Small engine
Encyclopedia
A small engine is the general term for a wide range of internal combustion engine
s used to power lawn mower
s, generator
s, concrete mixer
s and many other machines that require independent power sources. Most small engines are single-cylinder, V-twin, or flat-twin units. Although much less common, there have been small Wankel
(rotary) engines manufactured for use on lawn mowers and other such equipment."Now You Can 'Mow' with a Wankel!" Popular Science, July 1973 p. 18
The engines, which may be of two- or four-stroke design, are small in both physical dimensions and power output, relative to larger automobile engines. Power output ranges from less than 1 to about 20 horsepower
. The smallest of all are used in handheld garden machinery, such as string trimmer
s and chainsaw
s, which may be as small as 25 cc piston displacement
. The most common are four-stroke air-cooled
single-cylinder engines equipped with a carburettor and running on petrol. Engines for small machinery that must be hand-carried, such as string trimmer
s and chainsaw
s, are usually of the two-stroke design, which is lighter for any given power output. However, two-stroke engines create a relatively large amount of air pollution
and noise pollution
, and so are beginning to be supplanted by four-stroke units.
Compared to modern vehicle engines, small engines are relatively simple in design. Capital cost
is usually more important than fuel economy
, running costs or longevity, thus encouraging simple designs.
In contrast to automotive engines which have long all been of overhead valve
configuration, many of the small four-stroke engines are still of the sidevalve design owing to its simplicity for both manufacture and basic maintenance. An increasing number of small engines have overhead valves, which allow greater efficiency for reduced fuel consumption and air pollution
. A few engines now use overhead cam valvegear, driven by a timing belt
.
Electrical systems for these engines are usually simple and minimal, mostly to avoid the need for a battery. Many uses require the engines to be ready for immediate use, even after long standby storage and the risk of a flat battery. Most small engines are equipped with capacitor discharge ignition
, which—like the magneto that preceded it—is an ignition system
that does not require a battery.
Electric starting is available for small engines and is found primarily on high-feature garden and snow-removal machinery, larger generators, and welders particularly where there is already a complex electrical system and there may be a need for auto-starting on demand. However, a self-retracting rope-pull mechanism called a recoil starter is the predominant method of starting small engines; it does not require a battery to power a starter motor, nor an alternator
to keep the battery charged. Before the advent of the recoil starter, a notched pulley was attached to the engine's flywheel
; the operator would manually wind a rope around the pulley then jerk the rope to rotate the engine so that it would start. Another starting method briefly popular in the 1960s was the "impulse" or "wind-up" starter. These were operated by winding a heavy spring by means of a rotating crank handle equipped with a sprag clutch, then releasing the spring's tension by means of a lever or knob so that it would spin the engine. These were discontinued when safety problems became apparent: it was possible to leave the starter wound up and ready to start the engine unintentionally, even long after the crank was wound up, if the release were jarred.
Fuel systems are usually simple, at least for petrol engines. The fuel tank may be placed above a float-type carburettor so that gravity is adequate to bring fuel to the carburettor, avoiding the need for a fuel pump
. Alternatively, the tank may be placed below a carburettor which uses engine vacuum or crankcase pressure pulsations to lift fuel into the carburetor. Float-type carburettors can only be used when the engine is reliably vertical and without excessive vibration.
Most small engines have a governor
to regulate and maintain the engine speed as constantly as possible despite changing load. Some engines also have an adjustable throttle
by which the operator or the machine itself can alter the engine speed. Generally, the nominal "throttle control" does not directly control the opening of the carburetor throttle, but rather increases or decreases spring tension
on the governor, which in turn regulates the engine speed higher or lower.
Small engines may be configured with the crankshaft
horizontal or vertical, according to the intended application. Vertical axis engines were developed for rotary lawnmowers, but the size of this large market has encouraged a supply of cheap engines and they are now used for other purposes too, such as generators.
s, where the engines were made by the same manufacturers.
Major manufacturers today
Past UK manufacturers
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
s used to power lawn mower
Lawn mower
A lawn mower is a machine that uses a revolving blade or blades to cut a lawn at an even length.Lawn mowers employing a blade that rotates about a vertical axis are known as rotary mowers, while those employing a blade assembly that rotates about a horizontal axis are known as cylinder or reel...
s, generator
Engine-generator
An engine-generator is the combination of an electrical generator and an engine mounted together to form a single piece of equipment. This combination is also called an engine-generator set or a gen-set...
s, concrete mixer
Concrete mixer
A concrete 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...
s and many other machines that require independent power sources. Most small engines are single-cylinder, V-twin, or flat-twin units. Although much less common, there have been small Wankel
Wankel engine
The Wankel engine is a type of internal combustion engine using an eccentric rotary design to convert pressure into a rotating motion instead of using reciprocating pistons. Its four-stroke cycle takes place in a space between the inside of an oval-like epitrochoid-shaped housing and a rotor that...
