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., 675 Mass Ave, Cambridge, MA 02139, USA.
22 #include <simgear/compiler.h>
32 #include "LaRCsim/ls_constants.h"
34 #include <Main/fg_props.hxx>
36 //*************************************************************************************
37 // Initialise the engine model
38 void FGNewEngine::init(double dt) {
40 // These constants should probably be moved eventually
41 CONVERT_CUBIC_INCHES_TO_METERS_CUBED = 1.638706e-5;
42 CONVERT_HP_TO_WATTS = 745.6999;
44 // Properties of working fluids
45 Cp_air = 1005; // J/KgK
46 Cp_fuel = 1700; // J/KgK
47 calorific_value_fuel = 47.3e6; // W/Kg Note that this is only an approximate value
48 rho_fuel = 800; // kg/m^3 - an estimate for now
52 p_amb_sea_level = 101325; // Pascals
54 // Control inputs - ARE THESE NEEDED HERE???
55 Throttle_Lever_Pos = 75;
56 Propeller_Lever_Pos = 75;
57 Mixture_Lever_Pos = 100;
63 // Engine Specific Variables that should be read in from a config file
64 MaxHP = 200; //Lycoming IO360 -A-C-D series
65 // MaxHP = 180; //Current Lycoming IO360 ?
66 // displacement = 520; //Continental IO520-M
67 displacement = 360; //Lycoming IO360
68 displacement_SI = displacement * CONVERT_CUBIC_INCHES_TO_METERS_CUBED;
69 engine_inertia = 0.2; //kgm^2 - value taken from a popular family saloon car engine - need to find an aeroengine value !!!!!
70 prop_inertia = 0.05; //kgm^2 - this value is a total guess - dcl
71 Max_Fuel_Flow = 130; // Units??? Do we need this variable any more??
73 // 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.
74 Max_Manifold_Pressure = 28.50; //Inches Hg. An approximation - should be able to find it in the engine performance data
75 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
77 Min_RPM = 600; //Recommended idle from Continental data sheet
78 Mag_Derate_Percent = 5;
80 n_R = 2; // Number of crank revolutions per power cycle - 2 for a 4 stroke engine.
82 // Various bits of housekeeping describing the engines initial state.
85 crank_counter = false;
88 // Initialise Engine Variables used by this instance
94 Manifold_Pressure = 29.96; // Inches
95 Fuel_Flow_gals_hr = 0;
98 CHT = 298.0; //deg Kelvin
99 CHT_degF = (CHT_degF * 1.8) - 459.67; //deg Fahrenheit
101 Oil_Pressure = 0; // PSI
102 Oil_Temp = 85; // Deg C
103 current_oil_temp = 298.0; //deg Kelvin
104 /**** one of these is superfluous !!!!***/
107 Torque_Imbalance = 0;
109 // Initialise Propellor Variables used by this instance
111 // Hardcode propellor for now - the following two should be read from config eventually
112 prop_diameter = 1.8; // meters
113 blade_angle = 23.0; // degrees
116 //*****************************************************************************
117 // update the engine model based on current control positions
118 void FGNewEngine::update() {
121 // Hack for testing - should output every 5 seconds
122 static int count1 = 0;
124 // cout << "P_atmos = " << p_amb << " T_atmos = " << T_amb << '\n';
125 // cout << "Manifold pressure = " << Manifold_Pressure << " True_Manifold_Pressure = " << True_Manifold_Pressure << '\n';
126 // cout << "p_amb_sea_level = " << p_amb_sea_level << '\n';
127 // cout << "equivalence_ratio = " << equivalence_ratio << '\n';
128 // cout << "combustion_efficiency = " << combustion_efficiency << '\n';
129 // cout << "AFR = " << 14.7 / equivalence_ratio << '\n';
130 // cout << "Mixture lever = " << Mixture_Lever_Pos << '\n';
131 // cout << "n = " << RPM << " rpm\n";
132 // cout << "T_amb = " << T_amb << '\n';
133 // cout << "running = " << running << '\n';
134 cout << "fuel = " << fgGetFloat("/consumables/fuel/tank[0]/level-gal_us") << '\n';
135 // cout << "Percentage_Power = " << Percentage_Power << '\n';
136 // cout << "current_oil_temp = " << current_oil_temp << '\n';
137 cout << "EGT = " << EGT << '\n';
144 // Check parameters that may alter the operating state of the engine.
