A plenum chamber is a pressurised housing containing a

fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear ...

(typically air) at

positive pressure Positive pressure is a pressure within a system that is greater than the environment that surrounds that system. Consequently, if there is any leak from the positively pressured system it will egress into the surrounding environment. This is in ...

. One of its functions is to equalise pressure for more even distribution, compensating for irregular supply or demand. It is typically relatively large in volume and thus has relatively low velocity compared to the system's other components. In wind tunnels, rockets, and many flow applications, it is a chamber upstream on the fluid flow where the fluid initially resides (approximately at rest). It can also work as an acoustic silencer. Examples of plenum chambers include those used with: *

Superchargers In an internal combustion engine, a supercharger compresses the intake gas, forcing more air into the engine in order to produce more power for a given displacement. The current categorisation is that a supercharger is a form of forced ind ...

Hovercraft A hovercraft, also known as an air-cushion vehicle or ACV, is an amphibious Craft (vehicle), craft capable of travelling over land, water, mud, ice, and other surfaces. Hovercraft use blowers to produce a large volume of air below the hull ...

Corliss steam engine A Corliss steam engine (or Corliss engine) is a steam engine, fitted with rotary valves and with variable valve timing patented in 1849, invented by and named after the American engineer George Henry Corliss of Providence, Rhode Island. Engines ...

s * Raised floors and false ceilings in equipment rooms * Some organs (to supplement the

bellows A bellows or pair of bellows is a device constructed to furnish a strong blast of air. The simplest type consists of a flexible bag comprising a pair of rigid boards with handles joined by flexible leather sides enclosing an approximately airtigh ...

) * A number of

aerophone An aerophone () is a musical instrument that produces sound primarily by causing a body of air to vibrate, without the use of strings or membranes (which are respectively chordophones and membranophones), and without the vibration of the instru ...

s, such as the bag of

bagpipes Bagpipes are a woodwind instrument using enclosed reeds fed from a constant reservoir of air in the form of a bag. The Great Highland bagpipes are well known, but people have played bagpipes for centuries throughout large parts of Europe, No ...

and the ''slow air chamber'' of the

Native American flute The Native American flute is a flute that is held in front of the player, has open finger holes, and has two chambers: one for collecting the breath of the player and a second chamber which creates sound. The player breathes into one end of the ...

* Plenum chamber anesthetic vaporizers * Rocket motor combustion chambers, which may have a section near the nozzle that is free of the propellant charge to improve mixing and combustion * Nuclear waste burning reactor rods

Etymology

The term "plenum" was coined in the 1670s, derived from Latin adjective ''plenus'' ("filled, full"). The usage originates from classical theories of physics and the notion that "

nature abhors a vacuum In physics, horror vacui, or plenism (), commonly stated as "nature abhors a vacuum", is a postulate attributed to Aristotle, who articulated a belief, later criticized by the atomism of Epicurus and Lucretius, that nature contains no vacuums be ...

". These gave rise to the notion of 17th century 'plenum' as the opposite of

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...

, and all things "being either Plenum or Vacuum". plenum By the 19th century, the development of mechanical fans and industrial machinery had provided another, more technical use. This referred to "a system of artificial ventilation", which used a pressure raised slightly above atmospheric pressure, in contrast to the "vacuum system" which used a pressure below atmospheric. At a time when high pressure steam or

hydraulic Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counter ...

systems were well established, these were a distinct set of systems based on low pressure and high volume flows.

Supercharging

Supercharged In an internal combustion engine, a supercharger compresses the intake gas, forcing more air into the engine in order to produce more power for a given displacement. The current categorisation is that a supercharger is a form of forced induct ...

piston engines typically use many cylinders arranged in-line and one or two superchargers. Superchargers deliver air at a relatively constant rate, while cylinders demand it in a varying manner, as the

valves 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 fitting ...

open and as piston speed varies through the

stroke A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop functionin ...

. Simple direct ducting would give problems where the nearest cylinders received more airflow. The pulsating demand from the cylinders would also show problems of either pressure waves in the duct, or a shortage of inlet air towards the end of the inlet phase. The solution is to provide a large-volume plenum chamber between the inlet and the cylinders. This has two benefits: it evens out the difference in path restriction between cylinders (distribution across space), secondly it provides a large-volume buffer against pressure changes (distribution over time). For non-supercharged / normally aspirated engines see

Airbox An airbox is an empty chamber on the inlet of most combustion engines. It collects air from outside and feeds it to the intake hoses of each cylinder. Older engines drew air directly from the surroundings into each individual carburetor. Modern ...

Norton Classic

The

Norton Classic The Norton Classic is a rotary-engined motorcycle built in 1987 by Norton as a special edition of just 100 machines. Engine development The Classic used an air-cooled twin-rotor Wankel engine that had been developed by David Garside at BSA's ...

was a

motorcycle 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 ...

whose

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 ...

twin-rotor

Wankel engine The Wankel engine (, ) is a type of internal combustion engine using an Eccentric (mechanism), eccentric rotary combustion engine, rotary design to convert pressure into rotating motion. It was invented by German engineer Felix Wankel, and desi ...

was developed by David Garside at BSA. Wankel engines run very hot, so Garside gave this air-cooled motor additional interior air-cooling. Air was drawn through a forward-facing filter situated to provide a ram air effect. This air passed through the interior of the rotors and then into a large pressed-steel plenum before entering the combustion chambers ''via'' twin carburettors. The plenum (which doubled as the bike's

semi-monocoque The term semi-monocoque or semimonocoque refers to a stressed shell structure that is similar to a true monocoque, but which derives at least some of its strength from conventional reinforcement. Semi-monocoque construction is used for, among ot ...

frame) enabled the transfer of much of the heat to the surrounding atmosphere. The carburation process further reduced temperatures ''via'' the

heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is al ...

of evaporation. Even so, at 50 °C the fuel-air mixture was still hotter than ideal, and the engine's

volumetric efficiency Volumetric efficiency (VE) in internal combustion engine engineering is defined as the ratio of the mass density of the air-fuel mixture drawn into the cylinder at atmospheric pressure (during the intake stroke) to the mass density of the same volu ...

remained somewhat impaired. The eccentric shaft's main bearings and the inlet manifolds were fed by oil-injection lubrication, and the fuel-air mix also carried residual mist of oil from the interior of the rotors, which helped to lubricate the rotor tips.Bill Murray monograph 1985: "The decline of the British motorcycle industry"

Hovercraft

Practical

hovercraft A hovercraft, also known as an air-cushion vehicle or ACV, is an amphibious Craft (vehicle), craft capable of travelling over land, water, mud, ice, and other surfaces. Hovercraft use blowers to produce a large volume of air below the hull ...

use a peripheral skirt system, where the air from the lift fans is routed to a narrow slot around the edge of the hull, and bound by a flexible skirt. Distribution of this air from the fans to the periphery is through a large-volume plenum chamber, so as to provide even distribution of airflow without sensitivity to the length of the direct path.

Nuclear Waste burning reactor rods

Molten salts consisting of chlorinated nuclear waste are contained in atomic fission reactor rods. This Alvin M. Weinberg et al design has a liquid fuel instead of a solid fuel. The gas plenum is above the fuel and below a diving bell style vent for the reaction products. This allows for convection currents and fluid flow eliminating the possibility for gas products to build up and rupture as in a light water reactor.

References

{{reflist Hovercraft Superchargers