1 #include "Atmosphere.hpp"
3 #include "PistonEngine.hpp"
6 const static float HP2W = 745.7f;
7 const static float CIN2CM = 1.6387064e-5f;
8 const static float RPM2RADPS = 0.1047198f;
10 PistonEngine::PistonEngine(float power, float speed)
17 _oilTemp = Atmosphere::getStdTemperature(0);
18 _oilTempTarget = _oilTemp;
21 // Presume a BSFC (in lb/hour per HP) of 0.45. In SI that becomes
22 // (2.2 lb/kg, 745.7 W/hp, 3600 sec/hour) 7.62e-08 kg/Ws.
23 _f0 = power * 7.62e-08f;
28 // We must be at sea level under standard conditions
29 _rho0 = Atmosphere::getStdDensity(0);
31 // Further presume that takeoff is (duh) full throttle and
32 // peak-power, that means that by our efficiency function, we are
33 // at 11/8 of "ideal" fuel flow.
34 float realFlow = _f0 * (11.0f/8.0f);
35 _mixCoeff = realFlow * 1.1f / _omega0;
38 _maxMP = 1e6; // No waste gate on non-turbo engines.
43 // Guess at reasonable values for these guys. Displacements run
44 // at about 2 cubic inches per horsepower or so, at least for
45 // non-turbocharged engines.
47 _displacement = power * (2*CIN2CM/HP2W);
50 void PistonEngine::setTurboParams(float turbo, float maxMP)
55 // This changes the "sea level" manifold air density
56 float P0 = Atmosphere::getStdPressure(0);
57 float P = P0 * (1 + _boost * (_turbo - 1));
58 if(P > _maxMP) P = _maxMP;
59 float T = Atmosphere::getStdTemperature(0) * Math::pow(P/P0, 2./7.);
60 _rho0 = P / (287.1f * T);
63 void PistonEngine::setDisplacement(float d)
68 void PistonEngine::setCompression(float c)
73 float PistonEngine::getMaxPower()
78 bool PistonEngine::isCranking()
83 float PistonEngine::getTorque()
88 float PistonEngine::getFuelFlow()
93 float PistonEngine::getMP()
98 float PistonEngine::getEGT()
103 void PistonEngine::stabilize()
105 _oilTemp = _oilTempTarget;
106 _charge = _chargeTarget;
109 void PistonEngine::integrate(float dt)
111 _oilTemp += (_dOilTempdt * dt);
113 // See comments in Jet.cpp for how this decay constant works
114 float decay = 1.5f * 2.3f / _turboLag;
115 _charge = (_charge + dt*decay * _chargeTarget) / (1 + dt*decay);
118 void PistonEngine::calc(float pressure, float temp, float speed)
120 if(_magnetos == 0 || speed < 60*RPM2RADPS)
122 else if(_fuel == false)
127 // Calculate the factor required to modify supercharger output for
128 // rpm. Assume that the normalized supercharger output ~= 1 when
129 // the engine is at the nominated peak-power rpm (normalised).
130 // A power equation of the form (A * B^x * x^C) has been
131 // derived empirically from some representative supercharger data.
132 // This provides near-linear output over the normal operating range,
133 // with fall-off in the over-speed situation.
134 float rpm_norm = (speed / _omega0);
135 float A = 1.795206541;
136 float B = 0.55620178;
137 float C = 1.246708471;
138 float rpm_factor = A * Math::pow(B, rpm_norm) * Math::pow(rpm_norm, C);
139 _chargeTarget = 1 + (_boost * (_turbo-1) * rpm_factor);
142 // Superchargers have no lag
143 _charge = _chargeTarget;
144 } else if(!_running) {
145 // Turbochargers only work when the engine is actually
146 // running. The 25% number is a guesstimate from Vivian.
147 _chargeTarget = 1 + (_chargeTarget - 1) * 0.25;
150 // We need to adjust the minimum manifold pressure to get a
151 // reasonable idle speed (a "closed" throttle doesn't suck a total
152 // vacuum in real manifolds). This is a hack.
153 float _minMP = (-0.008 * _turbo ) + 0.1;
155 _mp = pressure * _charge;
157 // Scale to throttle setting, clamp to wastegate
159 _mp *= _minMP + (1 -_minMP) * _throttle;
160 if(_mp > _maxMP) _mp = _maxMP;
162 // The "boost" is the delta above ambient
163 _boostPressure = _mp - pressure;
165 // Air entering the manifold does so rapidly, and thus the
166 // pressure change can be assumed to be adiabatic. Calculate a
167 // temperature change, and use that to get the density.
