Engine sputter / detonation with low power settings ...

[Anatoli-Kagari9]

The sounds are there, but I wonder if it's just that effect, or if there are actually the associeted effects being modeled in the engine system ?

[Crumpp]

Any engine with straight pipes will do it.

 

It is the change in pressure in the exhaust when you reduce the throttle. When the throttle is steady the pressure impulses are steady. When you make a drastic reduction in power, the pressure pulses cannot maintain integrity and begin to collapse and mix. You hear that pressure trying to equalize in a tube as a snapping sound.

Edited November 21, 2015 by Crumpp
It is not caused by excess fuel

[Anatoli-Kagari9]

Oh, I know that so well Crumpp, and everyone around me every time I decide to have one of those tasty / spicy recipes for my meal....

 

But one of the pipes in this case is really the furthest away from being straight ... while the other is more or less straight ...

 

:-)

Edited November 22, 2015 by jcomm

[Crumpp]

Thank you for that thought...

 

:cry:

 

 

:D

[sobek]

If it's after-fire (no pun intended) that you are referring to, that is caused by the engine running very rich when you throttle back, resulting in combustible fuel being left after combustion in the cylinder finishes, which then mixes with oxygen inside the exhaust, causing violent audible bangs (and sometimes visible flames) from the exhaust. If it's modelled, it's quite possible that there is a physical model underneath it.

[NeilWillis]

I have always thought this effect was missing. It is very good to hear the Merlin popping and banging on idle.

 

very very nice touch!

[Crumpp]

If it's after-fire

 

For the readers:

 

Afterfire is a form of backfire and generally the result of improper adjustments and damaged gaskets. Engines are generally not designed for it and it can damage the motor. Unburnt fuel exploding in the exhaust is not normal or good for an engine despite some myths floating around. An afterfire is a definitely possibility on a hot day when running full rich. That is an improper adjustment of the fuel air mixture and depending on the engine, something should be leaning the mixture whether it is the pilot manually or the automatics in the case of the Merlin.

 

In aircraft engines you see two basic kinds of flames:

 

Low energy flames are present generally at the start up due to excessive fuel. Because of the way aircraft engines are timed for the start, the valves maybe open in the start sequence when spark is delivered igniting fuel. It is not under-pressure and sends out the big dramatic looking flames which can sometimes continue to burn. As long as you keep motoring the engine or achieve a normal start it is not a big deal. Only if you stop motoring the engine and let it burn can it become an issue.

 

The second type are higher pressure flames again due to incomplete combustion and are normal for any internal combustion engine. This is not fuel ignited in the exhaust, it just flames still burning from the combustion cycle getting pushed out of the engine in the normal exhaust cycle.

 

That is normal for any engine but we just do not see them because of longer exhaust systems with baffles found on most internal combustion engines. The short exhaust stacks without baffles found on many large world war II piston engines allows a great view of them!

 

http://www.airliners.net/aviation-forums/tech_ops/read.main/184022/

 

That exhaust stream is high velocity and high pressure. When you back off the throttle, the pressure and velocity change. Make that pressure gradient large enough and it will create a shock wave as the high pressure collapses into the low pressure causing the popping noise.

 

In a straight exhaust stack, that popping noise is normal and not harmful other than the stresses of rapid throttle changes. It is not the result of improper adjustment, it just a property of that type of exhaust system and one reason why baffles were invented!

[sobek]

That exhaust stream is high velocity and high pressure. When you back off the throttle, the pressure and velocity change. Make that pressure gradient large enough and it will create a shock wave as the high pressure collapses into the low pressure causing the popping noise.

 

I strongly contest that theory as i doubt that any sane pilot would close the throttle fast enough to create a shockwave, yet the noise occurs quite frequently on some engines, e.g. at landing.

 

That is an improper adjustment of the fuel air mixture and depending on the engine, something should be leaning the mixture whether it is the pilot manually or the automatics in the case of the Merlin.

 

Given that the fuel regulator on the merlin is of a relatively simply pneumato-mechanic type, i find it much more plausible that the engine would remain running rich for the fraction of a second compared to that shockwave hypothesis of yours. Besides the exhaust of the Merlin is very short and rather sturdy, i doubt that the pressure gradient from a small yet rapid deflagration would cause any harm to the engine.

 

Edit: [ame]

[/ame]

 

If this is indeed what we are talking about, you can clearly make out the flames coinciding with the bangs from afterfire when he revs the engine up and down at the end of the vid before shutting it down.

Edited November 22, 2015 by sobek

[Crumpp]

Read what I wrote. It does not contradict anything you wrote in your first post.

 

any sane pilot would close the throttle fast enough

 

It is a fact that straight exhaust pipes will pop and snap when the throttle is reduced.

