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Diesel cycle

  1. Aug 12, 2018 #1
    In case of diesel cycle thermal efficiency is low, because heat addition takes place at constant pressure.so can we introduce some retractable plate which sits below the piston during end of compression stroke,and retracts back after heat addition,thus providing constant volume heat addition and increasing efficiency.These plates can be controlled by sensors connected to ECU.Does this idea works out
     
  2. jcsd
  3. Aug 12, 2018 #2
    You are studying mechanical engineering. That is certainly the right field of study for your interests. But you need to evaluate your own ideas.

    Here's how to start the evaluation of your idea. Make a sketch showing your idea. Include some approximate dimensions, calculate how far each part needs to move, how much time is available to make the moves, the necessary accelerations and resulting forces. Show us actuators that provide those forces at those speeds. If you get stuck, show us how far you got, and we will help.
     
  4. Aug 16, 2018 #3
    If I understand what you are suggesting, I believe this will not work. In a piston cylinder, the piston head provides an essentially air tight seal between the combustion chamber and the bottom portion of the bore. The heat addition (combustion) occurs at constant pressure because the cylinder is being driven down on the power stroke, thus changing the chamber volume. Any plates below the piston head would not affect the chamber pressure and therefore would not improve the efficiency. Keep thinking outside the box though!
     
  5. Aug 16, 2018 #4
    So that's what I think,if we can place plates under piston head we could restrict the movement of piston and thus causing constant volume heat addition
     
  6. Aug 16, 2018 #5

    CWatters

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    What do you mean by plates under the piston head? Can you make a sketch?

    Anything inserted into the cylinder to try and keep the volume constant would have to overcome the pressure in the cylinder. As far as I can see the power this would require is the same as that delivered to the piston.
     
  7. Aug 16, 2018 #6

    CWatters

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    Perhaps you could explode the fuel in a side chamber (of fixed volume) and then when it's fully burnt open a valve allowing the products of combustion into the cylinder head driving down the piston.

    You might even be able to use the piston itself as the valve.

    I'm sure there are disadvantages.
     
  8. Aug 16, 2018 #7

    JBA

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    The piston is linked to the crankshaft of the engine by the piston rod and therefore it is in constant motion as the crankshaft rotates so it is not possible to arrest its reciprocating motion at any point in its stroke.
     
  9. Aug 17, 2018 #8
    But in case of Otto cycle how could we achieve constant volume heat addition
     
  10. Aug 17, 2018 #9

    CWatters

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    See my post above.

    Edit: Another method might be to use a contra piston in the head that was moved up and down in sympathy with the regular piston to keep the volume constant for a short period around combustion time.

    The point is that it's likely to be possible, it's only engineering. The question is would it really make combustion more efficient?
     
    Last edited: Aug 17, 2018
  11. Aug 17, 2018 #10
    It is an approximation.From whatever I was taught during college,as the piston moves towards the TDC from the middle of the stroke, it is decelerated and its velocity changes direction at the TDC.So for a very short time, the piston velocity will be zero.
    Then the combustion process occurs so quickly that the piston would not have moved a lot during combustion.
    By aligning these two factors(i.e. choosing the right time to start the combustion), an approximately constant volume heat addition is achieved.
     
    Last edited: Aug 17, 2018
  12. Aug 17, 2018 #11

    JBA

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    Just as a comment: By the above theory, if constant volume expansion is more efficient; then, ignoring all other factors, an engine should be more efficient at lower rpm's because that maximizes the time during which the piston is transiting tdc, while the burn time is the same at all engine rpm's.
     
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