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flightgear/src/FDM/IO360.cxx

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// IO360.cxx - a piston engine model currently for the IO360 engine fitted to the C172
// but with the potential to model other naturally aspirated piston engines
// given appropriate config input.
//
// Written by David Luff, started 2000.
// Based on code by Phil Schubert, started 1999.
//
// This program is free software; you can redistribute it and/or
// modify it under the terms of the GNU General Public License as
// published by the Free Software Foundation; either version 2 of the
// License, or (at your option) any later version.
//
// This program is distributed in the hope that it will be useful, but
// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
// General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software
// Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
#include <simgear/compiler.h>
#include <math.h>
#include STL_FSTREAM
#include STL_IOSTREAM
#if !defined(SG_HAVE_NATIVE_SGI_COMPILERS)
SG_USING_STD(cout);
#endif
#include "IO360.hxx"
#include "LaRCsim/ls_constants.h"
#include <Main/fg_props.hxx>
//*************************************************************************************
// Initialise the engine model
void FGNewEngine::init(double dt) {
// These constants should probably be moved eventually
CONVERT_CUBIC_INCHES_TO_METERS_CUBED = 1.638706e-5;
CONVERT_HP_TO_WATTS = 745.6999;
// Properties of working fluids
Cp_air = 1005; // J/KgK
Cp_fuel = 1700; // J/KgK
calorific_value_fuel = 47.3e6; // W/Kg Note that this is only an approximate value
rho_fuel = 800; // kg/m^3 - an estimate for now
R_air = 287.3;
// environment inputs
p_amb_sea_level = 101325; // Pascals
// Control inputs - ARE THESE NEEDED HERE???
Throttle_Lever_Pos = 75;
Propeller_Lever_Pos = 75;
Mixture_Lever_Pos = 100;
//misc
IAS = 0;
time_step = dt;
// Engine Specific Variables that should be read in from a config file
MaxHP = 200; //Lycoming IO360 -A-C-D series
// MaxHP = 180; //Current Lycoming IO360 ?
// displacement = 520; //Continental IO520-M
displacement = 360; //Lycoming IO360
displacement_SI = displacement * CONVERT_CUBIC_INCHES_TO_METERS_CUBED;
engine_inertia = 0.2; //kgm^2 - value taken from a popular family saloon car engine - need to find an aeroengine value !!!!!
prop_inertia = 0.05; //kgm^2 - this value is a total guess - dcl
Max_Fuel_Flow = 130; // Units??? Do we need this variable any more??
// Engine specific variables that maybe should be read in from config but are pretty generic and won't vary much for a naturally aspirated piston engine.
Max_Manifold_Pressure = 28.50; //Inches Hg. An approximation - should be able to find it in the engine performance data
Min_Manifold_Pressure = 6.5; //Inches Hg. This is a guess corresponding to approx 0.24 bar MAP (7 in Hg) - need to find some proper data for this
Max_RPM = 2700;
Min_RPM = 600; //Recommended idle from Continental data sheet
Mag_Derate_Percent = 5;
Gear_Ratio = 1;
n_R = 2; // Number of crank revolutions per power cycle - 2 for a 4 stroke engine.
// Various bits of housekeeping describing the engines initial state.
