Why using nitromethan ?

Wednesday, 01 September 2004 23:25

Why using nitro ? (Article from Ian Mc Donald)

HIGH PERFORMANCE HELICOPTER FUELS Nitromethane: can truly be deemed "Liquid Horsepower" for model helicopters. For additional power and generally smoother running, our flying machines can't really afford to live without it! If the engine performance of your R/C helicopter is proving to be a bit of a problem for a whole tank of fuel, and you suffer from overheating as the tank empties during hover, or the idle is difficult to get "just right", it may be that a dose of good old fashioned "nitro" is what the engine doctor would order. Nitromethane is a liquid that has been around for a long time and is used to contribute towards smoother running and increasing power in varying amounts of many model fuels. In addition to altering the power output of an engine, it also contributes towards cooler, cleaner running, smoother idling, and easier starting. Of course, the positive affects of nitro must be weighed against the cost of the magic liquid, which can add from $2 to $4 per gallon per 5% of nitro. Other than the plain fact that nitro is probably the easiest way to increase the power output, or smoothen the run of a model aircraft engine, the whys and wherefore's of nitro are not well understood by most modelers (or anyone for that matter, based on the trouble I had finding information on the subject) so what follows will attempt to shed light on the mystery and help heli flyers decide whether they need use it, and if so, how much. Nitro is manufactured in production volumes by mixing nitric acid and natural gas (or other hydrocarbon base) under high temperature and pressure. It can be made in the laboratory by some complicated mixing and distilling of acetic acid, sodium carbonate, and sodium nitrate which is rather hazardous. The element that is most important is the oxygen which "disassociates" from the liquid at high temperatures.

While Methanol has almost the same amount of oxygen (50%) by weight, it is the overall "mix" that contributes to the unique nature of nitro, allowing a much higher fuel flow and the typically inert nitrogen which can serve to "soften" the shock of the combustion process and inhibit pre-ignition (this is not to say using nitro prevents pre-ignition). All fuels, whether gasoline, methanol or nitro (which incidentally can be burnt at 100% mix like most fuels) have a "stoichiometric" (I brought this word in in a wheelbarrow!) or chemically correct air to fuel ratio, at which they theoretically (as calculated by chemists on paper) burn the most efficiently in air. With gasoline it is 14.9:1 (air to fuel) with best power at 12.7 and best fuel consumption at 15-16:1. Gas puts out 2.78 kilo Joules of energy per kg. Stoichiometric methanol burns best at 6.5:1 or twice the liquid (by volume) for the same amount of air as gas and produces 2.67 kJ per kg, slightly less than petrol, but typically produces 10% more power due to the temperature drop of the mixture as it vaporizes, which produces a more dense mixture (higher density = more power). Methanol burns twice as much liquid as gasoline because it carries its own oxygen supply along with it (50% by weight). Methanol can also run 40% rich and still make good power because of this. This excess fuel contributes to cooler operating temperatures. Nitro burns at a big 1.7:1, or 37% liquid, 63% air, or nearly three times as much liquid as methanol. Energy at stoichiometric = 4.05 kJ per kilogram or 1.5 times that of methanol. This is where the effects of nitro become important. Getting fuel into an engine is never a problem. The problem with producing power from a given engine is getting the air in! Hence, the use of superchargers, turbochargers, special manifolding, porting and valving arrangements on modern car engines. With model engines in general, we don't have the luxury of supercharges, etc. (the 0S 120 Supercharged four stroke being the exception). So Nitromethane actually provides "chemical" supercharging, introducing up to 3.8 times more liquid overall or 5.5 times more oxygen per liter at 100% "stoichiometric" mix, meaning more fuel (methanol) can be burnt, because of all the extra oxygen (the oxidizing agent). For example, a methanol only mix provides 400 grams / liter of oxygen (gasoline has zero oxygen). At 20% nitro, there is 3.14 kJ/kg of energy and 436 g/l of oxygen, and because at 20% nitro the correct mixture or air / fuel ratio is about 4.2:1, a 35% increase in fuel flow will occur, which means around 47% more oxygen ends up in the engine when tuned correctly. I know this sounds complicated, but I did check my math repeatedly, and it all makes sense if you remember that we are talking about quantities here in two different situations: specific quantities per liter and quantities per liter at the "stoichiometric" mixture fuel flow! This increase in oxygen availability and fuel flow amounts to richer running. For example, the main needle has to be opened further to flow the correct amount of liquid to match the incoming air (which is pretty constant at any given throttle opening / rpm level). This also means that the tank may last up to 35% less than with straight methanol fuel. If you get 20 minutes with "straight" fuel, 20% nitro could only last 15 or so minutes. (In practice this is not a linear relationship. With more nitro, typically a smaller throttle opening is needed for the same amount of power, i.e. at hover. So it's generally more than 15 minutes mentioned here but less than the original 20 minutes.) With all this extra oxygen and fuel going into the engine, more power is available, as mentioned before, up to 50% at 80% nitro has been measured. So for every 5% nitro, a power increase of about 3-4% might occur if everything is adjusted correctly. Of course 3% is not much, but at 30% nitro which is common in the USA and Japan in choppers 15% to 20% power increases are easily within reach. More power equals a higher combustion pressure which equals more heat! Cooling: Of course with 42% more liquid going through the engine at 30% nitro much more heat can be soaked up - liquid absorbs heat much better than air. There is also 42% more oil going into the engine, almost flushing the internals continuously, which also helps take out more heat. So we have internal liquid cooling! All this extra liquid keeps the metal surface temperatures down and eliminates the burning of oil to carbon. So there are usually little or no carbon deposits in nitro fueled engines above 10% nitro. The cooling effect of nitro is further born out by the increased usage of high nitro fuels in fuselaged models which are almost totally enclosed for drag minimization. The nitro is used just as much for cooling as it is for good power! Nitro won't necessarily work the same wonders in engines from different manufacturers.

