Fuel Facts
What’s best for your engine?
[Editor’s note: As part of the Model Airplane News archives, this classic article contains a great deal of interesting information. However, since it was first published, some of the pricing may have changed.]
In today’s hobby industry, commercial fuel-blending companies are hardpressed to make a profit, and stay in business. Nitromethane is no longer made in the USA; our only refinery dedicated to its production was moved to India years ago. We now import nitro from China and are subject to interruptions in supply. As refineries shut down to reduce air pollution, the supply of nitro dwindled here, and its price soars. Later, when supply is restored the prices remained high. Although they are struggling, there is still stiff competition among fuel companies. In their advertising, a few come across boldly, verging on arrogance. One particular blender proclaims an almost divine knowledge of the discipline, predicting the fuel needs of all engine types and sizes; to him, the engine manufacturer’s recommendations should be dismissed as insignificant. In other words, some blenders attempt to persuade the modeler to disregard the engine’s instruction manual, and instead turn to them for guidance about fuel purchases.
ARE ENGINE MANUFACTURERS TOO CONSERVATIVE?
There is a concern throughout the fuel industry that many of the world’s engine manufacturers are too conservative when recommending lubricating oil percentages for their products. A high lubricating oil percentage never hurt an engine … or did it? A growing body of experimental and practical evidence suggests that modern engines are being impaired by excessive oil content in the fuel. Here are three examples:
The engine has difficulty maintaining a reliable, low-rpm idle.
The engine has difficulty obtaining a crisp throttle-up.
The engine exhibits diminished wide open throttle power.
THE TRADITIONAL MODELER
Suggest reducing the fuel’s oil content to a traditional modeler, and there’ll be an immediate objection, “What are you trying to do, ruin my engine?” Fuel blenders have discovered that change comes slowly when dealing with lifelong modelers. Faced with a traditionalist attitude, some blenders have ventured onto a new path: mix the fuel based on the latest technology and delete the label specifications. Lube percentage and sometimes the nitro content are often left off entirely, thus avoiding the inevitable criticism from engine manufacturers, engine repair centers and modelers comfortable with custom and tradition. Modelers are often suspicious that fuel blenders might substitute a less expensive component, such as methanol, for an expensive component such as nitromethane or a synthetic lubricant. When purchased in bulk, the fuel component costs to one commercial fuel company, minus the shipping charges (2006) are:
Synthetic lubricants: average $16 per gallon in multiple barrel lots (55 gallon).
Special synthetic lubricants: average $30 per gallon in multiple barrel lots (55 gallon).
Castor oil lubricant: $9.75 per gallon in multiple barrel lots (55 gallon). Traditional synthetic oils (UCON, etc.): less than $10 per gallon (55 gallon). Methanol: $1.49 per gallon in 5,000-gallon lots (tank truck). Nitromethane: $14 per gallon in 80-barrel lots (53 gallons/barrel, 2-gallon nitrogen space).
THE INCREASED COST OF REDUCING OIL CONTENT
A ring-less .40-ci ABC-type 2-strokecycle engine with a ball-bearingsupported crankshaft is a good example for comparing blending costs between traditional and non-traditional (reduced lubrication content) fuels. Traditional modelers generally agree that 18% oil (14% synthetic, 4% castor) is safe for this type and size of engine. Conversely, an honest commercial fuel blender knows that he can easily cut the total oil content to 14% (or less) with a mixture of 12% special synthetic and 2% castor oil, while improving the engine’s power, idle and throttling characteristics as well as maintaining its longevity.