running = false;
cranking = false;
crank_counter = false;
starter = false;
// Initialise Engine Variables used by this instance
if(running)
RPM = 600;
else
RPM = 0;
Percentage_Power = 0;
Manifold_Pressure = 29.96; // Inches
Fuel_Flow_gals_hr = 0;
// Torque = 0;
Torque_SI = 0;
CHT = 298.0; //deg Kelvin
CHT_degF = (CHT_degF * 1.8) - 459.67; //deg Fahrenheit
Mixture = 14;
Oil_Pressure = 0; // PSI
Oil_Temp = 85; // Deg C
current_oil_temp = 298.0; //deg Kelvin
/**** one of these is superfluous !!!!***/
HP = 0;
RPS = 0;
Torque_Imbalance = 0;
// Initialise Propellor Variables used by this instance
FGProp1_RPS = 0;
// Hardcode propellor for now - the following two should be read from config eventually
prop_diameter = 1.8; // meters
blade_angle = 23.0; // degrees
}
//*****************************************************************************
// update the engine model based on current control positions
void FGNewEngine::update() {
/*
// Hack for testing - should output every 5 seconds
static int count1 = 0;
if(count1 == 0) {
// cout << "P_atmos = " << p_amb << " T_atmos = " << T_amb << '\n';
// cout << "Manifold pressure = " << Manifold_Pressure << " True_Manifold_Pressure = " << True_Manifold_Pressure << '\n';
// cout << "p_amb_sea_level = " << p_amb_sea_level << '\n';
// cout << "equivalence_ratio = " << equivalence_ratio << '\n';
// cout << "combustion_efficiency = " << combustion_efficiency << '\n';
// cout << "AFR = " << 14.7 / equivalence_ratio << '\n';
// cout << "Mixture lever = " << Mixture_Lever_Pos << '\n';
// cout << "n = " << RPM << " rpm\n";
// cout << "T_amb = " << T_amb << '\n';
// cout << "running = " << running << '\n';
cout << "fuel = " << fgGetFloat("/consumables/fuel/tank[0]/level-gal_us") << '\n';
// cout << "Percentage_Power = " << Percentage_Power << '\n';
// cout << "current_oil_temp = " << current_oil_temp << '\n';
cout << "EGT = " << EGT << '\n';
}
count1++;
if(count1 == 100)
count1 = 0;
*/
// Check parameters that may alter the operating state of the engine.
// (spark, fuel, starter motor etc)
// Check for spark
bool Magneto_Left = false;
bool Magneto_Right = false;
// Magneto positions:
// 0 -> off
// 1 -> left only
// 2 -> right only
// 3 -> both
if(mag_pos != 0) {
spark = true;
} else {
spark = false;
} // neglects battery voltage, master on switch, etc for now.
if((mag_pos == 1) || (mag_pos > 2))
Magneto_Left = true;
if(mag_pos > 1)
Magneto_Right = true;
// crude check for fuel
if((fgGetFloat("/consumables/fuel/tank[0]/level-gal_us") > 0) || (fgGetFloat("/consumables/fuel/tank[1]/level-gal_us") > 0)) {
fuel = true;
} else {
fuel = false;
} // Need to make this better, eg position of fuel selector switch.
// Check if we are turning the starter motor
if(cranking != starter) {
// This check saves .../cranking from getting updated every loop - they only update when changed.
cranking = starter;
crank_counter = 0;
}
// Note that although /engines/engine[0]/starter and /engines/engine[0]/cranking might appear to be duplication it is
// not since the starter may be engaged with the battery voltage too low for cranking to occur (or perhaps the master
// switch just left off) and the sound manager will read .../cranking to determine wether to play a cranking sound.
// For now though none of that is implemented so cranking can be set equal to .../starter without further checks.
// int Alternate_Air_Pos =0; // Off = 0. Reduces power by 3 % for same throttle setting
// DCL - don't know what this Alternate_Air_Pos is - this is a leftover from the Schubert code.
//Check mode of engine operation
if(cranking) {
crank_counter++;
if(RPM <= 480) {
RPM += 100;
if(RPM > 480)
RPM = 480;
} else {
// consider making a horrible noise if the starter is engaged with the engine running
}
}
if((!running) && (spark) && (fuel) && (crank_counter > 120)) {
// start the engine if revs high enough
if(RPM > 450) {
// For now just instantaneously start but later we should maybe crank for a bit
running = true;
// RPM = 600;
}
}
if( (running) && ((!spark)||(!fuel)) ) {
// Cut the engine
// note that we only cut the power - the engine may continue to spin if the prop is in a moving airstream
running = false;
}
// Now we've ascertained whether the engine is running or not we can start to do the engine calculations 'proper'
// Calculate Sea Level Manifold Pressure
Manifold_Pressure = Calc_Manifold_Pressure( Throttle_Lever_Pos, Max_Manifold_Pressure, Min_Manifold_Pressure );
// cout << "manifold pressure = " << Manifold_Pressure << endl;
//Then find the actual manifold pressure (the calculated one is the sea level pressure)
True_Manifold_Pressure = Manifold_Pressure * p_amb / p_amb_sea_level;
//Do the fuel flow calculations
Calc_Fuel_Flow_Gals_Hr();
//Calculate engine power
Calc_Percentage_Power(Magneto_Left, Magneto_Right);
HP = Percentage_Power * MaxHP / 100.0;
Power_SI = HP * CONVERT_HP_TO_WATTS;
// FMEP calculation. For now we will just use this during motored operation.
