1 #include "Atmosphere.hpp"
3 #include "PistonEngine.hpp"
6 const static float HP2W = 745.7;
7 const static float CIN2CM = 1.6387064e-5;
9 PistonEngine::PistonEngine(float power, float speed)
13 // Presume a BSFC (in lb/hour per HP) of 0.45. In SI that becomes
14 // (2.2 lb/kg, 745.7 W/hp, 3600 sec/hour) 7.62e-08 kg/Ws.
15 _f0 = power * 7.62e-08;
20 // We must be at sea level under standard conditions
21 _rho0 = Atmosphere::getStdDensity(0);
23 // Further presume that takeoff is (duh) full throttle and
24 // peak-power, that means that by our efficiency function, we are
25 // at 11/8 of "ideal" fuel flow.
26 float realFlow = _f0 * (11.0/8.0);
27 _mixCoeff = realFlow * 1.1 / _omega0;
30 _maxMP = 1e6; // No waste gate on non-turbo engines.
32 // Guess at reasonable values for these guys. Displacements run
33 // at about 2 cubic inches per horsepower or so, at least for
34 // non-turbocharged engines.
36 _displacement = power * (2*CIN2CM/HP2W);
39 void PistonEngine::setTurboParams(float turbo, float maxMP)
44 // This changes the "sea level" manifold air density
45 float P0 = Atmosphere::getStdPressure(0);
46 float P = P0 * (1 + _boost * (_turbo - 1));
47 if(P > _maxMP) P = _maxMP;
48 float T = Atmosphere::getStdTemperature(0) * Math::pow(P/P0, 2./7.);
49 _rho0 = P / (287.1 * T);
52 void PistonEngine::setDisplacement(float d)
57 void PistonEngine::setCompression(float c)
62 float PistonEngine::getMaxPower()
67 void PistonEngine::setThrottle(float t)
72 void PistonEngine::setMixture(float m)
77 void PistonEngine::setBoost(float boost)
82 float PistonEngine::getTorque()
87 float PistonEngine::getFuelFlow()
92 float PistonEngine::getMP()
97 float PistonEngine::getEGT()
102 void PistonEngine::calc(float pressure, float temp, float speed)
104 // Calculate manifold pressure as ambient pressure modified for
105 // turbocharging and reduced by the throttle setting. According
106 // to Dave Luff, minimum throttle at sea level corresponds to 6"
107 // manifold pressure. Assume that this means that minimum MP is
108 // always 20% of ambient pressure.
109 _mp = pressure * (1 + _boost*(_turbo-1)); // turbocharger
110 _mp *= (0.2 + 0.8 * _throttle); // throttle
111 if(_mp > _maxMP) _mp = _maxMP; // wastegate
113 // Air entering the manifold does so rapidly, and thus the
114 // pressure change can be assumed to be adiabatic. Calculate a
115 // temperature change, and use that to get the density.
116 float T = temp * Math::pow(_mp/pressure, 2.0/7.0);
117 float rho = _mp / (287.1 * T);
119 // The actual fuel flow is determined only by engine RPM and the
120 // mixture setting. Not all of this will burn with the same
122 _fuelFlow = _mixture * speed * _mixCoeff;
124 // How much fuel could be burned with ideal (i.e. uncorrected!)
126 float burnable = _f0 * (rho/_rho0) * (speed/_omega0);
128 // Calculate the fuel that actually burns to produce work. The
129 // idea is that less than 5/8 of ideal, we get complete
130 // combustion. We use up all the oxygen at 1 3/8 of ideal (that
131 // is, you need to waste fuel to use all your O2). In between,
132 // interpolate. This vaguely matches a curve I copied out of a
133 // book for a single engine. Shrug.
135 float r = _fuelFlow/burnable;
136 if (burnable == 0) burned = 0;
137 else if(r < .625) burned = _fuelFlow;
138 else if(r > 1.375) burned = burnable;
140 burned = _fuelFlow + (burnable-_fuelFlow)*(r-.625)*(4.0/3.0);
142 // And finally the power is just the reference power scaled by the
143 // amount of fuel burned, and torque is that divided by RPM.
144 float power = _power0 * burned/_f0;
145 _torque = power/speed;
147 // Now EGT. This one gets a little goofy. We can calculate the
148 // work done by an isentropically expanding exhaust gas as the
149 // mass of the gas times the specific heat times the change in
150 // temperature. The mass is just the engine displacement times
151 // the manifold density, plus the mass of the fuel, which we know.
152 // The change in temperature can be calculated adiabatically as a
153 // function of the exhaust gas temperature and the compression
154 // ratio (which we know). So just rearrange the equation to get
155 // EGT as a function of engine power. Cool. I'm using a value of
156 // 1300 J/(kg*K) for the exhaust gas specific heat. I found this
157 // on a web page somewhere; no idea if it's accurate. Also,
158 // remember that four stroke engines do one combustion cycle every
159 // TWO revolutions, so the displacement per revolution is half of
160 // what we'd expect. And diddle the work done by the gas a bit to
161 // account for non-thermodynamic losses like internal friction;
164 float massFlow = _fuelFlow + (rho * 0.5 * _displacement * speed);
165 float specHeat = 1300;
166 float corr = 1.0/(Math::pow(_compression, 0.4) - 1);
167 _egt = corr * (power * 1.1) / (massFlow * specHeat);
170 }; // namespace yasim