Originally Posted by
http://modelenginenews.org/faq/index.html#qa5
Question:
Should I replace castor oil with modern synthetic oil?
Answer:
Tricky question, and not one I'm in any way qualified to answer. There's a lot of folk-lore and ju-ju thrown about on this topic. Instead of poking my vulnerable neck out, here's some words of real wisdom on castor from the master: Bert Striegler. Read it and decide for yourself!
Back in 1983 there was quite a controversy in Radio Control Modeller magazine about the tests that were necessary to measure the "lubricity" of various oils that might be useful in model engines. Castor oil was used as the benchmark, but it was obvious no one knew why this was so. They apparently got a lot of info on various industry tests of lubricants, but these were really designed for other purposes. This was my answer. I will remind you that I was a lubrication engineer and not a chemist, but I drew my chemical info from Bob Durr, the most experienced lubricant scientist in the labs at Conoco. Bob worked with my group on many product development projects and I can tell you that he is one smart hombre! Small changes were made in the text, but surprisingly very little has really changed since this was originally written. Here goes with the answer:
"I thought I would answer your plea for more information on castor oil and its "film strength", which can be a very misleading term. I have never really seen a satisfactory way to measure the film strength of an oil like castor oil. We routinely use tests like the Falex test, the Timken test or the Shell 4-ball test, but these are primarily designed to measure the effect of chemical extreme pressure agents such as are used in gear oils. These "EP" agents have no function in an IC engine, particularly the two-stroke model engine types.
You really have to go back to the basics of lubrication to get a better handle on what happens in a model engine. For any fluid to act as a lubricant, it must first be "polar" enough to wet the moving surfaces. Next, it must have a high resistance to surface boiling and vaporization at the temperatures encountered. Ideally the fluid should have "oiliness", which is difficult to measure but generally requires a rather large molecular structure. Even water can be a good lubricant under the right conditions.
Castor oil meets these rather simple requirements in an engine, with only one really severe drawback in that it is thermally unstable. This unusual instability is the thing that lets castor oil lubricate at temperatures well beyond those at which most synthetics will work. Castor oil is roughly 87% triglyceride ricinoleic acid, which is unique because there is a double bond in the 9th position and a hydroxyl in the 11th position. As the temperature goes up, it loses one molecule of water and becomes a "drying" oil. Castor oil has excellent storage stability at room temperatures, but it polymerizes rapidly as the temperature goes up. As it polymerizes, it forms ever-heavier "oils" that are rich in esters. These esters do not even begin to decompose until the temperature hits about 650 degrees F. Castor oil forms huge molecular structures at these elevated temperatures - in other words, as the temperature goes up, the castor oil exposed to these temperatures responds by becoming an even better lubricant!
Unfortunately, the end by-product of this process is what we refer to as "varnish." So, you can't have everything, but you can come close by running a mixture of castor oil with polyalkylene glycol like Union Carbide's UCON, or their MA 731. This mixture has some synergistic properties, or better properties than either product had alone. As an interesting sidelight, castor oil can be stabilized to a degree by the addition of Vitamin E (Tocopherol) in small quantities, but if you make it too stable it would no longer offer the unusual high temperature protection that it did before.
Castor oil is not normally soluble in ordinary petroleum oils, but if you polymerize it for several hours at 300 degrees F, the polymerized oil becomes soluble. Hydrogenation achieves somewhat the same effect.
Castor oil has other unique properties. It is highly polar and has a great affinity for metal surfaces. It has a flash point of only 445 degrees F, but its fire point is about 840 degrees F! This is very unusual behavior if you consider that polyalkylene glycols flash at about 350-400 degrees F and have a fire point of only about 550 degrees F, or slightly higher. Nearly all of the common synthetics that we use burn in the combustion chamber if you get off too lean. Castor oil does not, because it is busily forming more and more complex polymers as the temperature goes up. Most synthetics boil on the cylinder walls at temperatures slightly above their flash point. The same activity can take place in the wrist pin area, depending on engine design.
Synthetics also have another interesting feature - they would like to return to the materials from which they were made, usually things like ethylene oxide, complex alcohols, or other less suitable lubricants. This happens very rapidly when a critical temperature is reached. We call this phenomena "unzippering" for obvious reasons. So, you have a choice. Run the engine too lean and it gets too hot. The synthetic burns or simply vaporizes, but castor oil decomposes into a soft varnish and a series of ester groups that still have powerful lubricity. Good reason for a mix of the two lubricants!
In spite of all this, the synthetics are still excellent lubricants if you know their limitations and work within those limits. Used properly, engine life will be good with either product. Cooked on a lean run, castor oil will win every time. A mix of the two can give the best of both worlds. Most glow engines can get by with only a little castor oil in the oil mix, but diesels, with their higher cooling loads and heavier wrist pin pressures, thrive on more castor oil in the mix.
Like most things in this old life, lubricants are always a compromise of good and bad properties. We can and do get away with murder in our glow engines because they are "alcohol cooled" to a large degree. Diesels, though, can really stress the synthetics we use today and do better with a generous amount of castor oil in the lubricant mix. Synthetics yield a clean engine, while castor oil yields a dirty engine, but at least now you know why!"
