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Cox oil testing findings
About 2 years ago I started doing an investigation on the lubrication requirements for the Cox type engines using the "ball and socket" piston joint and steel piston/cylinder. This investigation was aimed at "sport" flyers using nitromethane contents 15% and below using un-modified reed valve engines. Two types of tests were run, one was a long term endurance test of 10 hours running at ideal mixture settings, turning a standard cox 6-3 "black" prop. The other test was a fligh simulation test where the engine was run on a small fuel supply that allowed a start, initial slightly rich run, lean out, and stop that would take approximately 6 minutes per cycle, again on the same propeller as the 10 hour endurance. The total time for the flight cycle testing was also 10 hours, or 100 cycles. All engines were broken in using standard procedures on their respective test fuels. The results were very interesting, and not what I originally expected. The base fuel used was 15% nitromethane, 20% castor oil by volume. The base fuel engine failed 8 hours into the 10 hour endurance test by wearing the connecting rod through the crown of the piston. The wearing surfaces of the ball and socket were not "varnished". The same test run with a synthetic oil in place of the castor ran for the full 10 hours without failure although the rod and piston joint showed .008" of wear. The flight cycle testing engine failed on the castor mix at 4.4 hours by the same piston failure, the rod had just broken the surface of the piston crown and affected starting, although the engine would still start via an electric starter. Again no "varnishing" was present in the joint. The synthetic oil engine passed the 100 flight cycles, but showed slightly more cylinder wear near the exhaust ports and a similiar .008" of wear in the ball/socket joint. So from the first basic tests it was clear that the ball/socket joint does not run hot enough with 15% nitro fuel to cause castor oil to polymerize, "varnish" and prevent metal to metal contact, which is of course castor's key strength as its anti-wear capability in liquid form is lower than other oils. The synthetics while better in the liquid form at anti-wear, do not have the capability to protect hot areas such as the cylinder near the exhaust port as well as the castor. A third test was run with 15% nitromethane fuel, 2% castor oil and 18% synthetic. The engines passed both the 10 hour endurance and 10 hours of flight cycle and both showed about .008" of wear in the ball/socket joint. Slight varnishing around the exhaust ports showed the castor was doing its job protecting that specific area, otherwise the engines were spotless inside. In a final effort to reduce the ball/socket wear an oil made up of 8% castor, 90% synthetic oil, and 2% tricresyl phosphate was used at the same 20% as previously with 15% nitromethane. The engines using this fuel passed both tests, with just under .002" ball/socket wear, slight varnishing around the exhaust ports, and overall excellent condition. This test was repeated with 16% oil volume with no degredation in wear and a 6% increase in RPM. To go back to determining the failure mode of the all castor fuel 30% nitro fuel was blended with 20% castor oil. The engine though needing to be de-varnished every 2 hours passed the 10 hours of testing without a ball/socket failure. The crankpin and crankcase wear was however higher than any other test. I did a final test using 20% of my synthetic/castor/TCP mix and 30% nitromethane, the engines ran the full 10 hours without needing a de-varnish, and the ball/socket and crankpin joint were in excellent condition. So from my little bit of testing, I have concluded that for my Cox design sport engines, a fuel mixture with castor/synthetic oil/and TCP will provide excellent service with as low as 16% total oil volume however I did not test the 30% nitromethane fuel at that oil level. This eliminates the gumming of engines in storage, sticking reeds etc.
Authors Additional information as the discussion progressed:
To clarify, no a reset was not done, I have made the tooling and have done so in the past but felt it was not part of this testing. I should also note that the wear in the ball/socket joint was not deformation from "pounding", it was strictly metal to metal contact, microwelding, and tearing of the piston material. If you put tension on the piston and rod assembly, no measurable change had taken place, so I feel fairly comfortable that there were little to no reversal loads with the 6x3 prop. Again I chose this as a "sport" flyers type cycle, not racing. All that being said, I personally feel a "reset" would have not changed the outcome as I see no way it would reduce the loading and wear on the upper surface of the ball and socket. In fact it might have restricted oil to the joint, which I believe is a weak point in the design to start.
The first cox engines DID have a real wristpin, and connecting rod. I have made pistons for a couple of mine along with aluminum connecting rods. Had to find a use for that box of old worn out cylinders that was taking up space. Anyway from what I found, on 15% or less fuel on a typical stock reed engine, it seems as if the piston and rod run just cool enough to prevent the varnishing. Now unfortunately its not easy to measure the temperature of that joint, so I dont know how close I was to the temperature needed to varnish the castor. Might be just a little, might be a lot. I think as a matter of course, it seems that the level of castor in the oil, could follow the nitro content and be a good "rule of thumb". I.E. 5% nitro=5% castor(95% synthetic) or 20% nitro = 20% castor (80% synthetic) etc. I think this would give engines that run cooler, a better lube package for their application and running conditions, and engines that run hotter a better lube package for them. Seems funny though that an oil that protects the best at extreme heat, underperforms at lower temperatures. I personally dont think I would ever go 100$ synthetic unless the engine cooled very well, which many of the AAC types do, just not the little steel wonders.
