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Old 08-23-2011, 08:43 AM   #3
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grambles423's Avatar
Drives: 2008.5 BMP GTI
Join Date: Jun 2009
Location: Balls Deep
Posts: 18,424
Engine Dynamics

Brake Horsepower (Watts) = N*Displacement(in cubic Meters)* (RPM/2)* nc*nm*nth*nv*Qhv*(F/A)*Air Density

N = Number of cylinders

Displacement = Bore*Stroke*pi/4

RPM/2 = Must be placed in Revs/Second = RPM/2 * (1/60 {s/min})

nc = Combustion efficiency = about .98-.99 on newer cars (can be calculated further, but these estimations are fine)

nm = mechanical efficiency = Function of RPM/heat/etc range of about .8-.93 (can be calculated further, but these estimations are fine)

nv = volumetric efficiency (what percentage is your TB open?)= 100% at WOT (Possibly a little more depending on the flow. Most I've seen is 108%)

nth = thermal efficiency = 0.8*(1-[compression ratio^(-.35)])

Qhv = Heating value of the fuel = 43,000,000 J/kg

Air Density = Pressure Entering / (287.2 J/kg-K * Intake Temperature)

Pressure Entering = Atmospheric Pressure * Pressure Ratio (This is a function of RPM) - J/m^3
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Last edited by grambles423; 01-18-2012 at 04:17 PM.
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