// Eventually we will calculate IMEP and use the FMEP all the time to give BMEP (maybe!)
if(!running) {
// This FMEP data is from the Patton paper, assumes fully warm conditions.
FMEP = 1e-12*pow(RPM,4) - 1e-8*pow(RPM,3) + 5e-5*pow(RPM,2) - 0.0722*RPM + 154.85;
// Gives FMEP in kPa - now convert to Pa
FMEP *= 1000;
} else {
FMEP = 0.0;
}
// Is this total FMEP or friction FMEP ???
Torque_FMEP = (FMEP * displacement_SI) / (2.0 * LS_PI * n_R);
// Calculate Engine Torque. Check for div by zero since percentage power correlation does not guarantee zero power at zero rpm.
// However this is problematical since there is a resistance to movement even at rest
// Ie this is a dynamics equation not a statics one. This can be solved by going over to MEP based torque calculations.
if(RPM == 0) {
Torque_SI = 0 - Torque_FMEP;
}
else {
Torque_SI = ((Power_SI * 60.0) / (2.0 * LS_PI * RPM)) - Torque_FMEP; //Torque = power / angular velocity
// cout << Torque << " Nm\n";
}
//Calculate Exhaust gas temperature
if(running)
Calc_EGT();
else
EGT = 298.0;
// Calculate Cylinder Head Temperature
Calc_CHT();
// Calculate oil temperature
current_oil_temp = Calc_Oil_Temp(current_oil_temp);
// Calculate Oil Pressure
Oil_Pressure = Calc_Oil_Press( Oil_Temp, RPM );
// Now do the Propeller Calculations
Do_Prop_Calcs();
// Now do the engine - prop torque balance to calculate final RPM
//Calculate new RPM from torque balance and inertia.
Torque_Imbalance = Torque_SI - prop_torque; //This gives a +ve value when the engine torque exeeds the prop torque
// (Engine torque is +ve when it acts in the direction of engine revolution, prop torque is +ve when it opposes the direction of engine revolution)
angular_acceleration = Torque_Imbalance / (engine_inertia + prop_inertia);
angular_velocity_SI += (angular_acceleration * time_step);
// Don't let the engine go into reverse
if(angular_velocity_SI < 0)
angular_velocity_SI = 0;
RPM = (angular_velocity_SI * 60) / (2.0 * LS_PI);
// And finally a last check on the engine state after doing the torque balance with the prop - have we stalled?
if(running) {
//Check if we have stalled the engine
if (RPM == 0) {
running = false;
} else if((RPM <= 480) && (cranking)) {
//Make sure the engine noise dosn't play if the engine won't start due to eg mixture lever pulled out.
