697 lines
30 KiB
C++
697 lines
30 KiB
C++
// IO360.cxx - a piston engine model currently for the IO360 engine fitted to the C172
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// but with the potential to model other naturally aspirated piston engines
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// given appropriate config input.
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//
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// Written by David Luff, started 2000.
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// Based on code by Phil Schubert, started 1999.
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//
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// This program is free software; you can redistribute it and/or
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// modify it under the terms of the GNU General Public License as
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// published by the Free Software Foundation; either version 2 of the
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// License, or (at your option) any later version.
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//
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// This program is distributed in the hope that it will be useful, but
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// WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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// General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License
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// along with this program; if not, write to the Free Software
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// Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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#include <simgear/compiler.h>
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#include <math.h>
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#include STL_FSTREAM
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#include STL_IOSTREAM
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#include <Main/fg_props.hxx>
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#include "IO360.hxx"
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#include "ls_constants.h"
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//*************************************************************************************
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// Initialise the engine model
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void FGNewEngine::init(double dt) {
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// These constants should probably be moved eventually
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CONVERT_CUBIC_INCHES_TO_METERS_CUBED = 1.638706e-5;
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CONVERT_HP_TO_WATTS = 745.6999;
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// Properties of working fluids
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Cp_air = 1005; // J/KgK
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Cp_fuel = 1700; // J/KgK
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calorific_value_fuel = 47.3e6; // W/Kg Note that this is only an approximate value
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rho_fuel = 800; // kg/m^3 - an estimate for now
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R_air = 287.3;
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// environment inputs
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p_amb_sea_level = 101325; // Pascals
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// Control inputs - ARE THESE NEEDED HERE???
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Throttle_Lever_Pos = 75;
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Propeller_Lever_Pos = 75;
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Mixture_Lever_Pos = 100;
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//misc
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IAS = 0;
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time_step = dt;
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// Engine Specific Variables that should be read in from a config file
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MaxHP = 200; //Lycoming IO360 -A-C-D series
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// MaxHP = 180; //Current Lycoming IO360 ?
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// displacement = 520; //Continental IO520-M
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displacement = 360; //Lycoming IO360
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displacement_SI = displacement * CONVERT_CUBIC_INCHES_TO_METERS_CUBED;
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engine_inertia = 0.2; //kgm^2 - value taken from a popular family saloon car engine - need to find an aeroengine value !!!!!
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prop_inertia = 0.05; //kgm^2 - this value is a total guess - dcl
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Max_Fuel_Flow = 130; // Units??? Do we need this variable any more??
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// 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.
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Max_Manifold_Pressure = 28.50; //Inches Hg. An approximation - should be able to find it in the engine performance data
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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
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Max_RPM = 2700;
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Min_RPM = 600; //Recommended idle from Continental data sheet
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Mag_Derate_Percent = 5;
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Gear_Ratio = 1;
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n_R = 2; // Number of crank revolutions per power cycle - 2 for a 4 stroke engine.
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// Various bits of housekeeping describing the engines initial state.
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running = false;
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cranking = false;
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crank_counter = false;
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starter = false;
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// Initialise Engine Variables used by this instance
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if(running)
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RPM = 600;
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else
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RPM = 0;
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Percentage_Power = 0;
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Manifold_Pressure = 29.96; // Inches
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Fuel_Flow_gals_hr = 0;
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// Torque = 0;
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Torque_SI = 0;
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CHT = 298.0; //deg Kelvin
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CHT_degF = (CHT_degF * 1.8) - 459.67; //deg Fahrenheit
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Mixture = 14;
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Oil_Pressure = 0; // PSI
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Oil_Temp = 85; // Deg C
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current_oil_temp = 298.0; //deg Kelvin
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/**** one of these is superfluous !!!!***/
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HP = 0;
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RPS = 0;
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Torque_Imbalance = 0;
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// Initialise Propellor Variables used by this instance
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FGProp1_RPS = 0;
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// Hardcode propellor for now - the following two should be read from config eventually
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prop_diameter = 1.8; // meters
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blade_angle = 23.0; // degrees
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}
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//*****************************************************************************
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// update the engine model based on current control positions
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void FGNewEngine::update() {
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/*
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// Hack for testing - should output every 5 seconds
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static int count1 = 0;
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if(count1 == 0) {
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// cout << "P_atmos = " << p_amb << " T_atmos = " << T_amb << '\n';
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// cout << "Manifold pressure = " << Manifold_Pressure << " True_Manifold_Pressure = " << True_Manifold_Pressure << '\n';
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// cout << "p_amb_sea_level = " << p_amb_sea_level << '\n';
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// cout << "equivalence_ratio = " << equivalence_ratio << '\n';
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// cout << "combustion_efficiency = " << combustion_efficiency << '\n';
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// cout << "AFR = " << 14.7 / equivalence_ratio << '\n';
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// cout << "Mixture lever = " << Mixture_Lever_Pos << '\n';
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// cout << "n = " << RPM << " rpm\n";
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// cout << "T_amb = " << T_amb << '\n';
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// cout << "running = " << running << '\n';
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// cout << "fuel = " << fgGetFloat("/consumables/fuel/tank[0]/level-gal_us") << '\n';
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// cout << "Percentage_Power = " << Percentage_Power << '\n';
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// cout << "current_oil_temp = " << current_oil_temp << '\n';
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// cout << "EGT = " << EGT << '\n';
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}
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count1++;
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if(count1 == 100)
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count1 = 0;
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*/
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// Check parameters that may alter the operating state of the engine.