(rotary) engines manufactured for use on lawn mowers and other such equipment."Now You Can 'Mow' with a Wankel!" Popular Science, July 1973 p. 18
The engines, which may be of two- or four-stroke design, are small in both physical dimensions and power output, relative to larger automobile engines. Power output ranges from less than 1 to about 20 horsepower
Horsepower
Horsepower is the name of several units of measurement of power. The most common definitions equal between 735.5 and 750 watts.Horsepower was originally defined to compare the output of steam engines with the power of draft horses in continuous operation. The unit was widely adopted to measure the...
. The smallest of all are used in handheld garden machinery, such as string trimmer
String trimmer
A string trimmer, also known as a strimmer, line trimmer, weed whacker, weed whip, weed eater, weedy, or whipper snipper, is a powered handheld device that uses a flexible monofilament line instead of a blade for cutting grass and other plants near objects...
s and chainsaw
Chainsaw
A chainsaw is a portable mechanical saw, powered by electricity, compressed air, hydraulic power, or most commonly a two-stroke engine...
s, which may be as small as 25 cc piston displacement
Engine displacement
Engine displacement is the volume swept by all the pistons inside the cylinders of an internal combustion engine in a single movement from top dead centre to bottom dead centre . It is commonly specified in cubic centimeters , litres , or cubic inches...
. The most common are four-stroke air-cooled
Air-cooled engine
Air-cooled engines rely on the circulation of air directly over hot parts of the engine to cool them.-Introduction:Most modern internal combustion engines are cooled by a closed circuit carrying liquid coolant through channels in the engine block and cylinder head, where the coolant absorbs heat,...
single-cylinder engines equipped with a carburettor and running on petrol. Engines for small machinery that must be hand-carried, such as string trimmer
String trimmer
A string trimmer, also known as a strimmer, line trimmer, weed whacker, weed whip, weed eater, weedy, or whipper snipper, is a powered handheld device that uses a flexible monofilament line instead of a blade for cutting grass and other plants near objects...
s and chainsaw
Chainsaw
A chainsaw is a portable mechanical saw, powered by electricity, compressed air, hydraulic power, or most commonly a two-stroke engine...
s, are usually of the two-stroke design, which is lighter for any given power output. However, two-stroke engines create a relatively large amount of air pollution
Air pollution
Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or cause damage to the natural environment or built environment, into the atmosphere....
and noise pollution
Noise pollution
Noise pollution is excessive, displeasing human, animal or machine-created environmental noise that disrupts the activity or balance of human or animal life...
, and so are beginning to be supplanted by four-stroke units.
Compared to modern vehicle engines, small engines are relatively simple in design. Capital cost
Capital cost
Capital costs are costs incurred on the purchase of land, buildings, construction and equipment to be used in the production of goods or the rendering of services, in other words, the total cost needed to bring a project to a commercially operable status. However, capital costs are not limited to...
is usually more important than fuel economy
Fuel efficiency
Fuel efficiency is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is...
, running costs or longevity, thus encouraging simple designs.
In contrast to automotive engines which have long all been of overhead valve
Overhead valve
An overhead valve engine, also informally called pushrod engine or I-head engine, is a type of piston engine that places the camshaft within the cylinder block , and uses pushrods or rods to actuate rocker arms above the cylinder...
configuration, many of the small four-stroke engines are still of the sidevalve design owing to its simplicity for both manufacture and basic maintenance. An increasing number of small engines have overhead valves, which allow greater efficiency for reduced fuel consumption and air pollution
Air pollution
Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or cause damage to the natural environment or built environment, into the atmosphere....
. A few engines now use overhead cam valvegear, driven by a timing belt
Timing belt
A timing belt, or cam belt , is a part of an internal combustion engine that controls the timing of the engine's valves. Some engines, such as the flat-4 Volkswagen air-cooled engine, and the straight-6 Toyota F engine use timing gears...
.
