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From: eric@amc.UUCP (Eric McRae)
Newsgroups: net.auto.tech
Subject: Re: Theory of Turbos (LONG)
Message-ID: <145@amc.UUCP>
Date: Fri, 3-Jan-86 13:44:29 EST
Article-I.D.: amc.145
Posted: Fri Jan  3 13:44:29 1986
Date-Received: Sun, 5-Jan-86 01:02:06 EST
References: <2@emacs.UUCP>
Reply-To: eric@amc.UUCP (Eric McRae)
Distribution: net
Organization: Applied Microsystems Corp.; Redmond, Wa.
Lines: 44
Keywords: Turbo Boost
Summary: Boost guage is behind throttle

In article <2@emacs.UUCP> pz@emacs.UUCP (Paul Czarnecki) writes:
>Take the case where you are charging down the road at 4,000 rpm and you
>let up on the throttle without depressing the clutch.  Under the above
>theory the turbo would continue spinning at tc(4000) since the engine is
>still at 4,000 rpm.  However the boost guage indicated an IMMEDIATE drop
>in boost....  why is this so?

The boost guage usually measures relative pressure/vacuum on the
engine side of the throttle.  The turbocharger is on the other side.
What you're seeing is the results of an engine that was breathing
freely but just had its air supply cut off.  If you stuck a pressure
guage on the compressor side of the throttle, you would see continued
high pressure as you expected.  (Well "continued" for a fraction of a
second or so anyway.

Regarding your tc(x) theory:  Remember that the power for the
compressor is supplied by expanding exhaust gasses.  An engine may be
turning at 4Krpm but if the throttle is closed, there is litle in the
way of exhaust gas to power the compressor.  At that point, the
significant energy source for the compressor is its rotational
inertia.  Most turbochargers are designed to have a minimum amount of
inertia.

Turbocharger output is a function of the power being generated by the
engine.  Thus, the power available from a turbo on a car that is doing
a steady 55 mph on a steep grade is much more than that from a car
doing a steady 55 on a level road.  Turbo lag is caused by a
combination of the rotational inertia  of the turbocharger and the
compressability of the air between the turbocharger and the engine and
the exhaust gasses between the engine and the turbocharger.

Finally, turbocharged engines are positive feedback systems.  A running
engine delivers power to the turbocharger which pushes more air at into
the engine which exhausts more gas at the turbocharger which....  The
negative feedback element is the wastegate, which causes exhaust gasses
to bypass the compressor when the pressure at the engine reaches a safe
maximum.  If your wastegate fails (stuck closed) you can blow up your
engine in a few seconds.

BTW, I own an intercooled Volvo turbo wagon.  I blew a pipe off the
compressor one day and can attest to the continued high pressure after
you let up on the throttle.  I and the fellow that I was passing at
the time, thought I had a rocket under the hood from the sound of the
escaping air :-).