running = false;
EGT = 298.0;
}
}
// And finally, do any unit conversions from internal units to output units
EGT_degF = (EGT * 1.8) - 459.67;
CHT_degF = (CHT * 1.8) - 459.67;
}
//*****************************************************************************************************
// FGNewEngine member functions
////////////////////////////////////////////////////////////////////////////////////////////
// Return the combustion efficiency as a function of equivalence ratio
//
// Combustion efficiency values from Heywood,
// "Internal Combustion Engine Fundamentals", ISBN 0-07-100499-8
////////////////////////////////////////////////////////////////////////////////////////////
float FGNewEngine::Lookup_Combustion_Efficiency(float thi_actual)
{
const int NUM_ELEMENTS = 11;
float thi[NUM_ELEMENTS] = {0.0, 0.9, 1.0, 1.05, 1.1, 1.15, 1.2, 1.3, 1.4, 1.5, 1.6}; //array of equivalence ratio values
float neta_comb[NUM_ELEMENTS] = {0.98, 0.98, 0.97, 0.95, 0.9, 0.85, 0.79, 0.7, 0.63, 0.57, 0.525}; //corresponding array of combustion efficiency values
float neta_comb_actual = 0.0f;
float factor;
int i;
int j = NUM_ELEMENTS; //This must be equal to the number of elements in the lookup table arrays
for(i=0;i<j;i++)
{
if(i == (j-1)) {
// Assume linear extrapolation of the slope between the last two points beyond the last point
float dydx = (neta_comb[i] - neta_comb[i-1]) / (thi[i] - thi[i-1]);
neta_comb_actual = neta_comb[i] + dydx * (thi_actual - thi[i]);
return neta_comb_actual;
}
if(thi_actual == thi[i]) {
neta_comb_actual = neta_comb[i];
return neta_comb_actual;
}
if((thi_actual > thi[i]) && (thi_actual < thi[i + 1])) {
//do linear interpolation between the two points
factor = (thi_actual - thi[i]) / (thi[i+1] - thi[i]);
neta_comb_actual = (factor * (neta_comb[i+1] - neta_comb[i])) + neta_comb[i];
return neta_comb_actual;
}
}
//if we get here something has gone badly wrong
cout << "ERROR: error in FGNewEngine::Lookup_Combustion_Efficiency\n";
return neta_comb_actual;
}
////////////////////////////////////////////////////////////////////////////////////////////
// Return the percentage of best mixture power available at a given mixture strength
//
// Based on data from "Technical Considerations for Catalysts for the European Market"
// by H S Gandi, published 1988 by IMechE
//
// Note that currently no attempt is made to set a lean limit on stable combustion
////////////////////////////////////////////////////////////////////////////////////////////
float FGNewEngine::Power_Mixture_Correlation(float thi_actual)
{
float AFR_actual = 14.7 / thi_actual;
// thi and thi_actual are equivalence ratio
const int NUM_ELEMENTS = 13;
// The lookup table is in AFR because the source data was. I added the two end elements to make sure we are almost always in it.
float AFR[NUM_ELEMENTS] = {(14.7/1.6), 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, (14.7/0.6)}; //array of equivalence ratio values
float mixPerPow[NUM_ELEMENTS] = {78, 86, 93.5, 98, 100, 99, 96.4, 92.5, 88, 83, 78.5, 74, 58}; //corresponding array of combustion efficiency values
float mixPerPow_actual = 0.0f;
float factor;
float dydx;
int i;
int j = NUM_ELEMENTS; //This must be equal to the number of elements in the lookup table arrays
for(i=0;i<j;i++)
{
if(i == (j-1)) {
// Assume linear extrapolation of the slope between the last two points beyond the last point
dydx = (mixPerPow[i] - mixPerPow[i-1]) / (AFR[i] - AFR[i-1]);
mixPerPow_actual = mixPerPow[i] + dydx * (AFR_actual - AFR[i]);
return mixPerPow_actual;
}
if((i == 0) && (AFR_actual < AFR[i])) {
// Assume linear extrapolation of the slope between the first two points for points before the first point
dydx = (mixPerPow[i] - mixPerPow[i-1]) / (AFR[i] - AFR[i-1]);
mixPerPow_actual = mixPerPow[i] + dydx * (AFR_actual - AFR[i]);
return mixPerPow_actual;
}
if(AFR_actual == AFR[i]) {
mixPerPow_actual = mixPerPow[i];
return mixPerPow_actual;
}
if((AFR_actual > AFR[i]) && (AFR_actual < AFR[i + 1])) {
//do linear interpolation between the two points
factor = (AFR_actual - AFR[i]) / (AFR[i+1] - AFR[i]);
mixPerPow_actual = (factor * (mixPerPow[i+1] - mixPerPow[i])) + mixPerPow[i];
return mixPerPow_actual;
}
}
//if we get here something has gone badly wrong
cout << "ERROR: error in FGNewEngine::Power_Mixture_Correlation\n";
return mixPerPow_actual;
}
// Calculate Cylinder Head Temperature
// Crudely models the cylinder head as an arbitary lump of arbitary size and area with one third of combustion energy
// as heat input and heat output as a function of airspeed and temperature. Could be improved!!!