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// (spark, fuel, starter motor etc)
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// Check for spark
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bool Magneto_Left = false;
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bool Magneto_Right = false;
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// Magneto positions:
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// 0 -> off
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// 1 -> left only
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// 2 -> right only
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// 3 -> both
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if(mag_pos != 0) {
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spark = true;
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} else {
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spark = false;
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} // neglects battery voltage, master on switch, etc for now.
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if((mag_pos == 1) || (mag_pos > 2))
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Magneto_Left = true;
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if(mag_pos > 1)
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Magneto_Right = true;
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// crude check for fuel
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if((fgGetFloat("/consumables/fuel/tank[0]/level-gal_us") > 0) || (fgGetFloat("/consumables/fuel/tank[1]/level-gal_us") > 0)) {
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fuel = true;
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} else {
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fuel = false;
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} // Need to make this better, eg position of fuel selector switch.
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// Check if we are turning the starter motor
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if(cranking != starter) {
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// This check saves .../cranking from getting updated every loop - they only update when changed.
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cranking = starter;
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crank_counter = 0;
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}
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// Note that although /engines/engine[0]/starter and /engines/engine[0]/cranking might appear to be duplication it is
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// not since the starter may be engaged with the battery voltage too low for cranking to occur (or perhaps the master
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// switch just left off) and the sound manager will read .../cranking to determine wether to play a cranking sound.
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// For now though none of that is implemented so cranking can be set equal to .../starter without further checks.
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// int Alternate_Air_Pos =0; // Off = 0. Reduces power by 3 % for same throttle setting
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// DCL - don't know what this Alternate_Air_Pos is - this is a leftover from the Schubert code.
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//Check mode of engine operation
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if(cranking) {
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crank_counter++;
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if(RPM <= 480) {
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RPM += 100;
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if(RPM > 480)
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RPM = 480;
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} else {
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// consider making a horrible noise if the starter is engaged with the engine running
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}
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}
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if((!running) && (spark) && (fuel) && (crank_counter > 120)) {
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// start the engine if revs high enough
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if(RPM > 450) {
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// For now just instantaneously start but later we should maybe crank for a bit
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running = true;
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// RPM = 600;
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}
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}
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if( (running) && ((!spark)||(!fuel)) ) {
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// Cut the engine
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// note that we only cut the power - the engine may continue to spin if the prop is in a moving airstream
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running = false;
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}
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// Now we've ascertained whether the engine is running or not we can start to do the engine calculations 'proper'
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// Calculate Sea Level Manifold Pressure
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Manifold_Pressure = Calc_Manifold_Pressure( Throttle_Lever_Pos, Max_Manifold_Pressure, Min_Manifold_Pressure );
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// cout << "manifold pressure = " << Manifold_Pressure << endl;
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//Then find the actual manifold pressure (the calculated one is the sea level pressure)
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True_Manifold_Pressure = Manifold_Pressure * p_amb / p_amb_sea_level;
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//Do the fuel flow calculations
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Calc_Fuel_Flow_Gals_Hr();
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//Calculate engine power
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Calc_Percentage_Power(Magneto_Left, Magneto_Right);
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HP = Percentage_Power * MaxHP / 100.0;
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Power_SI = HP * CONVERT_HP_TO_WATTS;
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// FMEP calculation. For now we will just use this during motored operation.
