like to add the real world friction coefficients of everything to possibly create a more accurate representation of "real world power" (perhaps for some type of software or hardware you are creating?)

To be absolutely accurate you would have to also accurately know wind speed and direction, as well as the coefficient of friction of virtually all the moving bearings and the road surface and tires. All of which is dependent on temperature and will actively change through your calculation sample window.

It's far too complicated for an average person with limited resources and will not be 100% applicable to anyone on the street. I'd suggest you draw a line somewhere and be happy that you are in the ballpark.

zoom44

or do a search for when this was all calculated years ago when we got the first gen tranny numbers

https://www.rx8club.com/general-auto...5980#post85980
https://www.rx8club.com/general-auto...673/#post29615

what the?!! i should go back in time and ban BUGER!!!!!DD:D

https://www.rx8club.com/rx-8-discuss...-promise-1660/

https://www.rx8club.com/newreply.php...wreply&p=32802

BlackBird05

Most of this mess is just me keeping busy before grad school starts in the Fall (clemson univ for automotive engineering). I'm not too interested in the friction of each individual component, but their summed final effect the engine has to push through. I did accomplish this via coasting and recording the rate of negative acceleration and my results did correspond to what Paul Lamar got, but I also have data for the 80 mph range. This would be useful if one was to attempt to put a really small rotary somewhere in the vehicle, let it run at high rpm, high power where the best BSFC is (fuel per HP), and enjoy the mpg benefits.

The WOT acceleration HP doesn't seem to be correct yet, but the coast down did correlate well, so maybe I should trust the data and actually finish the calc with the ratios removed.

Thoughts of a physics major turning engineer,
BB

BlackBird05

Here's the coast down data. Cell 800 has the graph in it. The first chart near cell 675 can be ignored.
And here are the references for the last post:
35 MPG RX8
Be sure to check out the bottom of that page with the cart engine.