Comparable oil percentage.

[Perry Rose]

If you run FAI fuel the engine has to be leaned to run good which means that the amount of oil going through the engine is reduced as well as the methanol. If I have 29% oil FAI fuel what  would be a comparable percentage of oil with 10% nitro fuel? The engine needs to be richened up to use the nitro rich fuel which means more oil is going through the engine. I'm guessing a 5 or 6% drop in oil content with the nitro fuel to equal the 29% FAI fuel. Does this make sense?
My hair is hurting.

[Howard Rush]

I've wondered that, too.  Maybe we should try to hold ounces of oil per minute constant, rather than ounces of oil per ounce of fuel.  For sports like combat and racing, where the engine runs at max power, temperature goes up with nitro concentration, so maybe the extra oil per minute is a virtue.  Stunt fliers use nitro to compensate for air density, holding engine temperature approximately constant.  For stunt, I would reckon that holding ounces of oil per minute constant might be the thing to do.  This might explain why Don McClave adds straight nitro to his fuel for low-air-density operation.

[tom hampshire]

     Those from warmer climes would not have experienced flying in cold weather, say below 35 deg F.  It has always been a bit of a surprise that after the needle is re set to match the higher air density, the 2-4 break seems to work about the same way.  IMO its a temperature dependent phenomenon, so if the engine were being cooled primarily by ambient (35 deg F) air, it should have a hard time breaking into a 2 stroke at all.  So the operating temperature doesn't seem to change much with a big change in ambient air temp.  Hence, the cooling must come from another source, oil flow through the engine.  This has always made me want to tune the system with the biggest possible outlet flow in the muffler, to get the best cooling in hot weather.  My IR thermometer shows that piped engines run a good bit hotter than mufflers, and are governed by the pipe in conjunction with head temp.  So let's omit pipes and their effect.  But for a muffler engine, should we start to experiment with both varying the nitro content, and lowering the oil content in cold weather?  Say with a mix of 2 stock fuels, one with 18% oil, 5% nitro and the second with 25% oil, 15% nitro.  (Both fuels with the same mix between castor and synthetic oils.)    This would add cooling oil when the added power and heat are needed, but only then.  Has anybody tried this?  Tom H.

[Howard Rush]

Looking at my post above, I see that I didn't try to answer Perry's question. I just read it and then restated it as if it were my own original idea.  

I'd use trial and error to see how much fuel it takes to get the same lap time and run time with 10% nitro and 25% oil as you got with no nitro and 29% oil.  Then, for the next iteration, try an oil fraction of .29 F1/(.75 F2 + .29 F1), where F1 is the amount of no-nitro, 29%-oil fuel needed for a flight, and F2 is the amount of 10%-nitro, 25%-oil fuel needed for a flight.   That ought to be close enough, but you can continue iterating if necessary, using the previous iteration's fuel quantity as F2 in the oil fraction formula above.  This method could be improved to make it converge quicker, but that is left as an exercise to the reader.  

Tom thickens the plot by considering what's doing the cooling.  My guess is that evaporation of the excess methanol and nitro does more cooling than heating the oil does, but I'm too lazy to calculate that.  I'll bet so is everybody else except Ron Burn.  Adding even more oil when you add nitro doesn't seem sensible to me, despite folk wisdom.  K&B .40s run happily on 78% nitro and 12% oil.  Show me some thermodynamics.   

[Brian Hampton]

I understand what's meant by the original post but it's hard to quantify an acceptable reduction in oil content when using nitro and the greater fuel flow needed for a correct mixture. I don't use nitro and I have a minimum oil content of 20% (castor) because of the leaner mixture used. In all my engines, except one, this has never caused a problem with overheating. The exception was an ST G51 that has a very low fuel consumption and needs 25% oil to keep the temperature in check or it'll break into a runaway 2 stroke that takes a couple of level laps to bring the temp back down and revert to a 4 stroke. This engine uses near enough to 10cc of fuel per minute so at 25% oil this means it needs 2.5cc of oil flowing through it each minute. With 20% oil there was only 2cc per minute so minimum flow is fairly critical. If nitro was used then fuel consumption increases and, by my figuring, if nitro increased fuel consumption to 12.5cc per minute then oil flow should be adequate with 20% oil.

Trying to calculate how much oil can be reduced when adding nitro gets complicated because of the hugely different acceptable air/fuel ratios between methanol and nitro. Methanol by itself ranges from about 4.5 to 6.5:1 while nitro has a range of around 0.5 to 2.5:1 which is a huge range and is why tuning is much less critical than using just methanol.