void FGNewEngine::Calc_CHT()
{
float h1 = -95.0; //co-efficient for free convection
float h2 = -3.95; //co-efficient for forced convection
float h3 = -0.05; //co-efficient for forced convection due to prop backwash
float v_apparent; //air velocity over cylinder head in m/s
float v_dot_cooling_air;
float m_dot_cooling_air;
float temperature_difference;
float arbitary_area = 1.0;
float dqdt_from_combustion;
float dqdt_forced; //Rate of energy transfer to/from cylinder head due to forced convection (Joules) (sign convention: to cylinder head is +ve)
float dqdt_free; //Rate of energy transfer to/from cylinder head due to free convection (Joules) (sign convention: to cylinder head is +ve)
float dqdt_cylinder_head; //Overall energy change in cylinder head
float CpCylinderHead = 800.0; //FIXME - this is a guess - I need to look up the correct value
float MassCylinderHead = 8.0; //Kg - this is a guess - it dosn't have to be absolutely accurate but can be varied to alter the warm-up rate
float HeatCapacityCylinderHead;
float dCHTdt;
// The above values are hardwired to give reasonable results for an IO360 (C172 engine)
// Now adjust to try to compensate for arbitary sized engines - this is currently untested
arbitary_area *= (MaxHP / 180.0);
MassCylinderHead *= (MaxHP / 180.0);
temperature_difference = CHT - T_amb;
v_apparent = IAS * 0.5144444; //convert from knots to m/s
v_dot_cooling_air = arbitary_area * v_apparent;
m_dot_cooling_air = v_dot_cooling_air * rho_air;
//Calculate rate of energy transfer to cylinder head from combustion
dqdt_from_combustion = m_dot_fuel * calorific_value_fuel * combustion_efficiency * 0.33;
//Calculate rate of energy transfer from cylinder head due to cooling NOTE is calculated as rate to but negative
dqdt_forced = (h2 * m_dot_cooling_air * temperature_difference) + (h3 * RPM * temperature_difference);
dqdt_free = h1 * temperature_difference;
//Calculate net rate of energy transfer to or from cylinder head
dqdt_cylinder_head = dqdt_from_combustion + dqdt_forced + dqdt_free;
HeatCapacityCylinderHead = CpCylinderHead * MassCylinderHead;
dCHTdt = dqdt_cylinder_head / HeatCapacityCylinderHead;
CHT += (dCHTdt * time_step);
}
// Calculate exhaust gas temperature
void FGNewEngine::Calc_EGT()
{
combustion_efficiency = Lookup_Combustion_Efficiency(equivalence_ratio); //The combustion efficiency basically tells us what proportion of the fuels calorific value is released
//now calculate energy release to exhaust
//We will assume a three way split of fuel energy between useful work, the coolant system and the exhaust system
//This is a reasonable first suck of the thumb estimate for a water cooled automotive engine - whether it holds for an air cooled aero engine is probably open to question
//Regardless - it won't affect the variation of EGT with mixture, and we can always put a multiplier on EGT to get a reasonable peak value.
