2 stroke terms


9 replies to this topic
  • Way Fast Whitey

Posted July 20, 2007 - 05:33 PM

#1

it has come to my attention that i have no clue what pinging is :thumbsdn:

could i be mistaking "a rattling noise" for pinging?

pinging is the same thing as detonation right? the air/fuel mix burns, or starts to before the piston is on its way back down?

sorry 2 stroke newbie here :bonk:

thanks guy one of my last newbi posts, honest.

  • HawkGT

Posted July 20, 2007 - 07:33 PM

#2

"Pinging" is another term for detonation--as is "knocking". These terms arose from being descriptors of the audible signal that can be heard (usually) when detonation is taking place. Detonation is known by a few other names as well, including spark knock and auto-ignition (not to be confused with PRE-ignition--that's a different phenomenon).

Detonation is when the end-gasses around the edges of the combustion chamber detonate (i.e. explode) rather then deflagrate (i.e. burn). "End-gas" is the term for the last part of the mixture to be consumed within the combustion chamber. In engines with a more or less centrally located spark plug these end-gases reside all around the parameter of the combustion chamber. Detonation is a type of combustion that is abnormal while deflagration is the normal burn that is suppose to happen.



Here's what happens:

1) The cylinder is filled with fresh air/fuel mixture and the piston is on it's way up (the compression stroke). Remember that the spark plug does not fire right at TDC like we sometimes think of it doing. The timing of ignition is advanced by about 15 deg of crank rotation before TDC (around there for a 250cc two stroke). So the spark plug fires and the piston is still compressing the mixture. Note: This is because it takes TIME for the a/f mixture to burn--it doesn't happen instantaneously. Ignition timing advance is designed so peak cylinder pressures occur somewhere around 10 or 15 degrees AFTER TDC. The difference between the moment when the plug fires and the moment peak cylinder pressures occur can be thought of as the time it takes for all the mixture to deflagrate (burn).

2) This element if time is important because during this period the end-gases around the edge of the combustion chamber are sitting there waiting for the deflagrating flame front to get there and consume them. The flame front can be thought of as an expanding ball--it starts at the plug and spreads out in three dimensions until everything is consumed.

3) While the end-gas is waiting it is being subjected to ever increasing heat and pressure from A) the piston still making its way to TDC on the compression stroke and B) the expansion of gases from the already burned mixture behind the advancing flame front. These forces initiate what are known as pre-flame reactions in unburned gasoline ahead of the flame front. The chemical kinetics can be complex but the jist of it is the gasoline can be "broken-down" into new chemicals. These new chemicals are so unstable they are able to light off (auto-ignite) all by themselves before the flame front has a chance to get there and consume them normally. That's what detonation is. There is a race of sorts happening between the advancing flame front and the pre-flame reactions. The flame has to get there and consume the end-gases before the pre-flame reactions have enough time to "beat-up" the gasoline to a point where it's able to detonate.

4) That's what octane ratings measure--how resistant a gasoline is to the pre-flame reactions that break it down into the new chemicals that are able to detonate. Make no mistake, octane ratings tell us NOTHING else about a gasoline--not burn speed, not energy content, nor any other thing--just detonation resistance.

6) What actually causes the "pinging" or "knocking" sound of detonation involves the interaction of pressure waves inside the combustion chamber. The normally deflagrating flame front sends out its own set of pressure waves. When spots of a/f mixture around the edges of the combustion chamber detonate they send out there own pressure waves. So you wind up with sets of waves that are traveling in opposite directions. They don't "clap together" to create the noise as some sources report. The two opposing waves constructively combine and create a standing wave that is stronger than either of the two waves individually.

It looks something like this; two waves moving in opposite directions constructively combining:
Posted Image

The spike in pressure this creates is greater than would occur if no detonation took place. The spike occurs so suddenly that the downward movement of the piston isn't fast enough to absorb the force (by way of pushing down on the piston). So the pressure spike gets absorbed right into the metal structure of the engine and causes it to audibly resonate.

This can damage rods, cranks, crank bearings, heads, valves, valve seats (obviously not found in a 2 stroke), etc. The other primary failure mode of a detonation problem is heat transfer. When a spot of a/f mixture detonates it blasts away the very thin thermal boundary layer of gas that clings (mostly undisturbed, normally) to the surfaces inside the combustion chamber. This layer insulates the metal parts from the extremely high burnt and burning gases. When this layer is compromised the hot gases can actually touch the metal and this burns it the metal away.

Here's an example of a piston with the heat transfer type of deto damage. Notice the top ring land.

Posted Image

Posted Image

If that piston had been allowed to continue to run while detonating the ring land and top edge of the piston would have been completely blasted away....like this one. The chicken scratch you see on the top face is from the broken ring bouncing around before the engine stopped spinning:

Posted Image

The top ring lands are particularly susceptible to this kind of deto damage because the space between the cylinder wall and the top ring land provides a little "nook" in which pressure waves can enter, bounce around, and create havoc with the protective boundary layer.


