Updates to IO360 engine model: Added a couple of guestimates for engine

and prop inertia and passed the timestep from LaRCsim in order to have
the engine rpm behaving according to the applied torque and the laws of
physics.

diff --git a/src/FDM/IO360.cxx b/src/FDM/IO360.cxx
index 3ee2e1ed2ac084831d7d2b239533b747d763a478..22e7f97f9d3ccef7cb6559adeb325117968b2f29 100644 (file)
--- a/src/FDM/IO360.cxx
+++ b/src/FDM/IO360.cxx
@@ -66,6 +66,10 @@
 // I've tried to match the prop and engine model to give roughly 600 RPM idle and 180 HP at 2700 RPM
 // but it is by no means currently at a completed stage - DCL 15/9/00
 //
+// DCL 28/9/00 - Added estimate of engine and prop inertia and changed engine speed calculation to be calculated from Angular acceleration = Torque / Inertia.  
+//              Requires a timestep to be passed to FGEngine::init and currently assumes this timestep does not change.
+//              Could easily be altered to pass a variable timestep to FGEngine::update every step instead if required.
+//
 //////////////////////////////////////////////////////////////////////
 
 #include <iostream.h>
@@ -128,7 +132,7 @@ float FGEngine::Lookup_Combustion_Efficiency(float thi_actual)
     neta_comb[8] = 0.63;
     neta_comb[9] = 0.57;
     neta_comb[10] = 0.525;
-    //combustion efficiency values from Heywood [1]
+    //combustion efficiency values from Heywood: ISBN 0-07-100499-8
 
     int i;
     int j;
@@ -194,7 +198,7 @@ static float Calc_Manifold_Pressure ( float LeverPosn, float MaxMan, float MinMa
 
 
 // set initial default values
-void FGEngine::init() {
+void FGEngine::init(double dt) {
 
     CONVERT_CUBIC_INCHES_TO_METERS_CUBED = 1.638706e-5;
     // Control and environment inputs
@@ -206,6 +210,7 @@ void FGEngine::init() {
     Cp_fuel = 1700;    // J/KgK
     calorific_value_fuel = 47.3e6;  // W/Kg  Note that this is only an approximate value
     R_air = 287.3;
+    time_step = dt;
 
     // Engine Specific Variables used by this program that have limits.
     // Will be set in a parameter file to be read in to create
@@ -220,6 +225,8 @@ void FGEngine::init() {
     MaxHP = 180;    //Lycoming IO360
 //  displacement = 520;  //Continental IO520-M
     displacement = 360;         //Lycoming IO360   
+    engine_inertia = 0.2;  //kgm^2 - value taken from a popular family saloon car engine - need to find an aeroengine value !!!!!
+    prop_inertia = 0.03;  //kgm^2 - this value is a total guess - dcl
     displacement_SI = displacement * CONVERT_CUBIC_INCHES_TO_METERS_CUBED;
 
     Gear_Ratio = 1;
@@ -491,7 +498,7 @@ void FGEngine::update() {
     // Engine Power & Torque Calculations
 
     // Loop until stable - required for testing only
-    for (num = 0; num < num2; num++) {
+//    for (num = 0; num < num2; num++) {
        // cout << Manifold_Pressure << " Inches" << "\t";
        // cout << RPM << "  RPM" << "\t";
 
@@ -530,7 +537,8 @@ void FGEngine::update() {
        //uses a curve fit to the data in the IO360 / O360 operating manual
        //due to the shape of the curve I had to use a 6th order fit - I am sure it must be possible to reduce this in future,
        //possibly by using separate fits for rich and lean of best power mixture
-       //first adjust actual mixture to abstract mixture - this is a temporary hack
+       //first adjust actual mixture to abstract mixture - this is a temporary hack in order to account for the fact that the data I have
+       //dosn't specify actual mixtures and I want to be able to change what I think they are without redoing the curve fit each time.
        //y=10x-12 for now
        abstract_mixture = 10.0 * equivalence_ratio - 12.0;
        float m = abstract_mixture;  //to simplify writing the next equation
@@ -710,15 +718,7 @@ void FGEngine::update() {
 //#if 0
 #ifdef PHILS_PROP_MODEL  //Do Torque calculations in Ft/lbs - yuk :-(((
        Torque_Imbalance = FGProp1_Torque - Torque; 
-#endif
-
-#ifdef NEVS_PROP_MODEL     //use proper units - Nm
-       Torque_Imbalance = prop_torque - Torque_SI;
-#endif
-
-       //  cout <<  Torque_Imbalance << endl;
 
