Model engine
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For the band (previously known as Black Eyed Sceva), see Model Engine.
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In radio-controlled modeling, a model engine is an internal combustion engine used to power a radio-controlled aircraft, radio-controlled car, radio-controlled boat, Free flight and Control line aircraft, and tether car models also use these engines.
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[edit] Overview
The fully-functional, albeit small, engines vary from the most common single cylinder two-stroke to the exotic single and multiple cylinder four-stroke, the latter taking shape in boxer, v-twin, inline and radial form, a few wankel engine designs are also used. Most model engines run on a blend of methanol, nitromethane, and lubricant (either castor or synthetic oil). Two stroke model engines range in typical size from .12 cubic inches (2 cubic centimeters) to .5 ci (8 cc) and generate between .5 horsepower (370 watts) to 5 hp (3.7 kW), can get as small as .010 ci (.16 cc) and as large as 3-4 ci (49–66 cc). Four stroke model engines have been made in sizes as small as 0.20 in3 (3.3 cm3) for the smallest single cylinder models, all the way up to 3.05 in3 (50 cm3) for the largest size for single cylinder units, with twin and multi-cylinder engines on the market being as small as 10 cm3 for opposed-cylinder twins, while going somewhat larger in size than 50 cm3 for some model radial engines. While the methanol and nitromethane blended "glow fuel" engines are the most common, it should be noted that many larger (especially above 15 cm3/0.90 cu.in. displacement) model engines, both two stroke and a few four stroke examples, are spark ignition, and are fueled with gasoline. This article concerns itself with the methanol engines; gasoline powered model engines are similar to those built for use in string trimmers, chainsaws, and other yard equipment.
The majority of model engines operate have, and continue to use, the two-stroke cycle principle to avoid needing valves in the combustion chamber, but a growing number of model engines use the four-stroke cycle design instead. Both reed valve and rotary valve type two strokes are common, with four stroke model engines using either conventional poppet valve, and rotary valve formats for induction and exhaust.
The engine shown to the right has its carburetor in the center of the zinc alloy casting to the left. (It uses a flow restriction, like the choke on an old car engine, because the venturi effect is not effective on such a small scale.) The valve reed, cross shaped above its retainer spring, is still beryllium copper alloy, in this old engine. The glow plug is built into the cylinder head. Large production volume makes it possible to use a machined cylinder and an extruded crank case (cut away by hand in the example shown). These Cox Bee reed valve engines are notable for their low cost and ability to survive crashes. The components of the engine shown come from several different engines.
[edit] Glowplug engines
Glow plugs are used for starting as well as continuing the power cycle. The glow plug consists of a durable, mostly platinum, helically wound wire filament, within a cylindrical pocket in the plug body, exposed to the combustion chamber. A small direct current voltage (around 1.5 volts) is applied to the glow plug, the engine is then started, and the voltage is removed. The burning of the fuel/air mixture in a glow-plug model engine, which requires methanol for the glow plug to work in the first place, and sometimes with the use of nitromethane for greater power output and steadier idle, occurs due to the catalytic reaction of the methanol vapor to the presence of the platinum in the filament, thus causing the ignition. This keeps the plug's filament glowing hot, and allows it to ignite the next charge. Since the ignition timing is not controlled electrically, as in a spark ignition engine or by fuel injection, as in an ordinary diesel, it must be adjusted by the richness of the mixture, the ratio of nitromethane to methanol, the compression ratio, the cooling of the cylinder head, the type of glow plug, etc. A richer mixture will tend to cool the filament and so retard ignition, slowing the engine, and a rich mixture also eases starting. After starting the engine can easily be leaned (by adjusting a needle valve in the spraybar) to obtain maximum power.
[edit] Diesel engines
Diesel engines are an alternative to methanol glow plug engines. These "Diesels" run on a mixture of Kerosene, Ether, castor oil or Vegetable Oil, and Amsoil Cetane booster. Despite their name, method of combustion, and the use of Kerosene, which is chemically identical to Diesel, model Diesels share very little with full size Diesel Engines.
Whilst a full size Diesel Engine, which may be found in a heavy duty vehicle such as a Truck, is Fuel Injected, and typically Four stroke, (with the exception of the Detroit Diesel and some Foden Diesels), a model Diesel engine is typically Carbureted, and is also usually Two stroke. In addition to this, model Diesels typically have continuously variable compression ratios, and are throttled by adding both more air and fuel, as opposed to only fuel, as in a fuel injected diesel. The variable compression is achieved by a "contra-piston", at the top of the cylinder, which can be adjusted by a screwed "T-bar".
Model Diesels are found to produce more torque than glow engines of the same displacement, and are thought to get better fuel efficiency, because the same power is produced at a lower RPM, and in a smaller displacement engine. However, the specific power may not be significantly superior to a glow engine, due to overbuilding, to assure that the engine can withstand the much higher compression, sometimes reaching ratios of as high as 30:1. Diesels also run significantly quieter, due to the more rapid combustion, unlike glow engines, in which combustion may still be occurring when the exhaust ports are uncovered, causing a significant amount of noise.
