09/12/2003

Roots Superchargers

The Internal Combustion Engine.  Volume 1 Principles, Second Edition 1937

The Internal Combustion Engine.  Volume 2 The Aero Engine.  1934

Two volume set by D.R. Pye.  Printed in england by the Oxford Press.

Mr Pye was a mate of Sir Henry Ricardo.  Close enough that Ricardo
"criticized" Pyes two volume set - in that he gave critical review prior to
publication and assisted in revising as needed for publication.  Pye was the
deputy director of scientific research, air ministry (RAF) and a former
lecturer at Trinity College, Cambridge and Fellow of New College, Oxford.

This is not the advertising of Corkshiite Bell, nor the crapola of comic books
by HRM and Ricky Racers.

Its that serious old dry reading of engineering principles and research by
some of the finest minds of the thirties, with many inventors such as Ricardo
directly referenced by the author who actually knew Ricardo and was present at
most of his presentations and spent time with him in the lab.  This man
watched the areo engine grow from the first fuel injected wright engine on the
first flight every grow into the rolls royce powered racers of the thirties.

On who I will chose to believe, I can guarantee you that there will be no
hesitancy on leaving brown stains on Corkshiite and other advertising
material.

He has some simple and disturbing facts about roots superchargers.  In case of
doubt, ILL or buy the book or argue with the author directly.  I am simply
reporting the bad news.

First, "A rate of supply of air equal to 1 lb per sec. would suffice for a
well-tuned engine of 540 bhp."

Chapter X - Superchargers Table 4.1 "Delivery temperature and h.p. required to
compress 1 lbs of air per second at various heights"

1.  Standard Height ft x 1000
2.  Initial temp degrees C absolute
3.  Isothermally Temp/hp
4.  Adiabatically Temp/hp
5   "In centrifugal supercharger of 65% efficiency"  Temp/hp
6   "In a roots type supercharger"

1.  2.    3.           4.          5.           6.
--------------------------------------------------------------+
0  | 288 | 288        |           |            |              |
10 | 268 | 268 / 17.5 | 299 /18.5 | 315 / 28.5 | 303 / 21.25  |
20 | 248 | 248 / 33.7 | 311 /37.8 | 343 / 58.2 | 332 / 51.0   |
30 | 228 | 228 / 48.5 | 325 /58.1 | 375 / 89.5 | 383 / 94.5   |
40 | 218 | 218 / 64.2 | 355 /82.7 | 426 / 127  | 492 / 167    |
--------------------------------------------------------------+

This is ground level supercharging - the ability to maintain ground level air
pressure at higher altitudes.

For 0.010 in tip clearance and .025 inch total end clearance, the volumetric
efficiency of a roots compressor at various pressure ratio's.  Rotor length
8.5 inches, cross section maximum 4.0 inches and at a speed of 3200 rpm.

P/R  1.15  Ve 92.6
P/R  1.36  Ve 90.7
P/R  1.67  Ve 88.1

"As regards to mechanical efficiency, this was found to vary within 82 and 92
per cent, according to the speed and load."

Bottom of Page 259

"It is possible to calculate an 'adiabatic temperature efficiency' for the
Roots supercharger if the temperatures at the entry and exit are measured, as
well as the pressure ratios. For the tests performed to in fig 95 this
efficiency has been plotted in fig 96, as well as the delivery temperatures,
for the same series of pressure ratios.  The efficiency is the ratio between
the temperature rise for an adiabatic compression in a certain ratio and the
actually measured rise under those conditions.  It will be seen to vary
between 80% and 70% with a maximum value at the ratio 1.3.  This ( Note
Adiabatic Efficiency for a Roots -ed ) is higher than can be shown for ANY
CENTRIFICAL SUPERCHARGER ( emphasis mine ed ), as will appear later, the lower
efficiency of the latter type being due to the eddy and friction losses which
accompany the very high speed of operation."

Blunt, simple, straight forward.  An roots by actual tests by unbiased
engineers with only the axe of more power to grind, is more efficient under
most tests and regimes than a centrifugal.  A summary is that Pye considers
that the Centrifugal advantage is simply size and weight period.

And a turbo, is an exhaust driven centrifugal supercharger.  And prior to
using centrifugal compressors, many diesels were equipped by WWI with "turbo
blowers" where an exhaust turbine spins a roots supercharger which takes less
power than a centrifugal to deliver the same air.