[Diy_efi] BHP calculations

[Mark Claywell]

Hello,
 
 While O2 levels will no doubtably effect torque/power/IMEP or whatever, I
think it is a non-issue here. If the horsepower is to be found at the
wheels, then you need to add the power absorbed by aero drag and rolling
resistance. If you are going to go beyond the wheels, and find the power at
the engine, frictional losses and slip losses must also be taken into
account. 

Back to the 02 issue at the risk of being a bit pedantic. First, this is a
car, so we can assume that we are running at the lower part of the
atmosphere, right? If so, correcting the power to STP conditions is a
matter of correcting for temperature, pressure, and humidity. The
percentage of 02 versus the rest of the molecules making up air should not
change signifigantly in the range of altitudes we are talking about. While
the amount of 02 getting to the engine matters, the percentage break down
will not. Thus 02 content never needs to be considered in a detailed sense.
If you don't wish to correct the power to STP conditions then, you do not
need to consider pressure, temp, and humidity either, at least in regards
to engine power. 

There is an SAE paper that was written a few years ago that goes into
getting power numbers from an accelerometer alone. The paper went through
some techniques on getting rolling road resistance, too. I'll see if I can
find the paper number. The results were compared to known results. 

If you plan on comparing the horsepower from your model to horsepower
numbers of a roller dyno, you might want to consider looking into figuring
out if there are any signifigant differences in long. slip values. 

mark

> 
> So you sticking to O2 levels having no effect on HP, interesting.
> Feel free to investigate the matter with a Top Fuel team.
> Bruce
> 
> 
> ----- Original Message -----
> From: "Bernd Felsche" <bernie at innovative.iinet.net.au>
> To: <diy_efi at diy-efi.org>
> Sent: Monday, July 29, 2002 10:22 PM
> Subject: Re: [Diy_efi] BHP calculations
> 
> 
> > Bruce tapped away at the keyboard with:
> > [top-post fixed]
> > > ----- Original Message -----
> > > From: "Bernd Felsche" <bernie at innovative.iinet.net.au
> > >> Bruce tapped away at the keyboard with:
> > >> [top-post fixed]
> >
> > >> > ----- Original Message -----
> > >> > From: "Bernd Felsche" <bernie at innovative.iinet.net.au
> >
> > >> >> Hugh Keir tapped away at the keyboard with:
> >
> > >> >> > I am working on a spreadsheet to calculate BHP from datalogged
> > >> >> > information.
> >
> > >> >> > The basic formula I am working to is:
> >
> > >> >> > Engine BHP = acceleration BHP + wind drag BHP + transmission
loss
> BHP.
> >
> > >> >> Don't forget about rolling resistance.
> >
> > >> > What about the humidity?.
> >
> > >> Accounted for in aero drag.
> >
> > >> > Variances in the gravitation pull of the earth?.
> >
> > >> Accounted for in rolling resistance.
> >
> > >> > Difference in weight for fuel used during the *event*.
> >
> > >> In rolling resistance.
> >
> > >> > Reduction in tire diameter for tire wear.
> >
> > >> In rolling resistance.
> >
> > >> > Increase in tire diameter due to heat, as applicable.
> >
> > >> In rolling resistance.
> >
> > >> > Variances in ambient O2
> >
> > >> Aero drag.
> >
> > > Hahahaha,
> >
> > > O2 variences have no effect on HP?.   That's interesting.
> >
> > Aerodynamic drag is largely determined by the density of air. (At
> > least for bluff-bodied cars.)
> >
> > > How do you take into count all these changes, in one event?.
> >
> > Too much to write here:
> > See Bosch Automotive Handbook - chapter titled "Motor Vehicle
> > Dynamics" - p338ff in 5th English edition or chapter titled "Dynamik
> > der Kraftfahrzeuge" - p330ff in 22nd German edition.
> >
> > >> Rolling resistance accounts for as much as 50% of power requirements
> > >> at speeds of up to 80kmh (50mph), depending on vehicle aerodynamics
> > >> and vehicle mass. For light vehicles, the break-even point is closer
> > >> to 60kmh (37mph).
> >
> > --
> > /"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
> > \ /  ASCII ribbon campaign | I'm a .signature virus!
> >  X   against HTML mail     | Copy me into your ~/.signature
> > / \  and postings          | to help me spread!
> >
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> 
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