Engine running with ignition off

[effte]

The two connection points with an arc between them? Circuit breaker.

[cichlidfan]

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?

 

If it were just a push button switch then it would be a straight line instead.

[flightace37]

That makes sense. Thanks.

[effte]

Allright, back home with a bit more reading done.

 

The booster coil is indeed activated through the starter switch, as far as I can tell. It is a coil with vibrating breakers, creating a constant flow of high power sparks to one bank of spark plugs*.

 

These sparks are fed to the right cylinders from the distributor, just as the normal sparks from the mags.

 

Normally, the spark is sent to the spark plug around 25 degrees before top dead center (TDC), that is 25 degrees before the piston reaches its top point and begins the downward stroke. This allows the flame fronts time to propagate throughout the combustion chamber and build up to peak pressure in time for the piston to milk the most of the energy out of the expansion of the mixture during its downward stroke.

 

At low RPMs, this is way too early and means the mixture would reach peak pressure way before TDC. The flame front propagation speed doesn't change with RPM. This will at best cause the combustion to decelerate the engine and subtract from the power output and at worst (often, with high-powered aircraft engines) cause kick-back - the engine violently coming to a halt and reversing direction. Kick-back isn't good at all. Lots of stress on engine parts. I guess don't need to point out that you should never attempt to hand-prop anything which can kick back... ? :cry:

 

To solve this, the booster coil sparks are retarded - delayed - at the distributor, causing them to fire the spark plugs later than the normal spark from the magneto. Booster coil ignition should happen right around TDC, even if I have not been able to find the exact figure for the retardation for the P-51 engine.

 

This allows the booster coil sparks to give good sparks during the starting, igniting the mixture and creating a pressure peak during the piston's downward travel, accelerating the engine towards normal RPMs. As RPMs increase, the magnetos start working and providing enough spark to ignite the mixture. When they do, they take over. The second spark being provided by the booster coil will not do anything, as the mixture is already ignited. The engine starts running normally.

 

However, with magnetos off, running off the starter and the booster coil, the retardation of the spark will be way too much. Peak pressure will be reached only once the engine is 30 degrees past TDC. I really can't see the engine running at all well under those conditions, on booster coil alone. Right now, there's no perceptible difference between running the engine on the mags and running it on the booster coil, apart from the fact that it will cut out as you release the starter with the mags off.

 

I don't suspect the operators of current warbirds, including those consulting for ED, will fancy trying to run their expensive and rare engines in any way except that specified by the checklists, so mags off starts is probably not something they have a lot of experience of. Please correct me if I'm mistaken. I do have a few leads to follow on my own though, so with a bit of luck I'll be able to get back with word from people with Merlin experience. It may be a while though.

 

Sidenotes:

A booster coil also prevents unburned mixture from filling up the exhaust manifolds, with the potential for spectacular backfires as the engine catches.

 

(An alternative method of providing sparks during startup, more common in modern aviation engines, is to have impulse couplings in the magnetos. At low RPMs, these delay the magnetos working against a spring. As they are released, the spring provides the increased velocity to give a good spark. Simultaneously, they retard the spark by a sufficient amount. As RPMs pick up, flyweights engage and the magnetos return to normal operation.)

 

 

Cheers,

/Fred

 

*) A poster earlier in the thread wondered about exhaus and intake plugs. Aviation engines almost exclusively have completely doubled ignition systems. Two magnetos, two harnesses, two plugs in each cylinder. This is for redundancy, obviously, as otherwise a magneto breaking would kill your engine Having suffered magneto failures in real life, I think this is an excellent design philosophy. It gives the added benefit of more efficient combustion as well, due to the two sources of ignition in each combustion chamber causing two flame fronts. That's why you get the RPM drop when checking mags before takeoff... as you always do, right? ;)

Edited May 17, 2012 by effte
30 degrees certainly not halfway

[sobek]

I really can't see the engine running at all well under those conditions, on booster coil alone. Right now, there's no perceptible difference between running the engine on the mags and running it on the booster coil, apart from the fact that it will cut out as you release the starter with the mags off.

 

Sorry if i again come across as argumentative, but that does not match with my experience in the sim. The engine, run with magnetos off, will repeatedly kick from isolated combustion events, but will definately not run smoothly, as it does after a few seconds with mags on.

 

Edit: Disregard, i need to retest before i'm certain. I think my memory stems from different circumstances.

 

It gives the added benefit of more efficient combustion as well, due to the two sources of ignition in each combustion chamber causing two flame fronts. That's why you get the RPM drop when checking mags before takeoff... as you always do, right?

 

Can this not be offset by ignition timing, at least to a degree?

 

If it were a detrimental factor to efficiency, i would assume that car engines would by now all be built with two plugs.

