Weird sound from the engine at part-throttle high-RPM

Salut!
sorry I don't speak French, I was addressed to Your forum as they told me here I'd find among the best experts of 2 stroke high performance engines in the world.

I have the following engine:

72mmx72mm 293cc by KTM
1.2mm squish
36mm PWK carburetor very well jetted to run clean (neither lean nor rich, in the whole range)

starting at mid-high RPM, the higher RPM the worst, at very part throttle only (say starting from 1/32 to 1/8) combustion stops being regular (it's carburated as to NOT do fourstroking) and I can hear a "chaff!! chaff!! chaff!!" (sorry, couldn't describe it better) sound from the engine.

It's not showing up at lower RPM, probably it's someway linked to the exhaust valve, but my suspect is that it may be detonation.

It's not much affected by carburetion: I have really good carburetion and anyhow even making it richer (to the point it starts fourstroking, or even worse) the problem is still there, it just sounds a bit mellower, but the "chaff!!" sound at very part throttle (which extends even to 1/4 throttle at very high the RPM) remains even if I go rich. Seems also to go worse if I retard the ignition (!), when I was expecting the opposite (I didn't try to advance it, though, only to retard).

I would like to ask.. what is it? Detonation? A sound of hell coming from the reed valve maybe?

The more technical-physical explanation You can give me, the better. I have very good knowledge of combustion and 2 stroke engines in general, so please be technical if You may help me!

Cheers,
Tony

Benvenuto Tony !

I can absolutely not help you with your technical issue.

BUT

I'm pretty sure spending a few minutes to introduce yourself in this area http://www.pit-lane.biz/f1-presentation
might help you get more responses  

Good luck with your chaff-chaffing engine  

Thank You Jarno, I've just done it.

Ciao Tony,
From your description I would think your chaff-chaff is detonation, caused by burnt gas that enters the transfer ducts when the scavenging pressure in the crankcase is low (part-throttle).
Retarding the ignition would yield an even higher temperature of this burnt gas.
An exhaust valve that opens too late or insufficiently would aggravate the problem because the cylinder pressure can't drop below the crankcase pressure before the transfer ports open, and that would become worse as the revs go up.
A quick way to find out would be to fix the exhaust valve in the fully-open position.
If that doesn't work, check the complete exhaust system for dents, carbon build-up or a broken perforated tube in the silencer. And while you're at it, check the cooling system: no enclosed air bubbles, no mud-clogged radiator? And maybe a colder type spark plug will make a difference.
Let me know what you discover.

Dear Frits,
thank You very much for Your answer.

The physical process You described was my fear too, made stronger after I (just) read parts of this forum where You, Jan Thiel and others were discussing, e.g. here (at the bottom of the page You propose also a solution, of course it's beyond me to implement it):
http://www.pit-lane.biz/t3173p60-gp125-all-that-you-wanted-to-know-on-aprilia-rsa-125-and-more-by-mr-jan-thiel-and-mr-frits-overmars-part-2

I understand the problem now, thank You.

I'd like to ask You, do I have to expect it to self-ignite if I cut all the sparks (when I am at high-RPM part-Throttle of course)? Or the the spark is anyway necessary for the detonation to happen, as the combustion (although slow and incomplete as it can only be at part-throttle) anyhow highers the pressure and the temperatures?

If no sign of ignition shows after I cut the spark, do I have to conclude that it may not be detonation then, or simply that my engine, being an enduro engine considerably less critical than the one of GP bikes, was now simply below the onset of the problem? (I'm oriented toward the latter conclusion, and I t recall I already made such a test without signs of auto-ignition, but I will test it again just to be sure).

I confirm that retarding (up to PMS+10) made things worse. I have no control on the exhaust valve, as it's centrifugal-mechanical on this engine, but clearly closing it would only make things worse.

By advancing the ignition from baseline instead (something I have not tested yet), I could lower the problem, maybe, as the gases would be a bit cooler when the transfer ports open.

I would like to ask You also if misfires (probable on a two stroke used at part throttle) may worsen the problem?

In this case I may try to increase the gap of the spark plug (I have a very powerful ignition which can output 40000V) from the regular 0.6mm to 1.0mm or beyond, this (in my mind) should increase the chances of combustion and thus create the conditions for regular scavenging and with the help of the pipe, better exhaust, burnt gases extraction.

Or may not help at all.. I will make some experiments and report, of course I don't want to risk with too much advance although I doubt that would be a problem at part-throttle, with such a slow combustion (due to low pressure and perhaps, not sure, also low turbulence).

As I wrote before, a richer carburation didn't help, I guess because it will only increase the chances of misfiring, and (also) will make combustion even slower. There's really no simple solution to the problem, as Jan pointed out.

But when exactly detonation happens? As the problem begins with the transfer ports opening, I'm not sure when exactly the mixture detonates then.

A sign of this part-throttle high-RPM detonation may be little dots on the head? Last time I checked the piston and the cylinder, there was no sign there of damage, but the head was full of little dots, very similar to this pic (not the pic of my head actually, but very very alike):
http://1.bp.blogspot.com/-DHr6su9o7X4/UQ6xCnXz9TI/AAAAAAAACFo/ic48LjjtvQU/s640/honda_head.JPG

Do You have a WAV or MP3 file where I can hear typical two stroke (expecially at part throttle) detonation sounds? I can't find anything useful on YouTube, for example.

