Fixing the mixture for JSBSim piston aircraft

[hbeni]

Hi,
David Megginson found (and fixed) a bug in the mixture control.

See: https://forum.flightgear.org/viewtopic.php?f=49&t=42055

Over on the flightgear-devel mailing list, I've been discussing a patch to fix the mixture input for JSBSim piston-engine aircraft.

Right now, the mixture control in src/FDM/JSBSim/models/propulsion/FGPiston.cpp is incorrectly clamped so that 1.0 (full rich) sets the mixture to peak power efficiency. In real life, the mixture control continues a ways rich of peak power to avoid predetonation at high power/low density altitude, which is why you have to lean a little for takeoff above 3,000 ft DA (or so). Anyone who's flown a simple trainer like a Cessna 172 or Piper PA-28 also knows how the RPM will initially increase as you start leaning, before it decreases again, which doesn't happen with our current JSBSim piston aircraft.

---

diff --git a/src/FDM/JSBSim/models/propulsion/FGPiston.cpp b/src/FDM/JSBSim/models/propulsion/FGPiston.cpp
index 1e73a503d..c1c75bd57 100644
--- a/src/FDM/JSBSim/models/propulsion/FGPiston.cpp
+++ b/src/FDM/JSBSim/models/propulsion/FGPiston.cpp
@@ -752,7 +752,8 @@ void FGPiston::doAirFlow(void)
 
 void FGPiston::doFuelFlow(void)
 {
-  double thi_sea_level = 1.3 * in.MixturePos[EngineNumber]; // Allows an AFR of infinity:1 to 11.3075:1
+  // use square root to increase sensitivity in the higher end
+  double thi_sea_level = 1.65 * sqrt(in.MixturePos[EngineNumber]); // Allows an AFR of infinity:1 to (no longer accurate) 11.3075:1
   equivalence_ratio = thi_sea_level * 101325.0 / p_amb;
   m_dot_fuel = (m_dot_air * equivalence_ratio) / 14.7;
   FuelFlowRate =  m_dot_fuel * 2.2046;  // kg to lb

[bcoconni]

Thanks @hbeni for submitting this issue.

I'm a bit curious about where this equation is coming from ? Can you document it ? At the moment, as far as I can tell (I'm no specialist in internal combustion engines) this diff is replacing some magic numbers by other magic numbers. Are the new values applicable to all piston engines under the sun ?

[seanmcleod]

So you're proposing to change thi_sea_level from f2 to f1 below? As @bcoconni asked, where is this equation coming from?

[bcoconni]

Yep, in addition I don't see the point in modifying the C++ code as the same result could be achieved using a <channel> in the aircraft FCS.

  <channel name="Mixture control">
    <fcs_function>
      <function>
        <product>
          <value>1.27</value> <!-- 1.65 / 1.3 -->
          <sqrt>
            <property>fcs/mixture-cmd-norm</property>
          </sqrt>
        </product>
      </function>
      <output>fcs/mixture-pos-norm</output>
    </fcs_function>
  </channel>

[bcoconni]

@dany93 could you comment on this topic ?

[bcoconni]

The same result could be achieved using a <channel> in the aircraft FCS.

I'm not inventing anything here as this is done in a number of our aircraft. See the 6 examples below:

jsbsim/aircraft/c310/c310.xml

Lines 521 to 551 in 92daf89

	<channel name="Automatic Mixture Control">
	<fcs_function name="systems/mixture-cmd-norm">
	<function>
	<quotient>
	<table>
	<independentVar lookup="row">atmosphere/P-psf</independentVar>
	<tableData>
	50 0.1
	990 0.5
	1300 0.675
	2117 1.0
	</tableData>
	</table>
	<value>1.0</value>
	</quotient>
	</function>
	</fcs_function>
	<pure_gain name="systems/mixture-pos-norm[0]">
	<input> fcs/mixture-cmd-norm[0]</input>
	<gain> systems/mixture-cmd-norm</gain>
	<output>fcs/mixture-pos-norm[0]</output>
	</pure_gain>
	<pure_gain name="systems/mixture-pos-norm[1]">
	<input> fcs/mixture-cmd-norm[1]</input>
	<gain> systems/mixture-cmd-norm</gain>
	<output>fcs/mixture-pos-norm[1]</output>
	</pure_gain>
	</channel>