[effte]

sobek,

by all means be as argumentative as you wish. A good discussion of these subjects is beneficial for all parties involved. Besides, discussing aircraft is fun. :)

 

Timing can offset it obviously, smaller cylinder volumes makes it less of a concern. There's cost, complexity and weight in the other balance. A lot of the work going into engine design these days is working on the airflow and mixing in the combustion chambers, which is really another means to the same end*. What you are really after is a controlled pressure rise as the flame front(s) propagate through the mixture. We can probably leave it at the fact that an engine designed with two plugs will perform worse on one? I'm approaching the limit of my knowledge here - I use engines and mess around with them, but so far have not found anyone willing to pay me to design them, unfortunately.

 

Cheers,

Fred

 

*) You can have a look at the length engine tuners will go to in order to clean up the airflow as well - it's rather extreme!

[Pikey]

Please God tell me this post is a publicity stunt?

[Teapot]

It blows my socks off listening to this fascinating discussoin, plus I learn a bit (I think!). About the only thing I can contribute to this thread is to let the knowledgable folk on this thread know that this is (well) very interesting stuff indeed ... and it seems to me you're very clever people too. Thanks!

 

@Pikey ... not sure what you mean?

[Yo-Yo]

 

I don't suspect the operators of current warbirds, including those consulting for ED, will fancy trying to run their expensive and rare engines in any way except that specified by the checklists, so mags off starts is probably not something they have a lot of experience of. Please correct me if I'm mistaken. I do have a few leads to follow on my own though, so with a bit of luck I'll be able to get back with word from people with Merlin experience. It may be a while though.

 

 

 

I am an operator of a very old car having a mechanical ignition distributor. :) After you replace something in it you have to adjust ignition timing. The right way is to set delayed ignition than too advanced - you have better chance to start the engine. The engine will run lacking the power and sometimes firing to a muffler but it will run.

[Pikey]

It's like the Alfa Romeo club in here sometimes...

@Teapot...I was being sarcastic, i'm saying (jokingly) that the conversation is so off the wall its like it was for another reason.

[flightace37]

I was being sarcastic, i'm saying (jokingly) that the conversation is so off the wall its like it was for another reason.

 

That's what happens when you get a bunch of propeller-heads and engineers together. Look at it as an opportunity to learn some cool stuff. I certainly have been.

[effte]

The right way is to set delayed ignition than too advanced - you have better chance to start the engine. The engine will run lacking the power and sometimes firing to a muffler but it will run.

 

Hehe, yeah, I have the dubious honour of operating an old bike or two with mechanical ignition. Retarded better, as it won't stop itself by firing too early during the compression stroke, but you'll notice it if it is delayed - which is my point. I'm guessing running the V-1650 off the booster coil retarded 25-30 degrees should be noticeable. Right now, it isn't.

 

Gonna try to find someone who knows his Merlins and ask.

 

Correct or not, it's hardly a high priority issue. :)

[sobek]

Sorry if i again come across as argumentative, but that does not match with my experience in the sim. The engine, run with magnetos off, will repeatedly kick from isolated combustion events, but will definately not run smoothly, as it does after a few seconds with mags on.

 

Edit: Disregard, i need to retest before i'm certain. I think my memory stems from different circumstances.

 

Finally had the time to test this and i agree, with the starter held, and the mixture in auto, the engine runs perfectly smooth at all RPM. I'm not certain of what magnitude the problems arising from doing so would be, but this seems unlikely. I'll reopen the bug report.

[sking06]

effte gets my vote for the most accurate response. My only comment would be that each magneto provides spark for all cylinders, so the spark from the booster coil would also be available to all cylinders. To help simplify.

 

1. Booster coils receive their voltage from the battery, usually though the starter switch.

 

2. Booster coils are used to provide high energy ignition spark for starting. (magnetos don't work well at low RPM's).

 

3. Booster coils provide a retarded ignition spark, usually close to Top Dead Center. (Aircraft engines do not have variable ignition timing like automobiles. The timing is fixed for optimum performance). Engines that have a high cranking RPM can start without a booster coil, but usually have one magneto retarded a few degrees more than the other. The Merlin with it's slow cranking speed would never start without a booster.

 

4. Would the Merlin run smoothly on the booster? I do not have direct experience here, but I seriously doubt it. On the radial engines that I work with that have a booster coil, you can get it to pop, but you had better have the mags on if you wanted to run.

[Tailspin45]

I do not have direct experience here...

 

You've had your hands in some very intimate places for someone who claims inexperience, my friend...

 

(More Su Su 2 here).

Edited June 2, 2012 by Tailspin45
(Nice shirt!)

