analyzing air/combustion systems

[thwlruss]

Im working through Otto, diesel, Brayton cycles ect. Is it proper to assume cold air standard analysis for these systems, i.e. constant specific heats? The MERM indicates that the ideal gas law makes this assumption implicitly. Does the PE exam indicate which approach we should take for each problem?

Thanks

 

[Clydeman]

Assuming constant specific heats (constant k - ideal gas) is appropriate. You can use the air tables if you prefer which are valid for low pressures and are still reasonable for higher pressures. They will not give you problems which force you to use the air tables.

What you will see on the test is a complex cycle. You will need to be able to quickly determine what they are asking for. They will never ask you to analyze a complete cycle.

I would focus on using the equations. I would prefer exquations over the risk of having to interpolate out of the air tables on a test.

 

[thwlruss]

Thanks. Thats the answer I wanted to hear.