Non A*E* Roots Numbers
Jim Davies
jimd at vcc.bc.ca
Sun Jun 20 17:11:45 GMT 1999
On Sun, 20 Jun 1999, Robert Harris wrote:
> Besides all the combined high performance sales of GMC superchargers probably
> would not total up to a line item in Detroit Diesels petty cash account.
>
> Another little tidbit - a roots type compressor is an external compression
> device. No compression takes place within the roots unit - its all done in
> the backpressure of the manifold.
>
> Why use an ancient idea - invented and in use a hundred years before saint
> otto and even before saint carnot for pumping air into mines ??
>
> First, its a positive displacement device. Within its normal range of
> operations, air gets pounded into the engine, unlike whirly things that sorta
> maybe move air into the system - if it really wants to. And the whirly things
> are extremely rpm dependent and take a finite time to speed up.
>
> And its not fair to compare an un-aftercooled roots to an extensively after
> cooled whirly thing. Roots systems respond as well or better to after
> cooling.
For those who may be historically inclined, the turbo vs supercharger
debate has been going on for over 50 years. I dont think there is anything
new to bring to the debate and the reason the debate still exists is
because the bottom line is this: its six of one and half a dozen of the
other.
> Then for the truly insane who covet Lancia technology, you can take a couple
> of whirly thingees and use them to pump into a roots. Its a little known
> factiod that roots blowers efficiency in all matters goes up with increasing
> air density and with twin turbos densifying the air prior to the roots, the
> Lancia got insane power and torque at any point of power curve.
Historically speaking, Lancia can hardly claim authorship of this [not
that they are...] Detroit Diesel uses this idea on many -53, -71 and -92
series diesels and its use was so common in WW2 American aircraft engines
that is too long a list to include here. Some basic aircraft types of WW2
that used a supercharger [single speed or two-speed] as a second stage
that was fed by a turbocharger first stage: B-29, B-17, B-24, P-38, P-47
etc.
Lest you wonder why old tech is relevent today; this stuff ran very high
boost pressures because the gasoline used allowed it.
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