1 // IO360.cxx - a piston engine model currently for the IO360 engine fitted to the C172
2 // but with the potential to model other naturally aspirated piston engines
3 // given appropriate config input.
5 // Written by David Luff, started 2000.
6 // Based on code by Phil Schubert, started 1999.
8 // This program is free software; you can redistribute it and/or
9 // modify it under the terms of the GNU General Public License as
10 // published by the Free Software Foundation; either version 2 of the
11 // License, or (at your option) any later version.
13 // This program is distributed in the hope that it will be useful, but
14 // WITHOUT ANY WARRANTY; without even the implied warranty of
15 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
16 // General Public License for more details.
18 // You should have received a copy of the GNU General Public License
19 // along with this program; if not, write to the Free Software
20 // Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
26 #include <simgear/compiler.h>
33 #include <Main/fg_props.hxx>
36 #include "ls_constants.h"
39 //*************************************************************************************
40 // Initialise the engine model
41 void FGNewEngine::init(double dt) {
43 // These constants should probably be moved eventually
44 CONVERT_CUBIC_INCHES_TO_METERS_CUBED = 1.638706e-5;
45 CONVERT_HP_TO_WATTS = 745.6999;
47 // Properties of working fluids
48 Cp_air = 1005; // J/KgK
49 Cp_fuel = 1700; // J/KgK
50 calorific_value_fuel = 47.3e6; // W/Kg Note that this is only an approximate value
51 rho_fuel = 800; // kg/m^3 - an estimate for now
55 p_amb_sea_level = 101325; // Pascals
57 // Control inputs - ARE THESE NEEDED HERE???
58 Throttle_Lever_Pos = 75;
59 Propeller_Lever_Pos = 75;
60 Mixture_Lever_Pos = 100;
66 // Engine Specific Variables that should be read in from a config file
67 MaxHP = 200; //Lycoming IO360 -A-C-D series
68 // MaxHP = 180; //Current Lycoming IO360 ?
69 // displacement = 520; //Continental IO520-M
70 displacement = 360; //Lycoming IO360
71 displacement_SI = displacement * CONVERT_CUBIC_INCHES_TO_METERS_CUBED;
72 engine_inertia = 0.2; //kgm^2 - value taken from a popular family saloon car engine - need to find an aeroengine value !!!!!
73 prop_inertia = 0.05; //kgm^2 - this value is a total guess - dcl
74 Max_Fuel_Flow = 130; // Units??? Do we need this variable any more??
76 // 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.
77 Max_Manifold_Pressure = 28.50; //Inches Hg. An approximation - should be able to find it in the engine performance data
78 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
80 Min_RPM = 600; //Recommended idle from Continental data sheet
81 Mag_Derate_Percent = 5;
83 n_R = 2; // Number of crank revolutions per power cycle - 2 for a 4 stroke engine.
85 // Various bits of housekeeping describing the engines initial state.
88 crank_counter = false;
91 // Initialise Engine Variables used by this instance
97 Manifold_Pressure = 29.96; // Inches
98 Fuel_Flow_gals_hr = 0;
101 CHT = 298.0; //deg Kelvin
102 CHT_degF = (CHT * 1.8) - 459.67; //deg Fahrenheit
104 Oil_Pressure = 0; // PSI
105 Oil_Temp = 85; // Deg C
106 current_oil_temp = 298.0; //deg Kelvin
107 /**** one of these is superfluous !!!!***/
110 Torque_Imbalance = 0;
112 // Initialise Propellor Variables used by this instance
114 // Hardcode propellor for now - the following two should be read from config eventually
115 prop_diameter = 1.8; // meters
116 blade_angle = 23.0; // degrees
119 //*****************************************************************************
120 // update the engine model based on current control positions
121 void FGNewEngine::update() {
124 // Hack for testing - should output every 5 seconds
125 static int count1 = 0;
127 // cout << "P_atmos = " << p_amb << " T_atmos = " << T_amb << '\n';
128 // cout << "Manifold pressure = " << Manifold_Pressure << " True_Manifold_Pressure = " << True_Manifold_Pressure << '\n';
129 // cout << "p_amb_sea_level = " << p_amb_sea_level << '\n';
130 // cout << "equivalence_ratio = " << equivalence_ratio << '\n';
131 // cout << "combustion_efficiency = " << combustion_efficiency << '\n';
132 // cout << "AFR = " << 14.7 / equivalence_ratio << '\n';
133 // cout << "Mixture lever = " << Mixture_Lever_Pos << '\n';
134 // cout << "n = " << RPM << " rpm\n";
135 // cout << "T_amb = " << T_amb << '\n';
136 // cout << "running = " << running << '\n';
137 // cout << "fuel = " << fgGetFloat("/consumables/fuel/tank[0]/level-gal_us") << '\n';
138 // cout << "Percentage_Power = " << Percentage_Power << '\n';
139 // cout << "current_oil_temp = " << current_oil_temp << '\n';
140 // cout << "EGT = " << EGT << '\n';
147 // Check parameters that may alter the operating state of the engine.
