4a683bed1e
Background are problems modeling the rotax 912 engine. The idle speed of the real engine is about half of the speed I could achieve with the default minimum manifold pressure. While on ground I can switch off the engine by pulling the throttle. The audible difference between the different minimum idle speed (real vs. simulated) is extreme. With the patch I get quite realistic sound. For the rotax engine I use min-throttle="0.05" which is half of the former default value.
279 lines
9 KiB
C++
279 lines
9 KiB
C++
#include "Atmosphere.hpp"
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#include "Math.hpp"
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#include "PistonEngine.hpp"
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namespace yasim {
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const static float HP2W = 745.7f;
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const static float CIN2CM = 1.6387064e-5f;
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const static float RPM2RADPS = 0.1047198f;
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PistonEngine::PistonEngine(float power, float speed)
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{
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_boost = 1;
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_running = false;
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_fuel = true;
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_boostPressure = 0;
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_hasSuper = false;
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_oilTemp = Atmosphere::getStdTemperature(0);
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_oilTempTarget = _oilTemp;
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_dOilTempdt = 0;
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// Presume a BSFC (in lb/hour per HP) of 0.45. In SI that becomes
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// (2.2 lb/kg, 745.7 W/hp, 3600 sec/hour) 7.62e-08 kg/Ws.
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_f0 = power * 7.62e-08f;
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_power0 = power;
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_omega0 = speed;
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// We must be at sea level under standard conditions
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_rho0 = Atmosphere::getStdDensity(0);
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// Further presume that takeoff is (duh) full throttle and
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// peak-power, that means that by our efficiency function, we are
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// at 11/8 of "ideal" fuel flow.
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float realFlow = _f0 * (11.0f/8.0f);
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_mixCoeff = realFlow * 1.1f / _omega0;
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_turbo = 1;
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_minthrottle = 0.1;
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_maxMP = 1e6; // No waste gate on non-turbo engines.
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_wastegate = 1;
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_charge = 1;
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_chargeTarget = 1;
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_turboLag = 2;
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// Guess at reasonable values for these guys. Displacements run
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// at about 2 cubic inches per horsepower or so, at least for
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// non-turbocharged engines.
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_compression = 8;
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_displacement = power * (2*CIN2CM/HP2W);
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}
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void PistonEngine::setTurboParams(float turbo, float maxMP)
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{
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_turbo = turbo;
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_maxMP = maxMP;
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// This changes the "sea level" manifold air density
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float P0 = Atmosphere::getStdPressure(0);
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float P = P0 * (1 + _boost * (_turbo - 1));
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if(P > _maxMP) P = _maxMP;
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float T = Atmosphere::getStdTemperature(0) * Math::pow(P/P0, 2./7.);
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_rho0 = P / (287.1f * T);
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}
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void PistonEngine::setDisplacement(float d)
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{
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_displacement = d;
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}
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void PistonEngine::setCompression(float c)
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{
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_compression = c;
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}
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void PistonEngine::setMinThrottle(float m)
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{
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_minthrottle = m;
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}
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float PistonEngine::getMaxPower()
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{
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return _power0;
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}
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bool PistonEngine::isCranking()
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{
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return _starter;
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}
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float PistonEngine::getTorque()
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{
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return _torque;
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}
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float PistonEngine::getFuelFlow()
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{
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return _fuelFlow;
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}
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float PistonEngine::getMP()
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{
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return _mp;
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}
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float PistonEngine::getEGT()
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{
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return _egt;
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}
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void PistonEngine::stabilize()
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{
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_oilTemp = _oilTempTarget;
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_charge = _chargeTarget;
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}
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void PistonEngine::integrate(float dt)
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{
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_oilTemp += (_dOilTempdt * dt);
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// See comments in Jet.cpp for how this decay constant works
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float decay = 2.3f / _turboLag;
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_charge = (_charge + dt*decay * _chargeTarget) / (1 + dt*decay);
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}
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void PistonEngine::calc(float pressure, float temp, float speed)
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{
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_running = _magnetos && _fuel && (speed > 60*RPM2RADPS);
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// Calculate the factor required to modify supercharger output for
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// rpm. Assume that the normalized supercharger output ~= 1 when
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// the engine is at the nominal peak-power rpm. A power equation
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// of the form (A * B^x * x^C) has been derived empirically from
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// some representative supercharger data. This provides
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// near-linear output over the normal operating range, with
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// fall-off in the over-speed situation.
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float rpm_norm = (speed / _omega0);
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float A = 1.795206541;
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float B = 0.55620178;
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float C = 1.246708471;
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float rpm_factor = A * Math::pow(B, rpm_norm) * Math::pow(rpm_norm, C);
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_chargeTarget = 1 + (_boost * (_turbo-1) * rpm_factor);
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if(_hasSuper) {
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// Superchargers have no lag
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_charge = _chargeTarget;
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} else if(!_running) {
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// Turbochargers only work well when the engine is actually
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// running. The 25% number is a guesstimate from Vivian.
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_chargeTarget = 1 + (_chargeTarget - 1) * 0.25;
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}
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// We need to adjust the minimum manifold pressure to get a
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// reasonable idle speed (a "closed" throttle doesn't suck a total
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// vacuum in real manifolds). This is a hack.