Oil by itself doesn't actually carry heat away from the engine, at least not to any noticeable amount. All oil does is reduce the heat of friction to the least possible level. Because oil doesn't (or certainly shouldn't!) vapourise then all it can do is heat up the liquid volume to some temperature below where it can vapourise. Taking my G51 as an example, if it flows 2.5cc of oil per minute at 8000 revs then each rev it flows 2.5/8000cc or .0003cc per rev which is a rather small volume to heat up .

[tom hampshire]

     Brian's comment about oil flow rate is understood, yet his observation about overheating is the same as mine - Adding oil does lower the temperature and help prevent thermal runaway.  Reducing the backpressure with some sort of free flow muffler may help more.

[Howard Rush]

"Trying to calculate how much oil can be reduced when adding nitro gets complicated because of the hugely different acceptable air/fuel ratios between methanol and nitro. Methanol by itself ranges from about 4.5 to 6.5:1 while nitro has a range of around 0.5 to 2.5:1 which is a huge range and is why tuning is much less critical than using just methanol. "

I take it you're not happy with the iterative method above.

I was thinking that holding the oil flow rate constant was a good idea, but most of the engine is being lubricated by the oil-fuel mixture, so maybe one wants a constant oil fraction in the fuel.  Where's Ron Burn when you need him?

[Dick Fowler]

Interesting article    http://www.lcrc.org/stu11.htm

[RandySmith]

"Trying to calculate how much oil can be reduced when adding nitro gets complicated because of the hugely different acceptable air/fuel ratios between methanol and nitro. Methanol by itself ranges from about 4.5 to 6.5:1 while nitro has a range of around 0.5 to 2.5:1 which is a huge range and is why tuning is much less critical than using just methanol. "

I take it you're not happy with the iterative method above.

I was thinking that holding the oil flow rate constant was a good idea, but most of the engine is being lubricated by the oil-fuel mixture, so maybe one wants a constant oil fraction in the fuel.  Where's Ron Burn when you need him?

Hi

I have ran and tested a lot on oil percentage in our model engines, and what I have found is it is much more critical to have a proper mixture of air\fuel.
 Running a engine just a little leaner will raise temps very fast, It is better ..within reason... to worry about the mixture ratio rather than adding additional or taking away oil. even in the high heat in south Ga, or Lubbock TX or  AZ , if your running a proper percentage of oil getting the engine to run a richer mixture will work better than running lean with an additional ounce or 2 of oil. this is one of the reasons adding nitro works so well

Regards
Randy

[Howard Rush]

I've heard the thing about oil cooling the engine before.  It is sounds so absurd that I'm sure folks demand proof before embarrassing themselves by passing it on.  The Web site Dick referenced, plus a tad of Googling, gave me the following:

Specific heat capacity of castor oil is .43 BTU/(lb. deg F).  Castor oil density is .0623 lb./fluid oz.  The Web site Dick referenced uses the example of a Supertigre .60 burning 1-1/8 oz. of 25%-castor, 75%-methanol fuel per minute.  Burning the methanol makes 350 BTU/min. and the Supertigre makes about .75 HP (= 31.8 BTU/min.), according to the same source.  Goran Olsson's reprint of George Aldrich's site cites the "burn point" of castor oil as 840 degrees F.  So if you raise the temperature of the castor oil from 70 degrees all the way to the "burn point", it absorbs 5.8 BTU per minute.  I'll go out on a limb here and say that the castor oil isn't doing much cooling, yet even the Web site Dick referenced says

"Castor oil at 10% would lubricate an engine, but it is raised to 25% so that the oil can also act as a coolant. It does not enter into combustion, just pumps through the engine the same as nitrogen. The castor "smoke" you see in the air is mostly fine droplets of oil. Traces of true smoke may be produced in a very hot engine."

So show me how.

[RandySmith]

""I've heard the thing about oil cooling the engine before.  It is sounds so absurd that I'm sure folks demand proof before embarrassing themselves by passing it on.  The Web site Dick referenced, plus a tad of Googling, gave me the following:
""

Hi Howard

It is true the oil/fuel charge will cool engines, you can see this very clearly with many examples, the FOX 35 we tried with 20 22  24  26  28 30 % oil content fuel,
 Running what sounds to be the same air fuel mixture with the motor in a 4-2 mode the engine will heat up fast with the 20 %, when you do for example a square 8  the engine will come out of the manouver and stay in a 2 cycle in level flight for up to a full lap before coming back to a 4 stroke,
 as you increase the oil percentage up to about 26% it comes back faster and faster with each increase untill you get to 26% it just snaps right back into a 4 stroke as soon as you enter level flight,
So the proper oil percentage is important, but as I stated above the mixture ratio seems to be much more important...IF.. you have the oil near correct to start.

