Compressed Air Engine Calculations

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Discussion Overview

The discussion centers around the calculations related to power and torque values for a single cylinder compressed air engine, specifically focusing on the effects of inlet pressure and the engine's operational characteristics. Participants are exploring the theoretical and practical aspects of engine performance, including the rate of air tank depletion at various rotational speeds.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant seeks validation of their calculations for power and torque at different RPMs, noting discrepancies with MATLAB and experimental results.
  • Another participant questions the type of engine being discussed, suggesting the need for clarification between a reciprocating machine and a continuous flow machine.
  • A later reply indicates that the use of a constant k = 1.4 suggests an assumption of adiabatic expansion, while a more accurate value for a polytropic expansion might be k = 1.35.
  • Concerns are raised about the idealized assumptions in the calculations, particularly regarding instantaneous fill and exhaust processes.
  • One participant suggests that charts could aid in understanding flow and expansion, indicating that the motor type may not significantly impact these evaluations.

Areas of Agreement / Disagreement

Participants have not reached a consensus on the calculations or the assumptions made. Multiple viewpoints regarding the engine type and the nature of the expansion process remain present.

Contextual Notes

There are unresolved assumptions regarding the idealized cycle and the specific characteristics of the engine type, which may affect the calculations and interpretations of results.

karan_s
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TL;DR
I have done some calculations to obtain power from the engine but don't think they are correct. Also need some help calculating torque at different rpms and air tank emptying time.
Hi, I posted a question a while ago which I have managed to do some more calculations for. I need a bit of help to see if they are correct. I am trying to obtain power and torque values at different rotational speeds of a single cylinder compressed air engine. The inlet pressure into the engine is a constant 9 bar and the air is discharged from the chamber as the piston reached BDC rather than a conventional cam valve design. My calculations below for power do no seem right compared to MATLAB and experimental results.

I have attached screenshot as it will not let me post equations into this.

I would like to get torque values at different rpms and also any formulas to calculate how quickly a air tank would empty at a specific rpm.

Much appreciate the help.
 

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What sort of engine do you have in mind? Is it a reciprocating machine employing a slider-crank mechanism or a continuous flow machine like an air turbine? You need to tell us more about what you are contemplating before much can be said about your calcs.
 
Dr.D said:
What sort of engine do you have in mind? Is it a reciprocating machine employing a slider-crank mechanism or a continuous flow machine like an air turbine? You need to tell us more about what you are contemplating before much can be said about your calcs.
It would like a reciprocating engine
 
You use of the constant k = 1.4 seems to imply an adiabatic expansion. In fact, it will be more nearly a polytropic expansion, k = 1.35 roughly.

The other problem I see is that you have assumed a very idealized cycle, with instantaneous fill and exhaust; this is not how it actually happens.
 

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