148 // (spark, fuel, starter motor etc)
151 bool Magneto_Left = false;
152 bool Magneto_Right = false;
153 // Magneto positions:
162 } // neglects battery voltage, master on switch, etc for now.
163 if((mag_pos == 1) || (mag_pos > 2))
166 Magneto_Right = true;
168 // crude check for fuel
169 if((fgGetFloat("/consumables/fuel/tank[0]/level-gal_us") > 0) || (fgGetFloat("/consumables/fuel/tank[1]/level-gal_us") > 0)) {
173 } // Need to make this better, eg position of fuel selector switch.
175 // Check if we are turning the starter motor
176 if(cranking != starter) {
177 // This check saves .../cranking from getting updated every loop - they only update when changed.
181 // Note that although /engines/engine[0]/starter and /engines/engine[0]/cranking might appear to be duplication it is
182 // not since the starter may be engaged with the battery voltage too low for cranking to occur (or perhaps the master
183 // switch just left off) and the sound manager will read .../cranking to determine wether to play a cranking sound.
184 // For now though none of that is implemented so cranking can be set equal to .../starter without further checks.
186 // int Alternate_Air_Pos =0; // Off = 0. Reduces power by 3 % for same throttle setting
187 // DCL - don't know what this Alternate_Air_Pos is - this is a leftover from the Schubert code.
189 //Check mode of engine operation
197 // consider making a horrible noise if the starter is engaged with the engine running
200 if((!running) && (spark) && (fuel) && (crank_counter > 120)) {
201 // start the engine if revs high enough
203 // For now just instantaneously start but later we should maybe crank for a bit
208 if( (running) && ((!spark)||(!fuel)) ) {
210 // note that we only cut the power - the engine may continue to spin if the prop is in a moving airstream
214 // Now we've ascertained whether the engine is running or not we can start to do the engine calculations 'proper'
216 // Calculate Sea Level Manifold Pressure
217 Manifold_Pressure = Calc_Manifold_Pressure( Throttle_Lever_Pos, Max_Manifold_Pressure, Min_Manifold_Pressure );
218 // cout << "manifold pressure = " << Manifold_Pressure << endl;
220 //Then find the actual manifold pressure (the calculated one is the sea level pressure)
221 True_Manifold_Pressure = Manifold_Pressure * p_amb / p_amb_sea_level;
223 //Do the fuel flow calculations
224 Calc_Fuel_Flow_Gals_Hr();
226 //Calculate engine power
227 Calc_Percentage_Power(Magneto_Left, Magneto_Right);
228 HP = Percentage_Power * MaxHP / 100.0;
229 Power_SI = HP * CONVERT_HP_TO_WATTS;
231 // FMEP calculation. For now we will just use this during motored operation.
232 // Eventually we will calculate IMEP and use the FMEP all the time to give BMEP (maybe!)
234 // This FMEP data is from the Patton paper, assumes fully warm conditions.
235 FMEP = 1e-12*pow(RPM,4) - 1e-8*pow(RPM,3) + 5e-5*pow(RPM,2) - 0.0722*RPM + 154.85;
236 // Gives FMEP in kPa - now convert to Pa
241 // Is this total FMEP or friction FMEP ???
