From: Larry.Mc.Donald@ofa123.fidonet.org
Newsgroups: rec.autos.tech
Subject: Re: supercharged vehicle
X-Sender: newtout 0.02 Aug 21 1992
Date: 13 Sep 92  09:13:00


seanr@lobo.rmhs.colorado.edu  
   (Sean Reifschneider) sez:

 > Since a Turbo runs off of the  exhaust, it's esentially running off of
 > waste energy.  It costs nothing, but takes a while for it to come
 > online when the exhaust pressure builds up.

   I've seen a lot of people here say that turbos are using 'free
energy'. Where is the turbo getting this 'free energy'? Turbos use 
about the same amount of energy as a supercharger (depending on the 
blower design) and they get it by creating back pressure on the
exhaust. This is definitely *not* free, since the engine has to
compress the exhaust gases through the turbine. There's no free
lunch.  Turbos are generally preferred because they are inherently 
sensitive to load conditions, whereas superchargers are not. During 
WW2 a lot of our planes were turbocharged for this reason and the 
Japanese planes were supercharged since they didn't have the 
technology to build the high speed turbines.
   The only waste energy produced by the engine is heat, lost through 
both the radiator and the exhaust.  The turbo does not use this heat, 
it uses the pressure created by the pistons.

 > An engine driven supercharger is driven by the engine, hence
 > it's always online.  

A turbo is driven by the engine and is always on line.

 > There is no supercharger lag like there is turbo lag.
 > But it's always drawing power from the engine, and so it 
 > uses up more gas.  

 It tends to use up more gas because it is insensitive to engine 
loading. Turbos use up more gas than normally-aspirated engines also.

 > I always thought they should use a clutch on it so that when you
 > don't want it on, it isn't.  Ot just have the clutch engage when the
 > throttle is open a cartain  percent...
 > Would an electronic clutch work for this?

   Here's where it gets fun. Back in the late '50s, Studebaker used 
Paxton superchargers on many of their 289ci V8 engines. One such setup 
had a variable v-belt drive (infinitely variable) which was controlled 
by a diaphragm hooked to the engine intake manifold. Essentially, it 
gave one all of the advantages of a belt driven blower (near-instant 
boost and no excess heat under the hood) and the advantages of an 
exhaust-driven turbine blower (sensitive to engine loading). I'm 
looking into designing such a system for my Volvo. 
   In 1960, GM experimented with a venturi based supercharger that 
was, essentially, 'off' most of the time. They used a small compressor 
driven through an electric clutch off of an air conditioning 
compressor to pressurise a small tank (about 2 - 3 liters). The 
compressor would pump the storage tank up to 3000 psi and then shut 
off. Under the accelerator pedal, there was a switch that would 
actuate when the pedal ws floored. There was a venturi (jet pump, 
Penberthy device, tank mixer, a lot of inaccurate names exist for this 
device) hooked to the inlet of the carb, through which the car would 
get it's air full time. Under normal conditions, the engine would pull 
air through the venturi from an air cleaner located at the other end 
of the venturi. When the boost system was activated, a small jet 
located in the center of the venturi would be fed with the 3000 psi 
air and, due to air entrainment in the high pressure stream, cause a 
total boost of about 5 psi at suitable volume to the engine (test 
engine was a 265 chevy six).  When the tank was depleted, the 
compressor would 'kick' back in and refill the tank. This gave about 
15 seconds of boost to the engine with about 10kg extra weight added 
to the car . 
   Chevy extensively tested this system, had great luck with it, 
then buried it.

                 The Venturi Pump:
                     Cylindrical, of course.

 _______
| A     |
| i  C  |***********                      *******************
| r  l  |           ********      ********
|    e  |                   ******
|    a  |     /=========                   5 psi boost to carb ==>
|    n  |    ||       ^     ******
|    e  |    ||     ********      ********
|    r  |***********  ^                   *******************
|_______|    ||       ^
             ||       ^
             ||       ^High pressure jet
             ||
             || <== 3000 psi from solenoid valve
             ||





... Larry Mc Donald, N6ZMB, Fullerton, Calif. (cough, cough)
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