When asked to write this feature I was very wary, if only because this is the one subject that’s almost guaranteed to put the cat amongst the amateur industrial chemists out there, with almost everyone having an opinion on the subject. The problem with glow fuel is that we think we know more than we actually do, and in truth reading the instructions supplied with most model engines does little to clear things up. For years the importer of one particularly well-respected range of four-stroke engines remained completely at odds with the manufacturer, countermanding their advice.
The supplied instructions were very clear in that they demanded only synthetic lubricants in the fuel, yet the factory instructions were rubber stamped ‘Use Castor Only’ in bold red typeface, leaving the buyer between a rather imposing rock and a very hard place indeed, with similar paradoxical examples to be found elsewhere.
This situation simply doesn’t occur with petrol engines, where you pull up at the pumps on your local garage forecourt on your way to the patch and fill both your fuel container and your motor car without a single worry about the chemical constituents of the liquor that you’re buying. You rightly trust the likes of BP and Texaco to sell you something fit for purpose, so topped up with unleaded and with a bottle of two-stroke oil tossed into your flight-box off you go for the day, happy as Larry. Why can’t we enjoy the same peace of mind with glow fuels?
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METHANOL
Let’s start with the basics. Why do we run methanol in our glow engines and not petrol? Methanol has only about half the calorific value of petrol, so taken at face value it seems a most unlikely candidate for model aeroplane fuel when we have to carry twice as much of it aloft for a given amount of run time. In reality there are several benefits that outweigh this very obvious shortfall.
Methanol runs cooler than petrol and if enough of it is burned then we can get a lot of power from our humble glow engines. And, of course, there’s nothing particularly clever needed in terms of an ignition system, since the platinum element used in our glow plugs does the job. There’s a chemical reaction ‘twixt the fuel and this platinum element that keeps the plug alight, and whilst it’s generally assumed that the heat from the previous combustion cycle keeps the plug glowing, this isn’t the whole story.
The platinum element needs to be bathed in methanol to create the catalytic reaction required to keep the fire burning; a small but usually overlooked detail. Petrol doesn’t have that effect on a glow element. Anyway, without turning this article into the whys and hows of the glow engine, let’s simply accept that model glow engines must burn methanol.
OIL
The next fuel ingredient, the lubricant, is probably the most controversial. Without lubrication all those bits of expensive engineering wouldn’t last very long at all. The job of any lubricant is to keep all the moving parts separated from one another, and in the case of an internal combustion engine that can be a tall order, given the stresses and strains at play plus the diverse range of temperatures encountered. Indeed temperature plays a major part in how any lubricant performs. Take a frying pan, drop a dollop of cooking oil into it, and you’ll see that it remains viscous until the heat changes its character, whereupon it runs to the sides of the pan and, if left unattended, will eventually burn and dissipate completely. This is bad news for your full English and very bad news indeed for your highly stressed glow engine if the same thing occurs inside the combustion chamber.
Lubricants fall roughly into two categories, these being natural vegetable or mineral oils and their synthetically derived counterparts. Despite all the rhetoric, good old castor oil is just that, a natural product derived from squeezing the humble castor bean until it squeaks, whereupon it releases its precious fluid and we bottle it. Terms like ‘first pressing’ are often uttered by the fuel makers and whilst this is a valid claim it’s only like saying ‘genuine coal’ or ‘real milk’, so don’t make too much of the sales pitch as no reputable fuel maker would use anything other than a first pressing quality castor oil.
Castor has all sorts of properties, some more laudable than others, and it does make a very good lubricant for model engines. However, in the end it kills with kindness, and it certainly used to have interesting side-effects on those early aviators who ingested a goodly amount during a sortie. On a more serious note the early aero engines had to be regularly dismantled and cleaned of castor residue, as do our own, because that horrid castor gum will shellac and clog model engines something rotten. For that reason alone I stopped using it years ago. Yes it’s cheap, it works, and you’ll be unlucky if you seize an engine using it, but it will completely destroy your four-stroke over time as it clogs up all the little valves and oil galleries, whereupon the valves will stick open and the bearings will skip and scuff. Castor harps back to the era where automobile engines were stripped and decarbonised every ten thousand miles or so, and any engine run on the stuff soon turns brown with that ‘dipped in toffee’ appearance!
So that’s the call for castor dealt with, what’s the deal with synthetic lubricants? Please read and inwardly digest the next bit, because this is crucially important and is often the whole crux of the ‘my fuel’s better than yours’ debate.
Not all synthetic lubricants are the same, in fact they aren’t even similar in many instances. There are some superb synthetic lubricants around that we can’t use because they’re not miscible (i.e. they don’t mix) with methanol and / or nitromethane. Now then, to suggest that one synthetic is akin to another is a bit like saying all alcohol is similar, so if you’re of that mindset I’ll drink a pint of Stella whilst you have a pint of single malt, and then we’ll have a race around the block!