243 Torque_FMEP = (FMEP * displacement_SI) / (2.0 * LS_PI * n_R);
245 // Calculate Engine Torque. Check for div by zero since percentage power correlation does not guarantee zero power at zero rpm.
246 // However this is problematical since there is a resistance to movement even at rest
247 // Ie this is a dynamics equation not a statics one. This can be solved by going over to MEP based torque calculations.
249 Torque_SI = 0 - Torque_FMEP;
252 Torque_SI = ((Power_SI * 60.0) / (2.0 * LS_PI * RPM)) - Torque_FMEP; //Torque = power / angular velocity
253 // cout << Torque << " Nm\n";
256 //Calculate Exhaust gas temperature
262 // Calculate Cylinder Head Temperature
265 // Calculate oil temperature
266 current_oil_temp = Calc_Oil_Temp(current_oil_temp);
268 // Calculate Oil Pressure
269 Oil_Pressure = Calc_Oil_Press( Oil_Temp, RPM );
271 // Now do the Propeller Calculations
274 // Now do the engine - prop torque balance to calculate final RPM
276 //Calculate new RPM from torque balance and inertia.
277 Torque_Imbalance = Torque_SI - prop_torque; //This gives a +ve value when the engine torque exeeds the prop torque
278 // (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)
280 angular_acceleration = Torque_Imbalance / (engine_inertia + prop_inertia);
281 angular_velocity_SI += (angular_acceleration * time_step);
282 // Don't let the engine go into reverse
283 if(angular_velocity_SI < 0)
284 angular_velocity_SI = 0;
285 RPM = (angular_velocity_SI * 60) / (2.0 * LS_PI);
287 // And finally a last check on the engine state after doing the torque balance with the prop - have we stalled?
289 //Check if we have stalled the engine
292 } else if((RPM <= 480) && (cranking)) {
293 //Make sure the engine noise dosn't play if the engine won't start due to eg mixture lever pulled out.
299 // And finally, do any unit conversions from internal units to output units
300 EGT_degF = (EGT * 1.8) - 459.67;
301 CHT_degF = (CHT * 1.8) - 459.67;
304 //*****************************************************************************************************
306 // FGNewEngine member functions
308 ////////////////////////////////////////////////////////////////////////////////////////////
309 // Return the combustion efficiency as a function of equivalence ratio
311 // Combustion efficiency values from Heywood,
312 // "Internal Combustion Engine Fundamentals", ISBN 0-07-100499-8
313 ////////////////////////////////////////////////////////////////////////////////////////////
314 float FGNewEngine::Lookup_Combustion_Efficiency(float thi_actual)
316 const int NUM_ELEMENTS = 11;
317 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
318 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
319 float neta_comb_actual = 0.0f;
323 int j = NUM_ELEMENTS; //This must be equal to the number of elements in the lookup table arrays
328 // Assume linear extrapolation of the slope between the last two points beyond the last point
329 float dydx = (neta_comb[i] - neta_comb[i-1]) / (thi[i] - thi[i-1]);
330 neta_comb_actual = neta_comb[i] + dydx * (thi_actual - thi[i]);
331 return neta_comb_actual;
333 if(thi_actual == thi[i]) {
334 neta_comb_actual = neta_comb[i];
335 return neta_comb_actual;
337 if((thi_actual > thi[i]) && (thi_actual < thi[i + 1])) {
338 //do linear interpolation between the two points
339 factor = (thi_actual - thi[i]) / (thi[i+1] - thi[i]);
340 neta_comb_actual = (factor * (neta_comb[i+1] - neta_comb[i])) + neta_comb[i];
341 return neta_comb_actual;
345 //if we get here something has gone badly wrong
346 SG_LOG(SG_AIRCRAFT, SG_ALERT, "error in FGNewEngine::Lookup_Combustion_Efficiency");
347 return neta_comb_actual;
350 ////////////////////////////////////////////////////////////////////////////////////////////
351 // Return the percentage of best mixture power available at a given mixture strength
353 // Based on data from "Technical Considerations for Catalysts for the European Market"
354 // by H S Gandi, published 1988 by IMechE
356 // Note that currently no attempt is made to set a lean limit on stable combustion
357 ////////////////////////////////////////////////////////////////////////////////////////////
358 float FGNewEngine::Power_Mixture_Correlation(float thi_actual)
360 float AFR_actual = 14.7 / thi_actual;
361 // thi and thi_actual are equivalence ratio
362 const int NUM_ELEMENTS = 13;
363 // 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.