Wear was measured by compressing the piston and rod in a jig to ensure everything was straight and measuring the distance from the top of the piston to the bottom of the rod. This was to measure pure wear, not "slop". Sig castor was used as it seems to be a fairly universal "standard".
I had a sample of the oils run through our materials lab to verify the material, the "Sig" castor is just castor, no EP, no metals, no polys, no esters, just bean oil. The synthetic oil chosen was Morgan Cool Power "blue" 2 cycle oil, which is the heavier of the 2 they sell and is comparable to the straight castor. The Cool Power oil was checked, and it was found to be a very common ester type with a proven EP additive from the industrial world. It would not be ethical for me to tell what it is though. I doubt the "looseness" had any influence on the wear, there was no evidence of any load reversal on the joint and the "extended length" of the piston/rod never changed. So the "crimp" never loosened, it just wore into the piston crown. Usually on a 2 stroke any load reversal is either a misfire, or running without a high enough load, so underpropping could cause it.
Nope not a fuel supplier. I work in engineering designing engines for a living, and not models. If you have read carefully through what I have written, you will note that I did say that the castor varnish ( real simple terms castor loses a water molecule and crosslinks forming the varnish that protects the bearing surfaces ) is one of the best for preventing wear. However....without a high enough temperature to cause the change, castor in its non-drying form does not have the anti wear capability of some others. So while being the best for high termperature areas, it may not be the best for lower temperature bearing areas of the engine. Not black magic, simple tribology. Nobody is asking you to change what you do, im just sharing what I have learned from a controlled test ( as controlled as it can be ). If someone wants to share similiar test results I would love to talk about it. Unfortunately much of the information that floats around in public channels is just empirical, and many conclusions are drawn and expanded to cover things not relevant to the original failure.
I have found many "claims" in the hobby lubrication world. In reality most of the "synthetics" are industrial lubricants from either Dow chemical , Chevron, or Shell Oil. The EP additives are mostly OTS additves like Vanlube 972 or others. My background is full size engine design and I work at probably the last big 2 stroke company in the US. Model engines are a hobby of mine, I have built a few, modified a bunch and crashed my share. One thing I have learned is that there is no ONE answer for everything, and that every design has its own needs, and those needs change with application. I was a "castor only" guy for a long time, until I decided to try some other lubricants. Every lubricant has its strong and weak points, and having the right balance can make something superb for a specific engine design. I have a test mule PAW .049RC that has over a gallon of fuel through it, being a 95/5 Synthetic/castor blend and 20% total oil volume. Someday I may pull it down for inspection, but it is just as snappy as when new, and I have never had a "stuck" engine in spring. I enjoy the freedom of blending my own fuels and actually knowing what my engines are feeding on.
The test I ran at 30% nitro with the same oil "varnished" and carboned up as I would expect. Most of my 1/2A models are draggy, high wing area designs that require a lot of blade area to perform. The 6x3 has been the standard cox prop for about 50 years I think. A smaller prop would deffinately reduce the loads at the joint, but would be unsuitable for a lot of models.
So my final thoughts on this are pretty much as follows:
1-The surestarts are inferrior to older product in the ball/socket area and require different lubricants if larger (6x3 or 7x3) props are used with low nitro fuels.
2-Castor only outperforms other common lubricants if the bearing surface runs hot enough to "varnish" it ( common knowledge in the tribology world ).
3-It does not take much castor percentage to protect the upper cylinder area on a reed valve cox (5% by volume) if low nitro fuel is used.
I am going to keep my model flying on my synthetic/castor/TCP fuel for a year and see how it survives in the "field". No after run oils are ever used.
Typically 375-400F, or about the temperature that PAG oils start to decompose. So in effect at the temperature that the PAG synthetics "give up" , the castor takes over to protect the parts. Unfortunately below this temperature the PAG's have better "lubricity". So when people talk about an oils ability to lubricate, they must be careful about what temperature they are talking about. As a note, the engine I was running on 100% PAG synthetic oil had nearly the same ammount of oil carryover (unburned oil) as the castor tests. And the engines themselves rarely got over 300F CHT. About the only area in the engine that seemed to run over 400F was the upper 2mm of the cylinder wall. An aluminum cylinder, or more heavily finned steel cylinder would probably drop this temperature.
Swinging a larger prop, I believe re-sets are a mute point. No signs of load reversal ( meaning no pulling on the crimp ), tells me that the crimp is literally along for the ride. Actually having a little load reversal would be good for preventing the failure I am seeing, as it would unload the top side of the ball and allow lubricantup into that joint. As it is now, if you swing a larger prop there is not much of a path for lubricant. I limited the castor as it was not really the better lubricant for the particular situation. Castor is great lubricant at high temperatures, but its not the greatest at lower temperatures. So since the ball and socket are running so cold ( probably under 300f ) the castor couldnt do as well at other oils...simple as that. I used the castor in the mix to protect the upper part of the cylinder. We will fly this for a year and see how it fares. Typically I burn about a half gallon of fuel in a year.
Anyway hope this is entertaining for people!