1 #include "Atmosphere.hpp"
3 #include "PistonEngine.hpp"
6 const static float HP2W = 745.7;
7 const static float CIN2CM = 1.6387064e-5;
8 const static float RPM2RADPS = 0.1047198;
10 PistonEngine::PistonEngine(float power, float speed)
16 // Presume a BSFC (in lb/hour per HP) of 0.45. In SI that becomes
17 // (2.2 lb/kg, 745.7 W/hp, 3600 sec/hour) 7.62e-08 kg/Ws.
18 _f0 = power * 7.62e-08;
23 // We must be at sea level under standard conditions
24 _rho0 = Atmosphere::getStdDensity(0);
26 // Further presume that takeoff is (duh) full throttle and
27 // peak-power, that means that by our efficiency function, we are
28 // at 11/8 of "ideal" fuel flow.
29 float realFlow = _f0 * (11.0/8.0);
30 _mixCoeff = realFlow * 1.1 / _omega0;
33 _maxMP = 1e6; // No waste gate on non-turbo engines.
35 // Guess at reasonable values for these guys. Displacements run
36 // at about 2 cubic inches per horsepower or so, at least for
37 // non-turbocharged engines.
39 _displacement = power * (2*CIN2CM/HP2W);
42 void PistonEngine::setTurboParams(float turbo, float maxMP)
47 // This changes the "sea level" manifold air density
48 float P0 = Atmosphere::getStdPressure(0);
49 float P = P0 * (1 + _boost * (_turbo - 1));
50 if(P > _maxMP) P = _maxMP;
51 float T = Atmosphere::getStdTemperature(0) * Math::pow(P/P0, 2./7.);
52 _rho0 = P / (287.1 * T);
55 void PistonEngine::setDisplacement(float d)
60 void PistonEngine::setCompression(float c)
65 float PistonEngine::getMaxPower()
70 void PistonEngine::setThrottle(float t)
75 void PistonEngine::setRunning(bool r)
80 void PistonEngine::setStarter(bool s)
85 void PistonEngine::setMagnetos(int m)
90 void PistonEngine::setMixture(float m)
95 void PistonEngine::setBoost(float boost)
100 bool PistonEngine::isRunning()
105 bool PistonEngine::isCranking()
110 float PistonEngine::getTorque()
115 float PistonEngine::getFuelFlow()
120 float PistonEngine::getMP()
125 float PistonEngine::getEGT()
130 void PistonEngine::calc(float pressure, float temp, float speed)
132 if(_magnetos == 0 || speed < 200*RPM2RADPS)
137 // Calculate manifold pressure as ambient pressure modified for
138 // turbocharging and reduced by the throttle setting. According
139 // to Dave Luff, minimum throttle at sea level corresponds to 6"
140 // manifold pressure. Assume that this means that minimum MP is
141 // always 20% of ambient pressure. But we need to produce _zero_
142 // thrust at that setting, so hold onto the "output" value
144 _mp = pressure * (1 + _boost*(_turbo-1)); // turbocharger
145 float mp = _mp * (0.2 + 0.8 * _throttle); // throttle
147 if(mp > _maxMP) mp = _maxMP; // wastegate
149 // Air entering the manifold does so rapidly, and thus the
150 // pressure change can be assumed to be adiabatic. Calculate a
151 // temperature change, and use that to get the density.
152 float T = temp * Math::pow(mp/pressure, 2.0/7.0);
153 float rho = mp / (287.1 * T);
155 // The actual fuel flow is determined only by engine RPM and the
156 // mixture setting. Not all of this will burn with the same
158 _fuelFlow = _mixture * speed * _mixCoeff;
160 // How much fuel could be burned with ideal (i.e. uncorrected!)
162 float burnable = _f0 * (rho/_rho0) * (speed/_omega0);
164 // Calculate the fuel that actually burns to produce work. The
165 // idea is that less than 5/8 of ideal, we get complete
166 // combustion. We use up all the oxygen at 1 3/8 of ideal (that
167 // is, you need to waste fuel to use all your O2). In between,
168 // interpolate. This vaguely matches a curve I copied out of a
169 // book for a single engine. Shrug.
171 float r = _fuelFlow/burnable;
172 if (burnable == 0) burned = 0;
173 else if(r < .625) burned = _fuelFlow;
174 else if(r > 1.375) burned = burnable;
176 burned = _fuelFlow + (burnable-_fuelFlow)*(r-.625)*(4.0/3.0);
178 // Correct for engine control state
184 // And finally the power is just the reference power scaled by the
185 // amount of fuel burned, and torque is that divided by RPM.
186 float power = _power0 * burned/_f0;
187 _torque = power/speed;
189 // Figure that the starter motor produces 20% of the engine's
191 if(_cranking && !_running)
192 _torque += 0.20 * _power0/_omega0;
194 // Also, add a negative torque of 10% of cruise, to represent
195 // internal friction. Propeller aerodynamic friction is too low
196 // at low RPMs to provide a good deceleration. Interpolate it
197 // away as we approach cruise RPMs, though, to prevent interaction
198 // with the power computations. Ugly.
199 if(speed > 0 && speed < _omega0)
200 _torque -= 0.05 * (_power0/_omega0) * (1 - speed/_omega0);
202 // Now EGT. This one gets a little goofy. We can calculate the
203 // work done by an isentropically expanding exhaust gas as the
204 // mass of the gas times the specific heat times the change in
205 // temperature. The mass is just the engine displacement times
206 // the manifold density, plus the mass of the fuel, which we know.
207 // The change in temperature can be calculated adiabatically as a
208 // function of the exhaust gas temperature and the compression
209 // ratio (which we know). So just rearrange the equation to get
210 // EGT as a function of engine power. Cool. I'm using a value of
211 // 1300 J/(kg*K) for the exhaust gas specific heat. I found this
212 // on a web page somewhere; no idea if it's accurate. Also,
213 // remember that four stroke engines do one combustion cycle every
214 // TWO revolutions, so the displacement per revolution is half of
215 // what we'd expect. And diddle the work done by the gas a bit to
216 // account for non-thermodynamic losses like internal friction;
219 float massFlow = _fuelFlow + (rho * 0.5 * _displacement * speed);
220 float specHeat = 1300;
221 float corr = 1.0/(Math::pow(_compression, 0.4) - 1);
222 _egt = corr * (power * 1.1) / (massFlow * specHeat);
223 if(_egt < temp) _egt = temp;
226 }; // namespace yasim