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Originally Posted by pik_d
i've had the crazy idea of writing a (c++) program which would make a pretty display of HP and torque based on some form of input.
the most obvious way i thought of was to input either the hp or torque at certain intervals along the powercurve. but i dont really like that idea, to much to input to make it look smooth.
my next idea was to find some calculation which would take "SOMETHING" and do some nifty calculations and give power/torque numbers. yay.
now, for all you guru's, is there a way to take bore, stroke, cylinder numbers, compression ratio, etc, and make a ROUGH guess of horsepower and torque?
i looked around a bit, and found something called RAChp ( royal automotive club, from england, also called taxable horsepower) which is this equation:
D^2 * (n*2.5)
where D is the bore in inches, and n is the number of cylinders.
now, this is obviously NOT going to be close at all, but it's all i've found. (by that, an SRT-4 gets 136hp.  )
so... is there anythin like what i'd be looking for? |
I think the following set of equations should get you started:
Summary –
EngineDisplacment * EngineSpeed = IdealAirFlowRate
IdealAirFlowRate * VolumetricEfficiency = ActualAirFlowRate
ActualFlowRate * AirDensity = ActualMassAirFlowRate [I suggest Using A Standard Density]
ActualMassAirFlowRate / AirFuelRatio = ActualMassFuelFlowRate
LowerHeatingValue * ActualMassFuelFlowRate = FuelEnergyRate [Assume Water Vapor]
1 – ( 1 / CompresionRatio^0.4 ) = ThermalEfficiency
ThermalEfficiency * FuelEnergyRate = PowerOutput
If you choose constants for VE, TE, etc the resultant power output will be linear with respect to speed.
Obviously, there are many other parameters that affect performance most of which can be ignored but you probably want to develop a few simple equations for heat loss and friction.
Heat loss will be inversely proportional to the square of the engine speed, while fiction will be directly proportional to the square or cube of the engine speed. In other words:
PowerOutput – [HeatLoss / ( EngineSpeed^2)] – Friction * EngineSpeed^2 = NetPowerOut