Brian Winch discusses myths surrounding petrol engines and conversions.
I often consider our great hobby to be teetering on the edge of a great abyss, wherein lurk all manner of hobgoblins, creatures of the dark, weird creepy crawlies and things that go 'boomp' in the night. This foreboding is bought to mind when I hear and read every so often the dreadful perpetuated myths of aeromodelling: advice and warnings that grew up with fungi on damp logs in dank forests long, long ago and still haunt the fringes of sensible instruction, ready to jump out in the hope of ensnaring a believer or influencing a beginner. Things like, 'If you store glow fuel in metal containers where there are vast temperature changes, moisture will seep though the container walls and contaminate the fuel during the coldness of the night' (when witches ride). What about, 'All model engines need a long, rich run-in on the bench, and castor oil must be used in the fuel'... 'If you fit a propeller in reverse (backwards) on an engine the model will fly in reverse' (and hit the flying witches)... 'ABC means the engine has no piston ring.' I'm sure you've encountered these and others; rest assured that they won't go away. Let's discuss a few that relate to petrol engines and conversions to petrol.
Getting the needle
'The con rod of a petrol engine has to have needle bearings'. Well, needle bearings (fine roller bearings) are certainly good, but definitely not necessary. A plain bearing - bronze or aluminium alloy - is quite okay, even with the lower oil content of the fuel. Some years back, petrol conversions had a good run due to an American company manufacturing a conversion kit that could be adapted to suit many glow engines of the time. Another company produced con rods with roller bearings, but these weren't as successful as they could have been. The size and spacing of the rollers (or pins) in a con rod is very important in order to prevent damage rather than prevent wear. If the rollers are too big and widely spaced then, as the bearing (as a whole) rotates, with rapid succession one roller will be on the peak of the diameter of the crankpin as the engine fires and the resultant force will push the hardened roller into the surface of the toughened crankpin. Mind you, we're talking about an imprint that would amount to probably only a fraction of a micron - three fifths of bugger all - but that minuscule impression is where the roller will rest for the next rotation. This happens with all the rollers, so after a reasonable amount of running the crankpin looks like it's been splined (grooves equidistant around its diameter) to no good result. A successful method is to use a roller bearing with an inner race, but the best is to use very fine needle rollers with no gap, known as 'crowded' rollers. By utilising this style the pressure during the firing stroke of the engine on the crankpin is over several rollers, thus preventing the aforementioned spline effect. OPS produced a 20cc four-stroke some years back that had a needle roller con rod which, from memory, had 22 fine needles in the big end - a very nice job.
No doubt we'll see more new spark ignition model engines from well-known manufacturers such as O.S. and Saito (others will join in sooner or later), and the use of needle roller con rods - on the big ends, at least - might be employed on a single cylinder engine or two. But if this never materialises, the use of standard plain bearing rods will prove to be reliable, as they have done over many years in glow and diesel engines. Whilst we're on the subject, to fit needle roller rods to a twin such as, say, an O.S. 300 Gemini or a Saito 200 inline twin would be a bit of a nightmare that would require a totally new design crankshaft, and I doubt that we'll see this in the near future.
Here's another one: 'The engine will lose power when converted to spark ignition and petrol fuel.'
This is a common misconception. As a fuel, petrol is quite efficient in power. Accordingly, for combustion you need only 1kg of petrol with 14.5kg of air, but a methanol mix requires 4.5kg of air per 1kg of methanol - i.e. a greater fuel rich mixture - hence more fuel is required for your bang. To fire-off a load of low octane petrol - regular unleaded, for example - the engine needs a compression ratio in the region of 6:1 up to 9:1, and for premium petrol - 98 octane - the absolute maximum is 12.5:1 without resorting to engine management controls of a type that are beyond our simple model engines. Engines running on methanol, to obtain peak performance, range from around 12:1 up to a maximum of 16:1. Higher compression ratios require that the engine's construction be quite robust to take the loads imposed, with the added disadvantage of higher vibration levels.
It's rare to have a glow engine that will run over its entire speed range on straight methanol fuel so the addition of nitromethane is called for, which greatly increases the cost of fuel and also raises consumption. Engines that will run well on straight methanol fuel are set with quite high compression and are generally specialist engines such as those used for pylon racing, which are running near maximum rpm during a flight... not much call for idling. I know there'll be the odd character who'll get on his soapbox and blast me about his engines running on straight methanol, " ...I've never used nitromethane." Yeah, okay, I know, and I can run engines on straight fuel too, but they start, idle and transition much better when a drop of nitro is added. To rub salt into the wound, if the weather is really cold, like snowy and horrible, a few percent of petrol in the methanol mix will greatly assist starting and idle. Petrol evaporates at much lower temperatures than methanol, and that's why a petrol engine will start quite readily in super-cold weather. Your car engine started okay to take you to the flying field when it was so cold you couldn't start your glow engine without almost flattening the starter battery... need I say more?
Before you start dipping your quill in the vitriolic ink to send me hate mail, no, I'm not knocking glow ignition engines, I like all types of engines and I have lots of glow engines that I love with a passion. I'm just putting paid to some of the worrisome information that floats around in the ether that appears to be of little use other than to knock a good idea on the head.
How about this pearl of wisdom: 'Spark ignition engines aren't as reliable as glow engines'.
