A description of fuel technology as applied to control line teamracing with 5cc glowplug engines in Australia around the early 1960s.

This is David Kidd's understanding of fuels for Class 2 teamracing, as remembered from the period of 1960 to 1964.  It will be interesting to compare with current thinking, as described on Lance Smith's 2007 Fuel Developments page.

Some of the properties that make various liquids suitable for consideration as fuel ingredients have been scientifically measured and published in various places, and of these the ones I considered important are:

Heat of combustion (called calorific value in the 1960's) - the energy released as heat when a fuel undergoes complete combustion with oxygen. Usually reported measured as calories per gm in the 1960's, and usually in Joules per gm or kg today, we teamracing fuel students are more interested in calories or Joules per cc or ml, since our fuel tanks are restricted to a particular volume, not weight of fuel. To be even more pedantic, it is the lower heat of combustion we need to consider, not the higher one.  Because glowplug engines are essentially heat engines, the more heat of combustion we can fit into our restricted fuel tanks the more work we can expect to get out of the engine (all other factors being correct). Any extra work we can get out of the engine may potentially be used to run longer (more laps) or (if the fuel's other properties allow) to run shorter at greater speed, or some combination of these. Liquids such as benzene, toluol, xylene, cumene, isopropyl alcohol (and numerous others) all have higher heat of combustion values than methanol, so should improve some aspect of a teamrace fuel (all other factors being correct).

Stoichiometric air-fuel ratio - the ratio at which air must be mixed with the liquid fuel to achieve complete burning. Although the exact ratio is rarely achieved the way we tune our engines, comparing the ratios of various liquids can explain why some (such as methanol) make good fuels regardless of their low heat of combustion. To understand this we need to consider that our engines are only capable of inducting a relatively fixed amount of air on each revolution, so fuels which will burn best when mixed richly with that fixed amount of air will deliver their energy contents faster than other fuels that run leaner. Methanol performs best when it is mixed with air at nearly twice the rate for things like benzene, enabling an engine to consume it more quickly, harness its calorific value more quickly, and therefore run more powerfully but for less time than on benzene or like liquids. Liquids such as benzene, toluol, xylene, cumene, isopropyl alcohol (and numerous others) all need more air than methanol in which to burn, so an engine will consume them more slowly (improving range) but probably with less power (slower speed)... although hopefully not much slower because of their greater total energy content, expressed as heat of combustion.

Auto-ignition temperature (also called spontaneous ignition temperature) - the minimum temperature at which a mixture of the fuel liquid in air will spontaneously (automatically) start to burn. Diesel engines that rely on nothing other than compression to ignite their fuels need fuels that have a low auto-ignition temperature (such as kerosene). Engines having a glowplug to assist the ignition can use fuels having much higher auto-ignition temperatures and are likely to suffer symptoms of pre-ignition (overheating, slowing down) if run on a fuel having a low auto-ignition temperature. Knowing that the fuel ingredients we might try have differing auto-ignition temperatures indicates the need for having some means of adjusting the ignition timing of our engines if we want them to perform at their best on the different fuels. I suspect that an engine having adjustable compression like we have on diesels would provide the best solution to this problem. In the absence of such an engine we messed around with glowplugs of different heat ranges, adding accelerants and retardants to the fuel, and similar half measures. In the early 1960's I had not seen anybody shimming engine heads to suit weather conditions the way speed competitors did later, or I might have tried shimming my teamracer engine head to suit various fuels.

Latent heat of vapourization - the energy required to transform a given quantity of a substance into a gas (for our purposes the heat absorbed in vapourizing fuel in a carburettor/engine, and measured as calories or Joules per cc or ml). The vapourizing fuel provides internal cooling of the engine and cools the incoming air/fuel mixture, resulting in a more dense and therefore more powerful charge (containing more air and fuel by mass than it could at outside temperature). Methanol has a very high latent heat of vapourization, which is why many glowplug engines designed to run on methanol have small cooling fins. The cooling fins on some glowplug engines are skimpy indeed when compared with those of an FAI teamrace diesel for example, and limit their suitablility for teamracing, in which for several reasons engines are run as hot as they can tolerate.

