Module: EngineTune::Calculator

Defined in:
lib/engine-tune/calculator.rb

Overview

A collection of formulas and calculations used to determine, among other things, the relative horsepower of an engine given different environmental conditions.

These calculations are modeled after Richard Shelquist’s Engine Tuner’s Calculator, but have been re-factored to be more ruby-like. I urge you to read his technical articles on Density Altitude and Corrected Horsepower for detailed explanations of these concepts.

In some cases I took a guess at what a method should be named based on the calculation it is returning, so any better suggestions are welcomed.

Class Method Summary collapse

Class Method Details

.absolute_pressure(altimeter, altitude) ⇒ Object 



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# File 'lib/engine-tune/calculator.rb', line 85

def absolute_pressure(altimeter, altitude)
  k1, k2 = 0.190263, 8.417286E-5       
  ((altimeter**k1)-(k2 * self.geopotential_altitude(altitude)))**(1/k1)
end

.altitude(altimeter, elevation, dew_point, temperature) ⇒ Object 



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# File 'lib/engine-tune/calculator.rb', line 69

def altitude(altimeter, elevation, dew_point, temperature)
  density = self.density(altimeter, elevation, dew_point, temperature)
  g, po, to, l, r, m = 9.80665, 101325, 288.15, 6.5, 8.314320, 28.9644
  density = density * 1000
  p2 = ( (l * r)/(g* m - l * r) ) * Math.log( (r * to * density) / (m * po) )
  h = -(to / l)*( Math.exp(p2) -1 )
  h * 1000
end

.altitude_z(altimeter, elevation, dew_point, temperature) ⇒ Object 



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# File 'lib/engine-tune/calculator.rb', line 78

def altitude_z(altimeter, elevation, dew_point, temperature)
  altitude = self.altitude(altimeter, elevation, dew_point, temperature)
  r = 6369E3
  ((r * altitude) / (r - altitude))
end

.celsius_to_fahrenheit(temp) ⇒ Object 



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# File 'lib/engine-tune/calculator.rb', line 124

def celsius_to_fahrenheit(temp)
  ((temp * 9.0) / 5.0) + 32.0
end

.density(altimeter, altitude, dew_point, temperature) ⇒ Object 



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# File 'lib/engine-tune/calculator.rb', line 58

def density(altimeter, altitude, dew_point, temperature)
  vapor_pressure = self.vapor_pressure(dew_point)
  absolute_pressure = self.absolute_pressure(altimeter, altitude) 
  
  rv, rd = 461.4964, 287.0531
  temperature_k = temperature + 273.15
  pv = vapor_pressure * 100
  pd = (absolute_pressure - vapor_pressure) * 100
  (pv / (rv * temperature_k)) + (pd / (rd * temperature_k))
end

.density_altitude(altimeter, altitude, dew_point, temperature) ⇒ Object 



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# File 'lib/engine-tune/calculator.rb', line 18

def density_altitude(altimeter, altitude, dew_point, temperature)          
    # Calculate geometric altitude Z (m) from geopotential altitude (m) H          
    densaltzm = self.altitude_z(altimeter, altitude, dew_point, temperature)
    unless (-5000..11000).include?(densaltzm)
        raise "Out of range for Troposhere Algorithm: Altitude =" + densaltzm.round + " meters"
    end
    densaltzm.round
end

.dyno_correction_factor(altimeter, altitude, dew_point, temperature) ⇒ Object 



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# File 'lib/engine-tune/calculator.rb', line 27

def dyno_correction_factor(altimeter, altitude, dew_point, temperature)        
  vapor_pressure_mb = self.vapor_pressure(dew_point)
  absolute_pressure_mb = self.absolute_pressure(altimeter, altitude)
  absolute_pressure_in = millibars_to_inches(absolute_pressure_mb)
  vapor_pressure_in = millibars_to_inches(vapor_pressure_mb)        
  p1 = 29.235 / (absolute_pressure_in - vapor_pressure_in)
  p2 = ((temperature + 273) / 298)**0.5
  (1.18 * (p1 * p2) - 0.18)
end

.geopotential_altitude(elevation_in_meters) ⇒ Object 



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# File 'lib/engine-tune/calculator.rb', line 53

def geopotential_altitude(elevation_in_meters)        
  r = 6369E3;
  ((r * elevation_in_meters) / (r + elevation_in_meters))
end

.inches_to_millibars(inches) ⇒ Object 



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# File 'lib/engine-tune/calculator.rb', line 112

def inches_to_millibars(inches)
  inches * 33.8637526
end

.meters_to_feet(meters) ⇒ Object 



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# File 'lib/engine-tune/calculator.rb', line 116

def meters_to_feet(meters)
  meters * 3.2808399
end

.millibars_to_inches(mb) ⇒ Object

Systems of measurement conversion methods



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# File 'lib/engine-tune/calculator.rb', line 108

def millibars_to_inches(mb)
  mb * (1/33.86389)
end

.relative_density(altimeter, altitude, dew_point, temperature) ⇒ Object 



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# File 'lib/engine-tune/calculator.rb', line 90

def relative_density(altimeter, altitude, dew_point, temperature)
  density = self.density(altimeter, altitude, dew_point, temperature)
  100 * (density / 1.225)
end

.relative_horsepower(altimeter, altitude, dew_point, temperature) ⇒ Object 



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# File 'lib/engine-tune/calculator.rb', line 99

def relative_horsepower(altimeter, altitude, dew_point, temperature)
  dyno_correction_factor = self.dyno_correction_factor(altimeter, altitude, dew_point, temperature)
  round((100 / dyno_correction_factor), 1)
end

.relative_humidity(temperature, dew_point) ⇒ Object 



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# File 'lib/engine-tune/calculator.rb', line 95

def relative_humidity(temperature, dew_point)
  (vapor_pressure(dew_point) / vapor_pressure(temperature)) * 100
end

.round(f, d) ⇒ Object 



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# File 'lib/engine-tune/calculator.rb', line 120

def round(f, d)
  (f * 10**d).round.to_f / 10**d
end

.vapor_pressure(temperature_c) ⇒ Object

Polynomial from Herman Wobus



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# File 'lib/engine-tune/calculator.rb', line 47

def vapor_pressure(temperature_c)                
  values = [ 1.1112018e-17, -1.7892321e-15, 2.1874425e-13, -2.9883885e-11, 4.3884187e-09, -6.1117958e-07, 7.8736169e-05, -0.0090826951, 0.99999683 ]
  pol = values.inject(-0.30994571E-19) { |result, v| result = v + temperature_c * result }        
  6.1078 / (pol**8)
end

.virtual_temperature(altimeter, altitude, dew_point, temperature) ⇒ Object 



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# File 'lib/engine-tune/calculator.rb', line 38

def virtual_temperature(altimeter, altitude, dew_point, temperature)        
  absolute_pressure = self.absolute_pressure(altimeter, altitude)
  vapor_pressure = self.vapor_pressure(dew_point)
  ((temperature + 273.15) / (1- (0.377995 * vapor_pressure / absolute_pressure)))-273.15
end