50:1 vs 40:1

[Slamm]

That's the Jenning's article I was referring too.

Bell found the same thing. His interpretation was not just about improved ring sealing, but improved heat transfer from the piston to the cylinder. More heat into the cylinder, less heat into the charge in the crankcase.

Less heat into the charge in the crankcase, less expansion of the charge, more charge for the same volume.

That is why I found TW's data so interesting. More heat transferred to the cylinder wall would explain the rise in cylinder temp, without a similar increase in exhaust temp.

Finally, you started to speak some form of english that I can understand.

Do you think that is is a good idea to run either no base gasket or a metal base gasket or a more insulative/fiber base gasket to keep the heat up in the cylinder and not let it travel down to the crankcase?

Wonder if it would make a nickels worth of difference.


Sam

 

[edisto]

Finally, you started to speak some form of english that I can understand.

Do you think that is is a good idea to run either no base gasket or a metal base gasket or a more insulative/fiber base gasket to keep the heat up in the cylinder and not let it travel down to the crankcase?

Wonder if it would make a nickels worth of difference.


Sam

Hadn't thought about it, but I think the big problem with transfer to the charge in the crankcase is from the piston itself. I recall reading a paper where they looked at heat flow in a cylinder using embedded thermocouples, and if I remember correctly, there was very little heat flow in the axial plane (towards the case), with most of the transfer being radial (towards the fins).

The idea behind ceramic piston coatings is, in part, to keep the charge in the case from being heated. I also have heard of (but haven't seen any data for) ceramic coatings on the intake to insulate the charge from cylinder heat, and coatings on the exhaust side to keep it from absorbing heat.

 

[Johny Utah]

40:1 vs. 50:1 ......curiously the same thread name from a few months ago back:

http://www.arboristsite.com/chainsaw/182877.htm

Yes around here we just like beating dead horses.

 

[Johny Utah]

"rc oil torture test was test" ......actually, main point was to determine oil deposits of various oils and wear (if any).

this link below shows effect of oil migration through engine, which is more in agreement with your first sentence.

Tech Tips - Maxima Racing Usa - Overkill

Interesting article but how applicable is that to a small 2-stroke engine that doesn't see the loads of bigger engines?

 

[Johny Utah]

Hadn't thought about it, but I think the big problem with transfer to the charge in the crankcase is from the piston itself. I recall reading a paper where they looked at heat flow in a cylinder using embedded thermocouples, and if I remember correctly, there was very little heat flow in the axial plane (towards the case), with most of the transfer being radial (towards the fins).

The idea behind ceramic piston coatings is, in part, to keep the charge in the case from being heated. I also have heard of (but haven't seen any data for) ceramic coatings on the intake to insulate the charge from cylinder heat, and coatings on the exhaust side to keep it from absorbing heat.

Interesting concept that in theory should work, but now how do you keep internal top end heat down with having less area for heat to dissipate because of the coating.

 

[Slamm]

Interesting concept that in theory should work, but now how do you keep internal top end heat down with having less area for heat to dissipate because of the coating.

????Should increase exhaust temps and that should exit with the exhaust.

You don't want the piston hot, there is no benefit to that beyond a normal amount of heat for expanding to proper running size and sealing.

Sam

 

[edisto]

Interesting concept that in theory should work, but now how do you keep internal top end heat down with having less area for heat to dissipate because of the coating.

Why would you want to? Heat makes power, as long as it isn't heating the next charge in line.

Crankcase scavenged 2-stroke engines have more tradeoffs than any other piece of machinery that I have played with.

You always have to rob Peter to pay Paul...which is why, as I believe Slamm pointed out earlier, these oil threads are worthless. There are benefits and costs to changing the ratio in either direction. One has to decide which way to go based on end-use.

I just like to jump in and rattle chains when someone jams a pole up their arse and starts talking in absolutes.

 

[aussie2]

I'm sorry, but doesn't sound like you have a grasp of lean/rich vs extra oil in the mix. A lean saw will certainly toast a saw. But a saw piston is cooled by the intake charge entering the cylinder: 50:1 oil provides plenty of lubrication.

i ment lean as in lean in oil lol .not enough

 

[Terry Syd]

Here's something I wrote in another thread -

"For decades I have believed that it was the piston crown temperature that heats the charge in the crankcase and drops the power of a two-stroke. That drop in power can be up to 20% between a flash cold dyno reading and a hot reading.

