#include "PistonEngine.hpp"
namespace yasim {
+const static float HP2W = 745.7f;
+const static float CIN2CM = 1.6387064e-5f;
+const static float RPM2RADPS = 0.1047198f;
+
PistonEngine::PistonEngine(float power, float speed)
{
+ _boost = 1;
+ _running = false;
+ _fuel = true;
+
// Presume a BSFC (in lb/hour per HP) of 0.45. In SI that becomes
- // (2.2 lb/kg, 745.7 W/hp, 3600 sec/hour) 3.69e-07 kg/Ws.
- _f0 = power * 3.69e-07;
+ // (2.2 lb/kg, 745.7 W/hp, 3600 sec/hour) 7.62e-08 kg/Ws.
+ _f0 = power * 7.62e-08f;
- _P0 = power;
+ _power0 = power;
_omega0 = speed;
// We must be at sea level under standard conditions
// Further presume that takeoff is (duh) full throttle and
// peak-power, that means that by our efficiency function, we are
// at 11/8 of "ideal" fuel flow.
- float realFlow = _f0 * (11.0/8.0);
- _mixCoeff = realFlow * 1.1 / _omega0;
+ float realFlow = _f0 * (11.0f/8.0f);
+ _mixCoeff = realFlow * 1.1f / _omega0;
_turbo = 1;
_maxMP = 1e6; // No waste gate on non-turbo engines.
+
+ // Guess at reasonable values for these guys. Displacements run
+ // at about 2 cubic inches per horsepower or so, at least for
+ // non-turbocharged engines.
+ _compression = 8;
+ _displacement = power * (2*CIN2CM/HP2W);
}
void PistonEngine::setTurboParams(float turbo, float maxMP)
// This changes the "sea level" manifold air density
float P0 = Atmosphere::getStdPressure(0);
- float P = P0 * _turbo;
+ float P = P0 * (1 + _boost * (_turbo - 1));
if(P > _maxMP) P = _maxMP;
float T = Atmosphere::getStdTemperature(0) * Math::pow(P/P0, 2./7.);
- _rho0 = P / (287.1 * T);
+ _rho0 = P / (287.1f * T);
}
-float PistonEngine::getPower()
+void PistonEngine::setDisplacement(float d)
{
- return _P0;
+ _displacement = d;
}
-void PistonEngine::setThrottle(float t)
+void PistonEngine::setCompression(float c)
{
- _throttle = t;
+ _compression = c;
}
-void PistonEngine::setMixture(float m)
+float PistonEngine::getMaxPower()
{
- _mixture = m;
+ return _power0;
}
-void PistonEngine::calc(float P, float T, float speed,
- float* torqueOut, float* fuelFlowOut)
+bool PistonEngine::isCranking()
{
- // The actual fuel flow
- float fuel = _mixture * _mixCoeff * speed;
-
- // manifold air density
- if(_turbo != 1) {
- float P1 = P * _turbo;
- if(P1 > _maxMP) P1 = _maxMP;
- T *= Math::pow(P1/P, 2./7.);
- P = P1;
- }
- float density = P / (287.1 * T);
-
- float rho = density * _throttle;
+ return _starter;
+}
+
+float PistonEngine::getTorque()
+{
+ return _torque;
+}
+
+float PistonEngine::getFuelFlow()
+{
+ return _fuelFlow;
+}
+
+float PistonEngine::getMP()
+{
+ return _mp;
+}
+
+float PistonEngine::getEGT()
+{
+ return _egt;
+}
+
+void PistonEngine::calc(float pressure, float temp, float speed)
+{
+ if(_magnetos == 0 || speed < 60*RPM2RADPS)
+ _running = false;
+ else if(_fuel == false)
+ _running = false;
+ else
+ _running = true;
+
+ // Calculate manifold pressure as ambient pressure modified for
+ // turbocharging and reduced by the throttle setting. According
+ // to Dave Luff, minimum throttle at sea level corresponds to 6"
+ // manifold pressure. Assume that this means that minimum MP is
+ // always 20% of ambient pressure. (But that's too much idle
+ // power, so use 10% instead!) But we need to produce _zero_
+ // thrust at that setting, so hold onto the "output" value
+ // separately. Ick.