Okay, let's look at some of the reasons a glow engine will stop without prior indication, and generally at the worst possible moment.
Water in the fuel - the air gap on top of your fuel bottle / can / caddie can be highly charged with water vapour, and this is absorbed by methanol. Water is also absorbed by straight methanol kept in a storage container that develops a large air gap over a period as the methanol is drawn off to blend fuel. The result of this is water in the fuel, which causes an unreliable idle or simply puts out the glow plug's fire at any speed.
The plug element can collapse or just cease to work due to a bit of hot running; there could be contaminants in the fuel, maybe the compression's too high, old age (the plug, that is - not the modeller). Metal flakes or dust particles can kill a plug, or reduce its heat to the point where it stops working. A rush of cold fuel, such as with a slightly flooded engine, will extinguish the plug.
Engine cooling resulting from a long approach or slow fly-by can cause the plug to lose interest. Insufficient back pressure (poor muffler design) will allow the plug to cool at low rpm. Incorrect idle mixture setting can cause the plug to be extinguished when the throttle is opened.
Tuning and reliability can be a problem when large fuel tanks are used due to the higher weight of the fuel load, and tuning can be inconsistent throughout the flight for the same reason. You start off with a litre of fuel - close to 1kg - pushing the fuel through to the engine. When the tank gets low this weight has decreased considerably, and the engine runs leaner as a result.
Unreliable running of glow plug ignition engines can often be attributed to their owners constantly fiddling with the needle valve. Does this sound familiar? A model completes a flight, during which the engine ran perfectly. After a short break, the modeller refuels the engine, re-starts it and re-tunes it - for heaven's sake! The needle is wound in and out a bit and, in most cases, returned to its previous position. The unreliability factor comes in when the needle isn't returned to the same position and the tuning is incorrect - mostly too lean. Generally, as the model just leaves the ground, the engine quits. "Why?" groans the modeller. Because the mixture was incorrect, and the engine quit as soon as it started to unload (increase rpm) in the air.
Most modellers using spark ignition petrol engines are generally weaned off this bad habit. Maybe they've learned that it's not needed, or maybe it's due to the fact that the carburettors of most petrol engines don't have a handy needle valve. To adjust the mixture requires the use of a small screwdriver for the tiny slot in the needle, making it very difficult to fit or maintain the screwdriver's position while the engine is running. Slowly but surely the bright light shines that the engine doesn't need constant needle fiddling. After all, how often do you tune your lawn mower / strimmer / leaf blower - these have the same type of engine and, in most cases, the same type of carburettor. This simple factor is one of the reasons a glow engine isn't as reliable as a petrol engine - user interference.
Muffle and pump
The question of muffler pressure often arises with petrol engines. It's a grey area that has no positives or negatives - nobody says no, and I've seen petrol engines with a pressure nipple fitted to the muffler. A similar problem beset coalminers many years back when they used flame lamps in the mines, and many sadly lost their lives when the lamp flame ignited gasses in the mine. It was the invention of the Davey Safety Lamp that signalled an end to that problem and we can borrow from that lamp principle. Briefly, fitting a mesh (filter) to the lamps prevented flame travel; so, fit a filter with a metal mesh in the pressure line and it might have the same effect as it did in the miner's lamps. In my opinion it seems unrealistic to consider the possibility of petrol being ignited in a tank - it's no more of a concern than it is with a methanol engine. If it does bother you, and you want the full box of tricks with your conversion, then fit a Perry / Conley fuel pump suitable for petrol - any fuel supply problems will cease to exist.
Concerns about pumped engines are genuine in cases where silicone valves and diaphragms are used as petrol will destroy these! Most modern pumps use Mylar (or similar) that are totally fuel proof for both methanol and petrol. Before you ask, sorry but I can't give you details of which engines have these 'safe' material in their pumps; if you want to convert a pumped engine then my advice is to do your own research as to its suitability. Often mentioned in a similar vein are 'O' rings, but these aren't generally a problem, except when used in a pumped engine when in contact with raw fuel. Apart from that, the 'O' rings in the majority of engines will never come into contact with raw fuel and, besides, many are of the fuel proof variety.
Well that's a few common stories put to bed, so let's move on and examine the reliability factors of a petrol engine.
Water (in small amounts) isn't the same problem for a petrol engine as it is for a glow engine. Oddly, in small, metered amounts it can actually improve a petrol engine's performance. I don't advise you to start tipping water into your petrol fuel - you'll certainly create a problem if you do - but, under controlled conditions, water mist introduced with the fuel flow can be of benefit. However you need the correct equipment and knowledge to carry out this procedure. As a point of interest, water isn't miscible (doesn't blend) in petrol, so can be seen in the fuel. The addition of a dose of methanol / ethanol / methylated spirit will blend the water - make it miscible - with the petrol, and the engine will generally run on this brew. You could probably achieve the same end using a high-grade kitchen detergent, but it's your experiment if you want to give it a try - I guarantee nothing with this! Whilst we're here, just in passing, methylated spirit is denatured ethanol - ethanol that's been contaminated so it can't be used in its original state (for any form of drinking), but its properties haven't been altered. This denaturing is achieved by the addition of methanol - wood alcohol - which is poisonous to ingest. As such, mixing a drop of 'cherry cheer' cordial with metho' to make cheap 'plonk' is not a good idea.