 

Listen to any Harley Davidson, car, or motorcycles equipped with straight pipes.

 

There is nothing about some insane throttle closure either.

 

That is one of the reasons why exhaust pipe baffles came to be in existence.

 

Crumpp says:

When you back off the throttle, the pressure and velocity change. Make that pressure gradient large enough and it will create a shock wave as the high pressure collapses into the low pressure causing the popping noise.

 

afterfire

 

Yes it does happen and it is the result of improper adjustment.

[Crumpp]

Besides the exhaust of the Merlin is very short and rather sturdy, i doubt that the pressure gradient from a small yet rapid deflagration would cause any harm to the engine.

 

It does not cause any harm in any engine. It is just a characteristic of straight exhaust pipes.

 

And watching the film....

 

I would have shut that engine down immediately just as the operator did!

 

There is something definitely out of adjustment and I would not be surprised if he did not have a gasket or too that needed replacement.

[Crumpp]

Read what I wrote. It does not contradict anything you wrote in your first post.

 

any sane pilot would close the throttle fast enough

 

It is a fact that straight exhaust pipes will pop and snap when the throttle is reduced.

 

Listen to any Harley Davidson, car, or motorcycles equipped with straight pipes.

 

There is nothing about some insane throttle closure either.

 

That is one of the reasons why exhaust pipe baffles came to be in existence.

 

Crumpp says:

When you back off the throttle, the pressure and velocity change. Make that pressure gradient large enough and it will create a shock wave as the high pressure collapses into the low pressure causing the popping noise.

 

afterfire

 

Yes it does happen and it is the result of improper adjustment.

[MiloMorai]

He was shutting down the Griffon and clearing the plugs.

[Anatoli-Kagari9]

But I really don't know if the sound sounds that realistic to me... It's a bit baffled ?

[sobek]

It is a fact that straight exhaust pipes will pop and snap when the throttle is reduced.

 

How do you explain that the condition sometimes persists until well after the throttle was closed?

 

Observe the following video:

[ame]

[/ame]

 

He starts closing the throttle at 8:02, the throttle is fully closed at 8:05, yet the poping noises continue until 8:10.

 

Either this shockwave theory is hokum or Kermit Weeks keeps his P-51 in awful shape.

 

Edit: Here's another video of a different P-51D afterfiring distinctly upon closing the throttle on landing (around the 1:50 mark).

 

[ame]

[/ame]

 

I'm sorry but i'm just not buying it.

Edited November 23, 2015 by sobek

[sobek]

But I really don't know if the sound sounds that realistic to me... It's a bit baffled ?

 

Yes, it doesn't have the impulse qualities one would expect, it also seems that the audio is abruptly cut out or in without regard to where playback of the audio sample is at, resulting in some very unnatural onsets and cutting out of the samples. WIP i guess.

[kripzoo]

How do you explain that the condition sometimes persists until well after the throttle was closed?

 

Observe the following video:

 

He starts closing the throttle at 8:02, the throttle is fully closed at 8:05, yet the poping noises continue until 8:10.

 

Either this shockwave theory is hokum or Kermit Weeks keeps his P-51 in awful shape.

 

Edit: Here's another video of a different P-51D afterfiring distinctly upon closing the throttle on landing.

 

 

I'm sorry but i'm just not buying it.

That's because this "pop pop" usually happens when throttle is closed and there is high enough rpm in the engine.

Normal thing for 4-stroke engines without "fuel cut out".

This "popping" is rapid burning fuel in the exhaust , mainly because of valve overlapping and strong low pressure in the cylinder due the closed throttle. :)

[sobek]

This "popping" is rapid burning fuel in the exhaust

 

I am well aware, please follow the thread. ;)

[Yo-Yo]

That's because this "pop pop" usually happens when throttle is closed and there is high enough rpm in the engine.

Normal thing for 4-stroke engines without "fuel cut out".

This "popping" is rapid burning fuel in the exhaust , mainly because of valve overlapping and strong low pressure in the cylinder due the closed throttle. :)

 

Yes, you got the point :). Additionally, I think, fuel/air ratio is not close to the optimal, so the burning velocity is lower than normal.

[Crumpp]

This "popping" is rapid burning fuel in the exhaust , mainly because of valve overlapping and strong low pressure in the cylinder due the closed throttle.

 

Then you know that valve overlapping is the valves remain open. The combustion occurs in the chamber NOT the exhaust system. It exits the exhaust system because the flow is moving that way.

 

If you detonated fuel in your exhaust...you will be replacing gaskets, cracking pipes, or the manifold. Look at the walls of a combustion chamber and look at the thickness of your exhaust pipe.

 

Sobek says:

How do you explain that the condition sometimes persists until well after the throttle was closed?

