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. Because of the square-cube law, the behaviour of many machines does not always scale up or down at the same rate as the machine's size (and often not even in a linear way), usually at best causing a dramatic loss of power or efficiency, and at worst causing them not to work at all. Methanol and nitromethane are proven solutions to enhance the power of an automobile engine (Top Fuel) and they can even get these small engines running.
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, most often designed since 1970 with Schnuerle 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 .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, and even upwards to well above 200 cm3 for some model radial engines. While the methanol and nitromethane blended "glow fuel" engines are the most common, 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. 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 have used, 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.
