Model Engine - Overview

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, 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 cc) for the smallest single-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 model radial 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. 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, unless they happen to be purpose-built for aeromodeling use, being especially true for four-stroke gasoline-fueled model engines.

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.

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.