Work Done By Engine/Heat Engine

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Discussion Overview

The discussion revolves around calculating the work done by a heat engine, specifically focusing on the Otto and Diesel cycles. Participants explore both graphical methods using P-v diagrams and numerical approaches involving equations of work, while seeking clarification on the nature of the processes involved.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant requests assistance in calculating work done by a heat engine using both graphical and numerical methods, mentioning specific state/process data.
  • Another participant suggests using air tables to look up enthalpy/energy values and calculating net work as a sum of differences in enthalpies for relevant points.
  • A participant identifies the cycles of interest as Otto and Diesel and lists formulas for work under constant volume, constant pressure, and constant temperature conditions, expressing uncertainty about whether the process is isothermal or polytropic.
  • One participant prompts a check on the previous post regarding compression and expansion, indicating that the same material is relevant to the current discussion and provides plots related to the performance of the Otto and Diesel cycles.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the processes involved, with some uncertainty about whether the processes are isothermal or polytropic. There is no consensus on the best approach to calculate the work done by the engine.

Contextual Notes

Participants mention specific formulas and processes but do not clarify the assumptions or conditions under which these formulas apply. The discussion lacks resolution on the nature of the processes involved.

EngNoob
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Hi

Can some one help me out on how to calculate the work done by a cycle/heat engine?

I am looking to calculate this in a graphical form, using P-v diagram and a numerical form, using an equation of work.

I have state/process data for for cycle for volume an pressure for point 1-2/2-3/3-4/4-1.

I have a compression and expansion index.

If anyone could help me out with the equation to use i would be real happy.

If anyone can point me in the direction of some further reading on this subject i would be greatful too.
 
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What kind of cycle? Get your air tables out and look up the enthalpy/energy values.
Then you calculate the net work as a sum of the difference in enthalpies for the points that generate or input work.
 
The Cycle would be "Otto" or "Diesel" heat engine.

I think i have the formulas.

Constant Volume: W = 0

Constant Pressure: W = P(V_2 - V_1)

Constant Tempreture: W = P_2 * V_2 * ln (V_2/V_1)

However, i am unsure if i am working with an ISOTHERMAL process vs POLYTROPIC process.

The pressure increases whilst the volume decreases, does this indicate Polytropic or Isothermal
 
Hi there:

Check out your post on compression and expansion.

The same material covers this post.

Here are two plots regarding the performance of Otto Cycle and Diesel Cycle.

Slide44.GIF


Slide51.GIF


Thanks,

Gordan
 
Last edited:

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