Sorry for the many questions!!
But very interesting discussion indeed! And lotsa new things to learn!

Cheers,
Tony

Maverock a écrit:

do I have to expect it to self-ignite if I cut all the sparks (when I am at high-RPM part-Throttle of course)? Or the the spark is anyway necessary for the detonation to happen, as the combustion (although slow and incomplete as it can only be at part-throttle) anyhow highers the pressure and the temperatures?

Self-ignition and detonation are two different phenomena. Detonation occurs in the course of a combustion event.

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If no sign of ignition shows after I cut the spark, do I have to conclude that it may not be detonation then

No; see above.

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I have no control on the exhaust valve, as it's centrifugal-mechanical on this engine

Can't you disconnect the operating mechanism and block the valve in the open position?

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By advancing the ignition from baseline instead I could lower the problem, maybe, as the gases would be a bit cooler when the transfer ports open.

Maybe, but you always need to be very careful when advancing the ignition timing.

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I would like to ask You also if misfires (probable on a two stroke used at part throttle) may worsen the problem?

I would not expect so, but remember that this whole discussion is solely based on a descripition of a strange sound;
I haven't even heard it.

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As the problem begins with the transfer ports opening...

The problem does not begin with the transfer ports opening; it begins with insufficient blowdown time.area for the circumstances.
The cylinder pressure cannot drop below the crankcase pressure before transfer opening when the revs are high and the crankcase pressure itself is low. Burnt gas entering the transfer ducts will heat up the fresh charge, making it more prone to detonate later on in the combustion process.

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A sign of this part-throttle high-RPM detonation may be little dots on the head? Last time I checked the piston and the cylinder, there was no sign there of damage, but the head was full of little dots, very similar to this pic (not the pic of my head actually, but very very alike):
http://1.bp.blogspot.com/-DHr6su9o7X4/UQ6xCnXz9TI/AAAAAAAACFo/ic48LjjtvQU/s640/honda_head.JPG

The discoloured egde at the right of your picture does show traces of detonation, but the little dots in the combustion chamber look more as if there has been some small foreign matter bouncing around in your engine.

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Do You have a WAV or MP3 file where I can hear typical two stroke (expecially at part throttle) detonation sounds? I can't find anything useful on YouTube, for example.

Sorry, no.

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Sorry for the many questions!

You're welcome,Tony. But to everbody else I would say: forget it . I can't usually afford to devote this much time to answering a question.

Hello,
In the programming software BPS ignition for Honda RS 125 NX4, what is the function of "mape exchange Thottle hi" and "mape rates low throttle" ? it is in percent with 70% for "hi" and 60% to "low". In Normal value.
This is what corresponds to "throttle roll off behind hi and low" in the settings.

Regards,
Charles

Hallo Frits,

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I have no control on the exhaust valve, as it's centrifugal-mechanical on this engine

Can't you disconnect the operating mechanism and block the valve in the open position?

I believe that because of the way it's made, it could be reliably blocked only in the fully closed position.. but I'll see. However, in the RPM range where the detonation is most noticed the valve is already fully open, so I doubt there's any gain possible here.

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Citation :

As the problem begins with the transfer ports opening...

The problem does not begin with the transfer ports opening; it begins with insufficient blowdown time.area for the circumstances.
The cylinder pressure cannot drop below the crankcase pressure before transfer opening when the revs are high and the crankcase pressure itself is low. Burnt gas entering the transfer ducts will heat up the fresh charge, making it more prone to detonate later on in the combustion process.

OK, now I understand why it would be beneficial a device able to "raise" the exhaust port. That would give us more blowdown time.

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Sorry for the many questions!

You're welcome,Tony. But to everbody else I would say: forget it . I can't usually afford to devote this much time to answering a question.

I appreciated it much, thanks again! No need to spend Your time here, sometimes just a stimulus like the one You gave me (and I am sure also to others) may have helped us to be motivated enough and addressed to walk then by ourselves a new road of knowledge. Most of us are self-taught anyway, so we just need to be ignited.

Tot ziens! (I hope I spelled it correctly )

Maverock a écrit:

... in the RPM range where the detonation is most noticed the valve is already fully open

It should be fully open. I just wanted to make sure that it is.

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Tot ziens! (I hope I spelled it correctly)

You spelled it perfectly. But how should I reply? With Arrivederci or with See you later? Your english is too good for an Italian....

Dear Frits,
only my written English is good.. my spoken one is truly a disaster.

In other words, You write in French way better than I am able to pronounce English words.

Anyhow, I am now reading the whole 3-parts RSA 125 thread. Very interesting discussion indeed! And I am still only at page 18 of thread 1. Can't wait to read more.. like in a mystery novel.

What are You working on now? Are You still developing Your FOS Scavenging concept? Or You retired after the foul-stroke became the law?

About the "chaff!!" sound, I will try to record it and then post the audio here, what You taught me/us has been very instructive.

Dank!

Arrivederci,
Tony