Randy

[Howard Rush]

Ha.  Why do suppose that is? Is the oil is keeping the plug from lighting the methanol? 

[Dick Fowler]

Just a general comment. From the amount of castor residue left on the plane after a flight along with the amount of castor that spews from the exhaust, I would offer that not much castor is consumed in the combustion process. Not scientific but logical.

Also when additional castor is added, there is a reduction in the total btu content of the fuel. Again that assumes little of the castor oil is consumed in the combustion process. This reduction in btu content should be reflected in a reduction in operating temperatures.

Probably the lighter fraction of the castor combusts creating the infamous varnish well know by all Fox Superfuel users.

[john e. holliday]

Far beyon my feeble mind when it comes to why more nitro or oil should be added to already mixed fuel.  I tried adding castor to a jug of fuel on someone's reccommendation.  Wound up pouring it out as I could not get decent needle settings or even get the engine to run after removing the battery.  Even tried hotter plugs to no avail.  So I learned to use the fuel I can get.  DOC Holliday

[RandySmith]

Ha.  Why do suppose that is? Is the oil is keeping the plug from lighting the methanol? 

No The plug is still lit, The oil is cooling the motor in the larger percentage for the FOX 35..up to a point, and enough oil is correct too much is NOT better. You see the same thing when breaking in an engine on the bench. It will run hotter and will be slow to come back to a 4 cycle when you lean it out on the bench, but after the breakin period you can see the engine will snap back to a 4 cycle very quickly when the breakin is finished.

On the FOX 35 I also have to run more nitro as I increase oil percentage to get the same run and power, if you just only add oil you have to run the motor harder to get the exact same power. The advantage of nitro in the motor is you can then run the mixture richer...which is a cooler run

Randy

[Jim Kraft]

This is all very interesting. I have often wondered on my spark motors about the amount of oil going through the engine. I run both glow fuel with 25 to 30% castor, or gasoline with generally 60wt. dino oil, also 25 to 30%. Since you are only running about 1/2 of the amount of fuel/oil through the engine on gas, it seems they would need more oil on gas. They do run hotter with gas, however they do run just as well as they do on fuel except not quite as much power. I am guessing the spark ign. versus glow ign., makes the difference as we can also alter the timing manually, and do not depend on glow/compression for ign.  With the cost of fuel going up all the time, and the battery tech. on the rise, we may all be running gasoline again in the near future as the ign. goodies get smaller and lighter. I would bet a PA 61 would run just great on ign.

[Howard Rush]

The oil's obviously not cooling the engine directly.  This appears to be another of those Mysteries of Stunt, where we know what happens, but explanations of how it happens get kinda farfetched. 

[Dick Fowler]

The oil's obviously not cooling the engine directly.  This appears to be another of those Mysteries of Stunt, where we know what happens, but explanations of how it happens get kinda farfetched.  

I feel that way about the "2 - 4 Break". Or is it 4 - 2 break?

[RandySmith]

Lets see  the oil goes into the motor cold and comes out blazing hot..in the same time frame if you heat more oil,rather than less oil, does this remove anymore heat from the motor??? Or does it just lube the motor better so you don't make as much friction???


I would also ask why when you run 20% 10% fuel and then 26% 10% fuel, The 35 will hang-on to a 2 cycle for a much longer time than the same with the 26% oiled fuel??
Randy

[Howard Rush]

By my cyphering, the amount of heat that goes into the oil is insignificant.  Something else is certainly happening, as your experiments show.  I sure don't know what it is.

[RandySmith]

I think there isn't much doubt the oil must be carrying out some heat, how much is debatable .
You certainly  do NOT need that much oil for lubrication, but to run the 35 in a 4-2 mode you have to cool the motor,
if the 35 gets hot it will NOT come back to a 4 cycle, the bench test will prove that, you have to physically reach down and turn the needle to a richer setting to get the motor back to a 4 cycle. The only thing that is being changed here is the amount of oil running thru the motor in a given time period.

Randy

[Dick Fowler]

I think there isn't much doubt the oil must be carrying out some heat, how much is debatable .
You certainly  do NOT need that much oil for lubrication, but to run the 35 in a 4-2 mode you have to cool the motor,
if the 35 gets hot it will NOT come back to a 4 cycle, the bench test will prove that, you have to physically reach down and turn the needle to a richer setting to get the motor back to a 4 cycle. The only thing that is being changed here is the amount of oil running thru the motor in a given time period.