Typically the Japanese have been heavy nitro "users" and produce engines which work well on high nitro and have relatively low compression ratios. In contrast, the European manufacturers, with nitro being more expensive and hard to get in Europe (not that it is all that cheap in Japan), appear to have engineered their engines with higher compression ratios for little or no nitro usage. It is not unusual to have to "decompress" European engines for satisfactory high nitro operation. Nitro may not tune the same on different engine types, even from the same manufacturer, because the combustion process in a glow engine is triggered by a catalytic reaction of the compressed mixture with the platinum compound of the glow plug. Many different factors affect the "timing" of the ignition of the fuel / air mix (i.e. fuel quality and mixture setting, ambient air temperature, engine temperature, fuel temperature actual compression ratio, inlet & exhaust timing, muffler/type, etc.). This is in contrast to a gasoline spark engine where the ignition timing is influenced predominantly by the spark timing, which can always be optimized for best engine running at any rpm. Further, the tuning with nitro can also be affected by the ability of the carburetor to deliver the fuel in sufficient quantity/accuracy thru various speed ranges. On some engines, the engine may not run well in hover above, say 12% nitro, but merely changing to a different carb, with better mid range flow adjustability may fix the problem. Also, too high a compression ratio with too much nitro may have combustion on the edge of pre-ignition , and cause unreliable running and may be difficult to tune. Decreasing nitro content or decreasing compression ratios may very well cure the problem, as trying glow plugs with different heat ranges. Hooked on Nitro: Many people will have heard that some engines are "hooked" on nitro, i.e. on nitro they run great, straight fuel they run like a dog, or have less power. While not necessarily "hooked" on nitro, they are used to running at a particular temperature, and all the clearances (especially piston to bore clearance in an ABC engine) in the engine are run in at that temperature. Changing the nitro content changes the operating temperature and you have a different engine on your hands. While researching this article it became evident that information on nitro is very scarce. So if anyone out there has more information or would like to comment on or "discuss" any part of this article, please let me know c/o Rotory or at my CompuServe address: 100240,2265. IAN McDONALD

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