Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Thermo Question P-V diagram

  1. Apr 21, 2010 #1
    If you were to combine a diesel cycle and a steam cycle (crower cycle) in 1 p-v diagram, what would it look like?
  2. jcsd
  3. Apr 21, 2010 #2


    User Avatar

    Staff: Mentor

    Welcome to PF.

    Sorry, but the question doesn't make a whole lot of sense. What, physically, are you describing?
  4. Apr 21, 2010 #3
    Thanks for the welcome.

    I'm talking about a 6-stroke engine, more specifically a Crower cycle.

    There will be a intake stroke, compression, combustion, exhaust... and at TDC of the exhaust stroke, water will be injected to harness the otherwise wasted heat and convert to steam... expanding and creating work, then exhausted again and the cycle starts over.
  5. Apr 22, 2010 #4
    I would imagine it would simply be 2 superimposed cycles (otto/diesel, and rankine) analyzed separately, rather than one cycle since a cycle depicts what is done to a single working fluid. The duty cycles of each are reduced to producing power 1/6 of the time, which might or might not be enough to justify it.

    In addition, I'm not sure you could create superheated steam in sufficient quantities in such a short period of time with those temperatures. Rankine cycle steam temperatures are roughly around 800* F, which will never occur in a typical IC engine.
    Last edited: Apr 22, 2010
  6. Apr 22, 2010 #5
    Thanks for the reply.

    There will be 2 power strokes every 6 strokes.

    The temps are there to phase change to steam at 1atm, but we can still play with the pressures by adjusting the timing at which it's injected, bringing down the pressure below 1atm.

    Now the temps are not 800F, but I don't know what pressures you thinking of when you come up with that number. Are you saying that the steam will not be able to do work unless the temps are at 800F?

  7. Apr 22, 2010 #6
    The quality of steam goes down when pressure goes up (it wants to condense), so you need a higer temperature to maintain a superheated steam (>100% quality) at effective pressures.

    And yes, you have 2 power strokes every 6 strokes for a duty cycle of 33%, whereas a 4 stroke has a duty cycle of 25%, so you are right there. However, It seems that the steam power stroke will not produce enough added benefit, not to mention the possibility of residual moisture during combustion since your condensing step is done in the cylinder (if i understand correctly).
    Last edited: Apr 22, 2010
  8. Apr 23, 2010 #7

    jack action

    User Avatar
    Science Advisor
    Gold Member

    It would look like two superpose traditional cycles (one smaller that the other).

    1- Traditional compression stroke
    Traditional heating by combustion of fuel
    2- Traditional power stroke
    3- Traditional exhaust stroke (flat line @ Patm)
    Heating of water injected by residual exhaust heat (creating pressure rise when liquid change to steam)
    4- Steam power stroke
    5- Steam exhaust stroke (flat line @ Patm)
    6- Traditional intake stroke (flat line @ Patm)

    Of course, when I say "flat line @ Patm", that's in theory. In reality, there are always some pumping losses that will make these strokes deviate slightly from Patm.

    There might also be some gain in transforming the traditional exhaust stroke (#3) into a slight compression stroke (early valve closing) to help the quality of steam.
    Last edited: Apr 23, 2010
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook