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Amodel engine is a smallinternal combustion engine[1] typically used to power aradio-controlled aircraft,radio-controlled car,radio-controlled boat,free flight,control line aircraft, or ground-runningtether car model.
Because of thesquare–cube law, the behaviour of many engines does not always scale up or down at the same rate as the machine's size; usually at best causing a dramatic loss of power or efficiency, and at worst causing them not to work at all.Methanol andnitromethane are common fuels.
The fully functional, albeit small, engines vary from the most common single-cylindertwo-stroke to the exotic single and multiple-cylinderfour-stroke, the latter taking shape inboxer,v-twin,inline andradial form, a fewWankel engine designs are also used. Most model engines run on a blend ofmethanol,nitromethane, and lubricant (eithercastor orsynthetic oil).
Two-stroke model engines, most often designed since 1970 withSchnuerle porting for best performance, range in typical size from .12 cubic inches (2 cubic centimeters) to 1.2 ci (19.6 cc) and generate between .5horsepower (370watts) 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).[2]Four-stroke model engines have been made in sizes as small as 0.20 in3 (3.3 cc) for the smallestsingle-cylinder models, all the way up to 3.05 in3 (50 cc) for the largest size for single-cylinder units, with twin- and multi-cylinder engines on the market being as small as 10 cc for opposed-cylinder twins, while going somewhat larger in size than 50 cc, and even upwards to well above 200 cc for some modelboxer opposed-piston,inline andradial engines. While the methanol and nitromethane blended "glow fuel" engines are the most common, many larger (especially above 15 cc/0.90 ci displacement) model engines, both two-stroke and a growing number of four-stroke examples are spark ignition, and are primarily fueled with gasoline — with some examples of both two and four-stroke glow plug-designed methanol aeromodeling engines capable, with aftermarket upgrades, to having battery-powered, electronically controlled spark ignition systems replacing the glow plugs normally used. Model engines refitted in such a manner often run more efficiently on methanol-based glow plug engine fuels, often with the ability to exclude the use of nitromethane altogether in their fuel formulas.
This article concerns itself with the methanol engines; gasoline-powered model engines are similar to those built for use instring trimmers,chainsaws, and other yard equipment, unless they happen to be purpose-built for aeromodeling use, being especially true for four-stroke gasoline-fueled model engines. Such engines usually use a fuel that contains a small percentage of motor oil as a two-stroke engine uses for lubrication purposes, as most model four-stroke engines — be they glow plug or spark ignition — have no built-in reservoir for motor oil in their crankcase or engine block design.
The majority of model engines have used, and continue to use, thetwo-stroke cycle principle to avoid needing valves in thecombustion chamber, but a growing number of model engines use thefour-stroke cycle design instead. Bothreed valve androtary valve-type two-strokes are common, with four-stroke model engines using either conventionalpoppet 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 theventuri 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). TheseCox 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.
Images of a glowplug engine and a "diesel" engine are shown below for comparison. The most obvious external difference is seen on top of the cylinder head. The glowplug engine's glow plug has a pinlike terminal for its center contact, which is an electrical connector for the glowplug. The "diesel" engine has a T-bar which is used for adjusting the compression. The cylindrical object behind the glowplug engine is an exhaust silencer ormuffler.
Glow plugs are used for starting as well as continuing the power cycle. The glow plug consists of a durable, mostlyplatinum, helically wound wirefilament, within a cylindrical pocket in the plug body, exposed to thecombustion chamber. A smalldirect currentvoltage (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/airmixture in a glow-plug model engine, which requiresmethanol for the glow plug to work in the first place, and sometimes with the use ofnitromethane for greater power output and steadier idle, occurs due to thecatalytic 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 aspark ignition engine or byfuel injection, as in an ordinarydiesel, it must be adjusted by the richness of the mixture, the ratio of nitromethane to methanol, thecompression ratio, the cooling of thecylinder 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 aneedle valve in the spraybar) to obtain maximum power. Glowplug engines are also known as nitro engines. Nitro engines require a 1.5 volt ignitor to light the glow plug in theheat sink. Once primed, pulling the starter with the ignitor in will start the engine.
Diesel engines are an alternative to methanol glow plug engines. These "diesels" run on a mixture ofkerosene,ether,castor oil orvegetable oil, andcetane oramyl nitrate booster. Despite their name, their use ofcompression ignition, and the use of a kerosene fuel that is similar todiesel, model diesels share very little with full-sizediesel engines.
Full-size diesel engines, such as those found in atruck, arefuel injected and eithertwo-stroke orfour-stroke. They use compression ignition to ignite the mixture: the compression within the cylinder heats the inlet charge sufficiently to cause ignition, without requiring an applied ignition source. A fundamental feature of such engines, unlike petrol (gasoline) engines, is that they draw in air alone and the fuel is only mixed by being injected into the combustion chamber separately. Model diesel engines are instead acarburetedtwo-stroke using thecrankcase for compression. Thecarburetor supplies amixture of fuel and air into the engine, with the proportions kept fairly constant and their total volume throttled to control the engine power.
Apart from sharing the diesel's use of compression ignition, their construction has more in common with a small two-stroke motorcycle or lawnmower engine. In addition to this, model diesels have variablecompression ratios. This variable compression is achieved by a "contra-piston", at the top of the cylinder, which can be adjusted by a screwed "T-bar". The swept volume of the engine remains the same, but as the volume of the combustion chamber attop dead centre is changed by adjusting the contra-piston, the compression ratio (swept volume + combustion chamber / combustion chamber) changes accordingly.
Model diesels are found to produce moretorque than glow engines of the samedisplacement, and are thought to get betterfuel efficiency, because the same power is produced at a lowerrpm, and in a smaller displacement engine. However, thespecific power may not be significantly superior to a glow engine, due to the heavier construction needed to assure that the engine can withstand the much highercompression ratio, sometimes reaching 30:1. Diesels also run significantly quieter, due to the more rapid combustion, unlike two-stroke glow engines, in which combustion may still be occurring when the exhaust ports are uncovered, causing a significant amount of noise.
Recent developments in model engineering have produced true diesel model engines, with a traditional injector and injector pump, and these engines operate in the same way as a large diesel engine.