A model engine is a small-scale motor, often designed to power a model vehicle such as a model automobile, locomotive, boat or aircraft. There are several types of model engines. Some provide power to models – that is, small-scale replicas of full-sized motorized vehicles. Another group of model engines, built for display, are actual replicas of the larger engines that power real vehicles, but do not themselves power anything. Some of these, especially model steam engines, are fully operational, while others, especially replicas of gas engines built from kits, are not.
Model engines in general have become so popular that they've become a popular hobby on their own, with websites, newsletters and hobbyists who attend regular meetings. Some model engine enthusiasts specialize in the internal combustion engines that power model vehicles, while others prefer the operational display replicas of steam or internal combustion engines.
The model engines that power model cars, trains, boats and aircraft generally are not scaled-down versions of the large engines that power the full-sized vehicles represented by the models &emdash; that is, the model engine that powers a model automobile is not a miniature V-8 internal combustion gasoline, and the power produced by model engines is not proportional to that which powers the full-sized version of the vehicle. Some model vehicles are powered by electric engines, and the model will have a compartment built in for a battery. Others are powered by engines that rely on a number of different fuels, and the vehicle they power must carry the engine, the fuel supply, and a source of electricity to ignite the fuel. These are most often two-stroke, one-cylinder internal combustion engines built especially to power a model vehicle.
Another type of model engine, generally steam-powered, doesn't energize anything, but is actually a fully operational small-scale engine. Often this will be a replica of a larger engine, often with historic significance; sometimes it will be an original creation. Some of these engines can be relatively large &emdash; as much as 24 inches (61 cm) long by 18 inches (46 cm) tall, and operate with steam provided by a boiler.
A separate category of model engine is the special device used to power a model rocket. Some model rockets' thrust is provided by pressurized water or the chemical reaction resulting from mixing two substances. By far, the more popular option in powering model rockets, though, is the electrically-fired engine whose thrust is produced when a propellant, either black powder or a more stable mixture of dry chemicals, is ignited, very similar to the larger rockets they emulate.