364 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
365 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
366 float mixPerPow_actual = 0.0f;
371 int j = NUM_ELEMENTS; //This must be equal to the number of elements in the lookup table arrays
376 // Assume linear extrapolation of the slope between the last two points beyond the last point
377 dydx = (mixPerPow[i] - mixPerPow[i-1]) / (AFR[i] - AFR[i-1]);
378 mixPerPow_actual = mixPerPow[i] + dydx * (AFR_actual - AFR[i]);
379 return mixPerPow_actual;
381 if((i == 0) && (AFR_actual < AFR[i])) {
382 // Assume linear extrapolation of the slope between the first two points for points before the first point
383 dydx = (mixPerPow[1] - mixPerPow[0]) / (AFR[1] - AFR[0]);
384 mixPerPow_actual = mixPerPow[0] + dydx * (AFR_actual - AFR[0]);
385 return mixPerPow_actual;
387 if(AFR_actual == AFR[i]) {
388 mixPerPow_actual = mixPerPow[i];
389 return mixPerPow_actual;
391 if((AFR_actual > AFR[i]) && (AFR_actual < AFR[i + 1])) {
392 //do linear interpolation between the two points
393 factor = (AFR_actual - AFR[i]) / (AFR[i+1] - AFR[i]);
394 mixPerPow_actual = (factor * (mixPerPow[i+1] - mixPerPow[i])) + mixPerPow[i];
395 return mixPerPow_actual;
399 //if we get here something has gone badly wrong
400 SG_LOG(SG_AIRCRAFT, SG_ALERT, "error in FGNewEngine::Power_Mixture_Correlation");
401 return mixPerPow_actual;
404 // Calculate Cylinder Head Temperature
405 // Crudely models the cylinder head as an arbitary lump of arbitary size and area with one third of combustion energy
406 // as heat input and heat output as a function of airspeed and temperature. Could be improved!!!
407 void FGNewEngine::Calc_CHT()
409 float h1 = -95.0; //co-efficient for free convection
410 float h2 = -3.95; //co-efficient for forced convection
411 float h3 = -0.05; //co-efficient for forced convection due to prop backwash
412 float v_apparent; //air velocity over cylinder head in m/s
413 float v_dot_cooling_air;
414 float m_dot_cooling_air;
415 float temperature_difference;
416 float arbitary_area = 1.0;
417 float dqdt_from_combustion;
418 float dqdt_forced; //Rate of energy transfer to/from cylinder head due to forced convection (Joules) (sign convention: to cylinder head is +ve)
419 float dqdt_free; //Rate of energy transfer to/from cylinder head due to free convection (Joules) (sign convention: to cylinder head is +ve)
420 float dqdt_cylinder_head; //Overall energy change in cylinder head
421 float CpCylinderHead = 800.0; //FIXME - this is a guess - I need to look up the correct value
422 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
423 float HeatCapacityCylinderHead;
426 // The above values are hardwired to give reasonable results for an IO360 (C172 engine)
427 // Now adjust to try to compensate for arbitary sized engines - this is currently untested
428 arbitary_area *= (MaxHP / 180.0);
429 MassCylinderHead *= (MaxHP / 180.0);
431 temperature_difference = CHT - T_amb;
433 v_apparent = IAS * 0.5144444; //convert from knots to m/s
434 v_dot_cooling_air = arbitary_area * v_apparent;
435 m_dot_cooling_air = v_dot_cooling_air * rho_air;
437 //Calculate rate of energy transfer to cylinder head from combustion
438 dqdt_from_combustion = m_dot_fuel * calorific_value_fuel * combustion_efficiency * 0.33;
440 //Calculate rate of energy transfer from cylinder head due to cooling NOTE is calculated as rate to but negative
441 dqdt_forced = (h2 * m_dot_cooling_air * temperature_difference) + (h3 * RPM * temperature_difference);
442 dqdt_free = h1 * temperature_difference;
444 //Calculate net rate of energy transfer to or from cylinder head
445 dqdt_cylinder_head = dqdt_from_combustion + dqdt_forced + dqdt_free;
447 HeatCapacityCylinderHead = CpCylinderHead * MassCylinderHead;
449 dCHTdt = dqdt_cylinder_head / HeatCapacityCylinderHead;
451 CHT += (dCHTdt * time_step);
454 // Calculate exhaust gas temperature
455 void FGNewEngine::Calc_EGT()
457 combustion_efficiency = Lookup_Combustion_Efficiency(equivalence_ratio); //The combustion efficiency basically tells us what proportion of the fuels calorific value is released
459 //now calculate energy release to exhaust
460 //We will assume a three way split of fuel energy between useful work, the coolant system and the exhaust system
461 //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
462 //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.