145 // (spark, fuel, starter motor etc)
148 bool Magneto_Left = false;
149 bool Magneto_Right = false;
150 // Magneto positions:
159 } // neglects battery voltage, master on switch, etc for now.
160 if((mag_pos == 1) || (mag_pos > 2))
163 Magneto_Right = true;
165 // crude check for fuel
166 if((fgGetFloat("/consumables/fuel/tank[0]/level-gal_us") > 0) || (fgGetFloat("/consumables/fuel/tank[1]/level-gal_us") > 0)) {
170 } // Need to make this better, eg position of fuel selector switch.
172 // Check if we are turning the starter motor
173 if(cranking != starter) {
174 // This check saves .../cranking from getting updated every loop - they only update when changed.
178 // Note that although /engines/engine[0]/starter and /engines/engine[0]/cranking might appear to be duplication it is
179 // not since the starter may be engaged with the battery voltage too low for cranking to occur (or perhaps the master
180 // switch just left off) and the sound manager will read .../cranking to determine wether to play a cranking sound.
181 // For now though none of that is implemented so cranking can be set equal to .../starter without further checks.
183 // int Alternate_Air_Pos =0; // Off = 0. Reduces power by 3 % for same throttle setting
184 // DCL - don't know what this Alternate_Air_Pos is - this is a leftover from the Schubert code.
186 //Check mode of engine operation
194 // consider making a horrible noise if the starter is engaged with the engine running
197 if((!running) && (spark) && (fuel) && (crank_counter > 120)) {
198 // start the engine if revs high enough
200 // For now just instantaneously start but later we should maybe crank for a bit
205 if( (running) && ((!spark)||(!fuel)) ) {
207 // note that we only cut the power - the engine may continue to spin if the prop is in a moving airstream
211 // Now we've ascertained whether the engine is running or not we can start to do the engine calculations 'proper'
213 // Calculate Sea Level Manifold Pressure
214 Manifold_Pressure = Calc_Manifold_Pressure( Throttle_Lever_Pos, Max_Manifold_Pressure, Min_Manifold_Pressure );
215 // cout << "manifold pressure = " << Manifold_Pressure << endl;
217 //Then find the actual manifold pressure (the calculated one is the sea level pressure)
218 True_Manifold_Pressure = Manifold_Pressure * p_amb / p_amb_sea_level;
220 //Do the fuel flow calculations
221 Calc_Fuel_Flow_Gals_Hr();
223 //Calculate engine power
224 Calc_Percentage_Power(Magneto_Left, Magneto_Right);
225 HP = Percentage_Power * MaxHP / 100.0;
226 Power_SI = HP * CONVERT_HP_TO_WATTS;
228 // FMEP calculation. For now we will just use this during motored operation.
229 // Eventually we will calculate IMEP and use the FMEP all the time to give BMEP (maybe!)
231 // This FMEP data is from the Patton paper, assumes fully warm conditions.
232 FMEP = 1e-12*pow(RPM,4) - 1e-8*pow(RPM,3) + 5e-5*pow(RPM,2) - 0.0722*RPM + 154.85;
233 // Gives FMEP in kPa - now convert to Pa
238 // Is this total FMEP or friction FMEP ???
240 Torque_FMEP = (FMEP * displacement_SI) / (2.0 * LS_PI * n_R);
242 // Calculate Engine Torque. Check for div by zero since percentage power correlation does not guarantee zero power at zero rpm.
243 // However this is problematical since there is a resistance to movement even at rest
244 // Ie this is a dynamics equation not a statics one. This can be solved by going over to MEP based torque calculations.