 

The popping is the pressure impulses collapsing and mixing. It is a normal occurrence in straight exhaust pipes. That is why baffles were invented, btw.

 

You seem to think it is one or the other. That is not correct and you are attempting to invent an argument that does not exist.

 

Both afterfire and the normal acoustics of straight pipes occur. Afterfire is just an indication of something not being correctly adjusted. I have replaced a lot of aircraft exhaust gaskets for this very reason. All it takes is taxing around with the mixture control full rich and the right throttle manipulation.

 

Most commonly seen in the World War inverted and V engines is fresh air inversion. Basically the pipes are so large and short that fresh air gets drawn in after the exhaust impulse. It makes for a lot of popping and does contribute to afterfire because the cam is only optimized for a single rpm and the timing can be off at other rpm bands.

 

In fact most aircraft piston engine starting systems adjust the timing for first few power strokes to occur after TDC to make starting easier.

 

It comes down to popping is good after-firing or backfiring is not.

 

Ask an aircraft mechanic. I learned about all of this when I got my A&P license and a knowledgeable one will tell you the same.

 

He was shutting down the Griffon and clearing the plugs.

 

If he wanted to clear the plugs he would lean the mixture and run at high rpm for ~30 seconds.

 

I highly doubt the man is not knowledgeable on engines. The engine clearly backfires and he very appropriately shuts it down.

[Crumpp]

Either this shockwave theory is hokum or Kermit Weeks keeps his P-51 in awful shape.

 

 

Why?

 

Do you think the engine stops at idle?

 

No...it still produces pressure impulses.

[kripzoo]

Then you know that valve overlapping is the valves remain open. The combustion occurs in the chamber NOT the exhaust system. It exits the exhaust system because the flow is moving that way.

 

If you detonated fuel in your exhaust...you will be replacing gaskets, cracking pipes, or the manifold. Look at the walls of a combustion chamber and look at the thickness of your exhaust pipe.

"Detonation" is a little too violent word for this.

There's a "little" difference between a combust of fuel in the closed/pressurized cylinder than, a fuel combusting in open exhaust manifold.

 

Valve overlapping, high rpms with rapidly lifted/closed throttle can also be a reason for raw unburnt fuel being burnt in exhaust manifold.

 

I don't know what kind of exhaust manifolds you have seen, but im pretty sure that example, a casted iron manifold doesn't crack from fuel combusting inside it, since it's open from another end. Gaskets might hurt though...

[sobek]

If you detonated fuel in your exhaust...you will be replacing gaskets, cracking pipes, or the manifold. Look at the walls of a combustion chamber and look at the thickness of your exhaust pipe.

 

Please Crumpp, are you going to tell us that you will have the same peak pressure inside an exhaust that is only a few cm long and has a very low flow restriction compared to inside the cylinder where the charge is precompressed and can not expand? Who talked about detonation? You are aware that you need considerable pressure and temperature for the charge to detonate, yes? Both those conditions are surely not present in the exhaust. We are talking about deflagration here.

 

 

The popping is the pressure impulses collapsing and mixing.

 

How is that even possible in such a short exhaust system? I bet that the shockwave and even it's reflection are long gone out the pipe before a new exhaust cycle starts, especially at low RPM where this phenomenon occurs. Besides if it's just caused by the pressure gradient between subsequent exhaust cycles, why don't we see it at all RPM ranges? I'm perfectly aware that it should under no circumstances happen in a modern engine where the injection is metered by an electronic ECU, however, you can't hold these old engines to those standards and i'd be seriously surprised if it causes them any harm.

Edited November 23, 2015 by sobek

[Crumpp]

Listen to this motorcycle with straight pipes. When he backs off you can here the popping from the impulse collapse.

 

[ame]

[/ame]

 

 

Nice loud series of pops......nothing to do with anything being out of adjustment. Just the acoustics of straight exhaust.

[Crumpp]

"Detonation" is a little too violent word for this.

There's a "little" difference between a combust of fuel in the closed/pressurized cylinder than, a fuel combusting in open exhaust manifold.

 

Valve overlapping, high rpms with rapidly lifted/closed throttle can also be a reason for raw unburnt fuel being burnt in exhaust manifold.

 

I don't know what kind of exhaust manifolds you have seen, but im pretty sure that example, a casted iron manifold doesn't crack from fuel combusting inside it, since it's open from another end. Gaskets might hurt though...

 

Well then I should get my 790 dollars back for the Franklin engine exhaust manifold I had to buy in 2007.

[sobek]

Listen to this motorcycle with straight pipes. When he backs off you can here the popping from the impulse collapse.

 

How does that work, such an impulse collapse? You must have a basic understanding of the mechanism if you can say with such certainty that this is what's responsible and not afterfire.