Flash point - the minimum temperature at which a liquid evolves enough vapour to enable ignition with an external ignition source. This property probably does not affect the flying performance of a racer, but has caused races to be lost by teams using lots of low-flashpoint ingredients in their fuel and then wondering why they are plagued by fires of spilled fuel at pitstops. The common practice of priming a hot engine through its exhaust will often leave excess fuel dribbling down the cowling both outside and inside the model where, warmed by the hot engine, it is just waiting to burst into flame as soon as the slightest fire emanates from the exhaust. Dealing with a fire that threatens to destroy your valuable plane does nothing to promote a fast pitstop! Fuels containing lots of benzol (flashpoint minus11degC) are more fire prone than plain methanol and other alcohol fuels with flashpoints of 11degC and above.

In addition to the base fuel ingredients there are a couple of rather special ingredients:

Oil for lubrication. In the early 1960's I knew of nothing better than Castrol M (castor oil) or for the Eta29mk6c, Castrol R-40 (a castor/mineral oil blend). It is tempting to reduce oil content as much as possible to make way for more burnable base fuel but I never used less than 20% in glow fuels, and often used 25% in the Eta because that was what successful racers overseas recommended.  A matter to ponder with the benefit of later knowledge, is why 20% oil should be needed when FAI teamrace diesels can now win on less than 10%, gokarts race on 5%, and marine outboards survive on 2%.

Nitromethane to promote easier starting and faster burning of the base fuel, thereby increasing engine power output and speed at the expense of a bit of range. 10% to 15% was commonly used. Nitromethane has a pretty ordinary calorific value so it may seem to be a poor fuel, but it has the special property of releasing oxygen as it burns, thereby allowing more of the accompanying base fuel to be burned with every revolution of the engine. This same property enables nitromethane to be mixed very richly with the air entering an engine - 3 times more richly than methanol in fact, as reflected in its low stoichiometric air-fuel ratio and poor fuel economy in teamracers. It is this special property that makes the nitromethane useful for increasing the power output of an engine.

In the early 1960's nitromethane was virtually impossible for Australians to buy, presumably because of government regulation. The locally produced Keogh's racing fuel switched to nitroethane when that happened, but I never found it much good. To get the nitromethane content for my fuels I resorted to buying little cans of Wen Mac racing fuel imported from America, thought to contain 30% nitromethane. By mixing this 50-50 with the base fuels and oils I decided on a very good teamrace fuel resulted, although always containing whatever methanol and oil the Wen Mac fuel came with.

I was told by a chemical supplier of the 1960's that methanol was just a commercial grade of the more pure methyl alcohol, ethanol was just a commercial grade of the more pure ethyl alcohol, and xylol was a commercial grade containing a mixture of various kinds of pure xylenes. From that, I presumed that when I bought them from BP and Shell garages, benzol was a commercial product containing mostly benzene, and toluol was a commercial product containing mainly toluene.  It came as quite a surprise to learn later that benzol may have contained as much as 50% xylene!

Knowing all of the above did not provide an immediate answer to the question of what fuel is best, but it did indicate that there were several ingredients likely to produce similar good results if we could get them to burn properly.

Most of the potential substitutes for methanol have stoichiometric air-fuel ratios that dictate they can best improve the range (fuel economy) of teamracers, not the speed. Therefore, when formulating a teamrace fuel, a logical approach is to start by adding enough of our favourite methanol substitute(s) to just achieve the laps required to do the number of pitstops we are aiming at, preferably with a couple of laps extra. In 5cc racing the targets are 70 laps for 1 pitstop (difficult), 47 laps for 2 stops (common) or 35 laps for 3 stops (too easy). Then, if the engine is not getting distressed by the heat of doing that, we can try for more speed by opening up carburettor size to maximum, using a faster (higher revving) propeller, and increasing nitromethane content if it is not already around 15%. Doing all this might require a small increase in the percentage of our methanol substitute(s) to retain the same laps as before. Only then, if the engine is not getting distressed by the heat of performing at that level, should we consider trying to eliminate another pitstop by increasing our methanol substitutes as required.