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// Eventually we will calculate IMEP and use the FMEP all the time to give BMEP (maybe!)
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if(!running) {
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// This FMEP data is from the Patton paper, assumes fully warm conditions.
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FMEP = 1e-12*pow(RPM,4) - 1e-8*pow(RPM,3) + 5e-5*pow(RPM,2) - 0.0722*RPM + 154.85;
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// Gives FMEP in kPa - now convert to Pa
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FMEP *= 1000;
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} else {
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FMEP = 0.0;
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}
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// Is this total FMEP or friction FMEP ???
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Torque_FMEP = (FMEP * displacement_SI) / (2.0 * LS_PI * n_R);
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// Calculate Engine Torque. Check for div by zero since percentage power correlation does not guarantee zero power at zero rpm.
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// However this is problematical since there is a resistance to movement even at rest
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// Ie this is a dynamics equation not a statics one. This can be solved by going over to MEP based torque calculations.
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if(RPM == 0) {
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Torque_SI = 0 - Torque_FMEP;
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}
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else {
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Torque_SI = ((Power_SI * 60.0) / (2.0 * LS_PI * RPM)) - Torque_FMEP; //Torque = power / angular velocity
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// cout << Torque << " Nm\n";
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}
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//Calculate Exhaust gas temperature
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if(running)
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Calc_EGT();
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else
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EGT = 298.0;
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// Calculate Cylinder Head Temperature
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Calc_CHT();
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// Calculate oil temperature
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current_oil_temp = Calc_Oil_Temp(current_oil_temp);
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// Calculate Oil Pressure
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Oil_Pressure = Calc_Oil_Press( Oil_Temp, RPM );
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// Now do the Propeller Calculations
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Do_Prop_Calcs();
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// Now do the engine - prop torque balance to calculate final RPM
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//Calculate new RPM from torque balance and inertia.
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Torque_Imbalance = Torque_SI - prop_torque; //This gives a +ve value when the engine torque exeeds the prop torque
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// (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)
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angular_acceleration = Torque_Imbalance / (engine_inertia + prop_inertia);
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angular_velocity_SI += (angular_acceleration * time_step);
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// Don't let the engine go into reverse
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if(angular_velocity_SI < 0)
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angular_velocity_SI = 0;
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RPM = (angular_velocity_SI * 60) / (2.0 * LS_PI);
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// And finally a last check on the engine state after doing the torque balance with the prop - have we stalled?
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if(running) {
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//Check if we have stalled the engine
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if (RPM == 0) {
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running = false;
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} else if((RPM <= 480) && (cranking)) {
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//Make sure the engine noise dosn't play if the engine won't start due to eg mixture lever pulled out.
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running = false;
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EGT = 298.0;
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}
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}
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// And finally, do any unit conversions from internal units to output units
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EGT_degF = (EGT * 1.8) - 459.67;
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CHT_degF = (CHT * 1.8) - 459.67;
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}
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//*****************************************************************************************************
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// FGNewEngine member functions
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////////////////////////////////////////////////////////////////////////////////////////////
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// Return the combustion efficiency as a function of equivalence ratio
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//
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// Combustion efficiency values from Heywood,
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// "Internal Combustion Engine Fundamentals", ISBN 0-07-100499-8
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////////////////////////////////////////////////////////////////////////////////////////////
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float FGNewEngine::Lookup_Combustion_Efficiency(float thi_actual)
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{
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const int NUM_ELEMENTS = 11;
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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
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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
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float neta_comb_actual = 0.0f;
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float factor;
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int i;
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int j = NUM_ELEMENTS; //This must be equal to the number of elements in the lookup table arrays
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for(i=0;i<j;i++)
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{
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if(i == (j-1)) {
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// Assume linear extrapolation of the slope between the last two points beyond the last point
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float dydx = (neta_comb[i] - neta_comb[i-1]) / (thi[i] - thi[i-1]);
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neta_comb_actual = neta_comb[i] + dydx * (thi_actual - thi[i]);
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return neta_comb_actual;
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}
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if(thi_actual == thi[i]) {
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neta_comb_actual = neta_comb[i];
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return neta_comb_actual;
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}
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if((thi_actual > thi[i]) && (thi_actual < thi[i + 1])) {
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//do linear interpolation between the two points
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factor = (thi_actual - thi[i]) / (thi[i+1] - thi[i]);
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neta_comb_actual = (factor * (neta_comb[i+1] - neta_comb[i])) + neta_comb[i];
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return neta_comb_actual;
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}
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}
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//if we get here something has gone badly wrong
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SG_LOG(SG_AIRCRAFT, SG_ALERT, "error in FGNewEngine::Lookup_Combustion_Efficiency");
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return neta_comb_actual;
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}
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////////////////////////////////////////////////////////////////////////////////////////////
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// Return the percentage of best mixture power available at a given mixture strength
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//
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// Based on data from "Technical Considerations for Catalysts for the European Market"
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// by H S Gandi, published 1988 by IMechE
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//
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// Note that currently no attempt is made to set a lean limit on stable combustion
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////////////////////////////////////////////////////////////////////////////////////////////
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float FGNewEngine::Power_Mixture_Correlation(float thi_actual)
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{
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float AFR_actual = 14.7 / thi_actual;
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// thi and thi_actual are equivalence ratio
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const int NUM_ELEMENTS = 13;
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// 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.
