Talk:Plenum chamber

Poor quality stub

This is a poor quality stub that;s in need of expansion throughout. I created it with regret, solely because the previous content had been deleted (inadvisedly) from the old redir page. Andy Dingley (talk) 02:13, 8 July 2008 (UTC)

It's now just plain wrong. Congratulations Dragonwhatsit, you've followed a dab page that you broke and now started propagating the same error into this page too. You want to own the page, congratulations, it's your - enjoy. Andy Dingley (talk) 09:10, 14 July 2008 (UTC)

It's still wrong. You're confusing plenum chamber (which an organ does have, it's the box that distributes the air to the various pipes) with the notion of "plenum" (without chamber), one of several different usages (inlcuding the use of a plenum bellows in blacksmithing or a foot-pumped organ) related to a pressure itself over atmospheric. Andy Dingley (talk) 14:55, 14 July 2008 (UTC)

Definition of "Plenum Chamber"

I disagree with the introductory definition: "A plenum chamber is a pressurised housing containing a gas or fluid (typically air) at positive pressure (pressure higher than surroundings).". In relation to motor vehicles, a plenum (aka "airbox") is simply a reservoir of air between the air-filter and the carburetor (or equivalent); and the main reason for this reservoir is to ensure sufficient air is available for acceleration. If the filter were immediately before the carburetor, on acceleration the engine would be starved of air, and the fuel-air mix would be unnecessarily enriched. It follows that, for air to be drawn through the air-filter the pressure in the plenum will normally be lower than atmospheric pressure, so it is thus not generally true that "a plenum is a pressurised housing". Sometimes the vehicles air-intakes will be arranged to give a ram-air effect which could raise plenum pressure above ambient atmospheric pressure, but in this situation a standard carburetor (which relies on a venturi effect) might not function properly. Please consider amending the definition, or consider having a discussion on the topic. Arrivisto (talk) 14:42, 18 September 2011 (UTC)

AndyFixit (motor mechanic) agrees with this explanation. The plenum maybe presurised or vacuum depending on whether the engine is turbo charged or naturally aspirated. Note: Carbureted engine does not have a plenum, instead it has in intake manifold where petrol injected engines have a plenum infront of an intake manifold. — Preceding unsigned comment added by 202.159.158.130 (talk) 21:33, 25 January 2015 (UTC)

Rover 3.9 - a plenum chamber on a naturally aspirated V8

Rover V8, with plenum box removed to show individual trumpets

Airboxes (common) aren't necessarily plenums (relatively rare). A plenum chamber (in an automotive sense) is usually in conjunction with a forced induction system, so this is above atmospheric pressure anyway. It is unusual to see a plenum chamber used with a naturally aspirated engine - when they are, it's almost always where a many-cylinder engine (typically a V8) breathes through a long and restrictive inlet tract, and one that incorporates the throttle butterfly. The need here, and especially with the V8, is to balance demand between individual cylinders over time, as noted in the second sentence of the intro, "One function of the plenum can be to equalise pressure for more even distribution, because of irregular supply or demand. ". Yes, it's a reservoir of air, as you state. Note though that it's only a reservoir of air between cylinder intake pulses, not during acceleration (just consider the volumes involved for a moment). Nor for that matter will ram air affect a carburettor particularly (look at "blow through" carburettors for turbocharged engines, compared to "suck-through").

Even when naturally aspirated, the automotive plenum is above the pressure of its surroundings - if we take "surroundings" to be the downstream side of the individual inlets. There might be scope for improving the wording of the first sentence, but I still do not see it as demonstrably incorrect in any way.

Etymologically, "plenum" has a clear origin and history as being specifically a raised pressure. Although it's difficult to prove a negative, records of the 18th century experiments with vacuum pumps (at least as far as I've read) only ever use plenum in reference to a positive pressure. Andy Dingley (talk) 16:55, 18 September 2011 (UTC)

One can use etymology only so far. "Plenum" in Latin means "full". The Shorter OED simply declares a plenum to be, "a space completely filled with matter; opposite to vacuum". The latest Collins agrees with you: "an enclosure containing a gas at a higher pressure than the surrounding environment". However, your illustration of a naturally aspirated Rover V8 shows a plenum that must be below ambient pressure; and the Wankel engines in both the Norton Classic and the MidWest AE series have plenums below ambient pressure. I completely accept that in a "blown" engine, the plenum pressure will inherently be higher; my point is that a plenum is not inevitably above ambient pressure. Despite the etymology, my understanding is that current usage of the word is as I describe. Arrivisto (talk) 23:16, 18 September 2011 (UTC)

Whilst an automotive plenum is indeed likely to be lower than atmospheric pressure, the plenum and its whole function relies on it being at a higher pressure than the inlet throats. As for the simple organ chest plenum, the automotive plenum must still have this positive pressure, greater than its outlets. The point is not that it's higher than ambient (it's unlikely to be, unless supercharged) it's that it's higher than where it's flowing to. Andy Dingley (talk) 23:45, 18 September 2011 (UTC)

"The point is not that it's higher than ambient (it's unlikely to be, unless supercharged) it's that it's higher than where it's flowing to" I completely agree. Perhaps the definition can be rewritten to make this clear, or perhaps the article can be amended for clarity. Cheers! Arrivisto (talk) 10:01, 19 September 2011 (UTC)

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