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Fuel for Model Diesels - the Inside Story

By Adrian Duncan

The .15 cu. in. diesel engines used in Diesel Combat are actually not true diesels at all, since they don't have a fuel injection system but function exactly like a full-size two-stroke engine, the only difference being that they rely solely on compression-generated heat for ignition. As a result, they have certain fuel requirements in common with other two-strokes plus some special requirements dictated by their individualistic ignition system.

All two-stroke engines require two basic elements to their fuel - the base fuel which burns in the cylinder and provides the power, and a lubrication component. Model diesels are no different, but they require some additional components solely to aid the ignition process.
Before we go into the specifics of model diesel fuel, we wish to add a strong word of caution. We do NOT recommend that modellers try to mix their own diesel fuels! Good fuels are commercially available in customer-specified formulas at a cost less than that of home-brewing, and the materials involved are both hazardous to handle and potentially injurious to your health! Besides which, some of them are restricted substances and hence a hassle to get hold of. Our purpose here is strictly to provide an understanding of the roles of the various constituents of diesel fuel so that modellers can appreciate what is going on in their engines and make informed decisions about the suitability of a given fuel for use in those engines.

Briefly, a model diesel fuel has four major ingredients:

1. Kerosene, which is the base fuel and is the major power-producing ingredient
(equivalent to the gasoline in your weed whacker);

2. Oil, which performs the lubrication and sealing functions;

3. Ether, which has the function of reducing the temperature necessary in the cylinder for ignition to occur and also acts as a solvent to keep all the components in solution; and

4. an ignition improver, which further lowers the ignition temperature requirements and also reduces lag time for ignition to commence when the right temperature is reached.

Let's look at each of these in more detail!

Kerosene is actually a refined form of the diesel fuel used in road vehicles, and has very similar ignition and combustion properties. In fact, road diesel fuel can be quite successfully used as the base fuel in model diesels. However, kerosene burns cleaner than road diesel fuel, and is thus more suitable for model use. It has a relatively high calorific value (the amount of energy released for a given amount burned) and a relatively low self-ignition temperature (the temperature at which it will spontaneously self-ignite without any external flame source). Both of these features make it a very good base fuel for model diesels. But its self-ignition temperature (around 250 deg. C) is still too high for our miniature two-strokes to deal with, hence the need for other constituents as discussed below.

How much kerosene can or should be used? The more kerosene in the mix, the greater the power potential but also the greater the amount of heat generated. At some point, you get into a situation where the extra power benefits become susidiary to the resulting overheating problems. Between 40% and 45% appears to be the optimum range based on our many years of experience.

2. OIL
There is a lot of controversy about the best kind of oil to use in model diesels and indeed in model engines in general. Before throwing our hats in the ring, we'd like to remind the reader that the oil in a model diesel has three functions, not just one. As well as lubricating the engine under both start-up and working conditions, the oil has to provide a film of sufficient viscosity to fill the microscopic spaces between moving parts and thus provide a gas seal. Since there are no piston rings or O-rings, it is the oil which does the sealing. In addition, since all the other fuel components are burned as the engine runs, the oil is the only residual component when the engine stops. Therefore, it also has to guard against post-operative corrosion.

We also need to bear in mind that the very high compression ratios at which model diesels run compared to glow or ignition engines involve far higher gas pressures and place far greater loads on the bearings and wearing surfaces. This in turn places significantly greater demands on the oil, both as a lubricant and a sealant. A diesel is a real test of any lubricant!

Taking account of the above requirements, there is NO doubt whatsoever in our minds that castor oil remains by far the best lubricant for any model diesel. It has unparallelled film strength both hot and cold and maintains good viscosity for sealing purposes at all times. It is also an excellent corrosion inhibitor - you'll never find rust under all that gumminess! You will hear vehement arguments from oil manufacturers and fuel blenders in favour of synthetic oils, but the proof is in the every-day real-world experiences of modellers like us. We have in fact experimented extensively with state-of-the-art synthetic-based mixes, and have experienced a range of problems ranging from hard starting to engine overheating and even seizure. Our advice - stick firmly to castor oil, and don't accept any substitutes!

As far as the desireable oil content goes, experience shows that around 25% seems to give the best results both from the standpoints of lubrication and sealing. More than this appears unnecessary, and will reduce the power-generating qualities of the fuel by displacing the power-generating components from the mix. Much less than this seems to affect engine life over the long term and gives increasing sealing problems as engines become more worn, particularly in diesels used for combat where dirt ingestion is a fact of life.

One difficulty with castor oil is that it is insoluble in kerosene. This would preclude its use in model diesel engines except for the fortunate fact that both castor oil and kerosene are mutually soluble in ether! Hence in part the reason for our next ingredient.

