Turbo vs. Supercharger. (long, technical, blabla)

[amalventano at sc.rr.com (Allyn)]

Raffi,
ok, both you and Neal are correct...

however... (here we go)...

- the temperature rise from compressing the air far outweighs the heating
caused by conduction heat transfer from the turbine impeller.

lest take a "perfect" increase in pressure:
if you start at 25c (77f), and boost to 1 bar (15 psi), the resulting
temperature is 90c (194f)
if you take into account an (optimistic) turbine compressor efficiency of
70%, that resulting temperature jumps to 117c (242f). yes, your air
temperature has jumped by 92c (165f), and is now pretty damned hot.

while examining these numbers, it is important to note that the compressor
efficiency value takes into account conduction heat transfer (from the hot
compressor turbine to the cool air stream), as well as the heat build-up due
to pure work-related inefficiencies (due to air-blade friction, etc). that
70% value caused us another 27c (48f) jump in temperature. assuming half of
that temperature jump was due to the conduction heat transfer, our hot
turbine has only caused us a 13c (24f) temperature jump. comparing that to
the original 65c (117f) jump, you can see just how small that effect really
is compared to all of the other energy you are dumping into that intake air
stream.

- also of note, the supposed "cold pump" of a supercharger will quickly get
heated. not to turbo temperatures mind you, but it wont exactly be sitting
at ambient either. even with an externally compressing supercharger, it is
still doing work on the air, and this will still cause energy (heat) to be
absorbed by the supercharger itself, heating it.

- on the flip side (back to the turbo), cool air rapidly flying by a turbine
impeller blade will quickly cool that blade. the turbine impeller may be hot
at idle, but it quickly 'cools as it spools' (nice catch phrase, eh? heheh).
even with a 900f exhaust blade, the heat simply cant transfer across the
narrow coupling shaft as fast as the cooler intake air can strip heat from
the intake blading.

phew!
ok, enough geek-dom for today.

jeesh, you'd think i was doing calcs for the twin or something... <g> :)

hth
Al

Allyn Malventano, ETC(SS), USN
87 Rieger Scirocco GTO 2.0 16v (daily driver, 190k, rocco #6)
86 Kamei Twin 16V Turbo Scirocco GTX (30% complete, rocco #7)
86.5 Occo 16v Trailer (rocco #8)
87 Jetta 8v Wolfsburg 2dr (wifes daily, 285k)


----- Original Message ----- 
From: "Euroroc II" <flaatr@yahoo.com>
To: "Neal Tovsen" <nealtovsen@yahoo.com>; "Dave Ewing"
<MK1Scirocco16v@comcast.net>; <scirocco-l@scirocco.org>
Sent: Monday, July 07, 2003 5:31 PM
Subject: Re: Turbo vs. Supercharger.


> That is not correct... you can not say that having an air pump bolted
> directly to an exhaust manifold is not going to directly contribute to the
> air warming. It will.
>
> Compressing the air will cause it to get hotter, yes I agree but there is
a
> difference between compressing air in a cold pump or a really, really,
> really, hot pump.
>
> -Raffi
>
>
> At 02:19 PM 7/7/2003 -0700, Neal Tovsen wrote:
> >The heat in the intake of a turbo car has nothing to
> >do with being near the exhaust. Any time you compress
> >air, it heats up. Thus, superchargers create heat too.
> >
> >Neal
> >'77 G60
> >(now with air-to-water intercooled goodness!)
> >
> >
> >--- Dave Ewing <MK1Scirocco16v@comcast.net> wrote:
> > > I'm just curious here and coming into this
> > > conversation late (thanks AT&T
> > > for selling out to Comcast!!) but how the heck does
> > > the air in a
> > > supercharger get to the temps of that in a turbo
> > > charger when the air in a
> > > supercharger goes from atmosphere to charger to
> > > intake?  The front of the
> > > motor is not even close to the same temp as the back
> > > of the motor either.
> > > Try changing an exhaust manifold after you drove the
> > > car compared to
> > > changing an alternator.  I'm just curious as I don't
> > > know much about
> > > superchargers but after working on cars for 20
> > > years, I understand
> > > temperatures!
> > >
> > > Dave
> > >
> > >
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