TRADITIONAL BLEND:
18% lube, 15% nitromethane, and 67% methanol
14% traditional synthetic
($10 x 0.14 =$1.40)
4% castor oil ($9.75 x 0.04 = $0.39) 15% nitromethane ($14 x 0.15 = $2.10) 67% methanol ($1.49 x 0.67 = $1) Ingredient total: $4.89/gallon
SPECIAL SYNTHETIC BLEND:
14% lube, 15% nitromethane, and 71% methanol
12% special synthetic ($25 x 0.12 = $3) 2% castor oil ($9.75 x 0.02 = $0.195) 15% nitromethane ($14 x 0.15 = $2.10) 71% methanol ($1.49 x 0.71 = $1.055 Ingredient total: $6.35/gallon By removing all of the inexpensive traditional synthetic lube (16% at $10 per gallon) and replacing it with a special synthetic (12% at $25 per gallon) and methanol (4% at $1.49 per gallon), it should be clear that the reduced lubrication content fuel costs more to produce. Note: fuel blends are formulated by component volume, not component weight.
Commercial fuel blenders don’t always reduce the oil content of their fuels. Older engine designs that have lapped (ringless) ferrous (iron and/ or steel) pistons and cylinders, and/ or plain bearing (bushing) crankshaft support, require relatively high percentages of castor oil to provide adequate high-load (pressure) protection. For these engines, it’s common to find fuel blenders recommending up to 28% lube. RC helicopter fuel is another example of where the oil percentage (both special synthetic and castor) is often boosted several points (up to about 24%) due to the heavy loads and high cylinder head temperature conditions that are often encountered.
At the opposite end of the model fuel controversy, some engine companies are fighting against the commercial fuel blenders’ “secret” ingredients and percentages. Here’s a statement by NovaRossi, from the instruction manual of Serpent Engines: “Only use fuels which contain pure fuel elements like nitromethane, methanol and castor oil. We do not recommend using synthetic oils or any other fuel additives. Do not use after-run products. If you use high quality fuel then this is not necessary.”
This recommendation comes from a company that has won multiple European and World Championships with 2-stroke-powered RC model cars.
ENGINE CATEGORIES & LUBRICATION REQUIREMENTS
Ringed and ringless pistons represent the two broad categories of glowignition engines.
Ringed 2-stroke engines require lower castor oil percentages.
Ringless ABC (aluminum piston, brass/chromed cylinder), ABN (aluminum piston, brass/nickel cylinder), and AAC (aluminum piston, aluminum/chromed cylinder) engines need a bit more castor oil. Ringed pistons run best on higher quantities of synthetic oil, limiting varnish build-up. Although castor oil provides superior protection, it will varnish an engine when used in higher quantities. Varnish is not a problem until it begins to interfere with the ring’s ability to seal against the piston’s ringland and cylinder wall. Synthetic oils will not varnish, but they tend to flash off during the combustion process, limiting the lubricant’s protection. The best traditional strategy to maximize the qualities of both lubricant types in ringed engines is the following mix: 16% synthetic, 2% castor oil (18% total). Ringless pistons require higher percentages of castor oil than ringed pistons. These engines are designed with an interference fit (zero clearance) between the piston and cylinder near TDC (top-dead-center), requiring additional scuff protection. Of course, higher castor oil percentages varnish the piston/cylinder more rapidly, requiring more frequent cleaning. A good traditional combination of lubricants for ringless engines is: 14% synthetic, 4% castor oil (18% total).
BUSHING-SUPPORTED CRANKSHAFTS = HIGHER OIL PERCENTAGES
Ringed and ringless piston engines that use bushings (plain bearings) for crankshaft support require a higher castor oil percentage than engines utilizing ball bearings. Practical experience, over a long period of time, has shown that about 4% additional castor oil is correct for the traditional blends in question (e.g., ringed engine: 16% synthetic, 6% castor, 22% total oil; ringless engine: 14% synthetic, 8% castor oil, 22% total). Nostalgia glow-ignition engine designs (1948-1970) that use plain bearings forcrankshaft support, and a ringed or ringless iron/ steel piston/cylinder require additional castor oil lubricant. Duke Fox specified 28% oil content (all castor) for his famous Fox .35 Stunt engine. In continuous production for 60 years, it has a ringless iron piston, steel cylinder and a bronze bushing for crankshaft support.
