1

GLOW FUELS FOR SPORT FLYING

Roy Bourke

MAAC 204L

Now that weare into the flying season, a discussion of the composition of glow fuels might be appropriate. With so many different brands and formulae on the market, this is a subject that always invokes a lot of questions this time of year, particularly from new flyers.

We are going to limit our recommendations to average sizes of engines used for sport-type flying. There are additional and different considerations when flying in specialised competitiveevents such as racing, pattern, international FAI events, etc. and the flyers that fly these events are usually well aware of the best fuels for their purposes.

Combustion of fuel in an engine is a very complex process so we can only skim the surface here, but let's start with a discussion of alcohol as a fuel and the role of nitromethane ("nitro"). Of the three main "power producers" used in model enginefuels, alcohol in glow engines, kerosene in compression-ignition engines, and gasoline in spark ignition engines, alcohol is by far the coolest burning. The heat* from the combustion of fuel and the subsequent expansion of gas in the cylinder is what produces the power in an engine, and in general terms the more heat the more power. So at first glance it would appear that alcohol should produce the least power of the three. And it would if it were not for another important factor, the air-fuel ratio. To burn alcohol efficiently the carburettor needs to supply a much richer mixture of alcohol, (an air-fuel ratio of from 4:1 to 7:1), than the does carburettor of an engine running on kerosene or gasoline, (about 14:1). Did you ever notice how small the venturi is on a compression or spark ignition engine, and how much larger the venturi is on a glow engine? This means the volume of fuel flowing through a glow engine is much higher than through a comparable spark or compression ignition engine, and a high power level can be produced by the higher volume of combustible material. The cooler burning of the alcohol and the higher flow of cool liquid through the engine also prevents a dangerous build-up of heat in the engine.

(*Note: Don’t confuse "heat” with "temperature". Heat is energy, measured in calories, B.T.U.'s etc., produced by the combustion process. Temperature is a result of the ability or inability of an engine to dissipate excess heat, i.e. heat that isnot converted to work.)

A carburettor can supply only so much air (oxygen) so the main function of nitromethane (CH3NO2) is to increase the oxygen content of the fuel mixture, thereby aiding ignition and allowing the alcohol to produce more heat and burn more completely. The combination of easier ignition and higher heat helps an engine break in faster, produce more power, keeps the engine running steadier at idle, and produces better transitions from one RPM to another. The nitro also helps to keep castor oil in the fuel in suspension, preventing it from settling.

Sounds great eh? So the more nitro the better? Not necessarily! Let's look at the down side of high-nitro fuel.

If you think you are going to improve power substantially by using more nitro, you may be disappointed. In an experiment described in March '92 Model Airplane News, three typical engines of the time (Webra .61, OS.40 SF, and Fox .40) were run with fuels starting with 0% nitro and increasing in 5% increments. The first increment to 5% nitro produced an average increase of 577 rpm, the next increment only 100 rpm more and a third increment only 133 rpm. So the biggest increase of power occurred from adding the first 5% nitro, and thereafter the returns diminished. The conclusion is that there is really no need to go above 15% nitro content for most average or sport flying.

The added heat of combustion in high nitro fuels may also have detrimental effects on engines. It is harder on glow plugs and on other engine parts. In most cases, the engine will run hotter, meaning the engine will break in faster and may also wear out faster. And nitro content has an interesting effect on ABC engines (aluminum piston running in a chrome-plated brass sleeve) which are very common these days. ABC engines have a very tight fit between piston and cylinder, and are usually broken in on low nitro fuel. If after break-in you switch to a high-nitro fuel, the engine will break-in again at the higher heat level. Then if you try to switch back to a lower nitro fuel, the engine might not have enough compression at the lower heat level to give acceptable performance.

In the experiment described in M.A.N., it was found that increasing the nitro content also increased the noise level of the engine, in some cases taking it from an acceptable level to a few dB over a club's fieldlimit.

High nitro content also contributes to corrosion processes inan engine. Alcohol has a great propensity to collect water, which tends to remain in the engine during storage and corrode engine parts, and nitro helps to accelerate this process. With glow fuel, the use of a corrosion-fighting after-run oil such as LPS-2 or Rislone Engine Treatment is almost a must after every flying session.

