All About Fuels

by John Modistach

I have been mixing fuel commercially for model airplane engines for close to 20 years and have encountered numerous problems with the resultant product with various engines. When sport engines “act up” on a standard brew of 4:1 Methanol/Castor, there is a need to experiment. I have found that Methanol can vary from season to season and can some times cause motors to run extremely hot and inconsistent. The remedy is generally to do what Shell Oil Co has seen fit for as long as I’ve known their products and add 3% Acetone to the mixture. This action can turn an un co-operative engine into a well behaved engine. Recently I researched the topic further and found a very interesting section in Graham Bell’s book on Two Strokes. One of my fellow club members had contacted BP following his encounter with an Enya 40 SS BB and their Chemist sent a table of information which I have included.

In my experience with Four Stroke engines, I personally run a blend of 5% acetone and have never had a back-fire resulting in a prop being thrown. The following information supports my findings and will provide you with some FACTS, instead of listening to the local club member expressing a view and ultimately confusing the issue.

 
Extracts from Performance Tuning in Theory & Practice – Two Strokes by Graham Bell

Page 117
“I recommend the use of racing fuels in which the higher octane is obtained by blending additives such as acetone, toluol(methyl benzine), benzol, ethanol or methanol.”

Page 118
“Methanol ( Methyl Alcohol) and Ethanol (Ethyl alcohol) have an octane rating of 140 – 160, depending on mixture richness. These fuels therefore can be used with very high compression ratios.”

 Methanol and ethanol have very high latent heat of vaporisation, ie, it takes a lot of heat to convert them from a liquid into vapour. Petrol has a latent heat of evaporation of 135 Btu/lb., methanol 472 Btu/lb., and ethanol 410 Btu/lb.. This heat, required for proper atomisation, is removed from the piston crown, combustion chamber and the cylinder, resulting in an internally cooler engine.

The two stroke engine is a type of heat engine, ie.one that burns fuel to cause the expansion of gas and the subsequent movement of the piston. The more heat produced by the combustion process, the more pressure there will be exerted on the piston, which gives us a power increase.

Using Petrol, the fuel/air ratio for best power is 1 : 12.5. With methanol we can increase the fuel/air ratio to 1 : 4.5, although I usually prefer a ratio of 1 : 5.5. Less than 1:7 is too lean. One pound of Methanol delivers around 9800 Btu.

Methanol is extremely poisonous and, as it is an accumulative poison, it can build up over a period of time and oxidise to form formaldehyde, eventually causing blindness or even insanity. It is absorbed through the skin and lungs, either by direct contact or from the vapours. Inhalation of the exhaust gas can also be dangerous as vaporised methanol is usually present, especially when rich mixtures are being used.

Methanol and ethanol will absorb huge amounts of water out of the air, so they must always be kept in an air-tight container. The fuel will also have to be completely drained from the tank and the carburettor to prevent the formation of water-induced corrosion and oxidisation. In colder climates, starting difficulties may be encountered when pure alcohol is being burned. Some use other more volatile fuels blended in to help overcome this problem. Usually 5% acetone or a maximum of 3% ether is used.

Page 120
“Nitro methane, as such, isn’t really a good fuel but it can give two-stroke engines a power boost if used sensibly. Nitro’s only virtue is that it contains approximately 53% by weight oxygen, so in effect it is a chemical super-charger. In drag car engines it is blended 80-90% nitro to 10-20% methanol, but there is no way a two-stroke engine can hold together with more than a 20% nitro-80% methanol blend (recall that this was originally written for full size engines). To deter detonation, or other engine damage, it is always necessary to lower the compression ratio. Care is in order when handling nitro methane, as it may become explosive.”

The tables below indicate the basic fuels available to the racer. Some may be neat or blended, others are combustion accelerators and, as such, are blended in very small quantities with alcohol fuels only. When methanol or ethanol is the base fuel, propylene oxide may be added to increase the combustion flame speed. If you decide to use propylene oxide, be very careful to blend in not more than 3-5% by volume and ensure a rich fuel/air mixture of 1:4.5-5.0 is maintained.

Propylene oxide can become explosive if allowed to come in contact with rust particles or copper and its alloys. Therefore it must be stored in plastic or aluminium containers. Once blended with other fuels it is relatively stable. Acetone is often blended with alcohol to accelerate combustion flame speed, and also to reduce its tendency to pre-ignite when lean mixtures are used. Usually a 10% acetone/90% alcohol blend is all that is required for this purpose, although much higher percentages of acetone may be blended if desired.