168 // Note: need to model intercoolers here...
169 float T = temp * Math::pow(_mp/pressure, 2.0/7.0);
170 float rho = _mp / (287.1f * T);
172 // The actual fuel flow is determined only by engine RPM and the
173 // mixture setting. Not all of this will burn with the same
175 _fuelFlow = _mixture * speed * _mixCoeff;
176 if(_fuel == false) _fuelFlow = 0;
178 // How much fuel could be burned with ideal (i.e. uncorrected!)
180 float burnable = _f0 * (rho/_rho0) * (speed/_omega0);
182 // Calculate the fuel that actually burns to produce work. The
183 // idea is that less than 5/8 of ideal, we get complete
184 // combustion. We use up all the oxygen at 1 3/8 of ideal (that
185 // is, you need to waste fuel to use all your O2). In between,
186 // interpolate. This vaguely matches a curve I copied out of a
187 // book for a single engine. Shrug.
189 float r = _fuelFlow/burnable;
190 if (burnable == 0) burned = 0;
191 else if(r < .625) burned = _fuelFlow;
192 else if(r > 1.375) burned = burnable;
194 burned = _fuelFlow + (burnable-_fuelFlow)*(r-0.625f)*(4.0f/3.0f);
196 // Correct for engine control state
202 // And finally the power is just the reference power scaled by the
203 // amount of fuel burned, and torque is that divided by RPM.
204 float power = _power0 * burned/_f0;
205 _torque = power/speed;
207 // Figure that the starter motor produces 15% of the engine's
208 // cruise torque. Assuming 60RPM starter speed vs. 1800RPM cruise
209 // speed on a 160HP engine, that comes out to about 160*.15/30 ==
210 // 0.8 HP starter motor. Which sounds about right to me. I think
211 // I've finally got this tuned. :)
212 if(_starter && !_running)
213 _torque += 0.15f * _power0/_omega0;
215 // Also, add a negative torque of 8% of cruise, to represent
216 // internal friction. Propeller aerodynamic friction is too low
217 // at low RPMs to provide a good deceleration. Interpolate it
218 // away as we approach cruise RPMs (full at 50%, zero at 100%),
219 // though, to prevent interaction with the power computations.
221 if(speed > 0 && speed < _omega0) {
222 float interp = 2 - 2*speed/_omega0;
223 interp = (interp > 1) ? 1 : interp;
224 _torque -= 0.08f * (_power0/_omega0) * interp;
227 // Now EGT. This one gets a little goofy. We can calculate the
228 // work done by an isentropically expanding exhaust gas as the
229 // mass of the gas times the specific heat times the change in
230 // temperature. The mass is just the engine displacement times
231 // the manifold density, plus the mass of the fuel, which we know.
232 // The change in temperature can be calculated adiabatically as a
233 // function of the exhaust gas temperature and the compression
234 // ratio (which we know). So just rearrange the equation to get
235 // EGT as a function of engine power. Cool. I'm using a value of
236 // 1300 J/(kg*K) for the exhaust gas specific heat. I found this
237 // on a web page somewhere; no idea if it's accurate. Also,
238 // remember that four stroke engines do one combustion cycle every
239 // TWO revolutions, so the displacement per revolution is half of
240 // what we'd expect. And diddle the work done by the gas a bit to
241 // account for non-thermodynamic losses like internal friction;
243 float massFlow = _fuelFlow + (rho * 0.5f * _displacement * speed);
244 float specHeat = 1300;
245 float corr = 1.0f/(Math::pow(_compression, 0.4f) - 1.0f);
246 _egt = corr * (power * 1.1f) / (massFlow * specHeat);
247 if(_egt < temp) _egt = temp;
251 // Assume a linear variation between ~90degC at idle and ~120degC
252 // at full power. No attempt to correct for airflow over the
253 // engine is made. Make the time constant to attain target steady-
254 // state oil temp greater at engine off than on to reflect no
255 // circulation. Nothing fancy, but populates the guage with a
259 _oilTempTarget = 363.0f + (30.0f * (power/_power0));
261 // Reduce tau linearly to 300 at max power
262 tau -= (power/_power0) * 300.0f;
264 _oilTempTarget = temp;
267 _dOilTempdt = (_oilTempTarget - _oilTemp) / tau;
270 }; // namespace yasim