I don't know what 'expert' I read that information from back in the 60s/70s, but it stuck in my mind. However, I still questioned the concept. For example, a water-cooled two-stroke doesn't loose as much power as an air-cooled two-stroke, but the BMEP on the piston is higher. That means the piston crown gets even hotter than the air-cooled engine. Of course, there is conduction of the heat through the ring and piston skirt into the cooler cylinder, but the center of the piston crown is still getting hotter than the air-cooled piston. Hmmm

Blair's work clarified what is really happening in the two-stroke with actual measurments of the temps. Here's a quote, "The peak temperature of the crankcase air rises to 120°C and, as shown in Fig. 5.21, and can be heated within the cylinder up to 600°C during the early stages of scavenging. Not surprisingly, the majority of fuel vaporization occurs within the cylinder. Pg391-392"

Bingo! It is the remaining exhaust gases and cylinder temperature that raises the temperature of the incoming charge. The water-cooled two-stroke has a cooler cylinder, therefore it doesn't radiate as much heat into the incoming charge."

 

[husq2100]

That's the Jenning's article I was referring too.

Bell found the same thing. His interpretation was not just about improved ring sealing, but improved heat transfer from the piston to the cylinder. More heat into the cylinder, less heat into the charge in the crankcase.

Less heat into the charge in the crankcase, less expansion of the charge, more charge for the same volume.

That is why I found TW's data so interesting. More heat transferred to the cylinder wall would explain the rise in cylinder temp, without a similar increase in exhaust temp.

good stuff, something I can actually start to digest. I think the biggest problem with Brian's test was NOT retuning each mix. While it would show some results in a certain light, and give certain information, how much of it would be true, or change with tune change? I mean no one is going to set their saw tune to 50-1 and then run 32-1 mix and expect it to live.

Funny thing about oil mixs and manufactures is read any of the top brands owners manuals and they all state to use their brand oil at 50-1 and any other GOOD QUALITY AIR COOLED 2T mix at 25-30-1.....no mention of tuning to suit! Just today my neighbour comes over with his blower, says it needs a retune, says it si bogging down and no top end revs. I look at it and see the muffler is oozing oil gunk. Blower is pretty new. I ask him what mix he runs and he say 30-1 like the manual says. I look in his manual and it has the same crap as the others about oil brand.....so off with the muffler, clean it out, check air filter and clean, empty left overs from tank, new fresh mix using a non ethanol fuel and 50-1 mix....blower runs great without a retune.....factory set to 50-1

 

[edisto]

Blair's work clarified what is really happening in the two-stroke with actual measurments of the temps. Here's a quote, "The peak temperature of the crankcase air rises to 120°C and, as shown in Fig. 5.21, and can be heated within the cylinder up to 600°C during the early stages of scavenging. Not surprisingly, the majority of fuel vaporization occurs within the cylinder. Pg391-392"

Easy Terry! Those are not empirical data, just the output of a mathematical model. The purpose of the figure is to show the assumptions he is using, not test those assumptions.

He starts the section out saying:

The heat transfer characteristics in the crankcase of a crankcase compression two-stroke engine is not a topic found in the technical literature. Therefore, I present a logical approach to the analysis without any experimental proof that is accurate. That research will be conducted, but as yet has not been done.

 

[Slamm]

Easy Terry! Those are not empirical data, just the output of a mathematical model. The purpose of the figure is to show the assumptions he is using, ot test those assumptions.

He starts the section out saying:

You mispelled "to" with "ot".

Sam

 

[edisto]

You mispelled "to" with "ot".

Sam

Actually, I left the "n" off of "not".

Thanks for catching it...this stuff can get confusing enough without typos.

And you misspelled, "misspelled"...

 

[Terry Syd]

Easy Terry! Those are not empirical data, just the output of a mathematical model. The purpose of the figure is to show the assumptions he is using, not test those assumptions.

