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)
15 // Presume a BSFC (in lb/hour per HP) of 0.45. In SI that becomes
16 // (2.2 lb/kg, 745.7 W/hp, 3600 sec/hour) 7.62e-08 kg/Ws.
17 _f0 = power * 7.62e-08;
22 // We must be at sea level under standard conditions
23 _rho0 = Atmosphere::getStdDensity(0);
25 // Further presume that takeoff is (duh) full throttle and
26 // peak-power, that means that by our efficiency function, we are
27 // at 11/8 of "ideal" fuel flow.
28 float realFlow = _f0 * (11.0/8.0);
29 _mixCoeff = realFlow * 1.1 / _omega0;
32 _maxMP = 1e6; // No waste gate on non-turbo engines.
34 // Guess at reasonable values for these guys. Displacements run
35 // at about 2 cubic inches per horsepower or so, at least for
36 // non-turbocharged engines.
38 _displacement = power * (2*CIN2CM/HP2W);
41 void PistonEngine::setTurboParams(float turbo, float maxMP)
46 // This changes the "sea level" manifold air density
47 float P0 = Atmosphere::getStdPressure(0);
48 float P = P0 * (1 + _boost * (_turbo - 1));
49 if(P > _maxMP) P = _maxMP;
50 float T = Atmosphere::getStdTemperature(0) * Math::pow(P/P0, 2./7.);
51 _rho0 = P / (287.1 * T);
54 void PistonEngine::setDisplacement(float d)
59 void PistonEngine::setCompression(float c)
64 float PistonEngine::getMaxPower()
69 void PistonEngine::setThrottle(float t)
74 void PistonEngine::setStarter(bool s)
79 void PistonEngine::setMagnetos(int m)
84 void PistonEngine::setMixture(float m)
89 void PistonEngine::setBoost(float boost)
94 bool PistonEngine::isRunning()
99 bool PistonEngine::isCranking()
104 float PistonEngine::getTorque()
109 float PistonEngine::getFuelFlow()
114 float PistonEngine::getMP()
119 float PistonEngine::getEGT()
124 void PistonEngine::calc(float pressure, float temp, float speed)
126 if (_magnetos == 0) {
131 _egt = 80; // FIXME: totally made-up
138 // TODO: degrade performance on single magneto
140 // Calculate manifold pressure as ambient pressure modified for
141 // turbocharging and reduced by the throttle setting. According
142 // to Dave Luff, minimum throttle at sea level corresponds to 6"
143 // manifold pressure. Assume that this means that minimum MP is
144 // always 20% of ambient pressure. But we need to produce _zero_
145 // thrust at that setting, so hold onto the "output" value
147 _mp = pressure * (1 + _boost*(_turbo-1)); // turbocharger
148 float mp = _mp * (0.2 + 0.8 * _throttle); // throttle
150 if(mp > _maxMP) mp = _maxMP; // wastegate
152 // Air entering the manifold does so rapidly, and thus the
153 // pressure change can be assumed to be adiabatic. Calculate a
154 // temperature change, and use that to get the density.
155 float T = temp * Math::pow(mp/pressure, 2.0/7.0);
156 float rho = mp / (287.1 * T);
158 // The actual fuel flow is determined only by engine RPM and the
159 // mixture setting. Not all of this will burn with the same
161 _fuelFlow = _mixture * speed * _mixCoeff;
163 // How much fuel could be burned with ideal (i.e. uncorrected!)
165 float burnable = _f0 * (rho/_rho0) * (speed/_omega0);
167 // Calculate the fuel that actually burns to produce work. The
168 // idea is that less than 5/8 of ideal, we get complete
169 // combustion. We use up all the oxygen at 1 3/8 of ideal (that
170 // is, you need to waste fuel to use all your O2). In between,
171 // interpolate. This vaguely matches a curve I copied out of a
172 // book for a single engine. Shrug.
174 float r = _fuelFlow/burnable;
175 if (burnable == 0) burned = 0;
176 else if(r < .625) burned = _fuelFlow;
177 else if(r > 1.375) burned = burnable;
179 burned = _fuelFlow + (burnable-_fuelFlow)*(r-.625)*(4.0/3.0);
181 // And finally the power is just the reference power scaled by the
182 // amount of fuel burned, and torque is that divided by RPM.
183 float power = _power0 * burned/_f0;
184 _torque = power/speed;
186 // Now EGT. This one gets a little goofy. We can calculate the
187 // work done by an isentropically expanding exhaust gas as the
188 // mass of the gas times the specific heat times the change in
189 // temperature. The mass is just the engine displacement times
190 // the manifold density, plus the mass of the fuel, which we know.
191 // The change in temperature can be calculated adiabatically as a
192 // function of the exhaust gas temperature and the compression
193 // ratio (which we know). So just rearrange the equation to get
194 // EGT as a function of engine power. Cool. I'm using a value of
195 // 1300 J/(kg*K) for the exhaust gas specific heat. I found this
196 // on a web page somewhere; no idea if it's accurate. Also,
197 // remember that four stroke engines do one combustion cycle every
198 // TWO revolutions, so the displacement per revolution is half of
199 // what we'd expect. And diddle the work done by the gas a bit to
200 // account for non-thermodynamic losses like internal friction;
203 float massFlow = _fuelFlow + (rho * 0.5 * _displacement * speed);
204 float specHeat = 1300;
205 float corr = 1.0/(Math::pow(_compression, 0.4) - 1);
206 _egt = corr * (power * 1.1) / (massFlow * specHeat);
209 }; // namespace yasim