246 Torque_SI = 0 - Torque_FMEP;
249 Torque_SI = ((Power_SI * 60.0) / (2.0 * LS_PI * RPM)) - Torque_FMEP; //Torque = power / angular velocity
250 // cout << Torque << " Nm\n";
253 //Calculate Exhaust gas temperature
259 // Calculate Cylinder Head Temperature
262 // Calculate oil temperature
263 current_oil_temp = Calc_Oil_Temp(current_oil_temp);
265 // Calculate Oil Pressure
266 Oil_Pressure = Calc_Oil_Press( Oil_Temp, RPM );
268 // Now do the Propeller Calculations
271 // Now do the engine - prop torque balance to calculate final RPM
273 //Calculate new RPM from torque balance and inertia.
274 Torque_Imbalance = Torque_SI - prop_torque; //This gives a +ve value when the engine torque exeeds the prop torque
275 // (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)
277 angular_acceleration = Torque_Imbalance / (engine_inertia + prop_inertia);
278 angular_velocity_SI += (angular_acceleration * time_step);
279 // Don't let the engine go into reverse
280 if(angular_velocity_SI < 0)
281 angular_velocity_SI = 0;
282 RPM = (angular_velocity_SI * 60) / (2.0 * LS_PI);
284 // And finally a last check on the engine state after doing the torque balance with the prop - have we stalled?
286 //Check if we have stalled the engine
289 } else if((RPM <= 480) && (cranking)) {
290 //Make sure the engine noise dosn't play if the engine won't start due to eg mixture lever pulled out.
296 // And finally, do any unit conversions from internal units to output units
297 EGT_degF = (EGT * 1.8) - 459.67;
298 CHT_degF = (CHT * 1.8) - 459.67;
301 //*****************************************************************************************************
303 // FGNewEngine member functions
305 ////////////////////////////////////////////////////////////////////////////////////////////
306 // Return the combustion efficiency as a function of equivalence ratio
308 // Combustion efficiency values from Heywood,
309 // "Internal Combustion Engine Fundamentals", ISBN 0-07-100499-8
310 ////////////////////////////////////////////////////////////////////////////////////////////
311 float FGNewEngine::Lookup_Combustion_Efficiency(float thi_actual)
313 const int NUM_ELEMENTS = 11;
314 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
315 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
316 float neta_comb_actual = 0.0f;
320 int j = NUM_ELEMENTS; //This must be equal to the number of elements in the lookup table arrays
325 // Assume linear extrapolation of the slope between the last two points beyond the last point
326 float dydx = (neta_comb[i] - neta_comb[i-1]) / (thi[i] - thi[i-1]);
327 neta_comb_actual = neta_comb[i] + dydx * (thi_actual - thi[i]);
328 return neta_comb_actual;
330 if(thi_actual == thi[i]) {
331 neta_comb_actual = neta_comb[i];
332 return neta_comb_actual;
334 if((thi_actual > thi[i]) && (thi_actual < thi[i + 1])) {
335 //do linear interpolation between the two points
336 factor = (thi_actual - thi[i]) / (thi[i+1] - thi[i]);
337 neta_comb_actual = (factor * (neta_comb[i+1] - neta_comb[i])) + neta_comb[i];
338 return neta_comb_actual;
342 //if we get here something has gone badly wrong
343 cout << "ERROR: error in FGNewEngine::Lookup_Combustion_Efficiency\n";
344 return neta_comb_actual;
347 ////////////////////////////////////////////////////////////////////////////////////////////
348 // Return the percentage of best mixture power available at a given mixture strength
350 // Based on data from "Technical Considerations for Catalysts for the European Market"
351 // by H S Gandi, published 1988 by IMechE
353 // Note that currently no attempt is made to set a lean limit on stable combustion
354 ////////////////////////////////////////////////////////////////////////////////////////////
355 float FGNewEngine::Power_Mixture_Correlation(float thi_actual)
357 float AFR_actual = 14.7 / thi_actual;
358 // thi and thi_actual are equivalence ratio
359 const int NUM_ELEMENTS = 13;
360 // 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.
