Fuel: Choosing the right fuel is the last critical factor to ensure that your new two-stroke engine gets the best performance and longest life. The "right" fuel is also one of the most controversial, opinion-rich, and individualistic subjects in model aviation. But there are some useful guidelines to remember. 

Two-stroke fuel has three major ingredients: methanol, castor oil, and nitromenthane (nitro). Methanol comprises 60%-75% of most fuels. It burns completely and adds to the fuel's total energy output.

The lubricating oil comprises roughly 20% of the fuel. Most two-stroke fuels contain two types of oil; 4%-8% is usually castor and the remaining 12%-16% is a synthetic that varies by manufacturer. Oils typically do not burn completely, and what small percentage of the oil that does burn does so at lower energy levels than methanol.

Castor oil is used because it maintains a lubricating film at higher temperatures than most synthetic oils do. If the engine's high-speed fuel/air ratio is too lean—too much air—the engine will run at high temperatures. Castor oil will maintain lubrication in an overheated engine; most synthetic oils burn away.

Castor oil also helps remove heat from the combustion chamber better than most synthetics. Castor oil leaves a film residue in the engine that offers some rust protection. Most synthetic oils do not.

However, too much castor oil causes excessive residue buildup that can diminish an engine's performance. Because castor oil does not burn, it reduces the fuel's total energy output. For these reasons, the castor oil percentage is usually kept at less than 8%.

Synthetic oils do burn, but not well or completely. Little synthetic oil residue is left inside an engine. Synthetics also offer excellent engine lubrication when operating at normal engine temperatures.

Because high oil content detracts from the fuel's total energy output, the easiest way to increase an engine's apparent power output is to reduce the fuel's oil content. However, oil contents much less than 18% can cause long-term wear problems in .40-.60 sport engines.

Fuel manufacturers are studying new oils that produce more power and offer better protection with quantities as low as 16%. But for now, consider using fuels with 18%-20% oil content in newer or sophisticated (expensive?) .40-.60 engines.

The third fuel component is nitromethane. It burns at a higher energy level than methanol. However, it also produces higher combustion-chamber temperatures and therefore needs to be limited. Most sport fuels contain 5%-25% nitro. It prolongs the combustion event. The burning process takes longer, and that also produces more energy.

Many pilots overrate the power increase obtained by "upping the nitro" in their sport engine's fuel. Photos show how little effect higher nitro content has on sport engines. Raising its content by 5% in a fuel produced only a 300 rpm gain. But nitro does improve an engine's idling ability, permitting a lower reliable idle speed. Therefore, consider nitro contents in the 10%-15% range as a good sport-flying compromise.

There is such a thing as too much nitro content for a given engine. If the combustion event becomes too prolonged, detonation may occur. You may hear pinging, a sound like frying eggs, or the exhaust note may become very loose. Unless operating at high altitudes—exceeding 5,000 feet—there is no reason, and little gain, to use fuels with nitro contents higher than 20%.

Next month I'll cover some of the differences in and advantages of four-stroke engines. Since model engines do not run for long periods without an onboard fuel supply, I'll cover fuel-tank choices and installation requirements as well. MA