Randy

When the engine is set richer, more alcohol is being brought into the engine. I wonder if the aditional cooling comes from this. The problem with these little engines is that they are so inefficient, they probably pump nearly as much raw fuel through them as they burn. The calculations show enormous losses.

[RandySmith]

Yes when you set the engine richer more oil and nitro also goes they the motor, there is NO doubt  that a richer setting runs cooler, I was comparing 2 settings that seem to have the same needle setting , or at least as close as you can get them

Randy

[Howard Rush]

I think there isn't much doubt the oil must be carrying out some heat, how much is debatable .

My numbers are above.  So debate 'em.

[Dave Adamisin]

Oil has two main purposes in an engine, lubrication and cooling. The lub part is obvious the cooling less so. The oil cools an engine by conducting heat between adjacent parts like the piston and cylinder. Any heat carried out the exhaust by oil is just heat robbed from expansion work. The fuel portion of the mixture has at least an order of magnitude more cooling effect on the engine because it goes through a phase change. There are a few things that change when you add oil. The octane of the fuel goes down so it has the potential to have an effect on the exhaust gas temp (quicker burn means that the temp is usually lower by the time the piston opens the e-port) which could have an effect on the piston temp. and detonation. The needle valve should have to be opened so the hole size the fuel is going through has a lower resistance to flow so the engine can respond quicker to changes in  pressure. There are a lot of things going on but I would bet cooling has nothing to do with the change.

[RandySmith]

Running a 35 Fox for example,in a 4-2 style of run,is a balancing act, as it is with most motors, you must run it on or near the verge of a mixture where it will cycle back n forth, You must be able to heat and cool the motor to do this..or the motors heat and cool themselves,what ever way you care to see it. When I run my setup in cool weather..below 60 degrees, it goes thru the entire pattern in a 4 cycle, hotter weather it will 4-2 at the tops of the tricks.

It looks like the engines we run sometimes may run at the ragged edge of cooling, and the pityful amount of cooling that the oil does carry out is enough to sway the differance one way or the other. I do know for a fact that in a oil and air cooled  car engine, the oil can make a big differance in motor temp, I added a front end oil cooler to one of my 911s , brought down temps quite a bit.
So it maybe we are so close to the balancing act of heat and cool, a tiny amount does make a differance.

Maybe too the fuel mixture is seen as a richer mix in a model airplane engine when additional oil is added, Whatever is happening it works

Randy

[tom hampshire]

     Thanks to Howard for computing the specific heat of the oil... It does appear that losing waste heat to the extra oil, by itself, can't account for the demonstrated cooling which comes from added oil. The heat of vaporization of the combustible fuel ingredients can only cool the bottom end, and the top end is where our problem is.  So might it be that the extra oil is slowing the burn rate, thereby lowering the maximum temperature of the combustion products?  Or might it be that the extra oil, in combination with added nitro, slows the burn rate?  And i have no idea how to design an experiment to test this.  Just measuring the head temp is probably subject to other variables.  Maybe a pressure transducer coupled with temperature meaurement would give us some idea.  Tom H.

[RandySmith]

It seems that you are not realizing that the small decrease in temp is ...VERY.. substantial in cooling the engine, and just how close to the edge we are running in the heat balance when using a 4-2 setup. That is a real balancing act of heat-cool, most don't realize how "on the edge"  you are when setup this way.
I can tell a very significant differance in just a head that has a small amount of the fins shaved, or an engine that has a heat wrap on it, so it's not a stretch  to see that a small amount of cooling can be significant when running on the edge of a 4-2.
 No one is arguing Howard's numbers are wrong ,Knowing Howard I am sure they are correct.
Just what effect the extra oil has. Since It seems proved that the extra oil is doing the reduction in temps..whatever the dynamics are (since that is the only thing changed) there maybe other factors at work ..such as mixture change, however small, or Exhaust gas temps, or maybe the extra oil is coating the metal sufaces better preventing them from more friction and heat? etc?

Randy

[Dave Adamisin]

    Thanks to Howard for computing the specific heat of the oil... It does appear that losing waste heat to the extra oil, by itself, can't account for the demonstrated cooling which comes from added oil. The heat of vaporization of the combustible fuel ingredients can only cool the bottom end, and the top end is where our problem is.  So might it be that the extra oil is slowing the burn rate, thereby lowering the maximum temperature of the combustion products?  Or might it be that the extra oil, in combination with added nitro, slows the burn rate?  And i have no idea how to design an experiment to test this.  Just measuring the head temp is probably subject to other variables.  Maybe a pressure transducer coupled with temperature meaurement would give us some idea.  Tom H.