361 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
362 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
363 float mixPerPow_actual = 0.0f;
368 int j = NUM_ELEMENTS; //This must be equal to the number of elements in the lookup table arrays
373 // Assume linear extrapolation of the slope between the last two points beyond the last point
374 dydx = (mixPerPow[i] - mixPerPow[i-1]) / (AFR[i] - AFR[i-1]);
375 mixPerPow_actual = mixPerPow[i] + dydx * (AFR_actual - AFR[i]);
376 return mixPerPow_actual;
378 if((i == 0) && (AFR_actual < AFR[i])) {
379 // Assume linear extrapolation of the slope between the first two points for points before the first point
380 dydx = (mixPerPow[i] - mixPerPow[i-1]) / (AFR[i] - AFR[i-1]);
381 mixPerPow_actual = mixPerPow[i] + dydx * (AFR_actual - AFR[i]);
382 return mixPerPow_actual;
384 if(AFR_actual == AFR[i]) {
385 mixPerPow_actual = mixPerPow[i];
386 return mixPerPow_actual;
388 if((AFR_actual > AFR[i]) && (AFR_actual < AFR[i + 1])) {
389 //do linear interpolation between the two points
390 factor = (AFR_actual - AFR[i]) / (AFR[i+1] - AFR[i]);
391 mixPerPow_actual = (factor * (mixPerPow[i+1] - mixPerPow[i])) + mixPerPow[i];
392 return mixPerPow_actual;
396 //if we get here something has gone badly wrong
397 cout << "ERROR: error in FGNewEngine::Power_Mixture_Correlation\n";
398 return mixPerPow_actual;
401 // Calculate Cylinder Head Temperature
402 // Crudely models the cylinder head as an arbitary lump of arbitary size and area with one third of combustion energy
403 // as heat input and heat output as a function of airspeed and temperature. Could be improved!!!
404 void FGNewEngine::Calc_CHT()
406 float h1 = -95.0; //co-efficient for free convection
407 float h2 = -3.95; //co-efficient for forced convection
408 float h3 = -0.05; //co-efficient for forced convection due to prop backwash
409 float v_apparent; //air velocity over cylinder head in m/s
410 float v_dot_cooling_air;
411 float m_dot_cooling_air;
412 float temperature_difference;
413 float arbitary_area = 1.0;
414 float dqdt_from_combustion;
415 float dqdt_forced; //Rate of energy transfer to/from cylinder head due to forced convection (Joules) (sign convention: to cylinder head is +ve)
416 float dqdt_free; //Rate of energy transfer to/from cylinder head due to free convection (Joules) (sign convention: to cylinder head is +ve)
417 float dqdt_cylinder_head; //Overall energy change in cylinder head
418 float CpCylinderHead = 800.0; //FIXME - this is a guess - I need to look up the correct value
419 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
420 float HeatCapacityCylinderHead;
423 // The above values are hardwired to give reasonable results for an IO360 (C172 engine)
424 // Now adjust to try to compensate for arbitary sized engines - this is currently untested
425 arbitary_area *= (MaxHP / 180.0);
426 MassCylinderHead *= (MaxHP / 180.0);
428 temperature_difference = CHT - T_amb;
430 v_apparent = IAS * 0.5144444; //convert from knots to m/s
431 v_dot_cooling_air = arbitary_area * v_apparent;
432 m_dot_cooling_air = v_dot_cooling_air * rho_air;
434 //Calculate rate of energy transfer to cylinder head from combustion
435 dqdt_from_combustion = m_dot_fuel * calorific_value_fuel * combustion_efficiency * 0.33;
437 //Calculate rate of energy transfer from cylinder head due to cooling NOTE is calculated as rate to but negative
438 dqdt_forced = (h2 * m_dot_cooling_air * temperature_difference) + (h3 * RPM * temperature_difference);
439 dqdt_free = h1 * temperature_difference;
441 //Calculate net rate of energy transfer to or from cylinder head
442 dqdt_cylinder_head = dqdt_from_combustion + dqdt_forced + dqdt_free;
444 HeatCapacityCylinderHead = CpCylinderHead * MassCylinderHead;
446 dCHTdt = dqdt_cylinder_head / HeatCapacityCylinderHead;
448 CHT += (dCHTdt * time_step);
451 // Calculate exhaust gas temperature
452 void FGNewEngine::Calc_EGT()
454 combustion_efficiency = Lookup_Combustion_Efficiency(equivalence_ratio); //The combustion efficiency basically tells us what proportion of the fuels calorific value is released
456 //now calculate energy release to exhaust
457 //We will assume a three way split of fuel energy between useful work, the coolant system and the exhaust system
458 //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
459 //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.
