Engine sputter / detonation with low power settings ...

[Crumpp]

Often the cause in these aggressively cammed piston engines is the valve timing. When you start an aircraft engine, the idea is to ignite the either at TDC (top dead center) or just just past TDC when the piston starts its downstroke. This relieves stress on the engine and starter system and prevents starter kickback.

 

When the engine timing changes to normal logic after the start, the timing is changed to fire just before the piston reaches TDC. This increases the power as the piston then slams into the expanding charge at TDC producing more energy on the power stroke.

 

Our valve overlap is timed to work with normal piston logic.

 

If it is out of adjustment for the starting logic then backfires and afterfires can result.

 

Fouled plugs causing misfiring are also another common cause of afterfiring.

 

Read:

 

 

ASTM Manual for Rating Motor, Diesel and Aviation Fuels, 1973-74

 

After-Firing in Exhaust System:

 

After Fire may result in rupture of part of the exhaust system and should be corrected immediately. It is usually caused by one or more of the following:

 

Malfunctioning ignition system

Sticking intake valve

Misfiring engine

Leaks in the intake system

Incorrect valve timing

Preignition

 

https://books.google.com/books?id=SPI6AQAAMAAJ&pg=PA183&lpg=PA183&dq=afterfiring+valve+timing&source=bl&ots=c_jW5ZIjpi&sig=1IOdBi7f-Wj1CauromNLDryAVcU&hl=en&sa=X&ved=0ahUKEwinkPfdz6fJAhWK7CYKHbrOBa8Q6AEIKDAB#v=onepage&q=afterfiring%20valve%20timing&f=false

 

 

Having fuel detonate in the exhaust is neither normal nor is not harmful.

 

What is normal and not harmful is the acoustics of the collapsing pressure impulse. It is two different things.

 

You do not want DCS to model AFTERFIRING in our engines.

[Crumpp]

How does that work, such an impulse collapse?

 

The same way a balloon pops when you release the pressure.

 

The pressure wave then propagates away from that point as a weak shock wave, creating the telltale "Pop" sound.

 

[Crumpp]

BTW,

 

Afterfiring will become an issue if our P-51's get 100/150 grade fuel. The high lead content caused issues with plug fouling.

 

Plug fouling causes misfires which can result in afterfiring.

 

Aircraft engines have two spark plugs (dual ignition) for each cylinder. Fouling one plugs creates conditions that more easily foul the second plug. If both plugs foul, the engine now has a dead cylinder which creates conditions it is easier to foul additional plugs.

 

It can cascade which is why several 8th USAAF P-51's crashed and why such a big deal of it is made by the Technical Branches of both the RAF and the USAAF.

[M1Combat]

Look at the walls of a combustion chamber and look at the thickness of your exhaust pipe.

 

A combustion chamber contains an explosion. An exhaust pipe does not. It lets it just explode and the pressure wave travels towards the exit. SOMETIMES the backfire can push a closed exhaust valve open just a little but that's pretty rare and only under "odd" circumstances.

 

Backfires are simply unspent fuel/air mix being ignited by the hot exhaust manifold.

 

 

Also... referencing the sportster up there...

 

You can re-tune a fuel injected HD engine to not pop on deceleration/throttle chop...

 

By pulling fuel out at that throttle position/RPM location... You can't do that with a carburetor though...

Edited November 23, 2015 by M1Combat

[MiloMorai]

Afterfiring will become an issue if our P-51's get 100/150 grade fuel. The high lead content caused issues with plug fouling.

 

Fouling occurred at partial throttle. Opening up the throttle cleared the plugs.

[Friedrich-4B]

Fouling occurred at partial throttle. Opening up the throttle cleared the plugs.

 

As described in this 8th AF Tech Op memorandum, lead fouling on the V-1650-7 using 100/150 grade only became serious if the lead accumulation was left for too long, or if the engine was allowed to idle for too long while waiting for take-off or the pilot didn't use full power on take-off.

 

 

As it is, if the pilot followed the instructions on take-off, plug fouling only became evident after a couple of hours of cruising at low power, something that's not likely to happen with the DCS Mustang (should an option to use 100/150 grade be made available); if a DCS player chooses to cruise for (say) seven hours, the plugs can be cleared by running at high power for a couple of minutes after every two hours.

 

Also, spark-plug life would be irrelevant for the DCS P-51D and the Spitfire L.F. Mk. IX, because the engines are factory fresh at the start of every flight (DCS has very efficient ground crew).

 

Edited November 24, 2015 by Friedrich-4/B

[Echo38]

Fouling occurred at partial throttle. Opening up the throttle cleared the plugs.

 

I think I remember P-38 pilots being instructed to do this periodically when cruising, for this reason. No source at hand; sorry.