463 enthalpy_exhaust = m_dot_fuel * calorific_value_fuel * combustion_efficiency * 0.33;
464 heat_capacity_exhaust = (Cp_air * m_dot_air) + (Cp_fuel * m_dot_fuel);
465 delta_T_exhaust = enthalpy_exhaust / heat_capacity_exhaust;
467 EGT = T_amb + delta_T_exhaust;
469 //The above gives the exhaust temperature immediately prior to leaving the combustion chamber
470 //Now derate to give a more realistic figure as measured downstream
471 //For now we will aim for a peak of around 400 degC (750 degF)
473 EGT *= 0.444 + ((0.544 - 0.444) * Percentage_Power / 100.0);
476 // Calculate Manifold Pressure based on Throttle lever Position
477 float FGNewEngine::Calc_Manifold_Pressure ( float LeverPosn, float MaxMan, float MinMan)
481 //Note that setting the manifold pressure as a function of lever position only is not strictly accurate
482 //MAP is also a function of engine speed. (and ambient pressure if we are going for an actual MAP model)
483 Inches = MinMan + (LeverPosn * (MaxMan - MinMan) / 100);
485 //allow for idle bypass valve or slightly open throttle stop
489 //Check for stopped engine (crudest way of compensating for engine speed)
496 // Calculate fuel flow in gals/hr
497 void FGNewEngine::Calc_Fuel_Flow_Gals_Hr()
499 //DCL - calculate mass air flow into engine based on speed and load - separate this out into a function eventually
500 //t_amb is actual temperature calculated from altitude
501 //calculate density from ideal gas equation
502 rho_air = p_amb / ( R_air * T_amb );
503 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.
504 //calculate ideal engine volume inducted per second
505 swept_volume = (displacement_SI * (RPM / 60)) / 2; //This equation is only valid for a four stroke engine
506 //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
507 //Note that this is cylinder vol eff - the throttle drop is already accounted for in the MAP calculation
508 volumetric_efficiency = 0.8;
509 //Now use volumetric efficiency to calculate actual air volume inducted per second
510 v_dot_air = swept_volume * volumetric_efficiency;
511 //Now calculate mass flow rate of air into engine
512 m_dot_air = v_dot_air * rho_air_manifold;
516 //DCL - now calculate fuel flow into engine based on air flow and mixture lever position
517 //assume lever runs from no flow at fully out to thi = 1.3 at fully in at sea level
518 //also assume that the injector linkage is ideal - hence the set mixture is maintained at a given altitude throughout the speed and load range
519 thi_sea_level = 1.3 * ( Mixture_Lever_Pos / 100.0 );
520 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
521 m_dot_fuel = m_dot_air / 14.7 * equivalence_ratio;
522 Fuel_Flow_gals_hr = (m_dot_fuel / rho_fuel) * 264.172 * 3600.0; // Note this assumes US gallons
525 // Calculate the percentage of maximum rated power delivered as a function of Manifold pressure multiplied by engine speed (rpm)
526 // This is not necessarilly the best approach but seems to work for now.
527 // May well need tweaking at the bottom end if the prop model is changed.