460 enthalpy_exhaust = m_dot_fuel * calorific_value_fuel * combustion_efficiency * 0.33;
461 heat_capacity_exhaust = (Cp_air * m_dot_air) + (Cp_fuel * m_dot_fuel);
462 delta_T_exhaust = enthalpy_exhaust / heat_capacity_exhaust;
464 EGT = T_amb + delta_T_exhaust;
466 //The above gives the exhaust temperature immediately prior to leaving the combustion chamber
467 //Now derate to give a more realistic figure as measured downstream
468 //For now we will aim for a peak of around 400 degC (750 degF)
470 EGT *= 0.444 + ((0.544 - 0.444) * Percentage_Power / 100.0);
473 // Calculate Manifold Pressure based on Throttle lever Position
474 float FGNewEngine::Calc_Manifold_Pressure ( float LeverPosn, float MaxMan, float MinMan)
478 //Note that setting the manifold pressure as a function of lever position only is not strictly accurate
479 //MAP is also a function of engine speed. (and ambient pressure if we are going for an actual MAP model)
480 Inches = MinMan + (LeverPosn * (MaxMan - MinMan) / 100);
482 //allow for idle bypass valve or slightly open throttle stop
486 //Check for stopped engine (crudest way of compensating for engine speed)
493 // Calculate fuel flow in gals/hr
494 void FGNewEngine::Calc_Fuel_Flow_Gals_Hr()
496 //DCL - calculate mass air flow into engine based on speed and load - separate this out into a function eventually
497 //t_amb is actual temperature calculated from altitude
498 //calculate density from ideal gas equation
499 rho_air = p_amb / ( R_air * T_amb );
500 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.
501 //calculate ideal engine volume inducted per second
502 swept_volume = (displacement_SI * (RPM / 60)) / 2; //This equation is only valid for a four stroke engine
503 //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
504 //Note that this is cylinder vol eff - the throttle drop is already accounted for in the MAP calculation
505 volumetric_efficiency = 0.8;
506 //Now use volumetric efficiency to calculate actual air volume inducted per second
507 v_dot_air = swept_volume * volumetric_efficiency;
508 //Now calculate mass flow rate of air into engine
509 m_dot_air = v_dot_air * rho_air_manifold;
513 //DCL - now calculate fuel flow into engine based on air flow and mixture lever position
514 //assume lever runs from no flow at fully out to thi = 1.3 at fully in at sea level
515 //also assume that the injector linkage is ideal - hence the set mixture is maintained at a given altitude throughout the speed and load range
516 thi_sea_level = 1.3 * ( Mixture_Lever_Pos / 100.0 );
517 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
518 m_dot_fuel = m_dot_air / 14.7 * equivalence_ratio;
519 Fuel_Flow_gals_hr = (m_dot_fuel / rho_fuel) * 264.172 * 3600.0; // Note this assumes US gallons
522 // Calculate the percentage of maximum rated power delivered as a function of Manifold pressure multiplied by engine speed (rpm)
523 // This is not necessarilly the best approach but seems to work for now.
524 // May well need tweaking at the bottom end if the prop model is changed.
