What is model fuel made of? Why do I need such a special fuel? Can I use something cheaper?
Model fuel is a blend of methyl alcohol (methanol), Nitro-methane (Nitro), and oil. Methanol is the main ingredient, and provides most of the power. Nitro is added to assist the idle and acceleration, as well as increase power. The oil that's in the fuel is the source of lubricant for the engine.
Methanol is used for two main reasons: 1) It can be ignited with a platinum-element glow plug, and 2) It releases more energy per pound of air than gasoline. It's also very easy to obtain, and is inexpensive.
Nitro-methane is used to enhance power output. It acts as an oxidizer as well as a "hot" fuel in its own right. It's not used in large amounts in most model engines because it's too powerful a fuel for the way model engines are made...it's just too "hot". It can also be explosive if it's not handled correctly...ever see an AA-Fuel dragster or Funny Car explode?
Oil is used to lubricate all of the moving parts in the engine. Like all two-stroke engines, there's no oil sump, so you can't put oil into the engine and just add fuel. The oil is mixed into the fuel. Oil used in model fuels can be made from a single product, or a blend of products. The oil that used to be the most common was castor oil. This is a product refined from castor beans (like soybean oil comes from soybeans). It's the same oil you'll find in the drugstore, but it's been processed to make it less gummy and with fewer solids than the medicinal type. Lubricating castor oil is not certified for human internal use, though, so it's not a substitute for medicinal castor oil.
Castor oil has been replaced in most fuels by some kind of synthetic oil. The synthetic oils used in model fuels are basically synthetic versions of castor oil. The synthetic oils are used because they are: 1) less expensive than castor oil; 2) less gummy than castor oil; and 3) leave less mess on the model than castor oil. They are not "better" oils, but oils with different characteristics that are highly desirable. For "problem" engines, a fuel with some castor is highly desired, because it is actually a better lubricant at the operating temperatures a model engine can generate.
Nowadays, "Premium" fuels contain a blend of synthetic and castor oil, hoping to combine the best characteristics of each. "Sport" or "regular" fuels are usually 100% synthetic oil. Very few model fuels use 100% castor oil, but they are still available from some fuel manufacturers.
An "ideal" fuel blend for most model engines would be 20-22% oil, 10-15% Nitro, and the rest methanol. The various percentages of the ingredients are percentages of the volume of fully-mixed fuel. These numbers are controversial.
Some fuel manufacturers claim that their lubricant is so good that you need less of it, so they have lower oil content. Many advantages are claimed from this...some are even true. Lower oil will allow an engine to throttle up faster because there's less oil to get in the way of the combustion process. There's also less oil to give you a margin of error in case the engine gets a bit lean for some reason.
A lot of times, oil quantity is reduced so that the cost price of the fuel is lower, and the fuel manufacturer can increase profits by keeping the price at the same level as full-oil fuels. In fact, cost is the main reason most fuels are blended with about 18% oil. While it's lower than the "ideal" fuel, it still has enough oil to give good protection.
Just about the only thing that can be added to a basic fuel are some ingredients that help the glow plug fire off the mixture inside the engine...these are called "ignitors". Propylene oxide is an example of an ignitor that's been added to fuel in the past. Some rust-inhibiting compounds can be added to help slow down rusting of the bearings and crankshaft, but their effect is limited because only a small amount can be added to the fuel before the fuel's performance is affected.
Storage and fuel care
Yes, there are ways to care for fuel so that it will stay good while it's being stored.
First off, it should not be stored in unsealed containers. This allows air to get into the fuel container. Moisture in the air will be drawn into the fuel because methanol has a very strong attraction for water. The two will mix easily and readily. Once model fuel becomes contaminated with water, the engine's performance will suffer. It won't idle, it will be hard to set the needle properly, it will tend to run hot...all in all, it will be a mess.
When somebody has running problems, one of the first things to recommend is to try running the engine with brand new, fresh fuel.
Model fuel should be stored at a constant room temperature, if possible. If it's in an area with wide temperature swings, whatever moisture that's in the air in the bottle will tend to condense out and get into the fuel. Some oils will degrade faster if they are exposed to wide temperature swings.
Model fuel should not be stored in direct light. In fact, it should be kept away from light as much as possible. Light will cause the nitromethane to degrade. After a while, fuel kept in light will work just as if there was no nitro added in the first place.
Don't store the fuel for too long. Model fuel will last quite a while if it's kept in sealed a container, but nothing is forever. If it's over a few years old, you may not be able to count on it. Most of the fuel manufacturers don't guarantee their fuel for much more than a flying season's worth of time once you've purchased it.
Some additional notes:
Occasionally, we'll get a call with a complaint about getting a "bad batch of fuel". Because we purchase fuel in such large quantities from one of the largest and best model fuel manufacturers, the likelihood of anyone getting a bad case of fuel is about nil. One gallon could be bad because the jug was contaminated prior to filling, but not an entire case.
If a "bad batch" of fuel got out, there would be hundreds of complaints, not the few we actually do get.
The most likely reason for a complaint about bad fuel is either a problem in the engine itself, or something that happened to the fuel once it had been opened. If it's been a while since the fuel was opened, moisture could be the problem. It's also possible that the customer didn't de-fuel the model the last time it was flown. The residue will be mostly oil and stale methanol. The nitro would have evaporated.
Every time fuel's been returned as allegedly bad, test runs have proven that the fuel was just fine.