[louisv]

The A2A P-51D startup procedure stipulates that the magnetos (both) should be switched on only when 5 blades have passed, which pulls in oil throughout the engine...while you hold the starter switch, and the primer.

 

And, as per DCS P-51D startup procedure, switch mixture to Run. A pilot remarked somewhere that you needed 3 hands to start a Mustang...

[hitman]

The A2A P-51D startup procedure stipulates that the magnetos (both) should be switched on only when 5 blades have passed, which pulls in oil throughout the engine...while you hold the starter switch, and the primer.

 

And, as per DCS P-51D startup procedure, switch mixture to Run. A pilot remarked somewhere that you needed 3 hands to start a Mustang...

I think that turning the prop by hand is to get oil to circulate throughout the system. I could be wrong though, they always do this to rotary engines, since it helps prevent hydraulic lock.

[Tailspin45]

I think that turning the prop by hand is to get oil to circulate throughout the system. I could be wrong though, they always do this to rotary engines, since it helps prevent hydraulic lock.

 

You're right, radial engines are pulled through by hand to ensure that lower cylinders do not have a hydraulic lock. It does also spread some oil on cylinder walls, but that's not the real reason they're pulled through by hand--it's the hydraulic you worry about.

 

If oil leaks down into a lower cylinder you can bend a connecting rod when the piston tries to compress uncompressible liquid. The starters have clutches that are supposed to prevent this, but I personally know of three engine failures in flight caused by rods that were bent by oil in the cylinder on start. In fact, you can evn bend a rod when you're pulling a prop through by hand, if you aren't careful. If a cylinder has a hard resistance, pull a plug and let the oil drain out. But be careful or you'll get face full of oil if you don't release the compression first!

 

Early Wrights(J4, J5 etc) often had valves installed in the lower oil gallery so it was easy to drain the oil from the combustion chamber every morning or after a prolonged rest. They were loose engines and prone to hydraulic lock.

 

BTW,WW1 rotary engines (as distinct from radial engines) don't have a lubrication system, because the cylinders turn and the crank is fastened to the airframe. The oil (castor oil usually) is in the fuel. Rotary engines don't have a throttle for the same reason--everything is turning. They have a blip switch to kill the engine for ground use and landing. This is all modeled beautifully in the exquisite Rise Of Flight sim.

Edited June 2, 2012 by Tailspin45

[WildBillKelsoe]

the question is, will the finale Mustang release (the product proper) will feature any of the walk around stuff, check gear struts/bolts (in other words, a disembark/dismount keyboard command). Similarly, the rotation of the prop to circulate oil to engine, will we get extra animation?

[Vitesse2l]

Enjoying this thread.

 

As someone who runs old cars (and usually replaces mechanical ignition parts with solid state stuff) most of the discussion was understandable.

 

What I'm not sure about is it's timing - Effte's post describes the coil spark being timed by the distributor. Do the mags use the distributor too? Interesting engineering to get two differently timed sparks from the same distributor.

 

Just answered my own question... Fascinating.

 

"Ignition is by two high-tension rotating magnet magnetos mounted on each side of the wheelcase. The "A" bank magneto serves the intake spark plugs and the "B" side magneto serves the exhaust spark plugs. The magneto has an integral coil and breaker points operating off a four pole cam, each magneto is driven by a serrated coupling shaft turning at 1.5x engine speed. A distributor turning at 1/3 the magneto speed provides one complete cycle of 12 sparks every 2 revolutions of the engine. Provisions exist for an engine control mechanism to move the breaker base through 25 degrees of advance. In addition, the distributor features a two contacts on its rotor -- the leading finger is energized by the magneto and the trailing finger by an external terminal which can be connected to a booster coil to facilitate starting. The booster coils are essentially high tension coils with vibrating points which provide continuous spark -- when applied to the retarded finger they provide a high energy retarded spark which can greatly facilitate starting, especially with cold, slow turning (thick oil) engines which don't provide much speed for the magnetos. Merlins in aircraft only boosted the "A" bank, but I've seen boats with no boost, with booster coils on both banks, and both combinations in between. "

[sking06]

Curious.... where did the quote come from.... At first I thought it was an extract, except for the boat comments. It's been a lot of years since I've heard anyone speak of SK boats powered with Merlins and Allisons.

[Tailspin45]

http://www.thunderboats.org/history/history0324.html

[Vitesse2l]

Yes, that was the source.

 

I assume that as soon as the engine catches the more advanced spark from the magneto would make the starter spark redundant. If the mags are off the starter spark would be that much more retarded and the engine would run less well, as was mentioned earlier.

[effte]

Yeah, once the RPMs pick up to the point where the magnetos give enough spark to ignite the mixture, the booster spark will spark behind the flame front in mixture which has already burned and thus will not support combustion again, so it will have no effect.