Solving the Otto Cycle Ideal MEP Query

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Discussion Overview

The discussion revolves around the Otto cycle and its ideal mean effective pressure (MEP), with a focus on solving a specific problem related to thermodynamics. Participants engage in exploring the mathematical manipulation required to derive the ideal MEP, as well as discussing concepts of relative and absolute efficiency in the context of Diesel engines.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Conceptual clarification

Main Points Raised

  • Chris initially struggles with a past exam question regarding the ideal MEP of the Otto cycle, expressing frustration over the algebraic manipulation required.
  • Chris later claims to have solved the problem, attributing the difficulty to tricky algebra rather than a lack of understanding.
  • Another participant acknowledges Chris's success and inquires about his course of study, indicating a shared academic background.
  • Chris raises a new question regarding the concepts of relative efficiency and absolute efficiency in Diesel engines, noting confusion over how to apply specific ratios to determine absolute efficiency.
  • Chris reflects on a previous question where he misapplied relative efficiency as absolute efficiency, leading to incorrect results.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the definitions and applications of relative and absolute efficiency, as Chris expresses uncertainty and seeks clarification on these concepts.

Contextual Notes

The discussion includes unresolved mathematical steps related to deriving the ideal MEP and lacks clarity on the definitions of efficiency terms, which may depend on specific contexts or assumptions.

Who May Find This Useful

Students studying thermodynamics, particularly those focusing on the Otto and Diesel cycles, as well as those interested in efficiency concepts in engine performance.

ChrisHarvey
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Hi everyone. Sorry... I hope I don't bore you all - I fear this thread will be nowhere near as interesting as the most recent one on this forum on why aircraft fly! Anyway, I have a question relating to the Otto cycle - I'm revising my thermodynamics for an exam and on 1 of the past papers it asks me to, "Show that the ideal MEP of the Otto cycle is given by:"

(please follow link to see image)
http://wwwebworld.co.uk/photos/OttoMEP.gif

I have tried for many hours to get this (I'm not exaggerating) and to no avail. I'm on my easter break at the moment (from uni.) so I can't speak to my lecturer. Does anyone have a clue? I get the feeling I'm doing something stupid because it's not worth many marks and should therefore be easy.

Cheers,
Chris
 
Last edited by a moderator:
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I have actually just managed to solve it, and as I thought, it wasn't particularly difficult - just tricky algebraic manipulation. If you don't see your way through to the end from the beginning you can easily end up working the wrong way with it.

If anyone's interested, I have my solution:

http://wwwebworld.co.uk/photos/OttoMEPSoln.gif

Cheers,
Chris
 
Last edited by a moderator:
Good work Chris. You doing MechE at Loughborough perchance?
 
I am at Loughborough, but I'm doing the aero eng course there, not the mech eng although I think I'm right in saying the 1st years for both courses are relatively similar.
 
Does anybody know what relative efficiency is please (in the context of Diesel engines if it makes any difference)? I've done this [different] question, where I'm given relative efficiency, and I've treated it as absolute efficiency just to see what would happen (getting the wrong answer) but didn't have to use half the data given, i.e. compression ratio (rv) and cut-off ratio (rc). Now I'm looking back thinking "how could I possibly use these ratios to give me absolute efficiency?" In short: what is relative efficiency and how is different to absolute efficiency?

Thanks again,
Chris
 

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