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float _minMP = (-0.008 * _turbo ) + _minthrottle;
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_mp = pressure * _charge;
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// Scale to throttle setting, clamp to wastegate
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if(_running)
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_mp *= _minMP + (1 -_minMP) * _throttle;
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// Scale the max MP according to the WASTEGATE control input. Use
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// the un-supercharged MP as the bottom limit.
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float max = _wastegate * _maxMP;
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if(max < _mp/_charge) max = _mp/_charge;
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if(_mp > max) _mp = max;
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// The "boost" is the delta above ambient
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_boostPressure = _mp - pressure;
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// Air entering the manifold does so rapidly, and thus the
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// pressure change can be assumed to be adiabatic. Calculate a
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// temperature change, and use that to get the density.
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// Note: need to model intercoolers here...
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float T = temp * Math::pow((_mp*_mp)/(pressure*pressure), 1.0/7.0);
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float rho = _mp / (287.1f * T);
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// The actual fuel flow is determined only by engine RPM and the
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// mixture setting. Not all of this will burn with the same
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// efficiency.
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_fuelFlow = _mixture * speed * _mixCoeff;
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if(_fuel == false) _fuelFlow = 0;
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// How much fuel could be burned with ideal (i.e. uncorrected!)
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// combustion.
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float burnable = _f0 * (rho/_rho0) * (speed/_omega0);
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// Calculate the fuel that actually burns to produce work. The
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// idea is that less than 5/8 of ideal, we get complete
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// combustion. We use up all the oxygen at 1 3/8 of ideal (that
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// is, you need to waste fuel to use all your O2). In between,
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// interpolate. This vaguely matches a curve I copied out of a
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// book for a single engine. Shrug.
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float burned;
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float r = _fuelFlow/burnable;
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if (burnable == 0) burned = 0;
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else if(r < .625) burned = _fuelFlow;
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else if(r > 1.375) burned = burnable;
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else
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burned = _fuelFlow + (burnable-_fuelFlow)*(r-0.625f)*(4.0f/3.0f);
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// Correct for engine control state
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if(!_running)
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burned = 0;
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if(_magnetos < 3)
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burned *= 0.9f;
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// And finally the power is just the reference power scaled by the
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// amount of fuel burned, and torque is that divided by RPM.
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float power = _power0 * burned/_f0;
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_torque = power/speed;
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// Figure that the starter motor produces 15% of the engine's
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// cruise torque. Assuming 60RPM starter speed vs. 1800RPM cruise
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// speed on a 160HP engine, that comes out to about 160*.15/30 ==
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// 0.8 HP starter motor. Which sounds about right to me. I think
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// I've finally got this tuned. :)
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if(_starter && !_running)
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_torque += 0.15f * _power0/_omega0;
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// Also, add a negative torque of 8% of cruise, to represent
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// internal friction. Propeller aerodynamic friction is too low
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// at low RPMs to provide a good deceleration. Interpolate it
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// away as we approach cruise RPMs (full at 50%, zero at 100%),
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// though, to prevent interaction with the power computations.
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// Ugly.
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if(speed > 0 && speed < _omega0) {
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float interp = 2 - 2*speed/_omega0;
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interp = (interp > 1) ? 1 : interp;
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_torque -= 0.08f * (_power0/_omega0) * interp;
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}
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// Now EGT. This one gets a little goofy. We can calculate the
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// work done by an isentropically expanding exhaust gas as the
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// mass of the gas times the specific heat times the change in
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// temperature. The mass is just the engine displacement times
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// the manifold density, plus the mass of the fuel, which we know.
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// The change in temperature can be calculated adiabatically as a
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// function of the exhaust gas temperature and the compression
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// ratio (which we know). So just rearrange the equation to get
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// EGT as a function of engine power. Cool. I'm using a value of
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// 1300 J/(kg*K) for the exhaust gas specific heat. I found this
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// on a web page somewhere; no idea if it's accurate. Also,
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// remember that four stroke engines do one combustion cycle every
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// TWO revolutions, so the displacement per revolution is half of
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// what we'd expect. And diddle the work done by the gas a bit to
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// account for non-thermodynamic losses like internal friction;
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// 10% should do it.
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float massFlow = _fuelFlow + (rho * 0.5f * _displacement * speed);
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float specHeat = 1300;
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float corr = 1.0f/(Math::pow(_compression, 0.4f) - 1.0f);
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_egt = corr * (power * 1.1f) / (massFlow * specHeat);
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if(_egt < temp) _egt = temp;
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// Oil temperature.
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// Assume a linear variation between ~90degC at idle and ~120degC
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// at full power. No attempt to correct for airflow over the
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// engine is made. Make the time constant to attain target steady-
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// state oil temp greater at engine off than on to reflect no
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// circulation. Nothing fancy, but populates the guage with a
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// plausible value.
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float tau; // secs
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if(_running) {
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_oilTempTarget = 363.0f + (30.0f * (power/_power0));
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tau = 600;
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// Reduce tau linearly to 300 at max power
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tau -= (power/_power0) * 300.0f;
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} else {
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_oilTempTarget = temp;
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tau = 1500;
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}
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_dOilTempdt = (_oilTempTarget - _oilTemp) / tau;
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}
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}; // namespace yasim
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