+ _mp = pressure * (1 + _boost*(_turbo-1)); // turbocharger
+ float mp = _mp * (0.1f + 0.9f * _throttle); // throttle
+ _mp *= _throttle;
+ if(mp > _maxMP) mp = _maxMP; // wastegate
+
+ // Air entering the manifold does so rapidly, and thus the
+ // pressure change can be assumed to be adiabatic. Calculate a
+ // temperature change, and use that to get the density.
+ float T = temp * Math::pow(mp/pressure, 2.0/7.0);
+ float rho = mp / (287.1f * T);
+
+ // The actual fuel flow is determined only by engine RPM and the
+ // mixture setting. Not all of this will burn with the same
+ // efficiency.
+ _fuelFlow = _mixture * speed * _mixCoeff;
+ if(_fuel == false) _fuelFlow = 0;
// How much fuel could be burned with ideal (i.e. uncorrected!)
// combustion.
// interpolate. This vaguely matches a curve I copied out of a
// book for a single engine. Shrug.
float burned;
- float r = fuel/burnable;
+ float r = _fuelFlow/burnable;
if (burnable == 0) burned = 0;
- else if(r < .625) burned = fuel;
+ else if(r < .625) burned = _fuelFlow;
else if(r > 1.375) burned = burnable;
- else burned = fuel + (burnable-fuel)*(r-.625)*(4.0/3.0);
+ else
+ burned = _fuelFlow + (burnable-_fuelFlow)*(r-0.625f)*(4.0f/3.0f);
+
+ // Correct for engine control state
+ if(!_running)
+ burned = 0;
+ if(_magnetos < 3)
+ burned *= 0.9f;
// And finally the power is just the reference power scaled by the
- // amount of fuel burned.
- float power = _P0 * burned/_f0;
+ // amount of fuel burned, and torque is that divided by RPM.
+ float power = _power0 * burned/_f0;
+ _torque = power/speed;
+
+ // Figure that the starter motor produces 15% of the engine's
+ // cruise torque. Assuming 60RPM starter speed vs. 1800RPM cruise
+ // speed on a 160HP engine, that comes out to about 160*.15/30 ==
+ // 0.8 HP starter motor. Which sounds about right to me. I think
+ // I've finally got this tuned. :)
+ if(_starter && !_running)
+ _torque += 0.15f * _power0/_omega0;
+
+ // Also, add a negative torque of 8% of cruise, to represent
+ // internal friction. Propeller aerodynamic friction is too low
+ // at low RPMs to provide a good deceleration. Interpolate it
+ // away as we approach cruise RPMs (full at 50%, zero at 100%),
+ // though, to prevent interaction with the power computations.
+ // Ugly.
+ if(speed > 0 && speed < _omega0) {
+ float interp = 2 - 2*speed/_omega0;
+ interp = (interp > 1) ? 1 : interp;
+ _torque -= 0.08f * (_power0/_omega0) * interp;
+ }
- *torqueOut = power/speed;
- *fuelFlowOut = fuel;
+ // Now EGT. This one gets a little goofy. We can calculate the
+ // work done by an isentropically expanding exhaust gas as the
+ // mass of the gas times the specific heat times the change in
+ // temperature. The mass is just the engine displacement times
+ // the manifold density, plus the mass of the fuel, which we know.
+ // The change in temperature can be calculated adiabatically as a
+ // function of the exhaust gas temperature and the compression
+ // ratio (which we know). So just rearrange the equation to get
+ // EGT as a function of engine power. Cool. I'm using a value of
+ // 1300 J/(kg*K) for the exhaust gas specific heat. I found this
+ // on a web page somewhere; no idea if it's accurate. Also,
+ // remember that four stroke engines do one combustion cycle every
+ // TWO revolutions, so the displacement per revolution is half of
+ // what we'd expect. And diddle the work done by the gas a bit to
+ // account for non-thermodynamic losses like internal friction;
+ // 10% should do it.
+
+ float massFlow = _fuelFlow + (rho * 0.5f * _displacement * speed);
+ float specHeat = 1300;
+ float corr = 1.0f/(Math::pow(_compression, 0.4f) - 1.0f);
+ _egt = corr * (power * 1.1f) / (massFlow * specHeat);
+ if(_egt < temp) _egt = temp;
}
}; // namespace yasim
projects / flightgear.git / blobdiff