Tom with all due respect (I'm not being a wise a!s) this whole discussion seems to be based on "the demonstrated cooling with added oil". How is this demonstrated? It manifests itself as an improvement in 4 stroke performance. It has been assumed that this is caused by cooling. How about some data like Howard suggests. There isn't any. You propose testing to prove it. I like that idea. The sticky part is what to measure and then deciding how the parts we measure affect the parts we don't measure. Without some serious lab equipment we will in the long run still be reduced to speculation and educated guess's because the process is still very complex. If you look at simple data like the boiling point of Methanol (148 deg. F) and Nitro (214 deg F) a simple explanation of the 4S effect of added nitro can be deduced as a smaller portion of the fuel is vaporized before it gets to the chamber. Is the air temp analogy relevant to charge preperation because higher air temps allow more of the methanol/nitro to change phase before it enters the chamber? I'd personally put my money on that but without data I'd be guessing too. Does adding oil mitigate the temp/vaporization effect? The amount of mixed charge (vapor plus oxy) to the amount that will be changed to vapor and mixed as the burn occurs and the amount that leaves the engine as heated liquid is a good suspect in the crime we are trying to solve. The fuel does have a cooling effect on the piston by comming in contact with the bottom during the cycle. This heats the charge before it enters the chamber.  This is why there are oil squirters on the big end of connecting rods on high performance 4 stroke  engines...... To use the oil as a coolant.......BUT there would be a lot more cooling if those squirters were dispencing something that could go through a phase change when it contacted the bottom of the piston. Sigh....

[RandySmith]

Another thing this may go a ways in explaining, if..we ever get to the real reasons why... is the burn down effect of 4 strokes, and why running more fuel thru the motor in the same timeframe may well help alleviate this condition, which is similar to thermal runaway effect that people talk about in 2 stroke, they are most likely , basically one in the same

Randy

[RC Storick]

I've heard the thing about oil cooling the engine before.  It is sounds so absurd that I'm sure folks demand proof before embarrassing themselves by passing it on.  

I guess the guys at Ferrari and McClaren and about 10 dozen car racers are wrong. They all run oil coolers for some reason? It must work or they wouldn't have it on the cars.

Oh not to forget Harley-Davidson. A twin cam with a oil cooler will run 20 degrees cooler than one without. Heat = loss in power and reliability.

[phil c]

I suspect Randy is right that the culprit is temperature.  I'm not at all convinced that adding a couple percent of oil to a gallon of fuel carries off significantly more heat.  Just from the general engine behavior(and being a chemist) I would lean towards the oil affecting the combustion process and reducing the amount of heat generated.  Trying some experiments with an LA 46 on hot summer days, adding a couple percent oil made a significant change in the sound of the engine run, even though it was running the same rpm.  The extra oil could easily be causing less burning to take place and leaving more residual exhaust in the cylinder on the off strokes of the 4 cycle.  The extra oil also made the engine sound much smoother, which is hard to quantify.  It just ran steadier, with less variation, and none of the hard, popping exhaust sounds.

[Howard Rush]

I guess the guys at Ferrari and McClaren and about 10 dozen car racers are wrong. They all run oil coolers for some reason? It must work or they wouldn't have it on the cars.

Oh not to forget Harley-Davidson. A twin cam with a oil cooler will run 20 degrees cooler than one without. Heat = loss in power and reliability.

I guess the guys at Ferrari and McClaren and about 10 dozen car racers are wrong. They all run oil coolers for some reason? It must work or they wouldn't have it on the cars.

Oh not to forget Harley-Davidson. A twin cam with a oil cooler will run 20 degrees cooler than one without. Heat = loss in power and reliability.

That has nothing to do with the subject of oil in the fuel cooling a two stroke.  The numbers are above.  Show me what's wrong with them. 

[RandySmith]

Hi Howard

I don't think anyone is saying your numbers are wrong, Just that it is the oil that in some manner is cooling the motors just enough to make a difference. Knowing you I would not bet on your numbers being wrong, as you are very thorough in your approach to things like this.
 From my test, which have been many and often over the years, that is the only factor that has any significant change.
 I am conviced that the extra oil, in a marginally cooled motor that is running at the edge of what it can disspell is affected by the cooling of the additional 5 or 6% of oil.
Wether it is changing the mixture that is burned enough to do this, I don't know, however I think not. In my test I set the needle back to the ..same.. run it had before. The oil maybe cooling and plating the surfaces, and alleviating just enough friction just enough to make that little difference that will stop the thermal run away.
And I am not saying other things are not happening, The dynamics of how this effects the engine maybe 3 or 4 other things, but I think they are very small, or at least much smaller than the change in oil percentage.