[Friedrich-4B]

I think I remember P-38 pilots being instructed to do this periodically when cruising, for this reason. No source at hand; sorry.

 

Source at hand:

 

[Echo38]

DCS has very efficient ground crew. [picture]

 

[adjusts goggles]

 

That ground crew can harmonize my Brownings anytime!

[Crumpp]

Fouling occurred at partial throttle. Opening up the throttle cleared the plugs.

 

And?

 

You must taxi to the runway which why some of the 8th USAAF P-51 accidents occurred at take off.

 

Unlike gamers, real aircraft operate at cruise power settings and do not fly around with the throttle fire walled.

 

The majority of the time an aircraft is in flight, it is at partial throttle.

[sobek]

The same way a balloon pops when you release the pressure.

 

And why does that happen only so sporadically and not at every exhaust stroke?

 

I'm getting the popcorn, this theory of yours keeps getting better and better.

[sobek]

Having fuel detonate in the exhaust is neither normal nor is not harmful.

 

I'd say having it detonate in the exhaust is impossible under the conditions prevalent there.

 

The harmful effects you describe would only manifest themselves if afterfire were present while the engine is running at power. When throttling down, it's normal for such an old engine.

Edited November 24, 2015 by sobek

[Friedrich-4B]

You must taxi to the runway which why some of the 8th USAAF P-51 accidents occurred at take off.

 

Naturally some 8th AF Mustangs crashed at take-off, which is one of the trickiest parts of flying. In DCS one normally doesn't keep the engine idling for ages while waiting for other aircraft to take-off, which is when lead fouling could became a problem in real life. Full take-off power normally cleared any fouling that occurred while taxiing.

 

Unlike gamers, real aircraft operate at cruise power settings and do not fly around with the throttle fire walled.

 

Nor do gamers cruise at low power for hours, such that lead fouling becomes a problem if the pilot forgets to run the engine at high power to clear the plugs...:music_whistling:

 

Anyway just for interest, here are some notes on exhaust flame patterns from the Service Instructions for the V-1650-3, -7 and Merlin 68 & 69

 

 

And the Merlin 66/67/70/71/76/85 Maintenance Manual A.P. 1590 P S & U (1944)

 

[Crumpp]

Naturally some 8th AF Mustangs crashed at take-off, which is one of the trickiest parts of flying.

 

Yes. However that does not change the fact that the 8th USAAF Mustangs crashed due to lead fouling caused by 100/150 grade. That was the cause of their deaths.

 

It is just a fact that 100/150 grade fuel anti-knock properties came from the fact it had large amounts of lead. That large amount of lead leads to a greatly increased chance of lead fouling.

 

Not modeling that property would be like modeling MW50 without its cooling properties. It is just how the system works.

 

Nor do gamers cruise at low power for hours, such that lead fouling becomes a problem if the pilot forgets to run the engine at high power to clear the plugs...

 

You have to clear the plugs with 100/150 grade because of the fouling, Fredrich.

 

The warning to run at high power was issued because real aircraft spend most of their time at reduced throttle settings because real engines failure rates go way up if you run around at full throttle all the time.

 

It is not fun to be an airplane when the engine quits in flight.

[Crumpp]

And why does that happen only so sporadically and not at every exhaust stroke?

 

Depends on the frequencies of the impulses.

 

That is why baffles work to stop it. I think you are familiar with harmonics, right?

 

I'm getting the popcorn, this theory of yours keeps getting better and better.

 

Really?

 

So I guess the aviation mechanics are all taught wrong in this "theory" and we should go with your idea? I will let Aviation Maintenance Technician Society know they are teaching it all wrong and you can write American Society for Testing and Materials to straighten them out.

 

 

ASTM Manual for Rating Motor, Diesel and Aviation Fuels, 1973-74

 

After-Firing in Exhaust System:

 

After Fire may result in rupture of part of the exhaust system and should be corrected immediately. It is usually caused by one or more of the following:

 

Malfunctioning ignition system

Sticking intake valve

Misfiring engine

Leaks in the intake system

Incorrect valve timing

Preignition

 

https://books.google.com/books?id=SPI6AQAAMAAJ&pg=PA183&lpg=PA183&dq=afterfiring+valve+timing&source=bl&ots=c_jW5ZIjpi&sig=1IOdBi7f-Wj1CauromNLDryAVcU&hl=en&sa=X&ved=0ahUKEwinkPfdz6fJAhWK7CYKHbrOBa8Q6AEIKDAB#v=onepage&q=afterfiring%20valve%20timing&f=false

 

I'd say having it detonate in the exhaust is impossible under the conditions prevalent there.

 

I would agree. Modeling it in DCS would be modeling mechanics who do not know their job.

