Subthread 32:1 vs 50:1 (heat data)

[c5rulz]

OK guys something else to consider.

It has been stated that more oil slows down the combustion process. Some have questioned this and I don't know but consider this analogy:

A piston/cylinder in operation is very similar to how pressure in a gun being fired is generated. You have a given amount of gunpowder that will be burned, (not detonated) after it has been ignited by a primer. The pressure will build but that can be altered greatly by the type of powder being used. The bullet moves from the mouth of the case much like a piston and the volume increases until the bullet exits the barrel. Efficient powder charges burn completely prior to the bullet exit or a flash will occur at the muzzle.

Now this is a fact. Slow burning magnum powders often result in faster muzzle velocities than quicker ones.

The reason for this is slow burning powders continue to build pressure longer and the bullet accelerates for a longer period of time. Fast burning powders will reach peak pressure very quickly and then fall off prior well before the bullet clears the barrel and consequently small amounts of powder must be used and still stay under peak pressure which would be the limit of the gun. Both fast and slow powders still have the constraint of staying under the peak pressure limit of the barrel.

The point of this is perhaps the richer mix of oil more efficiently accelerates the piston prior to top dead center.

 

[edisto]

Old thread and still a lot of deep thoughts on the subject.
Early on I believe it was established no carb adjustments were done to tune the saw to the mix ratio and 200 to 300 rpms was gained with more oil and higher temps was a result. I think 1 thing overlooked that would add heat is your spark plug is firing each revelation, thats 200 to 300 times more combustions. per minute burning, certain to make your exhaust hotter and transfer more heat.

But the muffler temp didn't really change...only the cylinder temp did.

"But wouldn't it push heat out of the exhaust as well?"

Nope, think of when an engine goes into detonation. The cylinder head temp rises and the egts drop. The heat is being forced into the engine components.

But during detonation, it has nowhere else to go.

I know the oil drops the octane by a couple of points, but I can't find anything on the spread of the flame front. The doubling of hydrocarbons suggest a much less efficient burn, but that does not necessarily mean the flame front is slower.

Bell's and Jenning's observations do suggest better transfer from the piston with increased oil in the mix. This would account for at least some of the heat in the cylinder without an increase in egt but, like weimedog said, without knowing the effects of added oil on flame propagation, it's hard to get an idea of the relative influences.

 

[weimedog]

The following article I linked at the bottom of this book I'm about to post..(sorry!) pretty much sums up what my understand of two stroke oils is, and also where my understanding came from. The prevailing theory at the time was more oil increased lubricity and ring seal therefore more power. No thermodynamic analysis was really needed because the empirical data was enough to just run the 28 to 32:1 ratios we did in the race bikes...got more power longer "seat of the pants" dyno testing during a race and the tear downs looked better as well..a no brainer! Empirical data was more than good enough at that level! I haven't seen anything to contradict those findings yet..so I STILL run 32:1! Might try 28:1 on my two 14k plus screamers I built.

BTW, in my travels in a prior life years ago I had viability to some pretty interesting places and I heard a discussion relative to power additions with oils in the fuel mix caused by the addition of more combustible hydrocarbons and BTU content of oil...sort of a "fuel air bomb" analogy...(yea...won't go there, thank you; for those who would ask) As was stated before usually adding hydrocarbons isn't the issue (terry syd?), its the addition of oxygen to burn those hydro carbons that's the typical issue ...BUT if you are over carburated and you add some in the form of more oil..and it gets into a volatile vapor as a result of being mixed in with a combustion process, can there be more power as a result? Bet the answer is yes. Almost has to be as there is more energy to be had with more oil in the mix. Just if the oil doesn't burnfast enough the latent energy is less as during the time of combustion as less of those hydrocarbons are burned (more deposits in the exhaust too) . Since I don't know this to be true...I can't say for certain there is a positive effect relative to power..so I'll keep as I have, and leave the door open to that as a possibility but NOT as a proven fact. I do know that more energy released for any reason will result in more heat. Could be from better compression, more BTU's in the fuel air mix being released as a result of a combination of things..etc..just don't know so why would I make a statement as if it was fact and base an entire hypothesis on something I can't prove...on the other hand its good from a research perspective to hold open the door to all possibilities and explore those as more variations may allow us to stumble into the real answer....so for me, more efficient heat transfer to through the rings would be a nice tidy answer to the additional pickup of heat to the cylinder...if that was only change then you would also see a reduction in exhaust temps....but there wasn't. Therefore along with the heat gain in the cylinder there has to be a heat gain in the system to keep the exhaust roughly the same...compression gains & resultant more efficient combustion is the easiest answer...but to get tangible heat gains at the end of a hear transfer system, my "order of magnitude scale" (akin to common sense) makes me think more is going on...don't know...but really suspect there is...that's it. No more no less. Just don't know and it might be fun to research...BTW its really tough to do research when you are trying to rationalize what you think you know...so I take an open mind approach to just leave all the possibilities on the table. Questions like are there temp drops in places due to increased lubricity? bet there is, Are there insulation changes based n more oil in the heat transfer system? Bet there are there as well. What about external factors? Where was the cylinder measurements taken? Are those repeatable results? Was there changes that may have effected the measuring process itself? All this stuff that has been discussed in combination add up to a measurable change, how much each contributes is an interesting research project...I don't have the answer but beginning to get curious..

This guy sums up my understanding and where I'm willing to base my mix decisions from:
http://dirtbike.off-road.com/dirtbike/tech/two-stroke-gasoil-ratios-20502.html

PS: for the semantic warriors among us, a change in pressure (From compression) is a change in the thermodynamic characteristics..therefore combustion characteristics.

 

[splitpost]

http://dirtbike.off-road.com/dirtbike/tech/two-stroke-gasoil-ratios-20502.html
i found the real engine test very interesting

 

[one.man.band]

hope it interests you enough to do some testing weimedog.

could the effect of increasing oil % equate to a small timing advance change? for a small change of 1 or 2 degrees, the plug temperature would rise rapidly and EGT might possibly go down. the flame front would begin earlier in the cycle. the total duration of combustion would increase. the piston top would see more heat due to the extra duration.

 

[WidowMaker1]

some interesting reading ?? :msp_smile: Dan's Motorcycle "2-Stroke Oil " ... Motorcycle Repair Course

 

[edisto]

....so for me, more efficient heat transfer to through the rings would be a nice tidy answer to the additional pickup of heat to the cylinder...if that was only change then you would also see a reduction in exhaust temps....but there wasn't.

That may have been the longest paragraph I have ever seen. You hitting the Red Bull again?

I wouldn't expect much of a decrease in the exhaust, just a relative shift in where the piston sheds heat, i.e., cylinder wall vs. charge in the crankcase. Adding crankcase temps to the variables measured would answer that. If there is more heat from the mix, the average crankcase temp should be elevated, if there is more efficient transfer, it should be reduced.

By itself, better sealing and better heat transfer would produce an increase in MEP, and therefore temperature. That's not to say there are no effects on combustion of changing the oil ratio, it's just that I don't know what they are, and don't need the additional variable to explain the general trend. As far as describing the exact magnitude of the changes, those variables would have to be understood.

 

[weimedog]

it's just that I don't know what they are, and don't need the additional variable to explain the general trend. As far as describing the exact magnitude of the changes, those variables would have to be understood.

yup.

 

[edisto]

yup.

LOL!

Now you have me wondering what it was about that post that needed editing!

 

[thomas1]

Bump

 

[HarleyT]

Here is a sister thread to the other oldie.