525 void FGNewEngine::Calc_Percentage_Power(bool mag_left, bool mag_right)
527 // For a given Manifold Pressure and RPM calculate the % Power
528 // Multiply Manifold Pressure by RPM
529 float ManXRPM = True_Manifold_Pressure * RPM;
532 // Phil's %power correlation
534 Percentage_Power = (+ 7E-09 * ManXRPM * ManXRPM) + ( + 7E-04 * ManXRPM) - 0.1218;
535 // cout << Percentage_Power << "%" << "\t";
538 // DCL %power correlation - basically Phil's correlation modified to give slighty less power at the low end
539 // might need some adjustment as the prop model is adjusted
540 // 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
541 // Calculate % Power for Nev's prop model
542 //Percentage_Power = (+ 6E-09 * ManXRPM * ManXRPM) + ( + 8E-04 * ManXRPM) - 1.8524;
544 // Calculate %power for DCL prop model
545 Percentage_Power = (7e-9 * ManXRPM * ManXRPM) + (7e-4 * ManXRPM) - 1.0;
547 // Power de-rating for altitude has been removed now that we are basing the power
548 // on the actual manifold pressure, which takes air pressure into account. However - this fails to
549 // take the temperature into account - this is TODO.
551 // Adjust power for temperature - this is temporary until the power is done as a function of mass flow rate induced
552 // Adjust for Temperature - Temperature above Standard decrease
553 // power by 7/120 % per degree F increase, and incease power for
554 // temps below at the same ratio
555 float T_amb_degF = (T_amb * 1.8) - 459.67;
556 float T_amb_sea_lev_degF = (288 * 1.8) - 459.67;
557 Percentage_Power = Percentage_Power + ((T_amb_sea_lev_degF - T_amb_degF) * 7 /120);
559 //DCL - now adjust power to compensate for mixture
560 Percentage_of_best_power_mixture_power = Power_Mixture_Correlation(equivalence_ratio);
561 Percentage_Power = Percentage_Power * Percentage_of_best_power_mixture_power / 100.0;
563 // Now Derate engine for the effects of Bad/Switched off magnetos
564 //if (Magneto_Left == 0 && Magneto_Right == 0) {
566 // cout << "Both OFF\n";
567 Percentage_Power = 0;
568 } else if (mag_left && mag_right) {
569 // cout << "Both On ";
570 } else if (mag_left == 0 || mag_right== 0) {
571 // cout << "1 Magneto Failed ";
572 Percentage_Power = Percentage_Power * ((100.0 - Mag_Derate_Percent)/100.0);
573 // cout << FGEng1_Percentage_Power << "%" << "\t";
576 //DCL - stall the engine if RPM drops below 450 - this is possible if mixture lever is pulled right out
577 //This is a kludge that I should eliminate by adding a total fmep estimation.
579 Percentage_Power = 0;
581 if(Percentage_Power < 0)
582 Percentage_Power = 0;
585 // Calculate Oil Temperature in degrees Kelvin
586 float FGNewEngine::Calc_Oil_Temp (float oil_temp)
588 float idle_percentage_power = 2.3; // approximately
589 float target_oil_temp; // Steady state oil temp at the current engine conditions
590 float time_constant; // The time constant for the differential equation
592 target_oil_temp = 363;
593 time_constant = 500; // Time constant for engine-on idling.
594 if(Percentage_Power > idle_percentage_power) {
595 time_constant /= ((Percentage_Power / idle_percentage_power) / 10.0); // adjust for power
598 target_oil_temp = 298;
599 time_constant = 1000; // Time constant for engine-off; reflects the fact that oil is no longer getting circulated
602 float dOilTempdt = (target_oil_temp - oil_temp) / time_constant;
604 oil_temp += (dOilTempdt * time_step);
609 // Calculate Oil Pressure
610 float FGNewEngine::Calc_Oil_Press (float Oil_Temp, float Engine_RPM)
612 float Oil_Pressure = 0; //PSI
613 float Oil_Press_Relief_Valve = 60; //PSI
614 float Oil_Press_RPM_Max = 1800;
615 float Design_Oil_Temp = 85; //Celsius
616 float Oil_Viscosity_Index = 0.25; // PSI/Deg C
617 // float Temp_Deviation = 0; // Deg C
619 Oil_Pressure = (Oil_Press_Relief_Valve / Oil_Press_RPM_Max) * Engine_RPM;
621 // Pressure relief valve opens at Oil_Press_Relief_Valve PSI setting
622 if (Oil_Pressure >= Oil_Press_Relief_Valve) {
623 Oil_Pressure = Oil_Press_Relief_Valve;
626 // Now adjust pressure according to Temp which affects the viscosity
628 Oil_Pressure += (Design_Oil_Temp - Oil_Temp) * Oil_Viscosity_Index;
634 // Propeller calculations.