528 void FGNewEngine::Calc_Percentage_Power(bool mag_left, bool mag_right)
530 // For a given Manifold Pressure and RPM calculate the % Power
531 // Multiply Manifold Pressure by RPM
532 float ManXRPM = True_Manifold_Pressure * RPM;
535 // Phil's %power correlation
537 Percentage_Power = (+ 7E-09 * ManXRPM * ManXRPM) + ( + 7E-04 * ManXRPM) - 0.1218;
538 // cout << Percentage_Power << "%" << "\t";
541 // DCL %power correlation - basically Phil's correlation modified to give slighty less power at the low end
542 // might need some adjustment as the prop model is adjusted
543 // 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
544 // Calculate % Power for Nev's prop model
545 //Percentage_Power = (+ 6E-09 * ManXRPM * ManXRPM) + ( + 8E-04 * ManXRPM) - 1.8524;
547 // Calculate %power for DCL prop model
548 Percentage_Power = (7e-9 * ManXRPM * ManXRPM) + (7e-4 * ManXRPM) - 1.0;
550 // Power de-rating for altitude has been removed now that we are basing the power
551 // on the actual manifold pressure, which takes air pressure into account. However - this fails to
552 // take the temperature into account - this is TODO.
554 // Adjust power for temperature - this is temporary until the power is done as a function of mass flow rate induced
555 // Adjust for Temperature - Temperature above Standard decrease
556 // power by 7/120 % per degree F increase, and incease power for
557 // temps below at the same ratio
558 float T_amb_degF = (T_amb * 1.8) - 459.67;
559 float T_amb_sea_lev_degF = (288 * 1.8) - 459.67;
560 Percentage_Power = Percentage_Power + ((T_amb_sea_lev_degF - T_amb_degF) * 7 /120);
562 //DCL - now adjust power to compensate for mixture
563 Percentage_of_best_power_mixture_power = Power_Mixture_Correlation(equivalence_ratio);
564 Percentage_Power = Percentage_Power * Percentage_of_best_power_mixture_power / 100.0;
566 // Now Derate engine for the effects of Bad/Switched off magnetos
567 //if (Magneto_Left == 0 && Magneto_Right == 0) {
569 // cout << "Both OFF\n";
570 Percentage_Power = 0;
571 } else if (mag_left && mag_right) {
572 // cout << "Both On ";
573 } else if (mag_left == 0 || mag_right== 0) {
574 // cout << "1 Magneto Failed ";
575 Percentage_Power = Percentage_Power * ((100.0 - Mag_Derate_Percent)/100.0);
576 // cout << FGEng1_Percentage_Power << "%" << "\t";
579 //DCL - stall the engine if RPM drops below 450 - this is possible if mixture lever is pulled right out
580 //This is a kludge that I should eliminate by adding a total fmep estimation.
582 Percentage_Power = 0;
584 if(Percentage_Power < 0)
585 Percentage_Power = 0;
588 // Calculate Oil Temperature in degrees Kelvin
589 float FGNewEngine::Calc_Oil_Temp (float oil_temp)
591 float idle_percentage_power = 2.3; // approximately
592 float target_oil_temp; // Steady state oil temp at the current engine conditions
593 float time_constant; // The time constant for the differential equation
595 target_oil_temp = 363;
596 time_constant = 500; // Time constant for engine-on idling.
597 if(Percentage_Power > idle_percentage_power) {
598 time_constant /= ((Percentage_Power / idle_percentage_power) / 10.0); // adjust for power
601 target_oil_temp = 298;
602 time_constant = 1000; // Time constant for engine-off; reflects the fact that oil is no longer getting circulated
605 float dOilTempdt = (target_oil_temp - oil_temp) / time_constant;
607 oil_temp += (dOilTempdt * time_step);
612 // Calculate Oil Pressure
613 float FGNewEngine::Calc_Oil_Press (float Oil_Temp, float Engine_RPM)
615 float Oil_Pressure = 0; //PSI
616 float Oil_Press_Relief_Valve = 60; //PSI
617 float Oil_Press_RPM_Max = 1800;
618 float Design_Oil_Temp = 85; //Celsius
619 float Oil_Viscosity_Index = 0.25; // PSI/Deg C
620 // float Temp_Deviation = 0; // Deg C
622 Oil_Pressure = (Oil_Press_Relief_Valve / Oil_Press_RPM_Max) * Engine_RPM;
624 // Pressure relief valve opens at Oil_Press_Relief_Valve PSI setting
625 if (Oil_Pressure >= Oil_Press_Relief_Valve) {
626 Oil_Pressure = Oil_Press_Relief_Valve;
629 // Now adjust pressure according to Temp which affects the viscosity
631 Oil_Pressure += (Design_Oil_Temp - Oil_Temp) * Oil_Viscosity_Index;
637 // Propeller calculations.
