Solving the Otto Cycle Ideal MEP Query

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In summary: Relative efficiency is the percentage of efficiency that a engine achieves when running under a specific load and a specific set of conditions. Absolute efficiency is the percentage of efficiency that a engine achieves when running under all possible load and conditions.Different engines will have different compression ratios (rv) and cut-off ratios (rc). To calculate absolute efficiency, you would need to know both the rv and rc.
  • #1
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
 
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  • #2
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
 
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  • #3
Good work Chris. You doing MechE at Loughborough perchance?
 
  • #4
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 similiar.
 
  • #5
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
 

1. What is the Otto Cycle Ideal MEP?

The Otto Cycle Ideal MEP (Mean Effective Pressure) is the average pressure exerted on the piston of an internal combustion engine during the power stroke. It is a measure of the efficiency of the engine in converting fuel energy into mechanical work.

2. How is the Otto Cycle Ideal MEP calculated?

The Otto Cycle Ideal MEP is calculated using the formula MEP = (P1 + P2)/2, where P1 is the pressure at the beginning of the compression stroke and P2 is the pressure at the end of the power stroke.

3. What factors affect the Otto Cycle Ideal MEP?

The Otto Cycle Ideal MEP is affected by several factors including engine design, fuel type, compression ratio, and ignition timing. A higher compression ratio and efficient combustion can result in a higher MEP, while factors such as engine friction and heat losses can decrease the MEP.

4. How can the Otto Cycle Ideal MEP be optimized?

To optimize the Otto Cycle Ideal MEP, engineers can make adjustments to the engine design, such as increasing the compression ratio or improving the combustion process. Realistic simulation and analysis can also help identify potential areas for improvement.

5. What are the practical applications of understanding the Otto Cycle Ideal MEP?

Understanding the Otto Cycle Ideal MEP is important for engine design and optimization, as well as for improving the overall efficiency and performance of internal combustion engines. It can also help in developing more efficient and environmentally friendly engines that produce lower emissions and consume less fuel.

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