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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
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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
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float mixPerPow_actual = 0.0f;
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float factor;
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float dydx;
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int i;
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int j = NUM_ELEMENTS; //This must be equal to the number of elements in the lookup table arrays
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for(i=0;i<j;i++)
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{
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if(i == (j-1)) {
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// Assume linear extrapolation of the slope between the last two points beyond the last point
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dydx = (mixPerPow[i] - mixPerPow[i-1]) / (AFR[i] - AFR[i-1]);
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mixPerPow_actual = mixPerPow[i] + dydx * (AFR_actual - AFR[i]);
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return mixPerPow_actual;
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}
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if((i == 0) && (AFR_actual < AFR[i])) {
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// Assume linear extrapolation of the slope between the first two points for points before the first point
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dydx = (mixPerPow[i] - mixPerPow[i-1]) / (AFR[i] - AFR[i-1]);
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mixPerPow_actual = mixPerPow[i] + dydx * (AFR_actual - AFR[i]);
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return mixPerPow_actual;
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}
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if(AFR_actual == AFR[i]) {
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mixPerPow_actual = mixPerPow[i];
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return mixPerPow_actual;
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}
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if((AFR_actual > AFR[i]) && (AFR_actual < AFR[i + 1])) {
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//do linear interpolation between the two points
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factor = (AFR_actual - AFR[i]) / (AFR[i+1] - AFR[i]);
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mixPerPow_actual = (factor * (mixPerPow[i+1] - mixPerPow[i])) + mixPerPow[i];
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return mixPerPow_actual;
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}
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}
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//if we get here something has gone badly wrong
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SG_LOG(SG_AIRCRAFT, SG_ALERT, "error in FGNewEngine::Power_Mixture_Correlation");
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return mixPerPow_actual;
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}
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// Calculate Cylinder Head Temperature
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// Crudely models the cylinder head as an arbitary lump of arbitary size and area with one third of combustion energy
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// as heat input and heat output as a function of airspeed and temperature. Could be improved!!!
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void FGNewEngine::Calc_CHT()
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{
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float h1 = -95.0; //co-efficient for free convection
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float h2 = -3.95; //co-efficient for forced convection
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float h3 = -0.05; //co-efficient for forced convection due to prop backwash
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float v_apparent; //air velocity over cylinder head in m/s
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float v_dot_cooling_air;
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float m_dot_cooling_air;
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float temperature_difference;
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float arbitary_area = 1.0;
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float dqdt_from_combustion;
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float dqdt_forced; //Rate of energy transfer to/from cylinder head due to forced convection (Joules) (sign convention: to cylinder head is +ve)
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float dqdt_free; //Rate of energy transfer to/from cylinder head due to free convection (Joules) (sign convention: to cylinder head is +ve)
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float dqdt_cylinder_head; //Overall energy change in cylinder head
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float CpCylinderHead = 800.0; //FIXME - this is a guess - I need to look up the correct value
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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
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float HeatCapacityCylinderHead;
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float dCHTdt;
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// The above values are hardwired to give reasonable results for an IO360 (C172 engine)
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// Now adjust to try to compensate for arbitary sized engines - this is currently untested
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arbitary_area *= (MaxHP / 180.0);
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MassCylinderHead *= (MaxHP / 180.0);
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temperature_difference = CHT - T_amb;
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v_apparent = IAS * 0.5144444; //convert from knots to m/s
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v_dot_cooling_air = arbitary_area * v_apparent;
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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';
|
|
}
|