638 void FGNewEngine::Do_Prop_Calcs()
640 float Gear_Ratio = 1.0;
641 float forward_velocity; // m/s
642 float prop_power_consumed_SI; // Watts
643 double J; // advance ratio - dimensionless
644 double Cp_20; // coefficient of power for 20 degree blade angle
645 double Cp_25; // coefficient of power for 25 degree blade angle
646 double Cp; // Our actual coefficient of power
649 double neta_prop; // prop efficiency
651 FGProp1_RPS = RPM * Gear_Ratio / 60.0;
652 angular_velocity_SI = 2.0 * LS_PI * RPM / 60.0;
653 forward_velocity = IAS * 0.514444444444; // Convert to m/s
658 J = forward_velocity / (FGProp1_RPS * prop_diameter);
659 //cout << "advance_ratio = " << J << '\n';
661 //Cp correlations based on data from McCormick
662 Cp_20 = 0.0342*J*J*J*J - 0.1102*J*J*J + 0.0365*J*J - 0.0133*J + 0.064;
663 Cp_25 = 0.0119*J*J*J*J - 0.0652*J*J*J + 0.018*J*J - 0.0077*J + 0.0921;
665 //cout << "Cp_20 = " << Cp_20 << '\n';
666 //cout << "Cp_25 = " << Cp_25 << '\n';
668 //Assume that the blade angle is between 20 and 25 deg - reasonable for fixed pitch prop but won't hold for variable one !!!
669 Cp = Cp_20 + ( (Cp_25 - Cp_20) * ((blade_angle - 20)/(25 - 20)) );
670 //cout << "Cp = " << Cp << '\n';
671 //cout << "RPM = " << RPM << '\n';
672 //cout << "angular_velocity_SI = " << angular_velocity_SI << '\n';
674 prop_power_consumed_SI = Cp * rho_air * pow(FGProp1_RPS,3.0f) * pow(float(prop_diameter),5.0f);
675 //cout << "prop HP consumed = " << prop_power_consumed_SI / 745.699 << '\n';
676 if(angular_velocity_SI == 0)
678 // 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
679 // Unlikely to happen in practice - but I suppose someone could move the lever of a stopped large piston engine from feathered to windmilling.
680 // This *does* give problems - if the plane is put into a steep climb whilst windmilling the engine friction will eventually stop it spinning.
681 // 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!!!
683 prop_torque = prop_power_consumed_SI / angular_velocity_SI;
685 // calculate neta_prop here
686 neta_prop_20 = 0.1328*J*J*J*J - 1.3073*J*J*J + 0.3525*J*J + 1.5591*J + 0.0007;
687 neta_prop_25 = -0.3121*J*J*J*J + 0.4234*J*J*J - 0.7686*J*J + 1.5237*J - 0.0004;
688 neta_prop = neta_prop_20 + ( (neta_prop_25 - neta_prop_20) * ((blade_angle - 20)/(25 - 20)) );
690 // Check for zero forward velocity to avoid divide by zero
691 if(forward_velocity < 0.0001)
693 // I don't see how this works - how can the plane possibly start from rest ???
694 // Hmmmm - it works because the forward_velocity at present never drops below about 0.03 even at rest
695 // We can't really rely on this in the future - needs fixing !!!!
696 // 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.
697 // It would be far more natural to work out the power consumed from the thrust - FIXME!!!!!.
699 prop_thrust = neta_prop * prop_power_consumed_SI / forward_velocity; //TODO - rename forward_velocity to IAS_SI
700 //cout << "prop_thrust = " << prop_thrust << '\n';