flightgear/src/FDM/YASim/PistonEngine.cpp

#include "Atmosphere.hpp"
#include "Math.hpp"
#include "PistonEngine.hpp"
namespace yasim {

const static float HP2W = 745.7;
const static float CIN2CM = 1.6387064e-5;

PistonEngine::PistonEngine(float power, float speed)
{
    _boost = 1;

    // 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) 7.62e-08 kg/Ws.
    _f0 = power * 7.62e-08;

    _power0 = power;
    _omega0 = speed;

    // We must be at sea level under standard conditions
    _rho0 = Atmosphere::getStdDensity(0);

    // 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;

    _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)
{
    _turbo = turbo;
    _maxMP = maxMP;

    // This changes the "sea level" manifold air density
    float P0 = Atmosphere::getStdPressure(0);
    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);
}

void PistonEngine::setDisplacement(float d)
{
    _displacement = d;
}

void PistonEngine::setCompression(float c)
{
    _compression = c;
}

float PistonEngine::getMaxPower()
{
    return _power0;
}

void PistonEngine::setThrottle(float t)
{
    _throttle = t;
}

void PistonEngine::setMixture(float m)
{
    _mixture = m;
}

void PistonEngine::setBoost(float boost)
{
    _boost = boost;
}

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)
{
    // 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.
    _mp = pressure * (1 + _boost*(_turbo-1)); // turbocharger
    _mp *= (0.2 + 0.8 * _throttle);            // 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 / (287.1 * 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;

    // How much fuel could be burned with ideal (i.e. uncorrected!)
    // combustion.
    float burnable = _f0 * (rho/_rho0) * (speed/_omega0);

    // Calculate the fuel that actually burns to produce work.  The
    // idea is that less than 5/8 of ideal, we get complete
    // combustion.  We use up all the oxygen at 1 3/8 of ideal (that
    // is, you need to waste fuel to use all your O2).  In between,
    // interpolate.  This vaguely matches a curve I copied out of a
    // book for a single engine.  Shrug.
    float burned;
    float r = _fuelFlow/burnable;
    if     (burnable == 0) burned = 0;
    else if(r < .625)      burned = _fuelFlow;
    else if(r > 1.375)     burned = burnable;
    else
        burned = _fuelFlow + (burnable-_fuelFlow)*(r-.625)*(4.0/3.0);

    // And finally the power is just the reference power scaled by the
    // amount of fuel burned, and torque is that divided by RPM.
    float power = _power0 * burned/_f0;
    _torque = power/speed;

    // 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 * 0.5 * _displacement * speed);
    float specHeat = 1300;
    float corr = 1.0/(Math::pow(_compression, 0.4) - 1);
    _egt = corr * (power * 1.1) / (massFlow * specHeat);
}

}; // namespace yasim