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XJet -> RE: mixing fuel 101 ? (8/19/2007 10:50:12 AM)
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25% is *way* too much for a modern RC engine and might do more harm than good.
A much better ratio is 17%-20% if you're using a traditional US model oil such as Morgans or Klotz.
One effect of using more oil is that it effectively raises the compression ratio and that can cause detonation (if you're also using nitromethane) or running in an engine that's tuned for low/no nitro.
Unlike air, or fuel-vapor, oil is incompressible so effectively reduces the volume of the combustion chamber which means that the ratio of swept volume to combustion chamber volue (ie: the compression ratio) goes up.
If you use more than 18% you're probably just throwing money out your exhaust pipe, making your plane really messy and losing performance.
I use just 12% of a high-quality oil and I get over 600RPMs more than using 20% of a "conventional" oil. In effect, it's like running an extra 10% nitromethane but without the cost and without the negative hit on fuel consumption.
Homebrewing is easy. Here are some tips (just off the top of my head, I'll try to organize them a bit better later):
1. Use the metric system.
It may seem unfamiliar if you've been bought up with fluid ounces, pints and gallons but it's a whole lot easier to calculate your ratios and it's easy enough to pick up measuring jugs marked in mL and liters -- look in the kitchen section of your local department store.
2. Use good quality components
Your methanol should be pure and dry. Always store your methanol in a suitably sized container. It's better to have a small container that's almost full than a large one that's nearly empty. I buy my methanol in 206 liter drums (thats 55 US-gals) and decant them into 20ltr (that's 5US-gal) plastic drums. A drum lasts me about 2-months in winter and a month in summer so the methanol in these drums never gets a chance to get wet.
I used to use low-tech oils such as Morgans (Coolpower) and Klotz at 18%-20% by volume but now I'm a convert to using less but better oils such as those from Cooper Fuels (or if you're in Europe there are other hi-tech oils such as Aerosave, Aerosynth and Motul Micro which are similarly advanced. I've been told you can run these oils as low as 8% by volume but I stick to a relatively conservative 12% and have had no problems at all -- even after running at least 20 gallons through some of my engines.
And don't forget the value of castor oil. Some folks say it will gum up your engine and make a mess but if you use just a few percent (2%-3%) by volume, you won't get any gumming and it will provide a valuable extra "last line" of defense against a lean run. Use only degummed AA castor oil or a reputable "modified" castor such as Klotz Belnol. Castor from the drug store may be better than nothing but chances are it *will* cause gumming up of your engine and excessive carbon build-up over time.
Another big bonus associated with the use of a small amount of castor is the added anti-rust protection this gives your bearings and crankshaft. Unlike most synthetics, castor oil does a wonderful job of protecting your engine's steel innards from rust -- especially if you don't religiously use an after-run oil.
In some parts of the world nitromethane can be hard to get and/or expensive. Many engines designed and made for the European markets don't *need* nitromethane in their fuel (and in fact, using too much can cause them damage). I've found that 5% is a good safe figure for 2-strokes and 10% is good for 4-strokes. This is enough to ensure a good idle and easy tuning but not so much as to risk engine damage from detonation.
3. Be safety conscious.
Make sure you mix your fuel in a well-ventilated place and well away from any ignition source. Methanol is a particularly dangerous substance for several reasons. Firstly, it's absorbed directly through the skin and repeated exposure can cause quit significant damage to your body's key organs and nervous system. Do not drink or bathe in this stuff! Secondly, its vapor ignites *very* easily and burns with an almost invisible flame. If you do get yourself in a methanol fire you probably won't even know until you feel the searing heat and pain that comes from bad burns. What's worse, because you can't see the flames you won't know where to run to get away from them!
Always keep a *SUITABLE* fire-extinguisher handy. I've tried foam extinguishers but found them sadly lacking against methanol-based fires. The foam effectively disappears because the methanol destroys the surface-tension necessary for its bubbles to remain intact. What's worse, the water in the foam mixes with the methanol and until you get below 100% proof, the resulting mixture still burns! Have a nice big CO2 extinguisher handy if you're mixing any significant volume of fuel. If you don't have or can't afford such an extinguisher, mix your fuel outdoors, well away from any flammable structure.
Of course it goes without saying that you should not be smoking while mixing this stuff :-)
4. Keep things clean
Make sure you use clean measuring jugs, funnels and containers. A single speck of dirt can ruin your day if your contaminated fuel causes an engine to stop at exactly the wrong time. Don't think that just because you've got a fuel filter in your fuel-line that you'll be safe. Dirt will eventually clog the filter and your engine will also stop then.
5. Be consistent.
Measure your ingredients accurately and try to mix up sufficiently large batches of fuel that any small errors won't make a big difference. This will save you having to re-tune your engine every time you start running a new batch.
6. Different brews for different engines
There is no "ideal* or "best" fuel. Different engines often respond better to different fuel blends.
I end up running three different fuel recipies...
My plain-bearing (bushed) engines such as my Thunder Tiger GP series (or OS LA series) demand a fuel which demand a higher percentage of castor oil than normal. My recipe for these engines is:
* 10% castor oil
* 10% synthetic oil (any synthetic, I use Coopers)
* 5 % nitromethane
* 75% methanol
The extra castor in this blend is to protect the relatively simple bushed crankshaft design from excessive wear. Some folks do run fuel with only synthetic oil in these engines but it *will* result in premature wear of the crankcase bushing. If you're not worried about engine-life then you can do the same -- but I treasure *all* of my engines and treat them accordingly.
For a regular ABC/ABN/ABL 2-stroke engine (OS46AX, GMS47, TT46Pro, Magnum 46/52, etc, etc) you can use this blend
* 15% regular model synthetic oil (klotz, Morgan's, etc)
* 3% castor oil
* 77% methanol
* 5% nitromethane
Despite what some (particularly in the USA) may consider a rather low nitro percentage, this fuel offers good performance, plenty of lubrication and relatively low-cost. Those who want a *little* more performance can increase the nitro up to 15% with a resulting reduction in the methanol content.
For a ringed 2-stroke engine you can use the same brew as above or you can use all-synthetic oil (ie: replace the 3% castor with just an extra 3% synthetic oil. Personally, I prefer to leave the castor in. It's *not* going to gum up your ring but it will provide valuable extra protection.
4-stroke engines:
Here's where things start getting a bit religious. Some folks swear that you should never use any castor oil in a 4-stroke engine because it'll gum up the lifters/rockers and cause carbon to build up on the exhaust valve. Well I've been running 2%-3% castor in my 4-strokes for years and *never* noticed any significant gumming or carbon build-up. What I *do* notice when using castor however, is that my camshart shows virtually no wear and there's no rust in my engines, despite not using an after-run oil.
What 4-strokes *do* like is a little more nitro. This extra nitro helps to keep the plug warm (since it's only re-heated once every two revolutions instead of every revolution as is the case with a 2-stroke). 10% is a good safe minimum for most 2-strokes -- except the YS which really runs quite poorly until you feed it with a fuel of at least 20% nitro.
I'm getting tired of typing now so I'll take a break.
Is this what the original poster was thinking when he started this thread???
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