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 // Presume a BSFC (in lb/hour per HP) of 0.45. In SI that becomes
18 // (2.2 lb/kg, 745.7 W/hp, 3600 sec/hour) 7.62e-08 kg/Ws.
19 _f0 = power * 7.62e-08f;
24 // We must be at sea level under standard conditions
25 _rho0 = Atmosphere::getStdDensity(0);
27 // Further presume that takeoff is (duh) full throttle and
28 // peak-power, that means that by our efficiency function, we are
29 // at 11/8 of "ideal" fuel flow.
30 float realFlow = _f0 * (11.0f/8.0f);
31 _mixCoeff = realFlow * 1.1f / _omega0;
34 _maxMP = 1e6; // No waste gate on non-turbo engines.
36 // Guess at reasonable values for these guys. Displacements run
37 // at about 2 cubic inches per horsepower or so, at least for
38 // non-turbocharged engines.
40 _displacement = power * (2*CIN2CM/HP2W);
43 void PistonEngine::setTurboParams(float turbo, float maxMP)
48 // This changes the "sea level" manifold air density
49 float P0 = Atmosphere::getStdPressure(0);
50 float P = P0 * (1 + _boost * (_turbo - 1));
51 if(P > _maxMP) P = _maxMP;
52 float T = Atmosphere::getStdTemperature(0) * Math::pow(P/P0, 2./7.);
53 _rho0 = P / (287.1f * T);
56 void PistonEngine::setDisplacement(float d)
61 void PistonEngine::setCompression(float c)
66 float PistonEngine::getMaxPower()
71 bool PistonEngine::isCranking()
76 float PistonEngine::getTorque()
81 float PistonEngine::getFuelFlow()
86 float PistonEngine::getMP()
91 float PistonEngine::getEGT()
96 void PistonEngine::calc(float pressure, float temp, float speed)
98 if(_magnetos == 0 || speed < 60*RPM2RADPS)
100 else if(_fuel == false)
105 // Calculate the factor required to modify supercharger output for
106 // rpm. Assume that the normalized supercharger output ~= 1 when
107 // the engine is at the nominated peak-power rpm (normalised).
108 // A power equation of the form (A * B^x * x^C) has been
109 // derived empirically from some representative supercharger data.
110 // This provides near-linear output over the normal operating range,
111 // with fall-off in the over-speed situation.
112 float rpm_norm = (speed / _omega0);
113 float A = 1.795206541;
114 float B = 0.55620178;
115 float C = 1.246708471;
116 float rpm_factor = A * Math::pow(B, rpm_norm) * Math::pow(rpm_norm, C);
118 // We need to adjust the minimum manifold pressure to get a
119 // reasonable idle speed (a "closed" throttle doesn't suck a total
120 // vacuum in real manifolds). This is a hack.
121 float _minMP = (-0.008 * _turbo ) + 0.1;
123 // Scale to throttle setting, clamp to wastegate
125 _mp = pressure * (1 + (_boost * (_turbo-1) * rpm_factor));
126 _mp *= _minMP + (1 -_minMP) * _throttle;
128 if(_mp > _maxMP) _mp = _maxMP;
130 // The "boost" is the delta above ambient
131 _boostPressure = _mp - pressure;
133 // Air entering the manifold does so rapidly, and thus the
134 // pressure change can be assumed to be adiabatic. Calculate a
135 // temperature change, and use that to get the density.
136 // Note: need to model intercoolers here...
137 float T = temp * Math::pow(_mp/pressure, 2.0/7.0);
138 float rho = _mp / (287.1f * T);
140 // The actual fuel flow is determined only by engine RPM and the
141 // mixture setting. Not all of this will burn with the same
143 _fuelFlow = _mixture * speed * _mixCoeff;
144 if(_fuel == false) _fuelFlow = 0;
146 // How much fuel could be burned with ideal (i.e. uncorrected!)
148 float burnable = _f0 * (rho/_rho0) * (speed/_omega0);
150 // Calculate the fuel that actually burns to produce work. The
151 // idea is that less than 5/8 of ideal, we get complete
152 // combustion. We use up all the oxygen at 1 3/8 of ideal (that
153 // is, you need to waste fuel to use all your O2). In between,
154 // interpolate. This vaguely matches a curve I copied out of a
155 // book for a single engine. Shrug.
157 float r = _fuelFlow/burnable;
158 if (burnable == 0) burned = 0;
159 else if(r < .625) burned = _fuelFlow;
160 else if(r > 1.375) burned = burnable;
162 burned = _fuelFlow + (burnable-_fuelFlow)*(r-0.625f)*(4.0f/3.0f);
164 // Correct for engine control state
170 // And finally the power is just the reference power scaled by the
171 // amount of fuel burned, and torque is that divided by RPM.
172 float power = _power0 * burned/_f0;
173 _torque = power/speed;
175 // Figure that the starter motor produces 15% of the engine's
176 // cruise torque. Assuming 60RPM starter speed vs. 1800RPM cruise
177 // speed on a 160HP engine, that comes out to about 160*.15/30 ==
178 // 0.8 HP starter motor. Which sounds about right to me. I think
179 // I've finally got this tuned. :)
180 if(_starter && !_running)
181 _torque += 0.15f * _power0/_omega0;
183 // Also, add a negative torque of 8% of cruise, to represent
184 // internal friction. Propeller aerodynamic friction is too low
185 // at low RPMs to provide a good deceleration. Interpolate it
186 // away as we approach cruise RPMs (full at 50%, zero at 100%),
187 // though, to prevent interaction with the power computations.
189 if(speed > 0 && speed < _omega0) {
190 float interp = 2 - 2*speed/_omega0;
191 interp = (interp > 1) ? 1 : interp;
192 _torque -= 0.08f * (_power0/_omega0) * interp;
195 // Now EGT. This one gets a little goofy. We can calculate the
196 // work done by an isentropically expanding exhaust gas as the
197 // mass of the gas times the specific heat times the change in
198 // temperature. The mass is just the engine displacement times
199 // the manifold density, plus the mass of the fuel, which we know.
200 // The change in temperature can be calculated adiabatically as a
201 // function of the exhaust gas temperature and the compression
202 // ratio (which we know). So just rearrange the equation to get
203 // EGT as a function of engine power. Cool. I'm using a value of
204 // 1300 J/(kg*K) for the exhaust gas specific heat. I found this
205 // on a web page somewhere; no idea if it's accurate. Also,
206 // remember that four stroke engines do one combustion cycle every
207 // TWO revolutions, so the displacement per revolution is half of
208 // what we'd expect. And diddle the work done by the gas a bit to
209 // account for non-thermodynamic losses like internal friction;
212 float massFlow = _fuelFlow + (rho * 0.5f * _displacement * speed);
213 float specHeat = 1300;
214 float corr = 1.0f/(Math::pow(_compression, 0.4f) - 1.0f);
215 _egt = corr * (power * 1.1f) / (massFlow * specHeat);
216 if(_egt < temp) _egt = temp;
219 }; // namespace yasim