Mistake by D.I.S.

[rbattelle]

Gotta get this off my chest... it bothers me as an engineer and dieselhead.



I was over at Diesel Injection Service's website awhile back and I took their ultimate diesel quiz version 2 (the latest one). Well, question 18 is as follows:



18. When air is compressed ____________?

A. The temperature increases

B. It's density is reduced

C. It is less efficient

D. All of the above



This is a boneheaded question. It's poorly worded, at best. Are we talking constant volume compression or what? So I figure they meant to refer to a piston/cylinder arrangement on the compression stroke and I answered "A", since according to the ideal gas law the temperature of a gas must increase if pressure increases, and density may also increase. Certainly the answer "it is less efficient" is complete rubbish, since there is no efficiency associated with air.



Well, DIS claims the correct answer is "D". So I wrote to them to point out their error, but they claim they're absolutely correct. The guy I spoke with cited some turbocharger book by Hugh MacInnis. Personally, I think MacInnis was either a moron for suggesting that compressing air reduces it's density and somehow makes it less efficient, or the guy writing the quiz misinterpreted.



Just a word to the wise if you indulge in the quiz. Thanks for reading my rant.

 

[illflem]

I also feel DIS is correct. I was reading an article on intercoolers a while back that had the question and three answers combined into one sentence. DIS almost could have taken it entirely from this article.

 

[rbattelle]

You could more correctly word it as:



"When air is compressed by a turbocharger it's temperature increases. As it passes into the compressor exit plumbing (where volume is greater), it's density decreases. Since the engine will run more efficiently on cooler, denser air, the air is passed through an intercooler to decrease its temperature (and increase density) before entering the intake manifold"



That sentence is correct. But the question posed by DIS is rubbish.

 

[RustyJC]

When air is compressed________?

B. Its density is reduced.

As shown above, this statement is patently incorrect. Density is defined as mass per unit of volume. From the 44th Edition of the CRC Handbook of Chemistry and Physics, page 2205, Density of Dry Air:



At 10*C and a pressure of 72. 0cm Hg, the density of dry air is 0. 001182 grams/milliliter.



At 10*C and a pressure of 77. 0cm Hg, the density of dry air is 0. 001264 grams/milliliter.



Therefore, as air is compressed, its density (mass per unit of volume) increases. In layman's terms, more air is contained in the same space, or the same amount of air is forced into a smaller space; therefore, it is more dense.



If option B had read "Its volume is reduced", that would have been a correct statement.



Rusty

 

[Doubleclutch]

I'm laughing at US!

Well guys, ya have to love it. If the idea of a test is to make people think, it was a good question! If you look at the simple question... When air is compressed. . it Does get hotter, and (uncompressed) hotter air is less dense. So if you're trying to combust hot, less dense air, don't we all agree, it's less efficient? How can you say they're wrong... . Most diesel owners don't think about things the way we do.

Greg

 

[Vaughn MacKenzie]

Doubleclutch, but the air in question is compressed. Even if it's a lot hotter, if it's compressed, it's still more dense, especially if you're talking about a diesel cylinder.







Vaughn

 

[rbattelle]

Ok, let's be more scientific about it, then. The ideal gas equation of state is:



P=rho*R*T



Such that rho is density, R is a constant, T is temperature, and P is pressure. For a constant volume, constant mass process, an increase in P will increase T and density will remain constant. For a piston/cylinder arrangement where the volume is decreasing (compression stroke), then density must increase (as well as temperature) because volume (which is in the denominator) is decreasing with mass held constant.



The only way for us to have a loss of density with an increase in temperature is to use a constant-pressure process, which we have already stated is not the case here.

 

[NVR FNSH]

Originally posted by rbattelle

You could more correctly word it as:



"As it passes into the compressor exit plumbing (where volume is greater), it's density decreases. "

Huh? Not sure I'm following you on this statement. Probably because I need to go to bed... .



Brian

 

[RustyJC]

Actually, the turbocharger impeller adds kinetic energy to the air by increasing the air's velocity. When the air exits the impeller, the kinetic (velocity) energy is converted to pressure energy as the air decelerates.



Rusty

 

[rbattelle]

I was just trying to point out that when the air is discharged from the turbocharger diffuser, it usually enters a duct of greater volume than the diffuser. Since the volume has increased, the density falls a bit.