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  1. #1
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    Model Glow Engine Fuels

    I have been involved in RC for quite a number of years now. I started off with model airplanes, and then added a Revo to the mix. I looked at various products from other manufacturers, but the Revo stood out head and shoulders above the rest!!

    From my airplane days, I had to mix my own fuel, because I just did'nt trust the fuels available locally. It amazed me to learn how oppinionated people are regarding what should go into model glow engine fuel and why. This led me to do research of my own, and I learned quite a lot of interesting facts. Nowadays I don't mix fuel anymore, because there is a local company that mixes a reliable coctail. (no Top Fuel available) To mix fuel yourself is not really worth it financially, and you end up with larger quantaties than what you can keep fresh.

    I am just curiuos to find out if there is enough interest from you guys to learn more about what that stuff is you put into your precious little engines. If there is, I would be happy to share and maybe even learn somthing new!
    Revo 2.5 that pops wheelies like nothing!

  2. #2
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    Model Glow Engine Fuel - Theory

    Model Glow Engine Fuels.

    This is the most controversial subject in the hobby industry. There are many myths and misconceptions regarding how a fuel should be made up and what the purpose of the ingredients are that is used in a fuel mix. Bad or wrong fuel mixes are certainly the most frequent cause of engine problems, poor performance, frustration and arguments over warranty work. I’ve decided to write this article to shed some light on this subject. All the information contained herein is based on some late-night Internet browsing, common sense, as well as personal experience and those of fellow club members.

    The most basic model glow engine fuel is made up of only two components namely methanol and castor oil. Other blends can also include special synthetic oils and nitromethane. All these components can be blended in certain ratios depending on the type of engine and/or the application or power requirements of an engine.

    A quick summary of what these chemicals are:
    The basis of any glow fuel is the methanol (methyl alcohol CH3OH). This is the flammable component that is burned in the engine to power the piston downwards. Chemical energy is converted into mechanical energy. Methanol have an octane rating of 140 - 160, and can thus be used with very high compression ratios. The castor and synthetic oil is there to lubricate the moving parts inside the engine, and the nitromethane is used as a ‘power booster’.

    A typical basic mix for aircraft use will be 80 % methanol and 18 % - 20 % castor oil. It must be noted that most engine manufacturers have very specific fuel requirements for their engines. Most of the time the specification will only state the minimum amount of oil the fuel must contain, as lubrication failure is the most common cause of engine problems. These specifications always err on the safe side, and while it will not necessarily produce the absolute peak performance extractable from the engine, it will however ensure the best balance between life expectancy and performance. Heed these instructions and the engine will last many, many years.

    Castor oil is extracted from the castor bean and is a totally natural product. There are different grades available, and some refineries put additives in the oil for model use. Castor oil's viscosity approximates SE 50 at ‘normal’ temperatures. There are numerous reasons why castor oils is used in glow fuels. The most important reason being that mineral oils will not mix with methanol. Castor oil is a good natural corrosion inhibitor, and has excellent lubricating properties. The only notable disadvantages from using castor oil is namely that when fuels are used containing more than 40 % nitromethane, miscibility becomes a problem between the nitro and the castor, and a move to synthetic oils becomes a necessity. This will however hardly ever be a problem, because one will seldom use a fuel containing more than 33% nitro. The higher percentages nitro is only used in high performance engines where peak performance is of utmost importance. Over a long priod of use, the castor that burns in the engine will develop a layer of varnish in the engine and some carbon build-up in the exhaust, also the castor that doesn’t (and shouldn’t) burn exits the exhaust and can make a bit of a mess of one’s model. This is something modellers have learnt to live with! Castor is however probably the best all round oil for use in model glow engines.

    Castor oil can be replaced or combined in the fuel mixture by using a good quality synthetic oil that was specially formulated for use in model glow engines. These oils contain special rust inhibitors to combat corrosion inside the engine. It is also cleaner burning and has a lower viscosity than castor oil. The lower viscosity means that there will be less oil drag inside the engine, hence a better performing engine. Synthetic oils have very good lubricating properties, however it burns off at much lower temperatures than castor oil. Most experienced modellers will be quick to tell you that if you don’t get oil droplets in your engine’s exhaust, there was no oil left after combustion to lubricate the piston. It does make sense.

    The best advice for deciding on oil types, would be to combine castor and synthetic oil in your fuel mix. This should give you the best of both worlds. This approach is particularly recommended for use in high performance engines. Some well known high performance engine manufacturers have this as a basic fuel requirement for their engines. If you consider the fact that these guys perform thousands of hours of exhaustive tests on their engines to ensure that they have a reliable product, it should be well worth keeping this in the back of your head when you decide to mix your own fuel or even when buying ready mixes of the shelf.

