Setting combustion off forces engine RPM to 0

When setting combustion to OFF using the A:var (reciprocal engine), engine RPM
instantaneously goes to 0 on the next frame. When combustion is stopped
through “regular means” such as turning the magnetos to OFF, the combustion
flag goes to 0 but the engine smoothly decelerates. Is it possible to make the
propeller behaviour consistent?

Hello @SWS-AlexVletsas I think you will
have this behaviour if you write to GENERAL ENG COMBUSTION EX1 instead of

I tried it and it did not work. The EX1 variable turns to 0 and is reset to 1
on the next cycle.

SWS-AlexVletsas can you be more specific about your sequence, so I could
reproduce it? There is no SimVar called A:var. I’m guessing your aircraft is
on the ground during this test. Can you reproduce this with a stock aircraft,
e.g. the Cessna 152? If so can you let me know the simplest plane? I think you
have an issue with the ENGINE behaviour, not the propeller behaviour, although
I accept those things appear bolted together in the model.

You can try with the Asobo TBM. Writing 0 to GENERAL ENG COMBUSTION will
instantly stop the propeller while writing 0 to GENERAL ENG COMBUSTION EX1
will have it decelerate smoothly.

Are you not writing to it somewhere else? I ran my tests on the Asobo TBM and
it worked.

I’ve realised this thread assumes the plane is on the ground, so my hope
someone might have found a way to stop the propeller rotating more generally
(i.e. including in the air) doesn’t apply. Raccoon is that a typo in your
comment? i.e. your “1” would be a zero hence 0 (>GENERAL ENG COMBUSTION:1,

This is an area were I am also struggling , both the slow down of the prop at
the end of a cranking start failure, as well as matching associated
appropriate sound, to that slowdown to stop transition. So watching with
interest to see how this develops :slight_smile:

There are no other instances where I’m setting the combustion variable, only
there. The aircraft uses a piston engine. When setting the magnetos to off,
either by code or using the switch, the engine decelerates smoothly. Cutting
fuel using the mixture lever or fuel selector also stops it smoothly. Setting
the combustion to 0 by code freezes it. Setting Combustion EX1 doesn’t do
anything and it resets back to 1 on the next frame.

Yes, I meant 0 obviously.

Ok, will try with a piston then. Maybe engine type related.

@SWS-AlexVletsas Some additional details:
The GENERAL ENG COMBUSTION EX1 simvar depends on other factors. On
turbines, if auto_ignition is enabled and ignition_auto_type is set to
always on (empty string or omitted parameter), the combustion will
logically be reset to 1 instantly. On piston engines, if you set the simvar to
0 but magnetos are on and engines are running fast enough to restart,
combustion will be set to 1 as well. May I ask what are you trying to achieve?
If you’re trying to stop combustion on a piston engine, I’d say using the
magnetos simvars is more appropriate. Regards, Sylvain

@Nocturne FYI

@flyingRaccoon I tried using the magneto simvars as well, but this resulted in
the key being turned with them. As the current engine/electrical systems do
not support PMAGs, the only way to do it is to null the combustion Simvar. To
explain a bit further: the plane with said problem is the RV-14 which is
equipped with P-MAGs and a test switch for them. Setting the switch to TEST
will disconnect the P-MAGs from the electrical system. If the engine has
sufficient speed to maintain a combustion it will, but if it drops below
1000rpm the engine will stop, regardless of the magneto position. As I cannot
set the rpm threshold through the engine file and setting the magnetos
triggers the animation, turning off the engine with one of the combustion
flags is the only way I can think of to shut down the engine.

@SWS-AlexVletsas What about using a custom
LVar instead of the magneto simvar to animate your key?

I am using the default Magneto (B:var based) so I don’t know if it can be
modified. It would also add an extra layer of complexity as the mags would
have to be synched to the L:var when the engine gets back over the threshold
and make it harder to reuse on other aircraft. It seems as if the game ignores
the propeller MOI & drag in this instance, I don’t see understand why the TBM
works but pistons don’t.

TBM has an auto ignition type set to Starter and the starter is probably off
when you write to the combustion simvar.