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A small engine is the general term for a wide range of small-displacement, low-powered
internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal co ...
s used to power lawn mowers, generators,
concrete mixer A concrete mixer (often colloquially called a cement mixer) is a device that homogeneously combines cement, aggregate such as sand or gravel, and water to form concrete. A typical concrete mixer uses a revolving drum to mix the components. F ...
s and many other machines that require independent power sources. These engines often have simple designs, for example an air-cooled single-cylinder petrol engine with a pull-cord starter, capacitor discharge ignition and a gravity-fed carburettor. Engines of similar design and displacement are also used in smaller vehicles such as
motorcycle A motorcycle (motorbike, bike, or trike (if three-wheeled)) is a two or three-wheeled motor vehicle Steering, steered by a Motorcycle handlebar, handlebar. Motorcycle design varies greatly to suit a range of different purposes: Long-distance ...
s, motor scooters,
all-terrain vehicle An all-terrain vehicle (ATV), also known as a light utility vehicle (LUV), a quad bike, or simply a quad, as defined by the American National Standards Institute (ANSI); is a vehicle that travels on low-pressure tires, with a seat that is stra ...
s, and go-karts.


Characteristics

The engines are small in both physical dimensions and power output, relative to larger
automobile engine , there were a wide variety of propulsion systems available or potentially available for automobiles and other vehicles. Options included internal combustion engines fueled by petrol, diesel, propane, or natural gas; hybrid vehicles, plug-in ...
s. Power outputs are typically less than . The smallest of all are used in handheld garden machinery, such as string trimmers and
chainsaw A chainsaw (or chain saw) is a portable gasoline-, electric-, or battery-powered saw that cuts with a set of teeth attached to a rotating chain driven along a guide bar. It is used in activities such as tree felling, limbing, bucking, pru ...
s, which have a displacement as small as . Production cost is often a key consideration for small engines, resulting in relatively simple designs (compared with automotive engines, for example). The most common configuration is a single-cylinder engines that is
air-cooled Air-cooled engines rely on the circulation of air directly over heat dissipation fins or hot areas of the engine to cool them in order to keep the engine within operating temperatures. In all combustion engines, a great percentage of the heat ge ...
. The combustion cycle can be either
two-stroke A two-stroke (or two-stroke cycle) engine is a type of internal combustion engine that completes a power cycle with two strokes (up and down movements) of the piston during one power cycle, this power cycle being completed in one revolution of ...
(which results in a lighter engine for a given power output) or
four-stroke A four-stroke (also four-cycle) engine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direct ...
(which produce lower levels of exhaust gas emissions). The fuel is usually either
petrol Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic c ...
or
diesel Diesel may refer to: * Diesel engine, an internal combustion engine where ignition is caused by compression * Diesel fuel, a liquid fuel used in diesel engines * Diesel locomotive, a railway locomotive in which the prime mover is a diesel engi ...
. In 1973, a small Wankel (rotary) engine manufactured by NSU was used in a lawn mower.


Design


Electrical system

When a manual starter system is used (such as a recoil starter with a pull-cord), only a basic electrical system is required, since the system's only purpose was to power the spark plug. Older engines used a magneto to achieve this, while newer engines often use a capacitor discharge ignition (CDI) system with an ignition coil. These systems do not require a battery or charging system. Before the invention of the recoil starter, a notched pulley was attached to the engine's
flywheel A flywheel is a mechanical device which uses the conservation of angular momentum to store rotational energy; a form of kinetic energy proportional to the product of its moment of inertia and the square of its rotational speed. In particular, as ...
; the operator would manually wind a rope around the pulley then jerk the rope to rotate the engine so that it would start. Following the introduction of the sprag clutch in the 1960s, the "impulse" or "wind-up" starter was popular for a brief period. These used a heavy spring which the operator placed under tension using a rotating crank handle. However, these systems were potentially dangerous as it was possible to leave the starter wound up and ready to start the engine unintentionally, even long after the crank was wound up. Electric starting, which has become more common over time, requires a more complex electrical system. These engines also require a starter motor, a battery to power it and an alternator to keep a battery charged.


Fuel system

Petrol engines often use simple fuel systems consisting of a float-type
carburetor A carburetor (also spelled carburettor) is a device used by an 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 tube in the main meteri ...
with a fuel tank located above it (so that the fuel is delivered by gravity, avoiding the need for a fuel pump). Sometimes, the fuel tank is located below the carburettor and fuel is delivered using engine vacuum or crankcase pressure pulsations. Diesel engines use
fuel injection Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of an injector. This article focuses on fuel injection in reciprocating piston and Wankel rotary engines. All com ...
.


Governor

Most small engines use a
governor A governor is an administrative leader and head of a polity or political region, ranking under the head of state and in some cases, such as governors-general, as the head of state's official representative. Depending on the type of political ...
to maintain a constant engine speed under varying loads. Some engines also have a mechanism for the user to adjust the engine speed. Rather than directly controlling the opening of the carburetor throttle, this is usually achieved by adjusting the governor, which in turn regulates the engine speed higher or lower.


Valvetrain

The most common design is an
overhead valve An overhead valve (OHV) engine, sometimes called a ''pushrod engine'', is a piston engine whose valves are located in the cylinder head above the combustion chamber. This contrasts with earlier flathead engines, where the valves were located be ...
configuration, as used by the Honda GX range since the 1980s for example. A sidevalve configuration is used instead by some engines, owing to its simplicity for both manufacture and basic maintenance. Some engines in recent years, for example the Honda GC series, now use an overhead cam configuration.


Crankshaft

Small engines can have the
crankshaft A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecti ...
oriented either horizontally or vertically, according to the intended application. Vertical axis engines were originally developed for rotary lawnmowers, but the size of this large market has encouraged a supply of cheap engines and they are now also used for other purposes such as generators.


Manufacturers

The four largest manufacturers of small engines for power equipment in 2019 were
Briggs & Stratton Briggs & Stratton Corporation is an American manufacturer of gasoline engines with headquarters in Wauwatosa, Wisconsin. Engine production averages 10 million units per year as of April 2015. The company reports that it has 13 large facili ...
,
Honda is a Japanese public multinational conglomerate manufacturer of automobiles, motorcycles, and power equipment, headquartered in Minato, Tokyo, Japan. Honda has been the world's largest motorcycle manufacturer since 1959, reaching a producti ...
, Kawasaki and Kohler. Other major players include: Kubota, Yamaha and Liquid Combustion Technology.


Repairs/maintenance

The repairing of small engines in a vocational occupation. While often touted as a separate job by commercial education institutions, it may also be a skill someone uses as part of a larger job, such as an automotive or agricultural mechanic. Various safety precautions need to be taken when working on small engines.


See also

* Motorcycle engine * Non-road engine
Article on Small SI Engines.


References

{{Reflist Internal combustion engine