635 void FGNewEngine::Do_Prop_Calcs()
637 float Gear_Ratio = 1.0;
638 float forward_velocity; // m/s
639 float prop_power_consumed_SI; // Watts
640 double J; // advance ratio - dimensionless
641 double Cp_20; // coefficient of power for 20 degree blade angle
642 double Cp_25; // coefficient of power for 25 degree blade angle
643 double Cp; // Our actual coefficient of power
646 double neta_prop; // prop efficiency
648 FGProp1_RPS = RPM * Gear_Ratio / 60.0;
649 angular_velocity_SI = 2.0 * LS_PI * RPM / 60.0;
650 forward_velocity = IAS * 0.514444444444; // Convert to m/s
655 J = forward_velocity / (FGProp1_RPS * prop_diameter);
656 //cout << "advance_ratio = " << J << '\n';
658 //Cp correlations based on data from McCormick
659 Cp_20 = 0.0342*J*J*J*J - 0.1102*J*J*J + 0.0365*J*J - 0.0133*J + 0.064;
660 Cp_25 = 0.0119*J*J*J*J - 0.0652*J*J*J + 0.018*J*J - 0.0077*J + 0.0921;
662 //cout << "Cp_20 = " << Cp_20 << '\n';
663 //cout << "Cp_25 = " << Cp_25 << '\n';
665 //Assume that the blade angle is between 20 and 25 deg - reasonable for fixed pitch prop but won't hold for variable one !!!
666 Cp = Cp_20 + ( (Cp_25 - Cp_20) * ((blade_angle - 20)/(25 - 20)) );
667 //cout << "Cp = " << Cp << '\n';
668 //cout << "RPM = " << RPM << '\n';
669 //cout << "angular_velocity_SI = " << angular_velocity_SI << '\n';
671 prop_power_consumed_SI = Cp * rho_air * pow(FGProp1_RPS,3.0f) * pow(float(prop_diameter),5.0f);
672 //cout << "prop HP consumed = " << prop_power_consumed_SI / 745.699 << '\n';
673 if(angular_velocity_SI == 0)
675 // 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
676 // Unlikely to happen in practice - but I suppose someone could move the lever of a stopped large piston engine from feathered to windmilling.
677 // This *does* give problems - if the plane is put into a steep climb whilst windmilling the engine friction will eventually stop it spinning.
678 // 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!!!
680 prop_torque = prop_power_consumed_SI / angular_velocity_SI;
682 // calculate neta_prop here
683 neta_prop_20 = 0.1328*J*J*J*J - 1.3073*J*J*J + 0.3525*J*J + 1.5591*J + 0.0007;
684 neta_prop_25 = -0.3121*J*J*J*J + 0.4234*J*J*J - 0.7686*J*J + 1.5237*J - 0.0004;
685 neta_prop = neta_prop_20 + ( (neta_prop_25 - neta_prop_20) * ((blade_angle - 20)/(25 - 20)) );
687 // Check for zero forward velocity to avoid divide by zero
688 if(forward_velocity < 0.0001)
690 // I don't see how this works - how can the plane possibly start from rest ???
691 // Hmmmm - it works because the forward_velocity at present never drops below about 0.03 even at rest
692 // We can't really rely on this in the future - needs fixing !!!!
693 // 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.
694 // It would be far more natural to work out the power consumed from the thrust - FIXME!!!!!.
696 prop_thrust = neta_prop * prop_power_consumed_SI / forward_velocity; //TODO - rename forward_velocity to IAS_SI
697 //cout << "prop_thrust = " << prop_thrust << '\n';