    Nitromethane is the final chemical to be put under the spotlight. Nitromethane is used in model glow fuel for the following reasons:

    To increase the power output of an engine.
    To make engine starting easier in cold weather.
    To make starting of a hot engine easier.
    To ensure a lower and more reliable idle.
    To increase the error margin for tuning an engine.

    It would be interesting to note that methanol is by far more flammable than nitromethane. This will now be explained : The power developed off an engine depends mainly on the weight of fuel it can burn per second, and this depends on the weight of oxygen it breathes per second. Air is 21 % oxygen and an engine’s piston(s) pump a fixed volume or weight of air per cycle for a certain throttle position. For optimum combustion to take place the air to fuel (A/F) ratio should be kept at the correct level by the carburettor. Different fuels require different A/F ratios. The lower the fuel's A/F ratio, the more fuel an engine can burn per second. For equal weights of petrol, methanol, and nitromethane, the combustion heat energy ratios are roughly 4 : 2 : 1. That is, petrol supplies twice the energy of methanol, and four times that of nitro methane. A fixed weight of air will burn roughly one unit by weight of petrol, 2.4 units of methanol, or 9 units of nitro. This means an engine can burn 3.8 times more nitro per second than methanol and 9 times that of petrol. With these higher fuel consumption rates the relative energy outputs become 1 : 1.2 : 2.3 So, although low in calories, for a given weight of air, nitro can supply almost twice the heat energy of methanol. The lower A/F ratio required for nitro combustion results from the available oxygen in it’s nitro or NO2 group. Effectively nitro is self oxidising, as it releases some oxygen when burned. An additional advantage of nitro is it’s higher density. The S.G. of methanol is 0.8 and that of nitromethane is 1.13 So, if replacing methanol with nitro volume for volume, we get 41 % more nitro by weight.
    Fuel is metered to the engine by volume, so although nitro has only 50 % the combustive energy of methanol by weight, volume for volume, it contains about 71 % the energy equivalent.

    Tuning your engine for higher nitro mixes

    If the methanol is progressively replaced with nitro and fed into an engine that was tuned for straight methanol, the engine will progressively burn leaner as the level of nitro is increased. This happens because of the lower A/F ratio the nitro has. To compensate for this the main needle valve must be opened to allow an increased fuel flow to reach the new correct A/F ratio. An engine that runs on nitro will be easier to tune, because slight miss adjustments of the high volume of fuel passing through the carburettor will not have a great effect on engine performance. More nitro means more power. More nitro also means a cooler running engine. The cooler running is a function of increased fuel flow through the engine. More fuel means more oil, and if optimal combustion timing is achieved, more energy is converted to mechanical work, and less is wasted as heat. This trend is not progressive and is not applicable to all types and sizes of engines, so it cannot be generalized. Methanol and nitromethane have a very high latent heat of vaporisation, i.e. it takes a lot of heat to convert it from a liquid into vapour. This heat, required for proper atomisation, is removed from the piston crown, combustion chamber and the cylinder, resulting in an internally cooler engine. Some engines are designed not to use any nitro at all and will not benefit as much from high nitro fuel blends as other engines would. The design parameters for different fuel specs are mainly the compression ratio (C.R.) and the intake timing. The same can be said about engines that were designed to run on high nitro blends. The compression is usually lower and it can happen that such an engine will have to be leaned out substantially to gain satisfactory performance on straight methanol/oil blends. This usually leads to engine overheating problems. Manufacturers usually make specifications available to convert an engine to run on high nitro blends. This simply entails putting a shim of the correct size for a certain nitro blend under the cylinder head to lower the C.R. More experienced engine tuners work this out for themselves by trial and error. Measurement of static prop speed vs. nitro content vs. C.R. and importantly engine temperature will at some point reach a “sweet spot”

    More things to keep in mind:

    If your engine yields satisfactory performance on low or even no nitro content, stick with it. Nitromethane can easily be 25 times more expensive than methanol and with the lower A/F ratio required by nitro, fuel consumption will progressively increase with the increase in nitro content. It is also important to make your mind up on what nitro percentage fuel the engine will be run on. Stick with this right from the start for the engine's life. This will ensure that the engine always operates in the same temperature range. You will be awarded with long engine life and consistent performance.

    When using nitromethane in your engine, stricter after-run procedures must be followed because nitro combustion gives of nitric acid leading to internal corrosion of engine components. The biggest source of this is the pressure line from the exhaust to the fuel tank. This is also the reason why it is sometimes stressed by some engine manufacturers to dispose of any fuel left in the fuel tank after running the engine.