Regards
Randy

[RandySmith]

by the way I have written several Engine and fuel articles where I stated this about the oil, and  NO I am not embarrssed of them.

Randy

[RandySmith]

Just as an added piece of info, I have checked several plain bearing and ball bearing motors running standard Hobby shop fuel with and without my oil additive, the ones with my additive have checked cooler running with a laser ir meter at the same RPMs than those without, many other have done the same, Larry Foster did this test a few years back and found the same things on his FOX 35s. So that would point to oil cooilng, plating, friction reductions or some other oil related cause in the engine.

Randy

[Dave Adamisin]

I suspect Randy is right that the culprit is temperature.  I'm not at all convinced that adding a couple percent of oil to a gallon of fuel carries off significantly more heat.  Just from the general engine behavior(and being a chemist) I would lean towards the oil affecting the combustion process and reducing the amount of heat generated.  Trying some experiments with an LA 46 on hot summer days, adding a couple percent oil made a significant change in the sound of the engine run, even though it was running the same rpm.  The extra oil could easily be causing less burning to take place and leaving more residual exhaust in the cylinder on the off strokes of the 4 cycle.  The extra oil also made the engine sound much smoother, which is hard to quantify.  It just ran steadier, with less variation, and none of the hard, popping exhaust sounds.

Phil nice to have a chemist comment on this because I agree it's all about the chemistry. I will ask a simple engineer question. If you are running the same rpm and generating less heat in each of those rpm, are you spinning a smaller prop or hitting on more cycles?

[RandySmith]

Hi All
I ran this by Dr. Scott Bair, It looks like a lot of what I thought was happening ,may really be going on, plus the plating I thought was helping was also manifesting itself in a manner I didn't have any idea of, Not only is it reducing heat by friction reduction and it carries some heat out Scott says it also shields the engine parts from heat by increasing the film coating...please see the below response. The numbers he is referring to are Howards.

"""Randy,

The numbers do indeed look correct.  I started from scratch using SI units and got a similar result although I used my dynamometer measurements (0.31 hp) for a 4-cycling Tigre 60 at 8000 rpm which is the rotational speed that I acoustically measured in flight.  Also, I used 300C as the exit temperature for the oil which comes from observing the short-time response (degree of coking) of castor oil on a hotplate at that T.   Using the same method of analysis as Mr. Rush I found that the rate of heat removal is about 1/3 of the mechanical power delivered.  While this fraction is small in comparison to the estimated heat rejection to the cooling air flowing through the cowl, it may be going out on a limb to say that it is not much.

The problem here is that the oil is being treated as an external coolant in the same way as the air flowing around the head and cylinder fins.  For an external coolant, the important thermal property is the volumetric heat capacity.  I believe that the cooling benefits of caster come from liquid-film-cooling.  Caster is a good choice here because it easily wets hot aluminum and steel, in contract to some polyglycols.  For film-cooling, the thermal conductivity is more important than heat capacity.  The conductivity of oil is about 100 times less than steel and about 1000 times less than aluminum.  A thin film of liquid insulates the metal from the hot gas and therebye redues the heat transfer to the structure internally.  The film is mechanically scraped away or blown away by the gasses to be replaced on the next cycle.

I used this liquid-film-cooling concept 33 years ago to address a fire problem in the bearing cavity in a GE 70 megaWatt gas turbine.  Hot gas from cooling of the burner cans was impinging on the metal walls surrounding and supporting a journal bearing.  The wall temperature would sometimes reach the ignition temperature of the mineral oil that lubricated the bearing.  The solution (that is still in use today, I believe) was to pull some of the oil flow from the bearing and direct it to flow in a film down the sides of the bearing cavity walls.  Although the wall temperature was significantly reduced, the heat rate from cooling the oil was very small in comparison to the theoretical heat delivery rate from the hot gas but that is not important as the cooling mechanism is different from, say, circulating the oil in cooling passages.

Scott Bair """

[Dave Adamisin]

So that's it then? Castor oil is a miracle coolant - to hell with physics. What I'm wondering is why we even waste room in the tank with methanol.