Electrical systems for these engines are usually simple and minimal, mostly to avoid the need for a battery. Many uses require the engines to be ready for immediate use, even after long standby storage and the risk of a flat battery. Most small engines are equipped with capacitor discharge ignition
Capacitor discharge ignition
Capacitor discharge ignition or thyristor ignition is a type of automotive electronic ignition system which is widely used in outboard motors, motorcycles, lawn mowers, chainsaws, small engines, turbine-powered aircraft, and some cars...
, which—like the magneto that preceded it—is an ignition system
Ignition system
An ignition system is a system for igniting a fuel-air mixture. Ignition systems are well known in the field of internal combustion engines such as those used in petrol engines used to power the majority of motor vehicles, but they are also used in many other applications such as in oil-fired and...
that does not require a battery.
Electric starting is available for small engines and is found primarily on high-feature garden and snow-removal machinery, larger generators, and welders particularly where there is already a complex electrical system and there may be a need for auto-starting on demand. However, a self-retracting rope-pull mechanism called a recoil starter is the predominant method of starting small engines; it does not require a battery to power a starter motor, nor an alternator
Alternator
An alternator is an electromechanical device that converts mechanical energy to electrical energy in the form of alternating current.Most alternators use a rotating magnetic field but linear alternators are occasionally used...
to keep the battery charged. Before the advent of the recoil starter, a notched pulley was attached to the engine's flywheel
Flywheel
A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia, and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed...
; the operator would manually wind a rope around the pulley then jerk the rope to rotate the engine so that it would start. Another starting method briefly popular in the 1960s was the "impulse" or "wind-up" starter. These were operated by winding a heavy spring by means of a rotating crank handle equipped with a sprag clutch, then releasing the spring's tension by means of a lever or knob so that it would spin the engine. These were discontinued when safety problems became apparent: it was possible to leave the starter wound up and ready to start the engine unintentionally, even long after the crank was wound up, if the release were jarred.
Fuel systems are usually simple, at least for petrol engines. The fuel tank may be placed above a float-type carburettor so that gravity is adequate to bring fuel to the carburettor, avoiding the need for a fuel pump
Fuel pump
A fuel pump is a frequently essential component on a car or other internal combustion engined device. Many engines do not require any fuel pump at all, requiring only gravity to feed fuel from the fuel tank through a line or hose to the engine...
. Alternatively, the tank may be placed below a carburettor which uses engine vacuum or crankcase pressure pulsations to lift fuel into the carburetor. Float-type carburettors can only be used when the engine is reliably vertical and without excessive vibration.
Most small engines have a governor
Governor (device)
A governor, or speed limiter, is a device used to measure and regulate the speed of a machine, such as an engine. A classic example is the centrifugal governor, also known as the Watt or fly-ball governor, which uses a rotating assembly of weights mounted on arms to determine how fast the engine...
to regulate and maintain the engine speed as constantly as possible despite changing load. Some engines also have an adjustable throttle
Throttle
A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...
by which the operator or the machine itself can alter the engine speed. Generally, the nominal "throttle control" does not directly control the opening of the carburetor throttle, but rather increases or decreases spring tension
Spring (device)
A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...
on the governor, which in turn regulates the engine speed higher or lower.
Small engines may be configured with the crankshaft
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
horizontal or vertical, according to the intended application. Vertical axis engines were developed for rotary lawnmowers, but the size of this large market has encouraged a supply of cheap engines and they are now used for other purposes too, such as generators.
History
Small engines date back to the early days of internal combustion engines. The first of a recognisably modern form date from the advent of motorcycleMotorcycle
A motorcycle is a single-track, two-wheeled motor vehicle. Motorcycles vary considerably depending on the task for which they are designed, such as long distance travel, navigating congested urban traffic, cruising, sport and racing, or off-road conditions.Motorcycles are one of the most...
s, where the engines were made by the same manufacturers.
Major manufacturers today
- Briggs & StrattonBriggs & StrattonBriggs & Stratton is the world's largest manufacturer of air-cooled gasoline engines primarily for outdoor power equipment. Current production averages 11 million engines per year.-History:...
- Honda
- KohlerKohler Company'The Kohler Company is a manufacturing company in Kohler, Wisconsin best known for its plumbing products. Kohler also manufactures furniture, cabinetry, tile, engines, and generators.-History:...
- RyobiRyobiRyobi Limited is a manufacturer of components for the automobile, electronics, and telecommunications industries. They also create printing equipment, power tools and builders' hardware....
- Tecumseh
Past UK manufacturers
- J.A.P.
- VilliersVilliers EngineeringVilliers Engineering was a manufacturer of motorcycles and cycle parts, and an engineering company based in Villiers Street, Wolverhampton, England....
- Clinton