-//  Some really crude engine speed calculations for now - we really need some moments of inertia and a dt in here !!!!
        if (Torque_Imbalance > 5) {
            RPM -= 14.5;
            // FGProp1_RPM -= 25;
@@ -730,8 +730,19 @@ void FGEngine::update() {
            // FGProp1_RPM += 25;
 //         FGProp1_Blade_Angle += 0.75;
        }
+#endif
+
+
+#ifdef NEVS_PROP_MODEL     //use proper units - Nm
+       Torque_Imbalance = Torque_SI - prop_torque;  //This gives a +ve value when the engine torque exeeds the prop torque
+
+       angular_acceleration = Torque_Imbalance / (engine_inertia + prop_inertia);
+       angular_velocity_SI += (angular_acceleration * time_step);
+       RPM = (angular_velocity_SI * 60) / (2.0 * PI);
+#endif
+
+
 
-       //DCL - This constant speed prop bit is all a bit of a hack for now
 /*
        if( RPM > (Desired_RPM + 2)) {
            FGProp1_Blade_Angle += 0.75;  //This value could be altered depending on how far from the desired RPM we are
@@ -757,11 +768,11 @@ void FGEngine::update() {
            RPM = 0;
 
 //     outfile << "RPM = " << RPM << " Blade angle = " << FGProp1_Blade_Angle << " Engine torque = " << Torque << " Prop torque = " << FGProp1_Torque << '\n';
-       outfile << "RPM = " << RPM << " Engine torque = " << Torque_SI << " Prop torque = " << prop_torque << '\n';    
+//     outfile << "RPM = " << RPM << " Engine torque = " << Torque_SI << " Prop torque = " << prop_torque << '\n';    
 
        // cout << FGEng1_RPM << " Blade_Angle  " << FGProp1_Blade_Angle << endl << endl;
 
-    }
+
 
     // cout << "Final engine RPM = " << RPM << '\n';
 }

diff --git a/src/FDM/IO360.hxx b/src/FDM/IO360.hxx
index 60f9405ed6edff22e65dc5070da259edbd38d413..71e63eb917b8f91c8592c409b80f0ce498076268 100644 (file)
--- a/src/FDM/IO360.hxx
+++ b/src/FDM/IO360.hxx
@@ -137,6 +137,10 @@ private:
     float enthalpy_exhaust;
     float Percentage_of_best_power_mixture_power;
     float abstract_mixture;    //temporary hack
+    float engine_inertia;      //kg.m^2
+    float prop_inertia;                //kg.m^2
+    float angular_acceleration;        //rad/s^2
+    double time_step;
 
     // Initialise Propellor Variables used by this instance
     float FGProp1_Angular_V;
@@ -189,16 +193,16 @@ public:
 
     //constructor
     FGEngine() {
-       outfile.open("FGEngine.dat", ios::out|ios::trunc);
+//     outfile.open("FGEngine.dat", ios::out|ios::trunc);
     }
 
     //destructor
     ~FGEngine() {
-       outfile.close();
+//     outfile.close();
     }
 
     // set initial default values
-    void init();
+    void init(double dt);
 
     // update the engine model based on current control positions
     void update();

diff --git a/src/FDM/LaRCsim.cxx b/src/FDM/LaRCsim.cxx
index b799988862fadaf29e929d06d21a386406ed923f..35fcb9d58e28c07aecd3f0fac347f5b3462616ea 100644 (file)
--- a/src/FDM/LaRCsim.cxx
+++ b/src/FDM/LaRCsim.cxx
@@ -45,7 +45,9 @@ int FGLaRCsim::init( double dt ) {
 
 #ifdef USE_NEW_ENGINE_CODE
     // Initialize our little engine that hopefully might
-    eng.init();
+    eng.init(dt);
+    // dcl - in passing dt to init rather than update I am assuming
+    // that the LaRCsim dt is fixed at one value (yes it is 120hz CLO)
 #endif
 
     // cout << "FGLaRCsim::init()" << endl;

diff --git a/src/FDM/engine.cxx b/src/FDM/engine.cxx
index 89758c8ac7c8ebe754e17975d41d058f07f34969..a87b3aace4996f0c62b25b1ed5566dd2dcf34065 100644 (file)
--- a/src/FDM/engine.cxx
+++ b/src/FDM/engine.cxx
@@ -1,11 +1,11 @@
 // 10520d test program
 
-#include "10520d.hxx"
+#include "IO360.hxx"
 
 int main() {
     FGEngine e;
 
-    e.init();
+    e.init( 1.0 / 120.0 );
 
     e.set_IAS( 80 );
     e.set_Throttle_Lever_Pos( 50.0 );