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I am working on a custom engine system for A320Family.

How do I tell JSB to start the engine? (the spinup, adding fuel)

I believe @jwocky told me how at one point, but I forgot.

Thanks,
Josh

Basically give them fuel, cut off and then connect fuel again to make them spin up.

Which property?

I need to find how to spin up the engines to the spinup point, and not go any higher until the fuel is added.

the rpm depend on throttle

You can see the whole procedure in the Ruslan for example. It hangs on the idle property to start it, but the main thing is the fuel cut-off and on thing. The spin-up point, I guess, you mean by that the idle rpm? That would be at zero throttle.

No,
N2 goes to about 20ish, then N1 to about 5ish, until fuel is added to engine. I need this. (I know it's possible, 707 does it, also A330-200, both JSB)
How to acquire this?

In Ya, I simulate engine startup via Nasal interpolate(), but I don't do this here unless has to.

J

I am still not sure what you mean here. The 707 (the Markus Krauss one) goes without fuel to some little rpm, that is from the starter, but the engine doesn't actually start without fuel at all. Then, it goes for a idle N1 of 30% and idle N2 of 60%. But to get there, the whole engine needs a moment to spin up and during this, it would roughly look like you describe: N2 goes up, at some point, the ignition comes in, then N1 goes first up with a boom and from there, it spins up to idle values. This is, give or take, the regular procedure, there seems to be nothing special done. Which plane are we talking about (as in which A320), so that I can look at it?

I am talking in real life. That is how it should work.

On starter, it goes only 20 N2 and 5 N1, and he does not go further Until fuel is added.

I stuck on the "until fuel is added". They won't go to 5% nor to 1% nor anywhere without fuel. So I assume, you mean additional throttle? If it is that, you want to look at the idleN1 and idleN2 properties in the engine files.

Okay, I took a moment to try. Set idlen1 and idlen2 in the engine file to 5 respectively 20%. But they need fuel to get there, otherwise they stay on 0% of course. What you set there is basically what they use at 0 throttle for idling.

Historically, a lot of JSB planes seem to use extreme high idle, that comes from the plane devs coming from the old pistons, which need obviously more. Some kind of bad tradition

No no no.
That's not how.

Look:
<milthrust> 28000.0 </milthrust> <!-- I'm using the IAE V2528-D5 version. 28000 Pounds Thrust. -->
<bypassratio> 2 </bypassratio>
<tsfc> 0.54 </tsfc>
<idlen1> 23.4 </idlen1>
<idlen2> 62.7 </idlen2>
<maxn1> 105.8 </maxn1>
<maxn2> 102.1 </maxn2>
<augmented> 0 </augmented>
<injected> 0 </injected>

From KC-137R, MK707s use the same thing

WHEN the fuel selector is off, and starter is on, JSB only spools up a little, not to those values.