# Chemical Forums

## Specialty Chemistry Forums => Chemical Engineering Forum => Topic started by: fkatzenb on June 14, 2005, 08:18:28 AM

Title: Finding Specific Heat Ratios...
Post by: fkatzenb on June 14, 2005, 08:18:28 AM
Hi all!  This is my first post, and looking forward to interacting with everyone for my little side project.  I am not a chemist or chem engineer.  To my partial dismay, I am an electrical engineer that happens to be a car performance nut who loves to learn new topics.  I did take 2 semesters of organic chemistry and a semester of thermo, however it didnt do much for me because I didnt care at the time.  Now I do. :(.

Anyway, I am currently writing a spreadsheet tool for learning about how engines are effected by geometery, rpm, fuel mixtures, compression ratio, etc.  One of the things that has gotten me started on this is the lack of learning guides that show how all of this interacts.  There are desktop dynos which cost lots of money which show this in a way, but its only meant for end to end information.  I wanted to create a free tool that anyone can use.  This partly came about when I finally had to the chance to pickup John B Heywood's "Fundamentals of Internal Combustion Engines."  Holy crap this book rocks.  While I have alot of the information I need from this book, a few things are still lacking.  These things that are lacking both in data and some understanding on my part.

This brings me to my question. 'Finding Specific Heat Ratios...'  Becaue the compression cycle is a isentropic (or that is what i assumed... no heat loss), I used the equations that would net me this:
p2 / p1 = (v1 / v2) ^ (gamma)
T2 / T1 = (p2 / p1) ^ [(gamma - 1)/gamma]
with gamma being the specific heat ratio - cp/cv.

Since I know my start data of pressure and temperature, I was using them to calculate the next crank angle (or volume position)... and using this progression until I get to the end of the compression cycle.

Now it is my understanding that specific heat (or heat capacity) is temperature dependent.  Isnt that a little recursive since my equations require me to calculate pressure first and then temperature?  Or is it allowable to substitue and come up with:
T2/T1 = ([v1 / v2]^[gamma])^[(gamma - 1)/gamma)]

This would allow me to calculate temperature first to factor into the specific heat?  I dunno.  This is where I get horribly confused as this is new to me.

I appreciate your help and patience with me.  Thanks!
Frank
Title: Re:Finding Specific Heat Ratios...
Post by: eugenedakin on June 15, 2005, 09:49:09 AM
Hello fkatzenb,

Personally having a mechanical background, I would be very interested in viewing your final product.  I like your idea of your work on this topic.

To help with your question, the typical unit for specific heat is Btu/lb*F.  In my interpretation of your question in this instance, the specific heat is related to the weight of fuel at the temperature of the air/mixed fuel inside the block during compression.  Typical compression ratio's for internal spark ignition systems are between 7.5 and 10.0 for conventional street engines.  During compression, there will be a very slight decrease in temperature due to boiling of rapidly condensed gases, and I think that this will be almost negligable when compared to the exothermic reaction of the controlled ignition of the fuel/air mixture.   In my opinion, I believe that a signifacently larger factor would be the heat loss during the ignition of the air/fuel during the power stroke that is transferred to the cylinder sleeve (a.k.a. block).

Since I do not have John B Heywood's excellent book, I am only taking an educated guess.  Feel free to post more information.  This is also a very interesting topic for me.

Sincerely,

Eugene Dakin Ph.D., P.Chem.

Title: Re:Finding Specific Heat Ratios...
Post by: Donaldson Tan on June 15, 2005, 10:00:40 PM
p2 / p1 = (v1 / v2) ^ (gamma)
T2 / T1 = (p2 / p1) ^ [(gamma - 1)/gamma]

i am not sure if the perfect gas assumption is valid for the conditions inside an internal combustion engine.
Title: Re:Finding Specific Heat Ratios...
Post by: fkatzenb on June 16, 2005, 08:29:40 AM
Thanks for the replies.  This spreadsheet has been great for working thru the system of an automobile and all the different effects everything has.  Below is a link for the spreadsheet I am building.  I added alot of notes, etc to help you see the overall picture I am trying to achieve.  There are still some more parameters and columns that will probably be needed... like entrapy, etc.  You will also see that I assumed gamma so that I could fill the pressure temperature columns to get a small idea.
http://www.squirrelpf.com/site/files/tech/dd.xl

As for geodome's comment, I got the equations at the link below.  What would you suggest I look at to determine what I need?  Thanks!
http://www.grc.nasa.gov/WWW/K-12/airplane/compexp.html

If I can use the equations, can I use a subsitution of the equations to do the following to make the pressure and temperature more independent?
T2/T1 = ([v1 / v2]^[gamma])^[(gamma - 1)/gamma)]

If I do use specific heat for all of this to determine gamma, I was going to use 5 pressures against 5 temperatures to create two tables (for cp and cv) to do look ups on and do a rough interpolation to get me close.

Thanks again everyone!