But wait! There's more (Bert again):
I have been thinking for a while about how to answer your seemingly simple question, "What is the flash and fire points of SAE 70 motor oil?". I was always told that engineers are people who can make something complicated out of something simple, and that is what I am going to have to do!
First of all, there is not now and has never been such a product as an SAE 70 oil. The SAE grading system used to stop at 60, and for all practical purposes, is now limited to 50. It is an arbitrary system based on the viscosity of lighter oils at 0 Degrees F. and heavier oils at 210 degrees F. The reasoning was that 0 degrees was about as low as you could expect to be able to start an engine, and 210 degrees was about as hot as you expected the oil to get in a crankcase. This scale was produced years ago, and today we routinely start gasoline engines at temps of -40 degrees F, and some of our crankcase oil testing is done in engines at oil temps of 330 degrees F and higher.
We used to make Conoco 70 oil, and it was basically a blend of bright stock and a little bit of 800 pale oil. These are the two products I will use as an example of why I cannot answer your question straightforwardly. Oil base stocks are usually produced in a vacuum distillation tower after all the lighter ends of the crudes are driven off. This is a process carried out under a 2MM vacuum. Our 5295 bright stock first starts boiling off in this tower at about 500F. 5% is boiled off at 560, 20% at 600, and finally 50% at 650, at which point the oil begins to "crack" into lighter and heavier products. The point is that, unlike synthetics, we are dealing here with a product that boils off in varying degrees in a range of temperatures from roughly 500F to 650F and then cracks. 800 pale oil is a lighter product which starts boiling at about 375F and finally starts cracking at about 630F. From the distillation curve, I would estimate the flash of bright stock at about 560 and 800 pale at about 475F. A blend of the two would flash closer to the 800 pale number that to the bright stock number. Different folks made 70 oils in different blending fashion, so no two brands would yield the same flash or fire points. We never used "dumbbell" blends at Conoco. By that I mean a blend of very heavy oil and very light oil to get a medium oil viscosity. We used to see a lot of this type blending and it is not good.
With petroleum oils, the fire point is usually 60 to 80 F above the flash point, very close to the cracking temperature. What does all this mean? Practically nothing, because in practice we don't usually reach these kind of temperatures EXCEPT where the oil is mixed into the fuels. Oil carried into the combustion chamber can be exposed to cracking temperatures. Have you ever had a really black exhaust with an ignition engine using petroleum oil lubes? I have, and it is even more common in diesels. What that tells you is that the oil is cracking, that burning of the oil is incomplete and the black is plain ole carbon. So, petroleum oil is a little bit like castor in that it comes apart over a wide range of temperature. Synthetics, on the other hand, boil, flash and then burn in a very narrow range but when they do burn, they normally burn completely with a clear and clean exhaust. That's why they leave less oil on the airplane.
Everything I just told you is in regards to paraffinic base oils, but there are also naphthenic base oils around. They produce less carbon, have a lower wax content and thin out more quickly with a rise in temperature than the paraffinic stocks. Naphthenic stocks are preferred in large gas engines and other operations that are sensitive to carbon buildup over a long period of time, but these stocks are more sensitive to oxidation and have a lower flash and fire point. Paraffinic oils are better lubricants in most operations. Paraffinic oils used to be called "bright" oils or "green" oils because in sunlight, a clear bottle filled with paraffinic oil had a bright green "bloom" when looked at with the light coming through it. Naphthenic stocks have a pale blue "bloom" and used to be sold as cheap engine oils in years past. I don't think a naphthenic based motor oil would be able to pass our modern engine tests and they are rarely seen anymore.
In a nutshell, you are not really going to burn a 70 oil in the combustion chamber and flash and fire points are a non-issue. I quit using 70 oils a long time ago because of the carbon problem. Modern outboard motor oils are SAE 40 oils with ashless additives and are diluted 20% with a high grade kerosene to assure easy mixing. One of the tests that they have to pass is a wide-open throttle test in a 100 HP outboard pulling about 60HP on the dyno using a 300 to 1 mix of the diluted oil in gasoline! This is a 100 hour test, or was when I was involved. The engines looked really good and all parts were measured before and after the testing was done. That is not a mistake, 300/1! Some of these oils are synthetic, blends with synthetic and petroleum oils, and straight petroleum oils. All have an elaborate, ashless additive system.
For our purposes, castor is still the best. The heavier petroleum oils share a wide distillation range so they offer some lean run protection like castor. The synthetics perform well in our engines unless you reach their boiling point, at which point you are dead meat. A good rule of thumb is "The messier the airplane, the better the lube is working." There is really nothing wrong with burning the synthetics in the combustion chamber. The only caution is that if the engine gets off too hot and the cylinder walls reach the boiling point of the lube, then all is lost. That is not likely to ever happen with petroleum or castor oils in the SAE 40 range of viscosity.
LOL. Fair enough Frits, thats some perspective. Expensive is relative of course. In the last couple of months I've bought a TD2 project bike, a TZR250 for one of my projects and more parts and services bike related than my marriage could stand were it openly discussed.
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). I've been told 30:1 is a good ratio in the water cooled 50's, would I increase the oil ratio for an air cooled motor on avgas? I understand more oil = more hp as per your post above but I was hoping theres an acceptable ratio that wont empty the wallet too quickly.
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