Having established that not all oils are equal, we can concentrate on the very real business of specific lubricant quantities – another thorny subject. If the lube isn’t the same then the quantity needed cannot possibly be the same, yet time after time, engine makers and suppliers bang on endlessly about percentages, stating (for example) that running your Acme ABC super whizzo on less than 18% oil will invalidate the warranty. Great, you run your example on 18% fish oil then and I’ll use half that quantity of the very best lubricant money can buy and we’ll see whose engine lasts the course. You don’t have to be an industrial chemist or an engine expert to see the folly of such logic. The synthetic lubricants used in popular brands of model engine fuels vary considerably, with some fuels specified as dedicated heli’ or ducted fan brews, the only difference between them being maybe the odd extra 1% of oil, or maybe a slight percentage increase in nitromethane, all much championed in the manufacturer’s blurb.
Beware the art of the marketing gurus who make much of little in that respect, as all model engine fuels err on the side of safety with their lubricant quantities due to the fact that many model fliers have no mechanical sympathy whatsoever and so a safety measure is factored in. A couple of percent here or there isn’t therefore necessarily all it may claim to be.
NITROMETHANE
Nitro (as it’s commonly called) is derived from propane and its inception by the Nazi war machine in W.W.II, when it was used as rocket propellant for the Me163, tells you a bit about it. Nitromethane has qualities that make it very desirable for use in a glow engine, and not wanting to get too technical, let’s agree that a healthy dollop of the stuff is all good news. There are downsides to nitro use, though. It’s expensive and heavy, and as it burns quickly you have to carry more of it, which of course means that the more you include in the mix, the shorter your engine run time will be. On the plus side you get more power and a far less fussy engine that throttles better and runs faster. What’s more, the idle performance will benefit greatly.
Running nitro in appreciable quantities will release a commensurate percentage of power. For example, running 30% nitro in your brew will give an approximate power hike of 30%, and that’s not to be sniffed at. Don’t unduly worry about the corrosive bi-products that nitro burners are supposedly afflicted with, given that you have to run methanol anyway, which is far worse in that respect. Actually, castor oil isn’t all good news in that department, either.
Beware the fuel soothsayers and the engine sellers, because the advertising and hype that surrounds nitromethane is almost mythical in its misrepresentation, to the extent where the model car world now talks about ‘nitro cars’, completely overlooking the fact that the things actually run on methanol with just a dash of the ‘good stuff’ in the tank. It’s a bit like the ‘turbo’ tag that afflicts anything and everything from cars and trucks to vacuum cleaners; a hit marketing term with much being made of little! Last but not least, remember that nitromethane is much heavier than methanol and the fuel sellers who quote percentages by weight are pulling our legs and getting away with it for the most part. This one is a real misnomer, because 30% nitromethane by weight is just a tad over 20% by volume, so it’s entirely likely that you could be comparing apples with pears when comparing one fuel against another and certainly, when comparing costs. Caveat emptor!
DORLING’S WUNDER BRAU
So, there we are. Different types of lube, different ways of measuring, different qualities of lubricant and different ways of marketing and selling the fuel. On top of that little lot you may well have an engine maker who’s at odds with the distributor regarding the advice given on fuels. All in all, this makes the model flier’s lot a difficult one. The bottom line on fuel is that no matter what the make or name on the can, no matter what the mix or type of fuel, if it’s got methanol and oil then your engine will run on it. Furthermore, provided you don’t run the engine impossibly lean then you’re pretty unlikely to damage anything.
Years ago I used to have four-stroke and two-stroke mixes, heli’ and ducted fan brews, big and little engine concoctions and old and new engine fuels, all complicating the very simple business of running glow engines strapped to the business end of my toy aeroplanes. These days, however, I do things rather differently. I run everything from my vintage Cox 049 free flight glows to my lovely Laser 300 V special 50cc four-stroke V-twin on one fuel mix only. This includes helicopters, high speed deltas and jet looky likeys running very high rpm power plants. I fly pattern aerobatic and fun fly 3D ships, cheap ‘n’ cheerful plain bearing sports 40 powered hacks and anything else in fact, all on just the one fuel. I never use castor these days as the mess and gunge is vile, and I certainly don’t flit from one brew to another.
What is my ‘wonder brew’ then? Well, without upsetting all the fuel sellers, I’ll say no more than it’s a fully synthetic methanol and nitromethane mix with a ‘sufficient’ (as Rolls Royce used to say) quantity of clean running, highly effective synthetic lubricant that seems well up to the job. I’ve run well over 100 new engines of various types on this fuel, and during extended trials for a four-stroke engine manufacturer it exceeded all expectations and proved eminently suitable and fit for purpose. Could I have achieved the same results with another brew? Possibly! In my experience it’s not the fuel that damages engines, it’s the operator.
CHECKLIST
So, how about some practical advice? Here’s a list of do and don’ts that will help you through the fuel minefield:
Fuel is a huge subject and a stand-alone abridged feature like this one can only gloss over the surface. My advice is not to get too hung up about it. At the end of the day all the clever stuff has been sorted out by the fuel alchemists, leaving you to enjoy their products and get down to the rather more important business of flying your aeroplanes. Every club will have its expert and opinions on fuel are as diverse as the fliers that use the stuff, but it ain’t rocket science. Mind you, thinking about nitromethane and its origins… perhaps it is!
