Re: [Flightgear-devel] Problem in FGPiston

[D Luff]

Frederic Bouvier writes:

 There is an invalid float operation in FGPiston when the engine is off :
 
 in void FGPiston::doAirFlow(void)
 ...
   double swept_volume = (displacement_SI * (RPM/60)) / 2;
   double v_dot_air = swept_volume * volumetric_efficiency;
   m_dot_air = v_dot_air * rho_air_manifold;
 ...
 RPM is zero so m_dot_air is zero, and in void FGPiston::doEGT(void)
 ...
   double heat_capacity_exhaust = (Cp_air * m_dot_air) + (Cp_fuel *
 m_dot_fuel);
   double delta_T_exhaust = enthalpy_exhaust / heat_capacity_exhaust;
 ...
 heat_capacity_exhaust that depends on m_dot_air is zero so we should have a
 div by zero error but enthalpy_exhaust is also zero
 
 I think we need a special case for engine off here.
 

Good catch.

Cheers - Dave
-- 
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RE: [Flightgear-devel] Problem in FGPiston

[Jon S. Berndt]

  I think we need a special case for engine off here.

 Good catch.

I have changed some things in FGPiston (fixed) based on my limited
understanding).

void FGPiston::doManifoldPressure(void)
{
  // DAVE: CHECK THIS
  if (Running ) {
ManifoldPressure_inHg = MinManifoldPressure_inHg +
(Throttle * (MaxManifoldPressure_inHg -
 MinManifoldPressure_inHg));
  } else if (Cranking) {
ManifoldPressure_inHg += (dt/2.0)*(MinManifoldPressure_inHg / 6.0 -
   ManifoldPressure_inHg);
  } else {
ManifoldPressure_inHg -= (dt/2.0)*ManifoldPressure_inHg; // fade
  }
}

void FGPiston::doEGT(void)
{
  ...
  ...
  // DAVE: CHECK THIS
  if (heat_capacity_exhaust = 0.001)
delta_T_exhaust = enthalpy_exhaust / heat_capacity_exhaust;
  else
delta_T_exhaust -= (dt/2.0)*delta_T_exhaust;


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Re: [Flightgear-devel] Problem in FGPiston

[Frederic Bouvier]

   I think we need a special case for engine off here.
 
  Good catch.

 I have changed some things in FGPiston (fixed) based on my limited
 understanding).

 void FGPiston::doManifoldPressure(void)
 {
   // DAVE: CHECK THIS
   if (Running ) {
 ManifoldPressure_inHg = MinManifoldPressure_inHg +
 (Throttle * (MaxManifoldPressure_inHg -
  MinManifoldPressure_inHg));
   } else if (Cranking) {
 ManifoldPressure_inHg += (dt/2.0)*(MinManifoldPressure_inHg / 6.0 -
ManifoldPressure_inHg);
   } else {
 ManifoldPressure_inHg -= (dt/2.0)*ManifoldPressure_inHg; // fade
   }
 }

 void FGPiston::doEGT(void)
 {
   ...
   ...
   // DAVE: CHECK THIS
   if (heat_capacity_exhaust = 0.001)
 delta_T_exhaust = enthalpy_exhaust / heat_capacity_exhaust;
   else
 delta_T_exhaust -= (dt/2.0)*delta_T_exhaust;

Compilers complains : delta_T_exhaust used without having been initialized

Why the '-=' in the else case of calculation of delta_T_exhaust ? What is
its previous value ?

-Fred



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RE: [Flightgear-devel] Problem in FGPiston

[BERNDT, JON S. (JON) (JSC-EX) (LM)]

 Compilers complains : delta_T_exhaust used without having 
 been initialized
 
 Why the '-=' in the else case of calculation of 
 delta_T_exhaust ? What is its previous value ?

Oy! Good catch. I screwed up. Fix to be committed shortly.

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Re: [Flightgear-devel] Problem in FGPiston

[Frederic Bouvier]

 void FGPiston::doEGT(void)
 {
   ...
   ...
   // DAVE: CHECK THIS
   if (heat_capacity_exhaust = 0.001)
 delta_T_exhaust = enthalpy_exhaust / heat_capacity_exhaust;
   else
 delta_T_exhaust -= (dt/2.0)*delta_T_exhaust;
 
 
Do you mean :
   if (heat_capacity_exhaust = 0.001)

because zero is less than 0.001 and it bombs anyway !

-Fred



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