Small engine in the context of "Mini bike"

A portable engine is an engine, either a steam engine or an internal combustion engine, that sits in one place while operating (providing power to machinery), but (unlike a stationary engine) is portable and thus can be easily moved from one work site to another. Mounted on wheels or skids, it is either towed to the work site or moves there via self-propulsion.

Portable engines were in common use in industrialised countries from the early 19th through early 20th centuries, during an era when mechanical power transmission was widespread. Before that, most power generation and transmission were by animal, water, wind, or human; after that, a combination of electrification (including rural electrification) and modern vehicles and equipment (such as tractors, trucks, cars, engine-generators, and machines with their engines built in) displaced most use of portable engines. In developing countries today, portable engines still have some use (typically in the form of modern small engines mounted on boards), although the technologies mentioned above increasingly limit their demand there as well. In industrialised countries they are no longer used for commercial purposes, but preserved examples can often be seen at steam fairs driving appropriate equipment for demonstration purposes.

A carburetor (also spelled carburettor or carburetter) is a device used by a gasoline internal combustion engine to control and mix air and fuel entering the engine. The primary method of adding fuel to the intake air is through the Venturi effect or Bernoulli's principle or with a Pitot tube in the main metering circuit, though various other components are also used to provide extra fuel or air in specific circumstances.

Since the 1990s, carburetors have been largely replaced by fuel injection for cars and trucks, but carburetors are still used by some small engines (e.g. lawnmowers, generators, and concrete mixers) and motorcycles. In addition, they are still widely used on piston-engine–driven aircraft. Diesel engines have always used fuel injection instead of carburetors, as the compression-based combustion of diesel requires the greater precision and pressure of fuel injection.