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Reciprocating Engines and Use of Vaccum

  1. Sep 5, 2014 #1
    Hi all,

    I have a random query that I suspect you guys will able to help me with on reciprocating engines (ala like steam engines rather than petrol motors).

    In the classic steam engines, obviously boiling water in a Pressure vessel and fed through a reciprocating engine is able to produce useful mechanical output, which relies on the expansive properties of steam and obviously pressure differential between the pressure vessel and the atmosphere.

    I have also seen on YouTube there are reciprocating engines that run on a vacuum being applied at the exhaust output, rather than apply greater-than-atmosphere pressure at the inlet. In this scenario, obviously pressure differential exists so that the mechanical output is able to be produced.

    My question is this:-

    In a vacuum reciprocating engine (assuming the vacuum is sufficiently effective tending towards zero atmospheres), is the inlet side of the engine better off to be exposed to standard atmosphere, or if you had a vessel with water in it (which at standard earth temperatures in a vacuum would become gaseous) having a feed of gaseous water - that is to say, does gaseous water also produce useful expansive properties in a reciprocating engine in a vacuum environment at atmospheric temperatures in the same way that a standard steam engine works as we currently know. Also, would using gaseous water be more effective / efficient than a standard feed of atmosphere in such a reciprocating engine?

    Hope this makes sense - any thoughts would be gratefully received!

  2. jcsd
  3. Sep 6, 2014 #2
    First off, most car engines are reciprocating.

    Steam engines work on the pressure differential between the two side of the piston.

    The pressure is generated by the raising of water to steam in the boiler. Water boils at 100 degrees celcius at 1 atm, because this is the vapour pressure of water at this temperature. As you raise the water temperature, the vapour pressure of the water increases, and water evaporates to steam until the pressure in the boiler is at the water vapour pressure.

    This high pressure steam is exposed to one side of the piston, expanding the chamber. At the full extent of the piston, a valve siwtches which side is exposed to high pressure steam, while the first chamber is vented to atmosphere.

    A vacuum engine would work in reverse, exposing vacuum (suction) to one side of the piston, while the other side was pushed with atmospheric air. Once the valve switches, the air is drawn into the vacuum, while the evacuated side is exposed to air. Unless you had an enormous vacuum chamber relative to the stroke volume, you would not get a large number of strokes out of this.

    This is why steam engines use water- it can form a huge volume of steam. You could just use a cylinder of compressed air.

    As to using room temperature water, on the air side, you could, but the vapour pressure of water is actually very low at 20 degrees C, and you would get only a small pressure differential. Also, as water vapourises off the water source, it pulls heat out of it through the latent heat of vapourization, further lowering the temperature of the water, and thus its vapour pressure.
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