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Why does heat addition happen at constant pressure in diesel cycle ?

  1. Apr 3, 2014 #1
    Why does heat addition happen at constant pressure in diesel cycle ? what is the basic concept behind it?? please do enlighten me on this topic.
    thanks in advance
  2. jcsd
  3. Apr 3, 2014 #2
    Because diesel takes a long time to burn and is injected and combusted during the power stroke. The progressive burn releases heat which would try to increase the pressure, but the cylinder is expanding which decreased pressure due to the increase in volume. In the ideal cycle these balance out. In reality they don't.

    It's like the ideal Otto cycle that had heat addition at constant volume. This assumed that the combustion event of a fixed homogenous charge of fuel and air takes place in an infinitely small amount of time. Where in reality it doesn't.
  4. Apr 3, 2014 #3

    thanks for the reply chris...but i would like to know why does it happen like that only ...is it something related to the volatility of the diesel ....or anything related to that ...please do explain....
  5. Apr 3, 2014 #4
    Diesel is a much longe chained hydrocarbon, and is ignited by pressure and temperature. So as long as the pressure and temperature are high enough diesel will combust.

    Diesel is directly injected into a hot chamber of compressed air. Which initially burns very rich at the injection spray location, the unburnt remainer of the fuel then mixes with air in the hot chamber and continues to burn (as the pressure and temperature are still high enough).

    Diesel combustion can take many crank degrees to complete, I'm not sure on an exact figure, but over 90 does not seem silly.

    Contrast this to petrol (port fuel injected to make the differences easier)

    A typical petrol charge is homogenous mix of fuel and air, that is then compressed. A spark is required to begin combusiton, this creases a flame front at the spark point. That then propogates through the entire charge. Any unburnt fuel after this will then not burn during the rest of the power stroke.

    This combustion happens rapidly, and is complete after maybe 15-20 crank degrees.
  6. Apr 7, 2014 #5
    Thanks for the reply chris .....i do agree with you ..but i am looking for some kind of derivation relating with the enthalpy factor during heat addition and i hope you could relate your answer with this and find a solution ... thanks in advance ...
  7. Apr 7, 2014 #6


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    In order to define a thermodynamic cycle it is necessary to minimise the number of changing variables on each phase of the cycle.
    The spark ignition model has all energy produced instantly, at constant volume.
    The compression ignition model has ignition over a time period, at constant pressure.
    They are the simplest ideal models that are close enough to reality to be useful in explaining the difference between SI and CI engines.
    Real systems are not ideal like the models, real systems are approximations.
  8. Apr 7, 2014 #7
    OK baluncore...but y does the compression ignition model has ignition over a time period at constant pressure ??? what is the need for that to happen at constant pressure ??
  9. Apr 7, 2014 #8


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    It is because the thermodynamic model mathematics is easier when a parameter is fixed.
    Eliminating pressure, temperature or volume from the differential equations makes for a tractable model.
  10. Apr 8, 2014 #9
    my question was why is the heat addition happening only at constant pressure and why cant it be done at constant volume just like in otto cycle! what is the difference in otto cycle and diesel cycle that makes the heat addition process different ...i was asked to derive this ans in an interview ,hope there is better way to approach for this answer...
  11. Apr 8, 2014 #10


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    If you look at the Otto and the Diesel cycles you will see they differ only in the combustion part of the cycle. If the Diesel cycle injected all fuel at TDC then it would be an Otto cycle. There would be no difference.

    A spark initiates the flame with all fuel available. An injection stroke takes significant time to atomise the injected fuel during which the piston is moving away.
  12. Apr 9, 2014 #11
    The reason, for constant pressure heat addition in a diesel cycle is that, when the fuel is injected into the compressed air inside the cylinder, question of maintaining a constant volume vanishes. We are adding more volume and igniting the mixture as heat addition. Hence, a constant volume is not possible. And secondarily, the fuel ignition process in a diesel engine is auto-ignition and is caused by pressurizing the Fuel and Air after mixing them.
  13. Apr 10, 2014 #12

    jack action

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    What is the difference in otto cycle and diesel cycle that makes the heat addition process different? Let see what the inventor himself (Rudolf Diesel) claim:

    Why cant it be done at constant volume just like in otto cycle? The question should be: What motivated Rudolf Diesel to invent an engine that doesn't have a constant-volume combustion? Again, according to the inventor himself:

  14. Oct 29, 2014 #13
    Ok i think i got it, and i had the same question.
    I think here are two posibilities:

    1-The combustion is constant pressure because diesel enters to the combustion chamber in a liquid state and have to reach "flash point" to ignite and maybe, as liquids, they need a not so high pressure to go on a vapour state, because remember, what ignites fuel is temperature, not the pressure, temperature increase with temperature, and maybe that constant pressure is the "vapour pressure of diesel".(diesel cant ignite in liquid state)

    2-Taking that i wrote recently, maybe diesel engine just needs more time to heat up with hot air every single diesel drop until they reach "flash point temperature" and as that process ocurrs the piston had already began to move down, expanding the chamber and that way counteracting the pressure gain of combustion, generating an aproximate constant pressure work...

    I apologize for the possible english mistakes...
  15. Dec 27, 2015 #14
    Why the term density came into picture in diesel engine's efficiency but not in otto cycle, please help me with this
  16. Dec 28, 2015 #15


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    Can you be more specific? The density of what?
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