It is important to bear in mind that ether (or diethyl ether as it is more properly termed) is a BAD diesel fuel on its own. It has a relatively low calorific value and tends to detonate unpredictably rather than self-ignite smoothly and cleanly. Detonation places enormous stresses on engine components, and is to be avoided at all costs. But ether does have the huge advantage of having an extremely low self-ignition temperature (around 188 deg. C). The ether's SOLE functions in a model diesel are to lower the temperature at which ignition of the fuel mixture will commence (and hence lower the required amount of compression required for starting and running) as well as to keep all the ingredients of the fuel in solution. So we need to use just enough ether to meet these two requirements AND NO MORE!

Ether is EXTREMELY flammable, and diesel fuel should therefore NEVER be handled anywhere near an external ignition source. Ether is also extremely volatile and evaporates readily. So diesel fuel should always be kept in a sealed container except when being transferred to a field container or to the model. Finally, ether is an excellent solvent and is highly reactive. It can also form highly unstable peroxides if exposed to light for any length of time. Diesel fuel should therefore be stored SOLELY in metal containers. If you try building a foam-core model for your diesel, it is the ether which will very efficiently dissolve your lovely foam core!! This is one reason why the British Columbia rules for nostalgia diesel combat do not allow foam cores - we want to protect our contestants against disappointments of this sort!!

The kerosene content greatly reduces the tendency of ether to detonate, so we do have some leeway to play with. Long experience shows that between 25% and 30% ether appears to be the optimum range. Mixtures at the higher end of this range are advantageous in that they contain some allowance for the inevitable evaporation of some of the ether as the fuel is used.

This is the one component that approaches the realm of "black magic" in terms of diesel fuel technology. It is a relatively minor component which has the effect of further lowering the self-ignition temperature of the fuel but more importantly reducing the lag time for ignition to commence once this temperature is reached. Both of these effects allow the use of still lower compression settings during starting and running, thus further reducing the stresses on the working components.

The most commonly-used ignition improvers today are amyl nitrate (mostly in North America) and iso-propyl nitrate (commonly used in Europe). Both of these substances are highly volatile, semi-explosive materials which require VERY respectful handling in their pure state. However, they become relatively stable once in solution with the other fuel ingredients. Amly nitrate is VERY dangerous from a health standpoint - it's used as a heart stimulant in medicine but can kill you if improperly used - a number of "recreational" users have found this out the hard way! We do NOT recommend that the average modeller become involved with these nasty critters, besides which they're restricted and very expensive. Leave it to the professional fuel blenders!

The effect of these improvers is most marked over the first few percent in the mix. After that, additional amounts cause erratic running, detonation and excessive overheating. About 3% seems optimal for amyl nitrate, and around 2% for iso-propyl nitrate.

One of the downsides of being a diesel user is "the smell"! Actually, most of this is caused by the amyl nitrate in the fuel. Iso-propyl nitrate does not create anywhere near the same proplem, so it's a pity that it does not seem to be generally available to North American blenders. If you ever go to Britain to fly your diesels, try some of their iso-propyl-based fuels - you'll be pleasantly surprised!

Well, there it is! That's what we've learned about fuels over the last 40 years or so (yes, we're that old!!). Our current basic mix for nostalgia diesel combat is:

Kerosene - 42%
Ether - 30%
Castor Oil - 24%
Amyl Nitrate - 3%

We get our fuel custom-blended by Red Max to the above formula, but there are other suppliers as well. On the above mix, direct measurements show that a well broken-in diesel will run typically on compression ratios of around 17 or 18 to 1 - quite manageable! If you're needing to use much more than this, chances are there's something wrong with your fuel!! In extremely hot weather, we find that the addition of a few extra percent of castor oil to the above mix can work wonders for consistency and dependability. It also helps in starting a worn engine by improving the seal. This is the only adjustment that we ever need to make - the above mix generally works well under the full range of conditions that we usually encounter. Anyway, we hope that the above information will help you select the best fuel mixture for you own needs.

Good flying!

Fuel Manufactures on the web:

Model Technics Diesel Fuel.
British Manufacture of quality diesel and other fuels.
D1000 is formulated for engines up to 1.5cc or as a running-in fuel for larger engines. D2000 is a general purpose fuel for engines over 1.5cc. D3000 is for high performance ball-race engines at speeds in excess of 14,000 rpm.

FHS Red Max Diesel Fuel. Red Max Diesel: Ball bearing, plain bearing, and custom blends available. FHS Supply, Inc. was started by J. Fred Wilson in 1969. He successfully built it up into the proud company that makes superior oils and fuels for people that want the best. You can find his sons and daughter-in-law [David, John+Serpil(Sera)] there, along with the other hard-working and dedicated people.


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This page was last updated on 02/21/04.