FACTS ABOUT FUEL - No. 1 - What’s the Oil Content?


(The following is the first in a series of articles exploring all facets of model engine fuel. The writer is Don Nix, founder and former owner of POWERMASTER FUEL. Readers are invited to contact Don directly via e-mail – FLYERDON@aol.com and to visit his website at http://www.rcauctioneer.com.)

Fact (A) - It’s quite likely that no other single facet of modeling generates as many myths, misconceptions, misunderstandings, errors (and more than a few lies), or as much outlandish goofiness as model fuel….one of our absolutely necessary, non-optional items for powered flight.

Fact (B) - Of all the above, the one fact that rouses the most questions - and without doubt the most wrong answers - is the ongoing nonsense about the amount of oil required in model fuel.

  • Myth: Model Glow Fuel must contain XX% oil to operate properly, perform well and protect the engine.

    Fact: There is no such fixed number….at least not a valid one.

    Why not? Think about it: In order for this to be true, all oils used in model fuel - all of them - would have to be identical in every characteristic. Does anyone honestly believe they are? I doubt it.

    While lubricants compounded for full-size engines - automotive, recreational vehicle or aircraft - are rarely, if ever, suitable for use in model engines (for many reasons), nevertheless, there are a number of base lubricants that are available for our highly specialized use. However, most of these must be modified slightly or extensively by the use of a variety of additives and modifiers.

    While Klotz model oils are perhaps the most well-known to the average user, and are quite good, they are by no means the only lubricants available to model fuel blenders, and there are currently a number in use. Each has its own "personality" - its own set of technical specifications and characteristics.

    At this point, we should point out that we’re speaking of the so-called "synthetic oils" popularly used in modern model fuels. Castor oil…the oil of choice, and, indeed, the only suitable model engine oil for many years, is more of a common and known factor. Assuming a good grade, if a fuel uses only castor as its lubricant, then we could give you a fixed percentage, at least for the various engine groups and types.

    However, few model fuels intended for R/C use today contain only castor oil as the lubricant. For the purposes of this discussion, we will only deal with fuels containing either straight synthetics, or a blend of castor and synthetics.

    So…what does all that mean?

    Let’s draw a little picture here: Suppose at some point in your life, you become concerned about living a long and healthy life, so you decide to consult a doctor for advice as to how to accomplish this. When you come to the subject of food, you say, "Well, tell me, Doctor….if I wanna still be healthy and virile at 90, how do I eat?" The good doctor replies, "M’boy, if you will eat two pounds of food a day, you’ll be fine!"

    My guess is your response would be something like, "well, what kind of food, Doc? After all, no two are exactly alike….is that two pounds of lettuce or two pounds of pork chops?" If he replied, "It doesn’t matter. Just as long as you eat that two pounds every day, you’ll probably outlive your kids." My bet is that you’d run, not walk, out of that quack’s office!

    Why, then do we blindly follow someone’s Word From On High when they say (in words engraved on stone tablets), Thou shalt use no fuel that does not contain XX% oil." It makes absolutely no sense to me, nor do I think it will to you, if you just stop to think about it. All foods are different; so are oils.

    If that’s true, why do the instructions with my engine specify a fixed percentage of oil? Simple - to protect themselves. All engine manufacturers have been burned (figuratively and literally) in recent years by "bargain priced" fuels containing either inferior oils, or insufficient amounts of oils. Every one that I’ve talked to will admit off the record that they know that fuels containing good oils won’t need as much as their instructions say. But they also say they know they have no control over that, so they are going to print a high number, in hopes that amount of even a cheap oil will be sufficient. Frequently, it isn’t.

    So why not just put a lot of oil - at least 20% or more - in fuel and not worry about it? A lot of reasons…all good ones. For example:

    1. Too much oil - any more than is necessary - makes the engine run really crappy. Think about it: methanol burns; oil doesn’t - or at least it shouldn’t. (Some do, but that will be dealt with in another installment.) Common sense would tell us that the less oil (nonburnable) we can safely use (to an irreducible minimum point, of course), the more methanol (burnable) we will have in our combustion chamber. More burnable ingredients = more power. One well-known magazine writer, with more than 50 years engine experience, tells me that in his experience, for every 1% oil removed from model fuel, the effect is about the same as adding 1% nitromethane. And it costs a lot less!
    1. By the same logic, the less oil we use (to the predetermined minimum, of course), the less the oil is going to be dousing the glow plug element, and we should be able to achieve a lower, smoother idle.
    1. Next to nitromethane, oil is the most expensive ingredient in model fuel. By not using an unnecessary amount of oil….especially if it’s just to satisfy some Great Guru’s edict….the manufacturer can keep the cost of the fuel down, which puts a smile on all modelers’ faces. Remember that even an additional 25 cents in manufacturing cost translates to an additional dollar….or more….at the retail level.

    So, what is the right amount?

    It all depends…on what kind of oils, in what combinations, with what additives, etc. And for what use? Sport airplanes… Racing… Helicopters… Boats… Cars… Ducted Fan? What size engines? (As engine size increases, they need progressively less oil. Why? Simple mathematics. Surface area of the combustion chamber increases at about half the rate as the displacement increases.) Most people know that the big T.O.C. and Unlimited racing engines use oil in the 4% to 5% range.

    Ducted fan and helicopter engines typically need more oil, 4-strokers less. It might be surprising to most airplane flyers to know that top competition model car engines use fuel with oil contents in the single digits, even though they are turning in the 40,000 - 50,000 rpm range, and have no fan in front to cool them! As matter of fact, they will hardly run on regular airplane fuel.