To avoid fooling ourselves when doing trial and error flight testing, we really need to ensure after any major change to fuel or whatever, that somehow we achieve the optimum ignition timing for the new combination (the equivalent of adjusting the compression of a diesel). Anyone who has raced diesels knows that worthwhile performance gains can be made with only small compression adjustments, and that usually some adjustment is beneficial when going to a different propeller, different fuel, or if the weather changes. Unless we adopt some way of adjusting for best ignition timing with our glowplug engines we can easily fool ourselves that some fuel ingredient or propeller is better than another when in fact the improved performance we have observed comes only from the fluke of having unknowingly achieved a better ignition timing. If anybody knows of a quick and easy way of adjusting the ignition timing of a glowplug engine I would be pleased to hear about it, as I suspect I fooled myself many times!

My starting point was a fuel formula given to me by Tony Farnan for the O.S. Max 3 engines we were using at the time. It contained (I think) 10% nitromethane, 5% nitrobenzene, 10% benzol, 20% oil, and 55% methanol, sufficient to achieve 50 laps per tankful without destroying the engine too rapidly. That amount of benzol caused glowplugs to blow with fair rapidity and the engine to run as hot as it reasonably could, so there was no point in decreasing the methanol content of the fuel any further. We needed all the internal engine cooling it could provide.

Later, we replaced the benzol with toluol which smelled better and seemed kinder to glowplugs. For one race we tried petrol instead, getting slightly worse results and with a worse smell. Eventually, by increasing the toluol content to around 20% we completed a race getting 70 laps (just) per tank with good speed but unreliable restarting when hot, a combination that resulted in the best race time I ever achieved with an O.S. Max 3.  The same combination also lost us a state championship a couple of months earlier, when the hot engine refused to restart for a full minute at a pitstop in the final.

When I started using an Eta29mk6c engine we tried some of the fuel ingredients being used in Etas overseas, and found that xylol seemed to produce about the same results as toluol.  It was then that we tried iso propyl alcohol, and kept using it because of the wonderful exhaust smell it produced. I could see no reason why iso propyl alcohol would deliver a performance any better than some combination of methanol and toluol or xylol, but we could afford to use it and it was less fire-prone than some of the other range extenders.

The Eta was essentially a thirstier and faster engine than the O.S. so I ended up using 20% xylol and 20% iso propyl alcohol in the fuel just to get 48 laps from a tankfull. I believe the O.S. would have been so hot after a run on such fuel that it would have been unstartable, but the Eta had piston rings to maintain its compression seal and started reliably in one flick, making it a winner.

We proved that there was no particular magic about my fuel one day in 1964 when Geoff Lawson, winner of class 2 teamrace at the previous Nats, wanted to try his fuel in my model. In his Enya 29 ballraced engine Geoff was using a blend of the Airspeed fuels that were commercially available then, believed to contain benzol and/or toluol, nitromethane, nitrobenzene, methanol, and very little oil. I added 5% extra oil to Geoff's fuel to protect my engine and then it produced a performance identical with my own fuel. It was comforting to know that my fuel was as good as that of my most able competitor, but disappointing that only the smell of mine was any better!  Formulator of the Airspeed fuels, Bill Evans, could justifiably feel proud to have been able to sell in hobby shops, canned fuel capable of winning Australian nationals and setting records.

Is there a more potent fuel?

Maybe.

Back in 1964 I was led to believe that a fuel high in cumene and nitromethane should be superior, if it could be made to burn properly in the engine and if the engine could then withstand the resulting heat. Such a fuel was used by the 1964 world record holders who apparently got 59 laps per tank from their Eta at 118mph, a performance I could only dream of.

Now, I'm not so sure.  Examining the properties of potential ingredients suggests that getting rid of the isopropyl alcohol and relying on toluene to get the laps should be the way to go.

Does anyone else have conflicting or supporting views?  Let me know, and I will be happy to add your contribution to this website for others to ponder.