 

The harmful effects you describe would only manifest themselves if afterfire were present while the engine is running at power. When throttling down, it's normal for such an old engine.

 

No, it is not normal for any engine and can be harmful anytime it occurs. "Old" engines are just more likely to have something out of proper adjustment.

 

The acoustics of straight pipes causes the popping sound in a correctly adjusted engine.

 

Afterfiring is just not normal and indicates something is not adjusted properly. Any mechanic is going to shut down the engine and troubleshoot.

 

No need for a 20 page thread, just crack a book on aircraft engine maintenance and it will tell you the same thing I just said.

[MiloMorai]

Yes. However that does not change the fact that the 8th USAAF Mustangs crashed due to lead fouling caused by 100/150 grade. That was the cause of their deaths.

 

Well, the pilots did not follow procedure. That is, they didn't run up the engine before beginning their take off run.

 

That is why baffles work to stop it.

My stock Harley has baffles and I can make it pop.

 

One thing I haven't seen mentioned is that the incoming fresh charge is used to help purge the combustion chamber of used charge. Some of this fresh charge ends up in the exhaust system.

[Crumpp]

Well, the pilots did not follow procedure. That is, they didn't run up the engine before beginning their take off run.

 

My stock Harley has baffles and I can make it pop.

 

One thing I haven't seen mentioned is that the incoming fresh charge is used to help purge the combustion chamber of used charge. Some of this fresh charge ends up in the exhaust system.

 

If it was pilot error milo....the 8th USAAF would have concluded it was pilot error and not withdrawn the fuel until a solution could be found.....

 

Go get your Harley fixed too.

 

:music_whistling:

[MiloMorai]

P-51 (V-1650 Engines).

 

The same type of lead fouling as described in a and b above happened in the case of the P-51 except that is was probably more serious than in either of the other two types. Using 130 grade fuel with 4½ cc. of lead, the average operational P-51 could last 5 missions (roughly 25 hours) before the fouling required plug change. With 150 grade fuel containing 6 cc. of lead, 10 to 12 hours, or normally 2 missions, was the average length of time between spark plug changes or cleaning. At various times in the six months of operation of P-51 aircraft on 150 grade fuel many other maintenance difficulties were attributed to the fuel, but final analysis proved that the only real effect of the fuel was the lead fouling. Some units maintained that they had some deteriorations of seals, but this was not borne our throughout the command, nor was there any concrete evidence that it existed in the units.

 

The excessive fouling of spark plugs usually exhibited itself in roughing up of engines after a couple of hours of low power cruising. Periodic bursts of high power in most cases smoothed the engine out. However, if the engine was allowed to go too long a period without being cleaned out, the accumulation of lead bromide globules successfully withstood any attempts to blow them out. In some instances, long periods of idling while waiting for take-off and a failure to use high power on take off resulted in loss of power during take-off run and in some cases caused complete cutting out with subsequent belly landing. The cases of cutting-out on take-off definitely attributed to excessive fouling were comparatively few, although numerous enough to list it as an effect of the extra lead.

 

As a result of several months operational use with the fuel, an SOP – designed to reduce power failures on take-off, leading troubles in flight, and other things which were causing early returns and abortive aircraft – was published. This is inclosure no. 1. Almost immediately after this section published this SOP practically all of the troubles then existing ceased, although it was necessary to change plugs after each two missions or thereabouts.

 

You are confusing PEP fuel with 100/150 fuel Crumpp.

[NineLine]

Stay on topic, this thread started about DCS World, that is what we should be discussing...

[BlackLion213]

One last thought on the matter (Sith...please bear with me :)).

 

This explanation is quite correct for a "pop" that is heard when throttling back:

 

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.

 

I have a new 2015 (fuel injection, variable valve timing, catalytic converters, etc - it's a bone stock new car with a warranty) car that does this on throttle lift (generally lifting between 2000-3500 rpms and more prevalent after periods of aggressive operations). You can't even change the air filter on this thing without triggering a check engine light and it is definitely running properly.

 

Even in a modern car with fuel injection and mass air sensors, there is a very brief lag between closing the throttle and the air-fuel mixture adjusting. A bigger concentration of unburned fuel travels through the exhaust and ignites upon reaching the exit (where oxygen is present). At night, you can see flames appear for a split second, but generally you just hear a pop.

 

It doesn't harm anything since it's happening at the exhaust exit (like firing blanks in a rifle) and no significant pressure is generated. Race cars do it all the time (more so than street cars) and you only see it during off throttle periods.

 

There may be other things that cause "pops" like this, but Sobek's explanation is definitely accurate and does not represent improper tuning or a bad gasket.

 

Not sure if this is the cause of of the sputter heard with the P-51, sounds like I might need to check things out.