Frank
Title: Re:Finding Specific Heat Ratios...
Post by: Donaldson Tan on June 16, 2005, 09:29:11 PM
the equations you got from http://www.grc.nasa.gov/WWW/K-12/airplane/compexp.html are all based on the perfect gas assumption, ie. PV = nRT. This assumption is only valid at conditions below 5bar. Any chance your internal combustion engine operates below 5bar?

I would recommend you to get hold on thermodynamic table of the fuel-air mixture. You may use perfect gas assumption to interpolate between the tabulated values, but not actually directly using perfect gas calculations.
Title: Re:Finding Specific Heat Ratios...
Post by: fkatzenb on June 16, 2005, 10:06:15 PM
Typically most boosted cars run under a 3bar setup as far as pre compression (I run around 2.5bar on my turbo car when racing it).  When you compress it, that is a different story.  That can be pretty dang high... well above 30bars before ingition, which is what I am currently concerned about.  I believe temps get to around 400 degrees before ignition.  So how would I calculate pressure and temperature as the cylinder compresses?  The good news is I obviously know the conditions right before compression.  :-\

Frank
Title: Re:Finding Specific Heat Ratios...
Post by: eugenedakin on June 16, 2005, 11:45:50 PM
Hello Frank,

I tried to open your excel sheet at: http://www.squirrelpf.com/site/files/tech/dd.xl
but only viewed your website page.  Any suggestions on how to view your excel sheet?

Eugene
Title: Re:Finding Specific Heat Ratios...
Post by: Borek on June 17, 2005, 03:08:31 AM
I tried to open your excel sheet at: http://www.squirrelpf.com/site/files/tech/dd.xl
but only viewed your website page.  Any suggestions on how to view your excel sheet?

Perhaps he lost an s at the end of url? Try

http://www.squirrelpf.com/site/files/tech/dd.xls
Title: Re:Finding Specific Heat Ratios...
Post by: fkatzenb on June 17, 2005, 08:44:10 AM
Opps!  Thanks Borek!

Frank
Title: Re:Finding Specific Heat Ratios...
Post by: eugenedakin on June 19, 2005, 06:54:45 PM
Hello Frank,

After reviewing your information and excel sheet, I am thoroughly impressed!! Well done.   You have assembled and calculated alot of information.

Currently, I am spending much time on the road (a little over 6000 miles in the last two-weeks), so there may be some signifacent time between my replies.

I attempted to view the http://www.grc.nasa.gov/WWW/K-12/airplane/compexp.html link, but they currently have a 'service interuption'.

You mentioned:

Now it is my understanding that specific heat (or heat capacity) is temperature dependent.  Isnt that a little recursive since my equations require me to calculate pressure first and then temperature?  Or is it allowable to substitue and come up with:
T2/T1 = ([v1 / v2]^[gamma])^[(gamma - 1)/gamma)]

This would allow me to calculate temperature first to factor into the specific heat?

Yes, with the available information, your suggestion is a logical approach.

I will look at the previously mentioned website and view the information once it is back on-line.

Once again, an excellent project.  Well done!!

Eugene

Title: Re:Finding Specific Heat Ratios...
Post by: fkatzenb on June 19, 2005, 08:33:27 PM
Thanks!  I still have a lot planned for it and there is still alot of work left... aka dealing with the Mass Fraction Burned once I get past the specific heat!  I will also probably then incorporate the negative work output (aka work used for compression, etc) to get an somewhat more accurate model.

Frank
Title: Re:Finding Specific Heat Ratios...
Post by: eugenedakin on June 21, 2005, 01:25:55 AM
Hello Frank,

This is a difficult one.  I spent quite a few hours on working on information from the website http://www.grc.nasa.gov/WWW/K-12/airplane/compexp.html, and did not reveal a common-sense answer.

I am going to look for a better reference on the internet.

Eugene
Title: Re:Finding Specific Heat Ratios...
Post by: eugenedakin on June 26, 2005, 10:41:50 PM
Hi Frank,

After many hours of searching, I have not been able to find any reasonable information.  Is there any chance that some information may exist in the reference book that you have?

Sincerely,

Eugene
Title: Re:Finding Specific Heat Ratios...
Post by: JimK on July 18, 2005, 08:24:03 PM
Hi,

What does this do for you  ;)

http://www.fs.isy.liu.se/~frisk/ISIS/SelPub/04P-14_MKlein.pdf
Title: Re:Finding Specific Heat Ratios...
Post by: fkatzenb on July 23, 2005, 09:57:16 AM
It might help some!  I am going to sit down one of these days and go thru my book some more and this document and re do some posting here.  Its been a crazy quick summer with car, racing, house, house, work, work, work.

Frank
Title: Re:Finding Specific Heat Ratios...
Post by: fkatzenb on September 14, 2005, 01:26:53 PM
Hey all.  Its been awhile.  Here is a wiki page I just created to help with the process of finishing this project.  Right now I am stalled on getting cp and cv values listed on the page.

http://www.squirrelpf.com/site/index.php?page=Desktop+Dyno

Frank