TRADITIONAL FUEL BLENDS: RINGED AND RINGLESS PISTONS
The following charts show recommended traditional fuels for both ringed and ringless piston engines fitted with ball bearings for crankshaft support. Although the fuel blends shown are formulated to work over a wide range of engine displacements (from approximately
.19 to 2.20ci), the total lubricating oil content is probably best suited to a .40ci engine (18%). The range of nitromethane percentages is provided to offer flexibility in performance, depending if the engine is designed for sport or racing type applications, or something in-between. Typically, the 5-, 10- or 15%-nitro content fuel would be used for sport flying.
REDUCED OIL CONTENT
I began experimenting with home-brew fuel and reduced oil content in the late ‘60s. The findings were applied to our RC pylon racing program, where there were no restrictions on fuel. Eventually, a summary of this work was published in the May 1974 edition of Model Airplane News (“Two-Stroke Oils: Their Analysis”). Briefly, I found that a racing 0.40ci engine would produce its best bhp (brake horsepower) with 14% oil content, using a blend of synthetics and castor oil; previously, conventional wisdom dictated that the safe minimum was 18%. By reducing the lubrication content by 4%, the fuel becomes less viscous (thinner), often allowing the engine to realize a modest power boost. This is due to:
Decreased pumping and bearingdrag losses.
Improved fuel and oxygen molecule contact within the engine’s inducted air.
Reduced energy loss (heating the excess oil) out of the exhaust.
When reduced oil content was tested in our RC pattern fuel, we found that the .60ci engines were better behaved; they idled steadily at a lower rpm, and throttled-up crisply without stumbling. Thirty years ago, a .60ci displacement 2-stroke glow engine was considered large. Over the decades, power requirements for giant-scale and pattern models enticed engine manufacturers to develop larger glow units, including: 1.2, 1.5, 1.8, 2.0, and 2.2ci 2-stroke single-cylinder designs.
FUEL REQUIREMENTS FOR LARGER ENGINES
As an engine’s size (displacement) increases:
It requires less lubricating oil percentage.
It demands less nitromethane percentage.
If you’re a traditional modeler who believes that high oil percentages are always needed throughout the engine displacement spectrum, take time to absorb the following two concepts.
LARGER ENGINES REQUIRE LESS LUBRICATING OIL PERCENTAGE
The following quote was excerpted from a paid advertisement (Duke’s Mixture) from the late engine manufacturer,
Duke Fox, (Fox Manufacturing Company) in the August 1989 issue of
Model Airplane News magazine: “… Larger motors need less oil, percentage-wise, than small ones.
The reason being that as the size of the motor increases, the displacement goes up as the cube, while the area to be lubricated goes up as the square. Thus a motor with a 1.5-inch bore would be as well lubricated on a 10% oil mix, as one with a 0.75-inch bore would be with a 20% oil mix.” This is known as the lubricating area to displacement ratio.