jsbsim/aircraft/Camel/Systems/automixture.xml

Lines 7 to 67 in 92daf89

	<channel name="Clerget9B">
	<fcs_function name="systems/mixture-cmd">
	<function>
	<product>
	<table>
	<independentVar lookup="row">atmosphere/P-psf</independentVar>
	<!-- This keeps the engine at 1250 rpm 2013/04/15 -->
	<tableData>
	<!--
	50 0.075
	990 0.52
	1078 0.62325
	1215.7 0.72225
	1364.6 0.824
	1679 0.714
	2048 0.846
	2110 0.86525
	2500 1.0
	-->
	<!--
	50 0.075
	990 0.52
	1208 0.5655
	1470 0.6425
	1671 0.725
	2043 0.846
	2111 0.86525
	-->
	50 0.075
	990 0.52
	1207 0.61775 <!-- wont reach 1250, about 1210 -->
	1469 0.843
	1684 0.78275
	2048 0.9065
	2113 0.934
	2500 1.0
	</tableData>
	</table>
	</product>
	</function>
	</fcs_function>
	<switch name="systems/mixture-pos-norm">
	<default value="fcs/mixture-cmd-norm"/>
	<test value="systems/mixture-cmd">fcs/automixture-enable == 1</test>
	</switch>
	<pure_gain name="systems/mixture-pos-gain-norm">
	<input>systems/mixture-pos-norm</input>
	<gain>fcs/mixture-gain</gain>
	<output>fcs/mixture-pos-norm</output>
	</pure_gain>
	</channel>
	</system>

jsbsim/aircraft/p51d/Systems/mixture-control.xml

Lines 27 to 61 in 92daf89

	<channel name="Mixture Control">
	<switch name="mixture/position">
	<default value= "atmosphere/delta" />
	<test value="atmosphere/delta"> <!-- emergency full rich -->
	fcs/mixture-cmd-norm GE 0.90
	</test>
	<test logic="AND" value="999999999999"> <!-- cut off -->
	fcs/throttle-cmd-norm LE 0.1
	fcs/mixture-cmd-norm LE 0.1
	</test>
	<test value="1.053"> <!-- lean -->
	fcs/mixture-cmd-norm LE 0.1
	</test>
	<test value="1.0"> <!-- normal run -->
	fcs/mixture-cmd-norm LT 0.90
	fcs/mixture-cmd-norm GT 0.1
	</test>
	</switch>
	<fcs_function name="mixture">
	<function>
	<product>
	<quotient> <!-- invert mixture/position -->
	<value> 1.0 </value>
	<property> mixture/position </property>
	</quotient>
	<property> atmosphere/delta </property>
	</product>
	</function>
	<output>fcs/mixture-pos-norm</output>
	</fcs_function>
	</channel>

jsbsim/aircraft/Short_S23/Systems/engines.xml

Lines 145 to 170 in 92daf89

	<fcs_function name="fcs/mixture/pos-norm[0]">
	<function>
	<product>
	<le>
	<property>fcs/outer-engines-cutoff-cmd-norm</property>
	<value>0.0</value>
	</le>
	<property>fcs/mixture/altitude-standard-pos</property>
	<sum>
	<value>1.0</value>
	<product>
	<property>fcs/mixture/manual-control-factor</property>
	<difference>
	<property>fcs/mixture-cmd-norm[0]</property>
	<value>0.5</value>
	</difference>
	</product>
	</sum>
	</product>
	</function>
	<clipto>
	<min>0.0</min>
	<max>1.0</max>
	</clipto>
	<output>fcs/mixture-pos-norm[0]</output>
	</fcs_function>

jsbsim/aircraft/ZLT-NT/Systems/engine-control.xml

Lines 263 to 278 in 92daf89

	<channel name="Mixture control">
	<switch name="fcs/etc/mixture-pos-norm[0]">
	<default value="fcs/mixture-cmd-norm[0]"/>
	<output>fcs/mixture-pos-norm[0]</output>
	</switch>
	<switch name="fcs/etc/mixture-pos-norm[1]">
	<default value="fcs/mixture-cmd-norm[1]"/>
	<output>fcs/mixture-pos-norm[1]</output>
	</switch>
	<switch name="fcs/etc/mixture-pos-norm[2]">
	<default value="fcs/mixture-cmd-norm[2]"/>
	<output>fcs/mixture-pos-norm[2]</output>
	</switch>
	</channel>

jsbsim/check_cases/piston_takeoff/aircraft/c172x/c172x.xml

Lines 237 to 257 in 92daf89

	<system name="Mixture control">
	<!--
	E = 1.3 * Mixture * P_std / P_amb
	Mixture = P_amb / P_std
	-->
	<channel name="Automatic Mixture Control">
	<fcs_function name="systems/mixture-pos-norm">
	<function>
	<table>
	<independentVar lookup="row">atmosphere/P-psf</independentVar>
	<tableData>
	0 0.0
	2117 1.0
	</tableData>
	</table>
	</function>
	<output>fcs/mixture-cmd-norm</output>
	</fcs_function>
	</channel>
	</system>

[hbeni]

I did post this back to the discussion on the forums: https://forum.flightgear.org/viewtopic.php?p=419011#p419011
Please note, I'm just the messenger here; David Megginson is the original source for this issue. I think he just has no access to github or so.

