Engine running with ignition off

[effte]

The engine will run happily with the ignition off, as long as you hold the starter engaged.

 

Once you disengage the starter, the engine stops again.

[Eddie]

Intended feature. I don't k ow the specifics (Yo-Yo is the man to ask) but the enginge starter system has it's own separate ignition coil.

[effte]

It has a booster coil on the magnetos, providing a more powerful spark on the R/H ignition system (the L/H booster coil is not connected to anything) to facilitate starting.

 

It's still the same magneto, so if it is grounded (as it is when the magneto switch is off), there's no spark and the engine should not run.

[Eddie]

I have no idea who's riight (I have less of an understanding of prop planes than I do quantum theory), all I can say is tha this was spotted and submitted as a bug during closed testing and Yo-Yo's response was pretty much as I said above.

[effte]

I don't expect the team has had much time to verify the interpretations made of the source documents before answering every individual question or report as of lately. :)

 

It is right there in the manuals though. I think the report needs to be reopened, but let's await an answer from the powers-that-be.

 

While the starter is cranking the engine, a booster coil aids the magnetos by intensifying the spark.

 

(My boldface)

 

There are also wiring charts showing how the booster coil is fed through the magneto on MM pg. 330/331.

 

Cheers,

/Fred

[26-J39]

I have to agree with effte here, but I'd have to see a wiring diagram to be sure. AFAIK the booster coil will "boost" the output voltage for RH magneto.

 

All the ignition switch really does is earth (neg -) itself to cut the spark when set to off, boosted or not the magneto is earthed hence no spark when off.

 

If current implementation is correct the booster coil would technically be the 3rd magneto system? :huh:

[sobek]

I submitted the bug report and was told that this is per specification from Yo-Yo. If you can provide a wiring diagram that proves different, please put it up for all to inspect.

Edited May 11, 2012 by sobek

[26-J39]

Thx Sobek, understandable.

 

Hmm need link to the manual effte refers... :music_whistling:

[flightace37]

Wiring diagram in P-51D maintenance manual here:

http://www.scribd.com/doc/34812228/P51D-Mustang-Maintenance-Manual

 

You'll have to run quite a ways down to find page 330/331 by real page number. Scribd is off a little.

 

I actually read it to indicate that the booster coil is wired in parallel, not series, with the ignition system.

[sobek]

Maybe this kind of diagram presumes notation conventions that i am unfamiliar with, but i have to say that i can't draw a definitive conclusion from that.

[flightace37]

Maybe this kind of diagram presumes notation conventions that i am unfamiliar with, but i have to say that i can't draw a definitive conclusion from that.

 

It's tough, and I'm not 100% on it either. Take a look at the earlier pages describing the ignition system, and you'll see a drawing that shows the high tension lead from the booster coil literally being wrapped around the lead of a spark plug, with power for the booster coil coming straight from the switch.

 

And the diagram probably presumes conventions from more than 60 years ago... I imagine there's been some change in that time. :\

Edited May 11, 2012 by flightace37

[sobek]

Take a look at the earlier pages describing the ignition system, and you'll see a drawing that shows the high tension lead from the booster coil literally being wrapped around the lead of a spark plug, with power for the booster coil coming straight from the switch.

 

Please provide a page or figure number, i can't find the diagram you mentioned.

[flightace37]

Scribd page 231, real page 204. Figure 249.

[26-J39]

Ok.. looked at the manual/diagrams, you can see clearly the boost coil HT outputs to the RH mag.

 

And here from the manual: Page 204-205

The low RPM of the engine during initial starting fails to actuate the magneto with enough speed to generate the required high-tension current for engine ignition: Consequently, the booster coil is actuated at the same time the starter switch is closed for cranking, and supplies the high voltage needed.

 

Personally cant see how the booster coil could act or provide spark independent of L or R Magneto.

 

The booster coil acts to "boost" RH magneto output to aid start.

[flightace37]

Yeah. I mis-read that figure. It's showing how to hook up the booster coil for testing. I didn't see how it could avoid being wired to the RH mag either.

 

So the the thing is, even with the ignition switch set to Off, according to the two diagrams (pp 330 and 331 like effte said), the booster coil is connected to the RH mag. Whenever the starter motor is actuating, the booster coil is also actuating. Wouldn't you expect to see current through the RH mag? It has two inputs: one from the ignition switch, and one from the booster coil. The electrons just flow to ground from either of those.

 

Basically, the diagram on pp. 331 says that the booster coil gets current any time the starter switch is held down. The one on page 330, to me, says that the booster is hooked directly to the RH mag, in parallel with the ignition system. For it to cut out when the ignition switch is off, the booster coil would have to somehow get it its electrons through the ignition switch, but it gets them straight through the starter switch.