When doubling the engine’s bore from 0.75-inch (.33ci, with a stroke of 0.75 inch) to 1.5-inch (2.65ci, with a stroke of 1.5 inches), displacement increases as the cube of the bore increase (0.75 in. x 2 = 1.5 in.); therefore 23 (2 x 2 x 2) = 8 times. Assuming similar design features, an engine that is 8-times larger than another (ci), will consume fuel about 8 times faster than the smaller engine. Conventional thinking suggests that 8 times the lubrication will also be needed for the larger engine. However, the large bore engine (1.5 inches) has only 4 times the lubricating area of the small bore engine (0.75 inch), since cylinder area increases as the square of the bore increase, or 22 (2 x 2) = 4 times. Consequently, the larger engine receives twice the lubrication of the smaller engine (8 ÷ 4 = 2). By reducing the larger engine’s lubrication content by half (from 20 to 10%), it will lubricate the same as the small engine. (Bore1 ÷ Bore 2 x Bore 1 % = Bore2 %), (0.75 ÷
1.5 x 20 = 0.5 x 20 = 10%). Based upon traditional lubrication content, here are a few engine displacements (bore = stroke) with their calculated lubrication percentages:
LARGER ENGINES DEMAND A LOWER NITROMETHANE PERCENTAGE
In 1948, three American engine manufacturers released their versions of the revolutionary 1/2A glow engine,
but the so called “baby engines” would soon cause problems for unsuspecting modelers. Initially, they were expected to run on fuel that was formulated for larger displacement glow ignition engines that contained mostly methanol. The tiny engines protested by being difficult to start and touchy to adjust; they vibrated, misfired and often quit cold. As it turned out “cold” was the operative word for understanding their balky operation. Small engines have a much higher *cooling area to displacement ratio when compared to larger engines; therefore they overcool, disrupting the normal combustion process. Adding 25- to 35% nitromethane solves the problem, since it provides additional heat to the tiny engine’s operating cycle – it also adds power. *Cooling area includes both the cylinder and the cylinder head.
The cold-running 1/2A experience helps to explain why engine designers enlarge the cooling fin area (head and cylinder) as displacement increases. Even with enhanced fins, acceptable head temperatures are often difficult to maintain, illustrating why big engines demand lower percentages of nitromethane. Elevated cylinder head temperatures often lead to potentially destructive combustion problems such as pre-ignition and detonation.
From the figures below, various ratios of cooling area (cylinder + head) to engine displacement are compared, ranging from the largest to the smallest engine; notice that the baby engine (0.049) has almost four times the cooling area per unit of displacement than the 2.65 ci engine (12.8 ÷ 3.3 = 3.88). Also note the approximate nitromethane percentages suggested for the given displacements; these are difficult to predict accurately because the engine’s design plays a significant role in its ability to cool:
NON-TRADITIONAL SPORT FUEL BLENDS
Ringed pistons, ball bearing supported crankshafts. As we have seen, larger engines require less lubrication and nitromethane content to attain their operational sweet spot. What can be expected? A lower, steadier idle, a quicker, crisper throttle-up, and a more powerful wide open-throttle performance, while enjoying the same level of engine component protection. The following fuel blends for various engine displacements are offered for your consideration: Note: the ratio of synthetic to castor oil (8/1) is maintained from the traditional blend for ringed, ball bearing engines.
The synthetic lubricant used for the all of these fuel blends is poly-alkylene glycol, the relatively inexpensive UCON oil. There are a multitude of other synthetics that are available including polypropylene glycol, polyesters, and polyol esters, but they are much more expensive. Fortunately, as confirmed by several lubricant experts, when castor oil is mixed with almost any synthetic, a superior lubricant is produced.
CASTOR OIL HELPS TO COOL A HOTRUNNING ENGINE
Another consideration for nontraditional fuels that use reduced lubricant percentages: Castor oil helps to cool any size engine, but it’s especially effective with larger displacement engines where the ratio of cooling area to cylinder displacement is limiting heat rejection. Castor oil has been proven to carry away more heat through the engine’s exhaust than any common synthetic. The reason? Castor oil doesn’t burn in the combustion chamber until extremely high temperatures are reached; most synthetics flash from hot internal surfaces, such as cylinder heads and upper cylinders; often, many synthetics simply burn, adding to the engine’s heat load. Several options are available to the engine tuner to alleviate high cylinder head temperatures:
Reduce the fuel’s nitromethane content.
Reduce the engine’s compression ratio (add a head shim).
Reduce the engine’s propeller load. Increase the fuel’s castor oil content. The first two suggestions will probably reduce the engine’s performance and should be used as a last resort. Reducing propeller pitch and/or diameter should probably be tried first. However, if overheating is still a problem, add a bit more castor oil to the existing fuel blend. How much? Start with 0.05% extra, and increase from there.