[RandySmith]

So that's it then? Castor oil is a miracle coolant - to hell with physics. What I'm wondering is why we even waste room in the tank with methanol.

Hi Dave

I didn't see anyone saying oil is miracle coolant, I did say that it didn't take much cooling to keep the engine on the edge of the cooling envelope. Or even to hell with physics, no one is saying that either. It is not  like we are having to add a massive amount of cooling to keep an engine in the range we need to run as described

Randy

[Dave Adamisin]

I will say I don't believe in the idea that we're on the fringe and that a tiny change suddenly puts things right. The systems' response slope can't be that steep or it would almost always be out of balance. If getting the oil to the bare minimum needed for stability was a good idea (and I'm not saying it isn't) than why not take the slight performance hit that going to 35 to 40 percent oil to safely move beyond the steep part of the slope? I also don't believe that the two cycle behavior is from a thermal run away. It just has too many holes in it as an analogy. It has always been cited as the culprit without any supporting data so we make up physics to support the idea. We don't need to make up physics, the real stuff works just fine. The engine is hot after a lean run ergo the problem was thermal run away. Sounds too much like the China syndrome. Course if it brought Jane Fonda out to watch me fly I might be willing to entertain the thought. I'm sorry if I came on as strident with my last post. I just was frankly insulted by the post with Dr. Bain's thoughts. I'll leave that at that. I will continue to believe what I believe and trust what I have learned and been taught.

[RC Storick]

This is real easy to prove or disprove. Get a test stand and run a engine at 10000 rpm with 20% syn/Castor one tank and use a lazier thermometer to see head temp while running. Same engine 10000 RPM with 25 % syn/Castor take reading. My bet is the 25% is going to be 10-20 degrees cooler.

[RandySmith]

I will say I don't believe in the idea that we're on the fringe and that a tiny change suddenly puts things right. The systems' response slope can't be that steep or it would almost always be out of balance. If getting the oil to the bare minimum needed for stability was a good idea (and I'm not saying it isn't) than why not take the slight performance hit that going to 35 to 40 percent oil to safely move beyond the steep part of the slope? I also don't believe that the two cycle behavior is from a thermal run away. It just has too many holes in it as an analogy. It has always been cited as the culprit without any supporting data so we make up physics to support the idea. We don't need to make up physics, the real stuff works just fine. The engine is hot after a lean run ergo the problem was thermal run away. Sounds too much like the China syndrome. Course if it brought Jane Fonda out to watch me fly I might be willing to entertain the thought. I'm sorry if I came on as strident with my last post. I just was frankly insulted by the post with Dr. Bain's thoughts. I'll leave that at that. I will continue to believe what I believe and trust what I have learned and been taught.

Dave

My post had nothing to do with you or insulting you, That certainly was not ,by any stretch the reason for my post.
 Dr. Scott Bair has a lot of experience in small motors and is has a PhD in tribology. he does this type of research all the time for a living, and a hobby. He is a natural person for me to ask questions as I did. I am sorry you took offense to it, but I can not see any reason why you would. Information posted here..at least  by me is for our curiosity. Not for insulting people.

Regards
Randy

[Dave Adamisin]

I'll go ya one better Bob. It seems that we need more data points. How about we start at 10% and go up in 5% steps to 35%. It'll give us more data and maybe even help us find a spot that doesn't work as well as the others. I might be the wrong or maybe even perfect guy to talk about this with because we have been flying stunt for 47 years and have .. never... added oil to stop "thermal run away", never added oil to improve 424 breaking, never added oil to "protect" the engine and believe me we have done some really nuts stuff to make engines give us what we needed. We added white gas to Vecos and st 40s to get the fuel economy we needed. You want heat, boy they ran hot, but they never ran away. My nephew Arch flew his silver and blue take apart on 10% nitro, 15% ucon fuel in all kinds of weather and at lot's of altitudes. No run aways, great break. I'm beginning to think that knowing why something happens isn't as important as individual experience. It's what most of us count on anyway. Peace

[Dave Adamisin]

Dave

My post had nothing to do with you or insulting you, That certainly was not ,by any stretch the reason for my post.
 Dr. Scott Bair has a lot of experience in small motors and is has a PhD in tribology. he does this type of research all the time for a living, and a hobby. He is a natural person for me to ask questions as I did. I am sorry you took offense to it, but I can not see any reason why you would. Information posted here..at least  by me is for our curiosity. Not for insulting people.