[dany93]

I have some experience on ultralights only, I fully trust David Megginson on the mixture subject.

However, I notice:
@bcoconni wrote

Are the new values applicable to all piston engines under the sun ?

Yep, in addition I don't see the point in modifying the C++ code as the same result could be achieved using a <channel> in the aircraft FCS.

I would rather agree with @bcoconni. Modifying the C++ code can have negative effects on current aircraft.
Using the FCS and adding some explanations in the wiki might be sufficient and less risky. With a piece of code as an example.
Moreover, it is more flexible.

[hbeni]

The issue with the current c++ code is, you cant go richer than the current 1.0 setting.
Thus, you cant fix it using a channel ☝️

Davids patch fixes this.

[hbeni]

Maybe another alternative, if you dont want to change it directly in the code for all aircraft, add in a factor read optionally from another new property, so aircraft can implement this.

[bcoconni]

The issue with the current c++ code is, you cant go richer than the current 1.0 setting.

Not sure what you mean by that. But the properties fcs/mixture-cmd-norm and fcs/mixture-pos-norm can be set to any real value, including higher than 1 or even lower than 0. AFAICS there is no code in JSBSim that caps the mixture properties to the [0;1] range.

I've checked by modifying the script scripts/c1721.xml as below. After having run the script, the properties fcs/mixture-pos-norm and fcs/mixture-cmd-norm return whatever value that they have been set to (-15.3 in the example below but 10.0 or 1.5345 work just as well).

diff --git a/scripts/c1721.xml b/scripts/c1721.xml
index 461cb93e..a02eea93 100644
--- a/scripts/c1721.xml
+++ b/scripts/c1721.xml
@@ -8,10 +8,19 @@
     <event name="event name">
       <condition>simulation/sim-time-sec  ge  0.25</condition>
       <set name="fcs/aileron-cmd-norm" action="FG_STEP" value="0.25" tc="0.25"/>
+      <set name="fcs/mixture-cmd-norm" action="FG_STEP" value="-15.3" tc="0.25"/>
+      <notify>
+        <property>fcs/mixture-cmd-norm</property>
+        <property>fcs/mixture-pos-norm</property>
+      </notify>
     </event>
     <event name="event name">
       <condition>simulation/sim-time-sec  ge  0.5</condition>
       <set name="fcs/aileron-cmd-norm" action="FG_EXP" type="FG_DELTA" value="0.5" tc="0.5"/>
+      <notify>
+        <property>fcs/mixture-cmd-norm</property>
+        <property>fcs/mixture-pos-norm</property>
+      </notify>
     </event>
     <event name="event name">
       <condition>simulation/sim-time-sec  ge  1.5</condition>

Script output:

event name (Event 0) executed at time: 0.258323
    fcs/mixture-cmd-norm = -15.300000
    fcs/mixture-pos-norm = 0.000000


event name (Event 1) executed at time: 0.508313
    fcs/mixture-cmd-norm = -15.300000
    fcs/mixture-pos-norm = -15.300000

[hbeni]

Okay, understood. Thank you for elaborating - but more info on the wiki would be good.
Just tested this on the 172p, works as described.

I think there just was a misconception about the clamping - the clamping in the 172p comes from the direct transmisison of the lever position (which is clamped to 1.0 for animation reasons).

---

In c172p.xml:

<fcs_function name="fcs/mixture-unit-cmd">
    <function>
        <product>
        <value>1.27</value> <!-- 1.65 / 1.3 -->
        <sqrt>
            <!--<property>fcs/mixture-cmd-norm</property>-->
            <property>/controls/engines/current-engine/mixture</property>
        </sqrt>
        </product>
    </function>
    <output>fcs/mixture-cmd-norm[0]</output>
    <output>fcs/mixture-cmd-norm[1]</output>
</fcs_function>

Does what the C++ patch would do.
(It mus be done to mixture-cmd-norm, because the priming code currently already occupies the pos-norm property)