stratmoto
Posts: 20
Joined: 12/1/2006
From: SydneyNSW, AUSTRALIA
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quote:
ORIGINAL: DarZeelon
quote:
ORIGINAL: Flyboy Dave
I see some new Internet myths developing here. 
Speedster 1919.... "A ringed engine will tolerate a lean run better than ABC."
DarZeelon...."I strongly disagree! A ringed engine, when run too lean and detonation sets in,
will first over-heat the ring, which will then butt its ends and continue to expand... it will eat
away the sleeve and itself."
Speedster 1919...."DAR you're talking extremely lean for ring to butt and chew liner and take
tension out of ring."
I was going to let this thread slide, but the "theories" have gotten way out there. 
...just keeping the record straight....
Let's look at DarZeelon's statement: "A ringed engine, when run too lean and detonation sets in"...
....what makes you say that detonation will "set in" on a ringed engine when it leans out ? Will an
ABC engine detonate if it leans out ?
Neither of these statements are true. One could argue that leanness is a cause of detonation, but
the truth is....high compression and too much nitro are the main causes of detonation, a too hot
plug will not help. Let's assume the engine is operating properly from the start. Just the fact that
the engine is running lean, will not cause it to start detonating. If that were the case, the engine
that was running properly during the entire run....would suddenly start detonating when it ran out
of fuel.
This, of course, does not happen....so we can chuck the statement....
...."RUN TOO LEAN AND DETONATION SETS IN".
The second part of DarZeelon's statement:
...."when run too lean and detonation sets in, will first over-heat the ring"...is another Internet
myth statement. The flame-front from the ignition process starts in the middle of the combustion
chamber....in the middle of the piston. The heat from normal combustion, and the excessive heat
from a detonation scenario are absorbed directly into the top of the piston.
The heat from the combustion process is absorbed into the aluminum piston. The job of the ring,
with it's seal to the cylinder wall....is to transfer the heat from the piston, to the cylinder
wall....where the heat passes through the wall....into the fins of the engine, where it is dissipated.
The ring will not, can not, and will never be any hotter than the piston, where all the heat is born.
Keep in mind, the ring....while transferring heat from the piston....to the cylinder wall, is cushioned
by a coating of oil. This oil is not only reducing the amount of friction between the ring and the
cylinder wall....it is acting as a conductor of heat. The piston ring is what conducts or
transfers the heat of combustion....to the outside of the engine.
The third part of our new Internet myth is:...."will first over-heat the ring, which will then butt its
ends and continue to expand"....
Piston rings will never "butt their ends" from expansion of heat, because of these reasons.
First....piston rings (iron) and cylinders (steel) expand at the same rate. There is no time where
the difference will result in a ring gap larger or smaller, enough to effect the operation of a ringed
engine. In fact....a ringed engine will usually have more compression, and a crisper feel
after a normal engine run.
Secondly....most ringed engines have a locating "pin" in the ring land (groove). Not only does this
not allow the ring to rotate and snag an end in a port....it does not allow the ends of the rings to
"butt its ends and continue to expand". Again....with the expansion rates of the ring and the cylinder
wall being the same....the ring, although it gets "longer" due to expansion heat...the cylinder is
expanding at about the same rate....the ring "gap" stays about the same.
Lastly....this Internet Myth...." will first over-heat the ring, which will then butt its ends and continue
to expand... it will eat away the sleeve and itself"....
This last Internet Myth....or a "DarZeelon Engine Theory"....excludes the reality of cast iron rings.
I can back this statement up with 25 years of two stroke racing experience.......rings, baby....rings,
rings, and more rings.
You cannot butt the ends of a ring together, and...." eat away the sleeve and itself"....
this is another load of....well, you know what. The reason is....if you close up the ends of a ring
(ring gap) and continue to expand the length of the ring....the ring will shatter !
Racers....people who race things....often try to gain advantage by using tight clearances to their
advantage. This advantage is a science. A racer can use a "first oversize" ring...on a standard
piston piston. The mechanic must grind the ends of the ring to his "specs". The ring gap must
be perfect....it has to start, and end, the race with maximum power....wear has to be considered.
Unfortunately for us, the ring gap cannot be changed on our peanut whistle aero-model engines.
So, ....I'm thinking...that you, DarZeelon....are either making this stuff up, are either grossly
misinformed ....or are just trying to BS somebody....I don't think it will be me. 
FBD. 
Dave,
The piston ring is a small, light part, which will reach a higher temperature faster than either the piston, or the sleeve.
The anti-rotation pin will not prevent the ring's ends from butting. It occupies only the inside part of the groove.
The ends of the ring are above it.
Please see the photos below, courtesy of MECOA.
Even if the pin is full height and the ring-ends butt with the pin, it will still expand to a point where it will press very strongly against the cylinder walls. The ring gap is designed not close when the engine is at proper running temperature.
All glow engines will reach the verge of detonation, or even detonate, if run too lean.
But when the mixture is leaned too far and maximum cylinder pressure comes too early, to the the verge of detonation, it causes the ring to over-heat more than do the piston, or the sleeve. You can bet there will be ring-end butting.
Will the piston not over-heat and over-expand? Yes, it also will, often to the point of seizure, but the ring will go first.
In an ABC engine, the similar expansion coefficient of hyper-eutectic (high silicon) alumimium and brass, will delay piston seizure and ultimately allow the engine to run again after being lean-run.
Even if you don't agree on how it happens, it simply does and one prolonged lean-run will do the ring in, even if nothing else must be replaced.
Please read Harry Higley's book again. It describes this very well.
No bovine excrement here...
When detonation occurs, it is the piston crown and cylinder head that are initially and mostly affected. The piston crown and cylinder head shape will determine where most of the detonation takes place. Most 2 strokes, with a pronounced squish band, will detonate around the circumference of the piston.
The heat then affects the clearance in the ring land. Localised overheating and distotion occur. When the ring land clearance becomes too little, the ring becomes trapped. The combustion pressure then is "blown by" past the piston ring. As it blows by, it also removes the lubricating oil. Without lubrication, things get ugly, very quickly.
Post mortem can be diagnosed as a ring failure. This is not normally the case with a lean run caused detonation. Chicken and egg.
FBD would have had similar experiences with his Bultaco. Years of 2stroke experience trains the left hand (clutch hand) reflexes to be lightning quick. By the time you think to yourself "hmm she's not pulling properly or what is that noise?" the back wheel could be locked.
< Message edited by stratmoto -- 2/20/2007 10:55:02 AM >
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RE: Ringed vs Non-ringed - 
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