 

Side-note: The right-hand magneto is the only one that actually provides ignition spark. The manual says the left-hand mag provides exhaust spark... Sounds a little strange to me, but there it is.

Edited May 11, 2012 by flightace37

[sobek]

Personally cant see how the booster coil could act or provide spark independent of L or R Magneto.

 

Coils have step response characteristics that lead to very high voltages if a current is sent through the coil and suddenly turned off. Essentially, the coil "seeks" to uphold the current and uses the energy stored in the magnetic field that it builds up during "charge" to do so. That means that, for a short time, the coil will act as essentially a current source, boosting the voltage until the current is again steady. By using a coil with a high inductivity, you can generate very high voltage sparks from quite low voltage (if you manage to turn it off fast).

 

Edit: Actually, by looking at the diagram of how the boosting coil is tuned, i am very much tempted to believe that it is indeed fed off of batt voltage and not from the magneto.

Edited May 11, 2012 by sobek

[26-J39]

Coils have step response characteristics that lead to very high voltages if a current is sent through the coil and suddenly turned off.

 

Could you clarify please?

 

Also: wonder if this is a case of the booster coil bypassing RH mag?

[26-J39]

Coils have step response characteristics that lead to very high voltages if a current is sent through the coil and suddenly turned off.

 

Could you clarify please?

Edit: rgr edit :)

 

Also: wonder if this is a case of the booster coil bypassing RH mag?

[flightace37]

From what material I could find on Google, the booster coil essentially sits there and builds up charge through magnetic induction until it is grounded. At that point, it discharges very rapidly, providing enough voltage for a spark.

 

It still has to be connected to the spark plug through the RH mag to prevent early spark.

 

With regards to the behavior effte witnessed in his tests, and given the electrical diagram clearly showing the coil wired to the RH mag, I think the only way that the coil could be prevented from generating a spark would be if the magneto somehow included a relay that opens the circuit from the coil unless there is power from the ignition switch.

 

That seems like a silly thing to do, especially given that the pilot could then just run the starter motor with the ignition off until the booster coil fails in spectacular fashion (ie, boom). It is far more likely that the design is simple, with both the booster coil and ignition hooked directly to one side of the magneto (together), and the spark plug hooked up to the other.

 

EDIT: Yeesh, lot of out of date edits going on tonight. I'm absolutely loving this discussion though.

Edited May 11, 2012 by flightace37

[flightace37]

Edit-sniped again.

 

Yes, sobek. I strongly believe the coil is fed directly from the 28.5V bus. The diagram on pp. 331 clearly states that is the case.

 

Edit: I'm going to bed. I'll see if there are any more posts (or edits) here tomorrow. ><

 

And Edit again: 28.5V bus, not battery.

 

Edit: Unless that's a battery symbol near the bottom of the right switch panel "box" in the diagram. I think we've determined that the booster coil is not fed from the ignition switch, at least. And... bed time.

Edited May 11, 2012 by flightace37

[sobek]

Edit: Unless that's a battery symbol near the bottom of the right switch panel "box" in the diagram. I think we've determined that the booster coil is not fed from the ignition switch, at least. And... bed time.

 

Well, at low RPM, the battery is the only thing supplying voltage to the bus, but yes, you are correct.

[Rotareneg]

The booster coil operates independently, notice the adjustment diagram at http://www.scribd.com/doc/34812228/P51D-Mustang-Maintenance-Manual#page=231 and the instructions on the next page. It appears to use a built-in motor or solenoid to trigger a contact breaker to fire the coil rapidly, "turn adjustment screw until there is a steady shower of sparks".

 

And, on page http://www.scribd.com/doc/34812228/P51D-Mustang-Maintenance-Manual#page=362 , figure 391, power from the 28.5 volt bus goes through the ignition switch to both the starter solenoid and the booster coil.

Edited May 11, 2012 by Rotareneg

[effte]

Rotareneg,

that prompted further reading. I'm finding documents stating that the booster coil is a high-tension coil with vibrating contact points, hooked up to a separate lead on the distributor to provide 30 degree retarded sparks for starting. I'll wait until I'm home at a computer rather than on a dumbphone before I call a good find and definitely put the issue to rest, but I thought I'd get it in early that I'm beginning to lean towards you (and ED) being right on this. :)

 

Cheers,

Fred

[159th_Viper]

I'm beginning to lean towards you (and ED) being right on this. :)

 

I have yet to see one of Yo-Yo's assertions be contradicted after he has expressly stipulated to the veracity thereof. That's not to say it will not happen....right up there with me waiting for the sky to fall on ma heed :D

[flightace37]

Anyone figure out what that bottom symbol is, on page 331, right switch panel? I asked someone, and they seem to think that it is a push button of some kind. Perhaps a circuit breaker?