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Old 06-06-2003, 08:20 PM   #1
LiquidForce
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Default Formula for calculating horsepower needed for AutoLogger

I have found several sites that have an equation for solving horsepower

horsepower = mep x displcement x rpm / 792,00

the only problem is I don't know how to solve for MEP (mean effective pressure). When I do a search I just get a solution that requires horsepower.

mep = hp x 792,000 / displacement x rpm


I am trying to include a road dyno in the Autologger program I wrote. The only piece I am missing is MEP. Is it possible the components of MEP from the UTEC log? IF so can someone give me a real world sample of the equation in use.


thanks
Mike K
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Old 06-09-2003, 11:16 AM   #2
RiftsWRX
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Default Re: Formula for calculating horsepower needed for AutoLogger

Quote:
Originally posted by LiquidForce
I have found several sites that have an equation for solving horsepower

horsepower = mep x displcement x rpm / 792,00

the only problem is I don't know how to solve for MEP (mean effective pressure). When I do a search I just get a solution that requires horsepower.

mep = hp x 792,000 / displacement x rpm


I am trying to include a road dyno in the Autologger program I wrote. The only piece I am missing is MEP. Is it possible the components of MEP from the UTEC log? IF so can someone give me a real world sample of the equation in use.


thanks
Mike K
Call and ask turboxs directly, maybe they can put you in touch with the right people.

Jorge (RiftsWRX)
www.ProjectWRX.com
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Old 06-09-2003, 01:37 PM   #3
Jon [in CT]
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Default

In addition to the logged vehicle speed and time data, if you're given additional information, you can calculate whp based on the forces necessary to overcome:
car's inertia (need:vehicle weight)
car's aerodynamic drag (need: frontal area, coefficient of drag, air density)
car's rolling resistance (need: coefficient of rolling resistance)

You also need to make some assumptions: level surface and no head- or tail-wind.
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Old 06-10-2003, 01:54 PM   #4
Dr Moreau
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Default cool thoughts

Hey y'all,

I sent the MEP question along to my pal Lars, hardcore engineer and Bronco nut. He's morphing into an aircraft engineer (some homebuilts use the Subie 2.5L powerplant) so this stuff interests him.


Hmmm... just had a look at the thread. The second reply pretty well describes what he needs to do the calculation. Mean effective pressure is an artificial number that is convenient to calculate for the purposes of comparing efficiencies of various internal combustion engines. There are lots of ways to calculate power. One is to multiply the volume which is swept (displacement of the engine) per unit time by the average pressure developed. Another way is torque X rpm. However there is no way to calculate mean effective pressure without having determined power somehow.

For the record, the British were obtaining astonishingly high mep's from aircraft engines- most notably, Rolls Royce with their V-12's- back in the late 20's. Roughly double the values seen in most engines today. Manufacturers today could match the values that RR were seeing without much trouble, but not without different fuels. RR's numbers were obtained from engines using 2 speed, 2 stage superchargers, and the fuels necessary to burn without detonation in those engines required rather more octane boosting ingredients (lead) than we would be willing to accept today.

One of the factors driving aircraft engine manufacturers' requirements for high mep was that, unlike auto engines, they couldn't just increase rpm to get more power. For one thing, the metallurgy was such that high rpm meant a guarantee of self-destruction. Another was that techniques for reducing friction between parts, which increases linearly with engine speed and robs power, was poorly understood. Most importantly though was the need to keep propeller tips from going supersonic, as at that point the prop efficiency falls off dramatically.

The bigger the prop (as needed to absorb the power developed) the lower the allowable speed. To some extent, overcome with reduction gearing, though even there, the larger the reduction the greater the gearbox losses, and the greater chance of the gearbox grenading. The Rolls Royce Merlin used in the P51 Mustang, Spitfire, Hurricane, Mosquito and others had a spur gear reduction box on the nose of the engine, which also conveniently relocated the propeller drive shaft higher for a better thrust line.

Daimler-Benz built the engines in the ME-109. Inverted V-12, with a planetary gearbox for speed reduction. Those are straight through, which was perfect, since, due to the inverted design, the engine crankshaft was already correctly located to provide the proper thrust line.
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Old 06-10-2003, 02:22 PM   #5
LiquidForce
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I am just looking for a rough estimate of power so I am will to make assumptions where necessary. So given that I am looking for a simple calcluation, something like the formula for deriving the power from a trap speed would be perfect. Is there any way to do this with the values provided by the UTEC? If there is, can someone post the equation (ex: x + y = z)?

Somehow they are managing to do this with the Delta Dash.


thanks
Mike K
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Old 06-10-2003, 03:52 PM   #6
Jon [in CT]
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Quote:
Originally posted by LiquidForce
Somehow they are managing to do this with the Delta Dash.
Ever notice that DeltaDash needs a table containing:
vehicle weight,
gear ratios,
tire size,
frontal area,
cd (drag coefficient)?
See http://forums.nasioc.com/forums/show...hreadid=310934.

Ever notice that Road Dyno needs vehicle weight, tire size, cd and frontal area, among other car-specific constants?
See http://forums.nasioc.com/forums/show...hreadid=193670.

They're both determining the torque necessary to overcome rolling resistance and aerodynamic drag. Then they add that to the torque necessary to accerlate a mass at the observed rate (F=m*a).

Take a look at these formulas from the Bosch Automotive Handbook. Usually, there's a method offered to estimate some of the missing constants. A particularly clever one is the "Coast Down" calibration, used to determine both a coefficient of rolling resistance and a coefficient of drag.



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Old 06-10-2003, 06:58 PM   #7
LiquidForce
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Default

Point made. This is exactly what I was looking for.

I should be able to incorporate this in the autologger app with a nice chart.

thanks
Mike K
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