enthalpy_exhaust = m_dot_fuel * calorific_value_fuel * combustion_efficiency * 0.33;
heat_capacity_exhaust = (Cp_air * m_dot_air) + (Cp_fuel * m_dot_fuel);
delta_T_exhaust = enthalpy_exhaust / heat_capacity_exhaust;
EGT = T_amb + delta_T_exhaust;
//The above gives the exhaust temperature immediately prior to leaving the combustion chamber
//Now derate to give a more realistic figure as measured downstream
//For now we will aim for a peak of around 400 degC (750 degF)
EGT *= 0.444 + ((0.544 - 0.444) * Percentage_Power / 100.0);
}
// Calculate Manifold Pressure based on Throttle lever Position
float FGNewEngine::Calc_Manifold_Pressure ( float LeverPosn, float MaxMan, float MinMan)
{
float Inches;
//Note that setting the manifold pressure as a function of lever position only is not strictly accurate
//MAP is also a function of engine speed. (and ambient pressure if we are going for an actual MAP model)
Inches = MinMan + (LeverPosn * (MaxMan - MinMan) / 100);
//allow for idle bypass valve or slightly open throttle stop
if(Inches < MinMan)
Inches = MinMan;
//Check for stopped engine (crudest way of compensating for engine speed)
if(RPM == 0)
Inches = 29.92;
return Inches;
}
// Calculate fuel flow in gals/hr
void FGNewEngine::Calc_Fuel_Flow_Gals_Hr()
{
//DCL - calculate mass air flow into engine based on speed and load - separate this out into a function eventually
//t_amb is actual temperature calculated from altitude
//calculate density from ideal gas equation
rho_air = p_amb / ( R_air * T_amb );
rho_air_manifold = rho_air * Manifold_Pressure / 29.6; //This is a bit of a roundabout way of calculating this but it works !! If we put manifold pressure into SI units we could do it simpler.
//calculate ideal engine volume inducted per second
swept_volume = (displacement_SI * (RPM / 60)) / 2; //This equation is only valid for a four stroke engine
//calculate volumetric efficiency - for now we will just use 0.8, but actually it is a function of engine speed and the exhaust to manifold pressure ratio
//Note that this is cylinder vol eff - the throttle drop is already accounted for in the MAP calculation
volumetric_efficiency = 0.8;
//Now use volumetric efficiency to calculate actual air volume inducted per second
v_dot_air = swept_volume * volumetric_efficiency;
//Now calculate mass flow rate of air into engine
m_dot_air = v_dot_air * rho_air_manifold;
//**************
//DCL - now calculate fuel flow into engine based on air flow and mixture lever position
//assume lever runs from no flow at fully out to thi = 1.3 at fully in at sea level
//also assume that the injector linkage is ideal - hence the set mixture is maintained at a given altitude throughout the speed and load range
thi_sea_level = 1.3 * ( Mixture_Lever_Pos / 100.0 );
equivalence_ratio = thi_sea_level * p_amb_sea_level / p_amb; //ie as we go higher the mixture gets richer for a given lever position
m_dot_fuel = m_dot_air / 14.7 * equivalence_ratio;
Fuel_Flow_gals_hr = (m_dot_fuel / rho_fuel) * 264.172 * 3600.0; // Note this assumes US gallons
}
// Calculate the percentage of maximum rated power delivered as a function of Manifold pressure multiplied by engine speed (rpm)
// This is not necessarilly the best approach but seems to work for now.
// May well need tweaking at the bottom end if the prop model is changed.
void FGNewEngine::Calc_Percentage_Power(bool mag_left, bool mag_right)
{
// For a given Manifold Pressure and RPM calculate the % Power
// Multiply Manifold Pressure by RPM
float ManXRPM = True_Manifold_Pressure * RPM;
/*
// Phil's %power correlation
// Calculate % Power
Percentage_Power = (+ 7E-09 * ManXRPM * ManXRPM) + ( + 7E-04 * ManXRPM) - 0.1218;
// cout << Percentage_Power << "%" << "\t";
*/
// DCL %power correlation - basically Phil's correlation modified to give slighty less power at the low end
// might need some adjustment as the prop model is adjusted
// My aim is to match the prop model and engine model at the low end to give the manufacturer's recommended idle speed with the throttle closed - 600rpm for the Continental IO520
// Calculate % Power for Nev's prop model
//Percentage_Power = (+ 6E-09 * ManXRPM * ManXRPM) + ( + 8E-04 * ManXRPM) - 1.8524;
// Calculate %power for DCL prop model
Percentage_Power = (7e-9 * ManXRPM * ManXRPM) + (7e-4 * ManXRPM) - 1.0;
// Power de-rating for altitude has been removed now that we are basing the power
// on the actual manifold pressure, which takes air pressure into account. However - this fails to
// take the temperature into account - this is TODO.