 

It's awesome that DCS depicts all of these nuances of piston engine operations, this is far more sophisticated than I would have expected. I might need to spend more time in the P-51D (I tend to prefer the FW190 D-9 and BF109 K-4).

 

-Nick

[Random]

 

No, it is not normal for any engine and can be harmful anytime it occurs. "Old" engines are just more likely to have something out of proper adjustment.

 

Oh it's normal for some... just not aviation ones ;)

[sobek]

Really?

 

So I guess the aviation mechanics are all taught wrong in this "theory" and we should go with your idea? I will let Aviation Maintenance Technician Society know they are teaching it all wrong and you can write American Society for Testing and Materials to straighten them out.

 

Yes, really. I have a degree in audio engineering and i do know a thing about resonance in open, half open and closed pipes. I can tell you with full certainty that the sound we are talking about is not principally caused by any form of resonance, period, even more so in the Merlin stack, because of its extremely short length.

 

That doesn't mean resonance doesn't occur or is not used to optimally scavenge the burned charge at certain RPMs in some engines, it just means that this sound is not caused by resonance.

 

The article you quote clearly lists faults that would lead to afterfire occurring during powered flight. Nobody contested that that would lead to damage.

 

I think you are familiar with harmonics, right?

 

Yes i am familiar with harmonics. Just a few questions for you before you blindly quote some textbook that doesn't apply:

 

Do these incidents occur at a fixed RPM and multiples of that RPM number? No they don't.

 

Do these incidents occur when revving up as well as when decreasing RPM? No they don't.

 

Please Crumpp, just this once consider that you are off base. This theory is cyclotron grade baloneyum.

Edited November 25, 2015 by sobek

[Friedrich-4B]

Just for interest, the P-51D's exhaust stacks were made of stainless steel. Is it possible that this contributes to the popping and crackling at idle?

 

[Solty]

If it was pilot error milo....the 8th USAAF would have concluded it was pilot error and not withdrawn the fuel until a solution could be found.....

 

Go get your Harley fixed too.

 

:music_whistling:

You do know that this is the second 150oct mixture they had? The first one had no problems what so ever, but the second one you are refering to was giving the problems with engines and so they had to quickly change to a fuel that was available.

 

You are talking about the new early 1945 blend of fuel called the PEP, while 44-1 was used up that point without problems. Please read:

 

150 grade fuel continued to be used by 8th AF units through 1944. 31 The WER engine limitation for the P-51 continued to be 72" Hg. 32 Eighth Air Force Fighter Groups converted to a new blend of 150 grade fuel, with increased amounts of ethylene dibromide (1˝ T) in early 1945. 33 P.E.P, as the new fuel was called, was tried in order to remedy lead fouling of spark plugs. While spark plug fouling was eliminated, PEP was found to have an undesirable effect on valve seats. As a result of excessive maintenance required on the V-1650 engines, General Doolittle of the Eighth Air Force decided in late March 1945 to revert to the normal 100/150 (1 T) grade fuel. 34

I agree everything should be modeled in DCS, but we should focus up to 1944, not on the fuel that was delivered in 1945, which changes nothing for DCS 1944.

 

Secondly, stop blowing things out of proportion. Reports say that standard usage of 44-1 fuel increased maintnance of P-51's engine. Stop thinking that US Army was stupid. They knew what they were dealing with and since may 1944 it was recomended to use 75'hg on P-51B, so D would have the same treatment.

 

A. It is recommended that the war emergency rating of the V-1650-7 engine as installed on the P-51B airplane and using 44-1 fuel be increased to 75 in. Hg. manifild pressure and 3000 RPM. 15

Reference:

http://www.wwiiaircraftperformance.org/150grade/150-grade-fuel.html

Edited November 25, 2015 by Solty

[Echo38]

You know, it's an invaluable resource, and I've relied heavily on it, myself, but, is there anywhere else that this sort of information is available, other than Mr. Williams' site? I have much respect for the people who visit national archives & such, and make that information available to the public, as a voluntary service. However, I get vaguely uneasy about one source being always referred to, and I'm unaware of any others which could be used to cross-reference these types of thing. I mean, some of it can be found elsewhere, here and there, but it's extremely rare to find a large repository of it.

 

I have no reason to suspect that Mr. Williams has an agenda, and I've seen nothing to suggest that he alters any of the document scans he posts on his site, or is selective in the inclusion thereof. However, if such were the case, or if any of the documents were erroneous for any other reason, how would any of us have any way of finding that out? My point is: are we overly reliant on a single source for the majority of our WWII aviation documents, on this subject? Cross-referencing is vital to ensure the accuracy of sourced information. Problem is, where else can we find this stuff?