internal temps

[bvaught]

CNY country try manually typing in the link or go to avweb.com. Click on columns, scroll down for the pelicans perch columns list, select the column labeled "mixture magic". It is an extremely good article that is backed up with data.

 

[klickitatsacket]

bvaught are you home right now. I was thinking if giving you that call if it is a good time

 

[techdave]

hi guys, just a few comments if I may. To understand why leaner than stoich fuel ratios do not ccause hotter gas temps try thinking of it this way: At stoich ratio (14.7 A:F)all the fuel combusts and makes heat, any air in excess of stoich must be heated by the combustionof the stoich amount, thereby lowering the temp. in the real world it may not be possible to get full mixing of A and F so slight increase in A my yield slightly higher temp.

As for boundry layers, the area aroiund the exhaust port has no such protection, and for this reason sutained ops with poor lube, lugging, dirty filter bypassing, etc may cause damage to piston or cyl before detonation occurs. Good maint and consistent use paying attention to vibration, sound, and smell of saw is the key tot longevity.

in a related topic the use of a SHARP chain with proper gage depth does a lot to protect the saw's engine too. Just my 2 cents, Dave.

 

[average joe]

While I am at it. I see a few saws (stock) that have severe errosion on the exhaust side of the piston crown. Does any one know the casue of this?

The sand blasted appearing errosion is caused by detonation.

What I don't understand is how NOX production is higher in leaner mixtures, doesn't that mean hotter combustion?

NoX forms at higher temperatures. That is why diesels have the worst nox emmisions.

the area aroiund the exhaust port has no such protection

Thats not true at all. That area burns first in most cases only because it is the hottest part of the piston and there fore takes less detonation induced heat before it starts to burn away.

 

[CNYCountry]

To understand why leaner than stoich fuel ratios do not ccause hotter gas temps try thinking of it this way: At stoich ratio (14.7 A:F)all the fuel combusts and makes heat, any air in excess of stoich must be heated by the combustionof the stoich amount, thereby lowering the temp. in the real world it may not be possible to get full mixing of A and F so slight increase in A my yield slightly higher temp.

This is exactly my understanding, plenty of O2 for the fuel available in the mixture ensures that O2 molecules are near every hydrocarbon molecule, ensuring their availability to complete the combustion process. Combustion is fast to the point of violent, complete and very hot. This is my understanding though...

The sand blasted appearing errosion is caused by detonation.

This was my thought exactly. If I remember correctly the documentation for my Stihl calls for at least 91 octane unleaded fuel. If someone is using standard (at least around here) 87 octane fuel and working a stock saw hard, that would easily explain your erosion around the exhaust area. Since that's going to be the hottest area it will most likely touch off preignition, leading to detonation and disturbance of the boundary layer which spot heats the aluminum to melting.

NoX forms at higher temperatures. That is why diesels have the worst nox emmisions.

Which is why I am saying increased formation of NOX at leaner mixtures is evidence of hotter combustion.

My mind is still open though, I am still trying to get to the avweb site to read that article...

Thats not true at all. That area burns first in most cases only because it is the hottest part of the piston and there fore takes less detonation induced heat before it starts to burn away.

Because it's the hottest part it's most likely to cause preignition leading to detonation, but this is getting into academics..

 

[CNYCountry]

This is really interesting:

http://media.corrado-club.ca/tech/detonation.pdf

 

[average joe]

Detonation can also start without pre ignition. Pre ignition often times leads to detonation, but not always.
As far as NOX formation goes. Think of compression ratios effect on combustion. More compression = more heat = more efficiant. The higher the heat of combustion, the more NOX that is formed. To lessson the NOX a EGR system is used to inject exhaust back into the combustion chamber to squelch the flames so to speak.

 

[average joe]

Another thing I would like to add is that poor fuel isnt always the cause of detonation. A faulty ignition or a air leak can set it off just as easy as low octane gas.

 

[techdave]

Hi Joe, there is no boundary layer on the cylinder wall in the area of the exhaust port because there is no cylinder wall in the area of the port. Because of that the boundary layer does not form immediately at the edge of the exhaust port . Perhaps we are talking about two different things here. USUALLY boundary layer refers to what is a typically thin portion of a fluid (gas) with radically different properties than the main mass of the fluid. In this particular case I am referring to a related effect where the cylinder wall and to a lesser extent the periphery of the piston are protected because the surface of the cylinder has been chemically altered during normal operations. The presence of the boundary layer is what enables that process to happen.

 

[average joe]

Off course there cant be a layer where there is no material.
Of use to this discussion is the period of time before the port is even opened. The boundry layer is as you described and protects the piston from heat generated by combustion.

 

[average joe]

As for boundry layers, the area aroiund the exhaust port has no such protection, and for this reason sutained ops with poor lube, lugging, dirty filter bypassing, etc may cause damage to piston or cyl before detonation occurs. Good maint and consistent use paying attention to vibration, sound, and smell of saw is the key tot longevity.

I think this is where the confussion on my part comes from. I thought you where reffering to the piston. not being protected by the boundry layer.
Many exhaust side seizures have nothing to do with detonation. most are caused by lean conditions. the exhaust side of the piston is the hottest so it seizes there first.

 

[bvaught]

Klikitatsacket,

Can you get your hands on one of those Carbon-Carbon pistons? I did some checking and sure enough, NASA has patented a process for manufacturing them, and it would be a blast to try one out. Judging from the patent I would imagine that they would be EXTREMELY expensive to manufacture.

Im willing to bet that they will also cause a host of problems and are of little practical value. If not they would probably be installed, at least, in every formula 1 car.

 

[klickitatsacket]

Bvaught- I wasn't intending to talk about this on here for awhile as I am still a little over my head with this, but yes I can get them. I have been working with the outfit that is in charge of the manufacturing process for 2 months now. When I brought the idea of doing a 2 stroke piston and the reasons why I wanted them; they became very interested and agreed to work with me. Yes they are very expensive. A typical small bore piston is around $800 (estimated cost) and take up to 3 months to grow. Yes I said grow. The process that they are using is some thing I can not begin to explain as I was lost when they went into explaining the process. Some of the features of this product that make it appealing are: controlled expansion (zero radial and axial expansion can be built right into the piston if desired) Working temperatures of up to 300 degrees c. The material actually becomes stronger at higher temperatures. Also these can be built to be self lubricating. There are several problems up front that need to be dealt with and I believe I have most of it worked out. The major hurdle right now will be the 14K - 16K for R&D costs to get the bugs worked out. If all goes well, I am hoping that I can have the first one built come next spring. I have actually learned alot on how some of this stuff works. Like how to deal with the academic types. I went into this thinking it was just a matter of saying this is what I want (shape and size). I got to say I had no idea what I was looking at. These guys have 100 different questions most of which I never even knew had to be asked.

 

[shtoink]

Now that kicks (_|_) !

This is something the definately interests me. Did they say how long the grow process takes to form a usable piston? I sure these guys had some serious brain-bending questions...