// Adjust power for temperature - this is temporary until the power is done as a function of mass flow rate induced
// Adjust for Temperature - Temperature above Standard decrease
// power by 7/120 % per degree F increase, and incease power for
// temps below at the same ratio
float T_amb_degF = (T_amb * 1.8) - 459.67;
float T_amb_sea_lev_degF = (288 * 1.8) - 459.67;
Percentage_Power = Percentage_Power + ((T_amb_sea_lev_degF - T_amb_degF) * 7 /120);
//DCL - now adjust power to compensate for mixture
Percentage_of_best_power_mixture_power = Power_Mixture_Correlation(equivalence_ratio);
Percentage_Power = Percentage_Power * Percentage_of_best_power_mixture_power / 100.0;
// Now Derate engine for the effects of Bad/Switched off magnetos
//if (Magneto_Left == 0 && Magneto_Right == 0) {
if(!running) {
// cout << "Both OFF\n";
Percentage_Power = 0;
} else if (mag_left && mag_right) {
// cout << "Both On ";
} else if (mag_left == 0 || mag_right== 0) {
// cout << "1 Magneto Failed ";
Percentage_Power = Percentage_Power * ((100.0 - Mag_Derate_Percent)/100.0);
// cout << FGEng1_Percentage_Power << "%" << "\t";
}
/*
//DCL - stall the engine if RPM drops below 450 - this is possible if mixture lever is pulled right out
//This is a kludge that I should eliminate by adding a total fmep estimation.
if(RPM < 450)
Percentage_Power = 0;
*/
if(Percentage_Power < 0)
Percentage_Power = 0;
}
// Calculate Oil Temperature in degrees Kelvin
float FGNewEngine::Calc_Oil_Temp (float oil_temp)
{
float idle_percentage_power = 2.3; // approximately
float target_oil_temp; // Steady state oil temp at the current engine conditions
float time_constant; // The time constant for the differential equation
if(running) {
target_oil_temp = 363;
time_constant = 500; // Time constant for engine-on idling.
if(Percentage_Power > idle_percentage_power) {
time_constant /= ((Percentage_Power / idle_percentage_power) / 10.0); // adjust for power
}
} else {
target_oil_temp = 298;
time_constant = 1000; // Time constant for engine-off; reflects the fact that oil is no longer getting circulated
}
float dOilTempdt = (target_oil_temp - oil_temp) / time_constant;
oil_temp += (dOilTempdt * time_step);
return (oil_temp);
}
// Calculate Oil Pressure
float FGNewEngine::Calc_Oil_Press (float Oil_Temp, float Engine_RPM)
{
float Oil_Pressure = 0; //PSI
float Oil_Press_Relief_Valve = 60; //PSI
float Oil_Press_RPM_Max = 1800;
float Design_Oil_Temp = 85; //Celsius
float Oil_Viscosity_Index = 0.25; // PSI/Deg C
// float Temp_Deviation = 0; // Deg C
Oil_Pressure = (Oil_Press_Relief_Valve / Oil_Press_RPM_Max) * Engine_RPM;
// Pressure relief valve opens at Oil_Press_Relief_Valve PSI setting
if (Oil_Pressure >= Oil_Press_Relief_Valve) {
Oil_Pressure = Oil_Press_Relief_Valve;
}
// Now adjust pressure according to Temp which affects the viscosity
Oil_Pressure += (Design_Oil_Temp - Oil_Temp) * Oil_Viscosity_Index;
return Oil_Pressure;
}
// Propeller calculations.
