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Polytropic exponent, thermodynamics question 
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#1
Nov3011, 06:51 PM

P: 38

1. The problem statement, all variables and given/known data
Air is compressed to 6% of its initial volume in the cylinder of a diesel engine. In the process, its pressure increased from 1 bar to 41 bar. Calculate the polytropic exponent for this process and comment on the associated heat transfer. 2. Relevant equations PV^λ = constant 3. The attempt at a solution Pinital (Vinital)^λ = Pfinal(Vfinal)^λ Pinitial = 1 bar P final = 41 bar V initial = 100% V final = 94% Well I'm pretty much not sure if I'm going about solving this (probably simple) problem in the right way. Anyone care to help? Also do you have to change the units from bar to pascals? Thanks dudes and dudettes 


#2
Nov3011, 07:00 PM

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Thanks
PF Gold
P: 4,903

Whew! "Polytropic exponent!" Ya live & ya learn!
OK, so you know p1 and V1, giving you one equation with unknowns C and n: p1V1^n = C Then, what are p2(p1) and V2(V1)? Write the equation involving pp2 and V2 in therms of p1 and V1. That's a second equation with two unknowns C and n. Solve pre h.s. algebra. Now for part 2: if you assume an ideal gas, you can compute T1 and T2 (assume w.l.o.g. 1 mole of air), use the 1st law to express ΔU = (const.)ΔT, then work = ∫pdV and finally ΔQ = ΔU + W. 


#3
Nov3011, 07:20 PM

P: 38

No volume is given in the question but can you just use percentage values instead? So I'll have P1V1^n = C and P2V2^n = C ? Thanks by the way rude man! 


#4
Nov3011, 08:18 PM

Sci Advisor
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PF Gold
P: 2,907

Polytropic exponent, thermodynamics question
Hi Studios,
Do you know how the polytropic exponent varies depending on heat flux? For an adiabatic (no heat transfer) process, the process is isentropic, so what do you think the polytropic exponent would be equal to in that case? For the case where temperature is constant, PV = mRT = constant, right? In that case, what would be the polytropic exponent and how would you describe the heat transfer to the air during the process? 


#5
Nov3011, 08:34 PM

HW Helper
Thanks
PF Gold
P: 4,903

Sorry, my post was totally incomplete.
Q Goest is giving you good leads. One thing that bothers me is I think we're all assuming an adiabatic process, which was not given but maybe was so defined in the relevant chapter of the textbook. I believe that assumption is generally made, so maybe I'm just being picky. For that matter we weren't given authority to assumean ideal gas either  but what can one accomplish without that? I suppose if n computes to cp/cv for air then we're entitled to assume isentropy. 


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