He starts the section out saying:

Good catch on it being a mathematical model and not actual measurements. I wonder if he ever got around to doing the measurements.

The significant drop in power of a two-stroke when it gets hot has always interested me. I've won a few drag races by using that information. You only get one race where it works, so it is unlikely that it is the crankcase components that heat up the intake charge. I think Blair's model is probably close to the truth. I note that a water-cooled two-stroke loses less power than a fan-cooled two-stroke which loses less than a finned cylinder - that is consistent with higher cylinder temps being the cause of the drop in power.

 

[one.man.band]

the idea of coatings on the piston top, combustion chamber and exhaust port..... (and for the exhaust pipe for that matter). is an aid/control/prevent heat from combustion gases to exit the motor without transferring heat into unwanted areas. ideal would be to burn and exit without heating up fresh intake charge. coatings help insulate and keep heat in for a purpose. hot gases travel much faster than cold gases through pipes. as the exhaust gas cools off it slows down. so, you want to keep as much exhaust heat in the flowing gasses. the quicker the gasses exit the motor, the cooler the motor will be.

example: heat on piston top transfers to piston skirt and ring(s). this should be reduced obviously. as the skirt expands, heats up or burns off the lubricating oil on it. too much heat on ring, in combination with the skirt heat can cause the ring to bind in the lands.

there are other benefits as well for coatings, but not in the context of direction this thread is taking.*

reference: swain tech coatings.
(not trying to plug this site, i don't agree with the sales pitch entirely). used their coatings on four strokes. do some research before if you decide to do this. motor may need to be additional clearance for top and skirt coating.

Why Coat a 2 Cycle Piston by Swain Tech for Automotive Coatings including engine piston coatings, ceramic coating services engine, automotive ceramic coating, ceramic header coating, dry film lubricant, race engine coatings, exhaust header coating


example: heat on piston top transfers to piston skirt and ring(s). this should be reduced obviously. as the skirt expands, heats up or burns off the lubricating oil on it. too much heat on ring, in combination with the skirt heat can cause the ring to bind in the lands.

heat is transferred primarily by the ring(s) in motors. too much oil insulates the ring, and it cannot transfer heat as readily.

reference: maxima oil tech doc.
http://www.tlr-online.com/yahoo_site_admin/assets/docs/Maxima_Tips.61180056.html


*since we are into 'spelling' at the moment this direction probably can't hurt.

-OMB

 

[Bushmans]

Woah! Wait a minute. Your supposed to put oil and gas in a chainsaw?
I've been running 40 yr old scotch mixed with extra virgin olive oil!
So I guess for me it would be 40-1. 40 yrs to one olive.

sorry!

udency:

 

[Gologit]

Woah! Wait a minute. Your supposed to put oil and gas in a chainsaw?
I've been running 40 yr old scotch mixed with extra virgin olive oil!
So I guess for me it would be 40-1. 40 yrs to one olive.

sorry!

udency:

Olive? Green or black?

 

[edisto]

Good catch on it being a mathematical model and not actual measurements. I wonder if he ever got around to doing the measurements.

I couldn't find anything in Heywood and Sher on it. For all of the work done on 2-strokes, the data I want to see never seems to be available!

The significant drop in power of a two-stroke when it gets hot has always interested me. I've won a few drag races by using that information. You only get one race where it works, so it is unlikely that it is the crankcase components that heat up the intake charge. I think Blair's model is probably close to the truth. I note that a water-cooled two-stroke loses less power than a fan-cooled two-stroke which loses less than a finned cylinder - that is consistent with higher cylinder temps being the cause of the drop in power.

I'm not talking about the crankcase components heating the charge, but rather, heat transferred from the piston to the charge. Transfer more heat out of the piston, and less of it warms the charge. Do a better job of cooling the cylinder, and you will increase heat transfer from the piston.

 

[edisto]

Olive? Green or black?

Pasty white...

 

[Bushmans]

Olive? Green or black?

Definatley green. The oil is purer which allows the saw to run at air temps of 120 degrees at WOT for at least 30 to 40......................wait for it..........................seconds!
Then while the saw is smoldering at your feet you sit back and drop the remaining olives in a big fat bloody mary and smile because now you get to rebuild a saw............again!