void FGNewEngine::Do_Prop_Calcs()
{
float Gear_Ratio = 1.0;
float forward_velocity; // m/s
float prop_power_consumed_SI; // Watts
double J; // advance ratio - dimensionless
double Cp_20; // coefficient of power for 20 degree blade angle
double Cp_25; // coefficient of power for 25 degree blade angle
double Cp; // Our actual coefficient of power
double neta_prop_20;
double neta_prop_25;
double neta_prop; // prop efficiency
FGProp1_RPS = RPM * Gear_Ratio / 60.0;
angular_velocity_SI = 2.0 * LS_PI * RPM / 60.0;
forward_velocity = IAS * 0.514444444444; // Convert to m/s
if(FGProp1_RPS == 0)
J = 0;
else
J = forward_velocity / (FGProp1_RPS * prop_diameter);
//cout << "advance_ratio = " << J << '\n';
//Cp correlations based on data from McCormick
Cp_20 = 0.0342*J*J*J*J - 0.1102*J*J*J + 0.0365*J*J - 0.0133*J + 0.064;
Cp_25 = 0.0119*J*J*J*J - 0.0652*J*J*J + 0.018*J*J - 0.0077*J + 0.0921;
//cout << "Cp_20 = " << Cp_20 << '\n';
//cout << "Cp_25 = " << Cp_25 << '\n';
//Assume that the blade angle is between 20 and 25 deg - reasonable for fixed pitch prop but won't hold for variable one !!!
Cp = Cp_20 + ( (Cp_25 - Cp_20) * ((blade_angle - 20)/(25 - 20)) );
//cout << "Cp = " << Cp << '\n';
//cout << "RPM = " << RPM << '\n';
//cout << "angular_velocity_SI = " << angular_velocity_SI << '\n';
prop_power_consumed_SI = Cp * rho_air * pow(FGProp1_RPS,3.0f) * pow(float(prop_diameter),5.0f);
//cout << "prop HP consumed = " << prop_power_consumed_SI / 745.699 << '\n';
if(angular_velocity_SI == 0)
prop_torque = 0;
// However this can give problems - if rpm == 0 but forward velocity increases the prop should be able to generate a torque to start the engine spinning
// Unlikely to happen in practice - but I suppose someone could move the lever of a stopped large piston engine from feathered to windmilling.
// This *does* give problems - if the plane is put into a steep climb whilst windmilling the engine friction will eventually stop it spinning.
// When put back into a dive it never starts re-spinning again. Although it is unlikely that anyone would do this in real life, they might well do it in a sim!!!
else
prop_torque = prop_power_consumed_SI / angular_velocity_SI;
// calculate neta_prop here
neta_prop_20 = 0.1328*J*J*J*J - 1.3073*J*J*J + 0.3525*J*J + 1.5591*J + 0.0007;
neta_prop_25 = -0.3121*J*J*J*J + 0.4234*J*J*J - 0.7686*J*J + 1.5237*J - 0.0004;
neta_prop = neta_prop_20 + ( (neta_prop_25 - neta_prop_20) * ((blade_angle - 20)/(25 - 20)) );
// Check for zero forward velocity to avoid divide by zero
if(forward_velocity < 0.0001)
prop_thrust = 0.0;
// I don't see how this works - how can the plane possibly start from rest ???
// Hmmmm - it works because the forward_velocity at present never drops below about 0.03 even at rest
// We can't really rely on this in the future - needs fixing !!!!
// The problem is that we're doing this calculation backwards - we're working out the thrust from the power consumed and the velocity, which becomes invalid as velocity goes to zero.
// It would be far more natural to work out the power consumed from the thrust - FIXME!!!!!.
else
prop_thrust = neta_prop * prop_power_consumed_SI / forward_velocity; //TODO - rename forward_velocity to IAS_SI
//cout << "prop_thrust = " << prop_thrust << '\n';
}
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