And let's not forget the significantly higher cost of high-nitro fuel. Nitro is an expensive component!

So how much nitro should be in the fuel? I consulted with Peter Woo (Modelhobbies) to get his views on this subject and here are his recommendations.

For 2-Cycle engines:

  • Small engines, under .10 cid tend to start better and run more reliably with about 10% nitro.
  • Engines from .10 to about .40 cid will operate well with less nitro, say 5%, but in colder weather a 10% nitro content is recommended. As a general fuel for most conditions, 10% nitro is recommended.
  • Engines from .40 to .60 cid could use at least 10% and up to 15% nitro for better reliability in running and idling.
  • Engines from .60 to .90 cid can fall back to about 10% nitro content.
  • Above .90 cid, these larger engines usually have relatively low compression ratios and the nitro content can fall back further to about 5%.

Four-stroke engines generally do not like less than 15% nitro in the fuel, but watch out for knocking. The higher nitro content is mainly for smoothness in running and improved RPM transitions, not for increased power. (Pattern engines use about 30% nitro content for smoothness)

Heli engines run at high RPM's and run richer than normal aircraft engines, so they also need about 15% nitro content in the fuel for smoother running and better transitions.

Now let's talk a bit about lubrication. Castor oil, specifically Baker's AA Degummed Castor Oil, has been the traditional lubrication in glow ignition engines ever since their beginnings in the late 1940's, but more recently synthetic lubricants have been used in glow fuels, either blended with castor oil or as the only lubricant in the fuel. There is still much difference of opinion as to which is the better lubricant. Castor oil has higher film strength, and does not burn during the combustion process, as does synthetic. Therefore castor retains its lubrication properties better at high temperatures than synthetics so it is more likely to protect a hot-running engine such as a racing engine, or an engine that has been leaned-out too much. On the other hand, castor oil tends to gum-up an engine, and hot castor bakes into a varnish on the piston and cylinder wall, and is very difficult to remove. In cold weather, castor turns to a molasses-like consistency, making the engine very sluggish. Synthetic oils are more "slippery", stay quite workable at cold temperatures, burn much cleaner, and are much easier to clean off the aircraft.

Most fuels today contain a blend of castor and synthetic oils ranging from about 1:5 castor/synthetic mix to about 1:1 mix. Some fuels (e.g. 2-cycle Cool Power) contain only synthetic oil. As the properties of synthetic oils tend to improve with development, the tendency is to use higher proportions of synthetic oil in these blended mixes. With synthetic oils comprising most if not all of the lubrication in a fuel, it is of vital importance that an engine isnever run too lean, particularly if it is an ABC engine. (With an ABC engine, it is usually a good idea to make sure the fuel contains some castor oil.)

If a quality grade of castor and/or synthetic oil is used, then the quantity of oil is a more important consideration than the type. The traditional view is that fuel for a 2-cycle engine should contain at least 20% lubricant, and a 4-stroke can use less lubrication (about 16%). However Peter Woo tells me that most fuel manufacturers have found that 16% to 18% lubrication is more than adequate for both 2-cycle and 4-stroke engines, and are now supplying most of their fuels with that level of lubrication. Because of the higher speeds that Heli engines run, Heli fuel has about 22% oil content. In some cases the manufacturer of the engine recommends specific lubricant levels (e.g. Super Tigre) so most fuel suppliers respond by marketing a fuel for specific makes of engine.

There is nothing magic about the formulation of glow fuels, and the ingredients of glow fuel are all readily available, so it is possible to mix your own fuel if you wish. But in comparison with the price of fuels on the market, you will probably find it is not economical to do so because of the higher price of buying ingredients in small quantities. However it is feasible and practical to keep a small stock of glow fuel ingredients on hand to modify the composition of commercial fuels to produce fuels for various special purposes. This avoids the problem of buying several bottles of fuel of different types if you have differing requirements.

References:

All About EnginesHarry Higley

2-stroke Glow Engines C. Davit Gierke

Model Airplane News March 1992

Peter Woo Modelhobbies