    To prepare your engine for storage the correct procedure is as follows: Empty the fuel tank and start the engine to run it 'dry'. Don't rev the engine high while doing this, because the engine will go into a lean condition as the last bit of fuel is used up. Try restarting several times after it stopped to make sure there is no fuel left inside the engine. Methanol is hygroscopic. That means that the moisture in the air will dissolve in the alcohol component in the methanol. If there is fuel left in the engine, the methanol will evaporate and the moisture will be left behind and cause corrosion on the metal parts, also the small amount of nitric acid left behind will concentrate in the moisture droplets producing a potent electrolyte, which greatly accelerates rusting. These droplets, surrounded by oil which dissolves less oxygen than water, act as tiny batteries powered by the oxygen concentration difference. Electrolytic corrosion starts around the droplets edge, at the steel-oil-water boundary. The next step would be to put a couple of drops of commercially available after-run oil in the carburettor opening and down the glow plug hole. Flip the engine over a couple of times to make sure that the oil is evenly distributed on all surfaces inside the engine. Tip: ordinary automatic transmission fluid (ATF) or WD-40 can be used for after run oil.

    Fuel storage:
    The most important thing to remember here is to keep the fuel dry. The moisture in the air will dissolve in the methanol and ruin the fuel if left to stand open. Use only smallish plastic containers with a lid that can seal properly. Steel containers warm and cool much more rapidly than plastic, and condensation is much more evident in this type of container. Large capacity containers trap a substantial volume of air each time the lid is opened and fuel is poured out. The deteriorative effect that U.V. light is said to have on nitromethane is so minor that, especially when diluted in a fuel mix, shouldn't be a reason for concern. It is however a good idea to store your pure nitromethane in a cool, dry, dark place away from fluorescent or sunlight.
    ---------------
    In my follow-up, I will discuss fuel safety and home-mixes.

    Any comments and suggestions are welcome !!

    I hope that most questions and misconceptions have been cleared up by this article, and that the modelling community will find this article of some value.

    Safe landings !!
    Revo 2.5 that pops wheelies like nothing!

  3. #3
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    Model Glow Engine Fuels - Home Brew

    I won't recommend to follow this route, as you will find that the financial benefit is almost zero. Also you have the risk of working with EXTREMELY dangerous chemicals.

    A basic home-made fuel:
    Get a source of fresh dry methanol. Find out if there are speed shops or oil depots in your area that stock methanol. Buying a 210 litre drum will considerably bring the price per litre down, but remember that you would have to pour it all out at once in smaller containers to prevent too much air to be trapped in the big drum as when you pour out just a few litres at a time. It might be a good idea to share this among other club members, but buying the smaller 20 litre containers should in general make more sense.

    The most well known and readily available synthetic oil is Klotz Techniplate, It is available in KL-200 (straight synthetic), and KL-100 (8o % synthetic and 20 % castor). Either will work, just remember to take the 20 % castor into consideration when working out your final volumes. The Klotz synthetic oil along with a good quality castor can be a little hard to come by. You would probably have to rely on your hobby shop as a source of these.

    Nitromethane is, like methanol, used in full size auto racing, but the nitro they use is not always the 99.9 % pure version as used in the model industry. For this too, you would probably have to resort to your hobby dealer.
    Get yourself a “Pyrex” or a good quality plastic measuring jug and make sure your fuel containers for your final mix is clean and dry and at hand. First pour the oil in the jug, and then the methanol. This will prevent the methanol from being exposed to air for not longer than what is absolutely necessary. Quickly stir and pour in your fuel can and seal it off.

    You might wonder why it would be necessary for anyone to go through all this trouble in mixing their own fuel when ready mixes are available off the shelf. The reasons are that you would most probably be able to end up with a much cheaper final product, and secondly that you would know exactly what went into your fuel. If a fuel manufacturer don't want to tell you what is in their fuel mix because it's a 'secret recipe', they are in my opinion covering up an inferior product. You can also make mixes to tailor your specific needs, that are maybe not available of the shelf.

    A good starting point for determining fuel content percentages for typical 2 stroke airplane engines:

    Methanol Nitro Castor Kl-100 Notes
    70 10 10 10 Good all round fuel
    Various Various 10 10 Other nitro contents for more or less power
    Various Various 20 0 Popular 'old timer' mix using castor only
    80 0 20 0 Good cheap fuel for engines not needing nitro

    Nitro cars and trucks typically require a lot more nitro, and less oil for performance reasons.

