I tried locating a calculator to try and determine the amount of heat generated by pressurizing air, but haven't had any luck. Anyone know a good formula to use for figuring out just how much heat is generated per a given amount of boost?

I'm more interested in a roots type charger than a centrifugal, so the efficiency should be the same throughout, correct?

 

Unfortunately, it's not a simple formula. To get started, you can figure out the adiabatic, i.e. no heat added or 100% efficient compressor, temperature rise due to compression using:

T2 = T1 * (P2/P1)^((k-1)/k)

Keep in mind that the temps and pressures must be in absolute, so you have to add 14.7 to the pressure and 460 to the temp. So, assuming that boost pressure is 10 psi and ambient temp is 90 degrees, you get:

T2 = 540 * (24.7/14.7)^((1.4-1)/1.4) = 626 R = 166 F

HOWEVER, centrifugal superchargers are closer to 70% efficient and small, street roots blowers are closer to 40%. My senior design project included testing the efficiency of a B&M 144 at 7 psi boost and it was only 38%. Anyway, this gets a bit more complicated. You first have to convert T1 and T2 for air into h, or internal energy (a Thermo book helps tremendously here). Then, use the following formula:

h2 = ((h2i - h1) / eff.) + h1, where h2i comes from T2 and h1 comes from T1.

For our example above, we get:

h2 = ((150 - 130) / .7) + 130 = 158.6 B/lbm, or 200 F for centrifugals or
h2 = ((150 - 130) / .4) + 130 = 180 B/lbm, or 290 F for roots

So, compressing 90 deg ambient air to 10 psig produces the following:

At 100% efficiency, 166 deg. F.
At 70% efficiency, 200 deg. F.
At 40% efficiency, 290 deg. F.

Mike

 

Thanks a lot! On the other hand, ouch, that's warm.

Would you happen to know of the compressor efficiencies of the hybrid roots style blowers (Eaton/Magnacharger)? I think the rotors have been changed as opposed to the old roots style. Are these called the twin screw blowers, or is that yet another variation?

 

For some reaseon, the Magnachargers seem to be more efficient than Eaton. That's why they sell $$$ upgrades for SVT products. I saw a test once that measured Lightning pre-intercooler temps to be over 300 degrees, so the Eaton may be <50% efficient. But I've also read articles stating that screw-chargers can get nearly the same efficiency as centrifugals (~70%).

Mike