68 lines
2 KiB
C++
68 lines
2 KiB
C++
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#include "PistonEngine.hpp"
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namespace yasim {
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PistonEngine::PistonEngine(float power, float speed)
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{
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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) 3.69e-07 kg/Ws.
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_f0 = power * 3.69e-07;
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_P0 = 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 = 1.225; // kg/m^3
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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 * (11.0/8.0);
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_mixCoeff = realFlow * 1.1 / _omega0;
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}
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void PistonEngine::setThrottle(float t)
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{
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_throttle = t;
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}
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void PistonEngine::setMixture(float m)
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{
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_mixture = m;
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}
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void PistonEngine::calc(float density, float speed,
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float* torqueOut, float* fuelFlowOut)
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{
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// The actual fuel flow
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float fuel = _mixture * _mixCoeff * speed;
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// manifold air density
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float rho = density * _throttle;
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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 = fuel/burnable;
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if (burnable == 0) burned = 0;
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else if(r < .625) burned = fuel;
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else if(r > 1.375) burned = burnable;
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else burned = fuel + (burnable-fuel)*(r-.625)*(4.0/3.0);
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// And finally the power is just the reference power scaled by the
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// amount of fuel burned.
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float power = _P0 * burned/_f0;
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*torqueOut = power/speed;
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*fuelFlowOut = fuel;
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}
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}; // namespace yasim
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