    4 Stroke glow engines usually require less oil. This fact can be attributed to the fact that they run at lower revs. so there is less friction. They also only have a power stroke every other stroke, so they should operate at lower temperatures. The glow plug cools down on the exhaust stroke and too much oil in the fuel will douse the plug even further. Methanol can vary from season to season and sometimes cause engines to run hot and inconsistent. 4 Stroke engines are prone to pre-ignite and throw a prop. This is not often a problem when tight fuel production tolerances are maintained at the fuel refinery. A remedy for suspect fuel quility would be to mix 3-10 % of Acetone with the methanol. Acetone blended with alcohol fuels will accelerate combustion flame speed, and reduce the tendency to pre-ignite, especially when lean mixtures are used.

    Some examples of 4 stroke mixes:

    Methanol Acetone Nitro Castor Kl-100
    72 3 10 5 10
    68 0 15 8 9
    Various 0 Various 6 9
    65 3 15 8 9
    Many more combinations can be made up. Remember to refer to manufacturers recommendations, and as a general rule, don't go below 2 % or 4 % castor. Manufacturers of synthetic oils say that you need not use any castor, but engine builders say that castor still has an important role to play as a lubricant. It also has some psychological benefits if you know it's there.
    Revo 2.5 that pops wheelies like nothing!

  4. #4
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    Model Glow Engine Fuels - Safety Information

    Fuel safety information:
    METHANOL (CH3OH) : Methanol is the simplest alcohol, containing one carbon atom. It is a colourless, tasteless liquid with a very faint odour. It burns only 25 % as fast as petrol and release heat at a eighth the rate of petrol fires. It burns with a light blue flame that is not easily seen in bright sunlight.

    When Methanol is applied for automotive use, the emissions are low in smog forming reactive hydrocarbons. It also emits almost no particulate matter witch causes smoke and odour that can be carcinogenic. Methanol can be manufactured from carbon based feed stocks such as natural gas, coal and biomass (e.g. wood).

    Methanol is highly toxic! It should never be taken orally. A few teaspoons can cause blindness and a few tablespoons can be fatal if exposure is not treated. The human body can metabolize and eliminate low concentrations of Methanol with no ill effects, but becomes poisonous when it overwhelms the body's capacity to remove it. Once absorbed into the body, it is very slowly eliminated.

    Indigestion and symptoms:
    Inhalation : A slight irritant to the mucous membranes. Toxic effects upon nervous system, particularly the optic nerve. Symptoms of overexposure may include headache, drowsiness, nausea, vomiting, blurred vision, blindness, coma and death. Do not mix fuel or operate an engine in confined spaces. A lot of unburnt vapour will exit the exhaust of an engine especially when running at rich settings.
    Ingestion : Parallel to inhalation. 100-125 ml is fatal.
    Skin contact : Methyl alcohol is a de-fatting agent and may cause skin to become dry and cracked. Skin absorption can occur. Symptoms parallel to inhalation exposure.
    Eye contact : Irritant. Continued exposure may cause eye lesions.

    Nitromethane (CH3NO2) :
    Nitromethane is a clear liquid fuel that contains it's own oxygen. Because of this, nitromethane uses less oxygen from the air than other fuels while it's burning, and is hard to put out. After combustion, mixtures containing nitromethane generate relatively large amounts of nitric acid in vapour form. When running large amounts of niromethane in full size drag racing, the use of a proper gas mask is essential. The use of nitromethane in model race cars and aircraft are much lower, both due to mixture strength and fuel usage. Nitrometane or fuels containing nitromethane can become destabilized and shock sensitive over a period of time. Never store nitromethane in a metal container, as the heat generated by the metal to metal friction when opening the lid can be enough to cause the nitro to explode once it is in a state of change. Fuel stored for over 5 years should be treated as suspect.

    Ingestion and symptoms :
    Inhalation : Irritation to the respiratory tract. A weak narcotic. Symptoms may include nausea, vomiting, diarrhoea and headaches.
    Ingestion : Parallel with inhalation.
    Skin contact : May cause irritation, redness and pain. May be absorbed through the skin, with symptoms similar to those of inhalation.
    Eye contact : May cause irritation and corneal damage.
    Chronic exposure : Repeated or prolonged skin exposure may cause dermatitis. Prolonged inhalation may cause liver damage.

    Synthetic and Castor oil :
    Not much to say here. Engine oil is not suitable for human consumption. Do not mistake the castor oil used for a lubricant for the castor used for medicinal purposes. Although it has the same origins, the medicinal version is much more refined and of a very high grade. It can be interesting to note the fact that the castor bean from which the oil is pressed, is highly toxic in it's natural state.

    Any comments and suggestions are welcome !!

    I hope that most questions and misconceptions have been cleared up by this article, and that the modelling community will find this article of some value.

    Safe landings !!
    Revo 2.5 that pops wheelies like nothing!

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