A petcock is a small shut-off

valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fittings ...

used to control the flow of

liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

. Historically, petcocks were threaded valves controlled by a butterfly handle; modern petcocks are typically

ball valve A ball valve is a flow control device which uses a hollow, perforated and pivoting ball to control fluid flowing through it. It is open when the ball's hole is in line with the flow inlet and closed when it is pivoted 90-degrees by the valve ...

s. Compared to the progressive control of a

gate valve A gate valve, also known as a sluice valve, is a valve that opens by lifting a barrier (gate) out of the path of the fluid. Gate valves require very little space along the pipe axis and hardly restrict the flow of fluid when the gate is fully ope ...

a ball valve style petcock's is coarse, confining their use principally to on-off applications.

Use

Motorcycle

Most older

motorcycles A motorcycle (motorbike, bike, or trike (if three-wheeled)) is a two or three-wheeled motor vehicle steered by a handlebar. Motorcycle design varies greatly to suit a range of different purposes: long-distance travel, commuting, cruising, ...

have a three-position fuel petcock valve (known as a "petrol tap" in the UK) mounted on or nearby the fuel tank to control the supply of

gasoline 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 co ...

: on, off, and reserve. The reserve position accesses the bottom portion of the fuel tank. Many older motorcycles lacked a fuel gauge, making reliance on a fuel reserve a relatively common experience. Most now have an automatic, vacuum operated petcock (that may include a prime position which bypasses vacuum operation and allows fuel to flow to the

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 ...

without the engine turning over).

Automobile

The most common application of a petcock in an automobile is as a drain plug for a radiator. Historically, petcocks were used on gravity-feed fuel systems found on automobiles. With universal adoption of

fuel pump A fuel pump is a component in motor vehicles that transfers liquid from the fuel tank to the carburetor or fuel injector of the internal combustion engine. Carbureted engines often use low pressure mechanical pumps that are mounted outside the f ...

s they have all but disappeared in that application. Some historic examples of gravity feed systems include the

Porsche 356 The Porsche 356 is a sports car that was first produced by Austrian company Porsche Salzburg, Porsche Konstruktionen GesmbH (1948–1949), and then by German company Porsche, Dr. Ing. h. c. F. Porsche GmbH (1950–1965). It was Porsche's first ...

and early Volkswagens, which have a petcock with off, on, and reserve positions, much like a motorcycle petcock. Petcocks may be used in vehicle heating and cooling systems, such as limousines and tractor trailers.

Heavy equipment

Many

John Deere Deere & Company, doing business as John Deere (), is an American corporation that manufactures agricultural machinery, heavy equipment, forestry machinery, diesel engines, drivetrains (axles, transmissions, gearboxes) used in heavy equipment, ...

and other tractors and stationary power units produced from the late 1920s to the early 1940s had compression relief petcocks on the engine block, one per cylinder. These were opened during starting to de-pressurize the cylinders when spinning the flywheel by hand. Once the engine started they were closed and the engine would begin running smoothly. Petcocks are used to periodically drain water from diesel fuel water separators on some trucks.

Marine engine

The large MAN six-cylinder diesel engines used on German

U-boat U-boats were naval submarines operated by Germany, particularly in the First and Second World Wars. Although at times they were efficient fleet weapons against enemy naval warships, they were most effectively used in an economic warfare role ...

s had petcocks that enabled the engineers to verify combustion in each cylinder. Opening one with the engine running would result in a long blue-white flame if the cylinder was firing correctly. The procedure is described in the novel

Das Boot ''Das Boot'' (, English: "The Boat") is a 1981 West German war film written and directed by Wolfgang Petersen, produced by Günter Rohrbach, and starring Jürgen Prochnow, Herbert Grönemeyer, and Klaus Wennemann. It has been exhibited both as ...

and shown in the film version.

References

{{Reflist Motorcycle technology Engine fuel system technology