Regards
Randy

[john e. holliday]

I in my experience say that if you don't get the cooling air over the parts of the engine that need cooling it will not matter how much or how little oil you have in your fuel.  Setting the needle too lean will cause an engine to run hot and get hotter.  Hopefully enough to seize up and quit.  My McCoy .40 Redhead did not do that.  It just kept cooking and cooking until the rehead was black.  I had flown that engine on a Midwest P-39 many times with no problems.  The plane that cooked the engine was the SIG Acrobat with that big plastic cowl.  Have fun, DOC Holliday

[billbyles]

Hi Dave,

It was good meeting you and talking with your Dad again at Keith Trostle's home at the VSC this year.  Big Art has quite the ability to remember names...I see him maybe once a year and he has never forgotten my name.  Hope to see you again one of these times.

For the life of me I can't understand how you think that Randy was insulting you with his post.  I know that you are an engineer, and as a mechanical engineer myself I try to keep an open mind about understanding these things.  Randy's friend Dr. Scott Blair is fairly conversant with thermodynamics, internal combustion engines, and turbines and I thought his input was an interesting part of this discussion, whether or not it was the final definitive statement.  Being able to quickly exchange information on a forum like this between people who live quite far apart is a huge resource that we did not have just a few years ago.  It's good to have your thoughts on this subject since you are both a modeler and an engineer.

Take care, Dave.

Bill Byles

[Dave Adamisin]

Hi Dave,

It was good meeting you and talking with your Dad again at Keith Trostle's home at the VSC this year.  Big Art has quite the ability to remember names...I see him maybe once a year and he has never forgotten my name.  Hope to see you again one of these times.

For the life of me I can't understand how you think that Randy was insulting you with his post.  I know that you are an engineer, and as a mechanical engineer myself I try to keep an open mind about understanding these things.  Randy's friend Dr. Scott Blair is fairly conversant with thermodynamics, internal combustion engines, and turbines and I thought his input was an interesting part of this discussion, whether or not it was the final definitive statement.  Being able to quickly exchange information on a forum like this between people who live quite far apart is a huge resource that we did not have just a few years ago.  It's good to have your thoughts on this subject since you are both a modeler and an engineer.

Take care, Dave.

Bill Byles

Thanks Bill I needed that. I'm a pretty intense guy and probably stepped over the line and if I caused any concern in other people I am truely sorry. I too enjoyed our conversation at Keith and Barb's house. It was a thrill for me to meet the people that came between my last national experience and today. I am putting a test method together in my head at the moment to try to quantify some of what we are talking about. I am also trying to get a couple combustion experts I worked with to think about what we are talking about in this thread. I still think that we owe Perry who started this thread a discussion on what was a good question. Does the engine respond to a constant oil flow rate.. Is the oil because it is such a large fraction of the mixture a dominant player in the burn.

[Dick Fowler]

Glad the discussion didn’t get ugly.

This discussion reminds me of three blind men examining an elephant…. Perspective is everything.

Most of us look at a problem from our own perspective, which is usually shaped by education and empirical experiences.

A chemist sees the problem from one point of view, a Physicist has another viewpoint, a mechanic has another. We tend to look at things with an enormous amount of prejudice based on our background.

I’ve thought about some of the variables that effect what we observed and there are a ton.

For example:

1.   BTU content of the fuel. This is not a real tough variable to nail down.

2 Thermal losses though radiation and convection of the engine head and case.

3 Thermal losses though the exhaust.

4 Thermal conductivity of internal components of the engine. Of importance is piston and sleeve material. Add to that the lubricating film characteristics.

5 Combustion efficiency with effects  flame front propagation rates, stoichiometric ratios physical shape of the combustion chamber, charge swirl, etc.

6 Friction Losses.

7. Power to the shaft .

8 Atmospheric conditions which influences prop loading, engine air charge.

9. I bet there are many more that could be added.

Scientifically we are failing miserably when we attempt to see the cause and effect of a single variable when we have not identified all the other variables but most important have the ability to hold all of these variables constant.

I’ll leave this to smarter people than me.

[Dave Adamisin]

You sound pretty smart to me Dick. I would add feedgas and residules. I was also worrying about the actual number of times the engine "hits" at a particular rpm. I was thinking about a microphone and computer to record and count the hits. It would be interesting to see how many hits there were so you could also count cooling cycles. Adjust the needle for the same number of hits not the same rpm. I'd also like o try hi and low compression head designs. I think (ymmv) that low compression heads like a lot of oil. I run as tight as I can get away with so maybe that keeps me out of the danger zone and explains some of the differences.Don't know without some numbers.....  I also believe that the crank timing has the biggest effect on run away. But that's another story.