I could go on :blah: but I've probably got way too far as it is! :blush: Hope at least some of that was more enlightening than confusing. Sometimes it's hard for me to "step-away" from my own interests in abnormal combustion and provide info that others can easily digest.

  • cr2fidy

Posted July 20, 2007 - 08:07 PM

#3

ya what he said :applause:

  • Way Fast Whitey

Posted July 21, 2007 - 05:31 PM

#4

thats a pretty solid post :applaus:

thanks HawkGT

  • hondacrf50f

Posted July 22, 2007 - 10:40 AM

#5

sweet post, you taught me more in 10 minutes then half a year in auto class.

  • Britincali

Posted July 22, 2007 - 10:45 AM

#6

Great post man!


The only thing I would say is in that last pic the bike must have been running really lean.

  • STILL_KICKIN

Posted July 22, 2007 - 12:11 PM

#7

Thanks for such an imformative explaination. :thumbsup:

Who says an old dog can't learn new tricks. :busted:

  • HawkGT

Posted July 22, 2007 - 03:53 PM

#8

Thanks for the comments. :thumbsup: I figure writing that stuff out not only helps others but it helps me too. Writing it out helps me bring information together and focus my ideas. Plus, any ensuing discussion helps me make sure I got it all right. So everyone wins! :applause:

...The only thing I would say is in that last pic the bike must have been running really lean.


That's what almost everyone says when I show that picture! :jawdrop: I can understand why though. Anything that compromises the thin boundary layer of gas that thermally protects the combustion chamber will lead to melted parts. Although I've yet to find the comprehensive research that explains the relationship between equivalency ratios (i.e. a/f ratios) and the boundary layer, it stands to reason that lean a/f ratios promote a thin and easily disturbed boundary layer. I look at it like this: If you're rich you may end up with a thick layer of carbon on the piston. That can't happen unless there is a relatively substantial space next to the piston's surface where the a/f mix won't burn. So if you go in the other direction--lean--is there a point where the boundary layer gets so thin it can't do it's job? Perhaps, but....

The precise mechanism that causes a part to fail can be hard to pin down. In my experience it's not usually a SINGLE thing, but rather a series of cascading events. For example: lean jetting causes a rise in cylinder temps, which over-works what the gasoline's octane rating can handle, which causes deto to take hold, which causes a piston to fail. So, did the engine fail because it was too lean? Or because it got too hot? Or because the fuel wasn't high enough octane? Or because it started deto'ing? All of them are correct, in a sense.


Here's why I think that piston had nothing to do with being lean and everything to do with the wrong fuel (octane rating):

  • Those two pistons came out of a 95' Kawasaki 750SXi jet ski (750cc twin). The pistons are a pair and were removed at the same time. The ski had been run for many hours with the same jetting without any issues. It was actually tuned quite well.

The damage on the first piston is without a doubt deto damage. It shows classic signs of the boundary layer disturbance (and resulting metal erosion) caused by deto. Notice in the first picture that lightened ring that runs all around the perimeter of the piston top? The metal in that area was being eroded away--it's rough with a sort of sandblasted look when examined up close. That ring nicely coincides with the end-gases locations most prone to detonation. This combustion chamber was experiencing detonation.
The second piston was going through the same thing (I believe) but was farther along in it's destruction. I believe the difference in the operating temperature of the two cylinders is to blame for the different progression of the pistons. The coolant routing ends up having one cylinder getting nice fresh water from the lake while the other cylinder only gets coolant after it's been heated up a bit by the first cylinder. I've never tried measuring the difference but I believe there is one and believe this accounts for why one piston was more damaged than the other. Heat has a HUGE influence on the likelihood and intensity of detonation.

Simply put, I believe both those pistons have the same damage--just one was degrading faster because that cylinder runs hotter.
After the engine blew I searched for the exact cause. I discovered that the fuel used was NOT the 100% premium that the ski demands. Instead, several gallons of 87 were accidentally mixed into the jug. :naughty: The head had been decked and I now realize that I was probably pushing my luck even with fresh 100% premium.
There are separate carbs for each cylinder so I can't deny the possibility that one carb may have suddenly developed a problem, or that the flywheel side crank seal pulled some air (that seal would pull air into that side of the crankcase). But no problem of that sort was found during the rebuild. The old crank seal wasn't tested--it was just replaced when the crank came out. It would seem awfully coincidental that one piston destroyed itself because it was lean while the other piston was on it's way to destroying itself because of too low octane rating. It seems more likely to me that BOTH pistons where going through the same thing--just in different stages because of slightly different cooling. That's my theory anyway. :ride:

  • redhornet

Posted July 22, 2007 - 07:48 PM

#9

i was going to say the exact same thing:thumbsup:

  • Scott_321

Posted July 22, 2007 - 11:39 PM

#10

I know it's not the same thing but this post reminds me of an old Rambler my mother had when I was a kid. It would keep running and knocking for about 10-15 seconds after turning off the ignition. Kind of embarrassing when we went places.





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