Patdaly, I had a look at the link you gave (procharger website), but I couldn't find out how they obtain a flat boost curve from 3200 to 7200 rpm. I understand the excess compressed air is blown off at high revs, as you speak of a BOV (blow off valve?). That may work for a drag racing car but it's certainly not a good means to increase engine efficiency and fuel economy !
I couldn't agree more with what Dan Barnes writes at:
Excerpt:
"Unfortunately, while lag is not an issue, dynamic characteristics remain a problem, to the extent that some pundits say a centrifugal supercharger combines the weaknesses of a turbocharger with the weaknesses of a supercharger. The mass flow rate of a centrifugal supercharger is roughly proportional to the square of the compressor's rotational speed.
This means that boost rises nonlinearly with rpm, and power is biased strongly toward the top end. This can be seen clearly in the dyno tests we have done on supercharged cars. The most extreme case was a 1.6-liter engine with which the torque curve rose steadily toward redline, the result being 272 hp at the wheels, the last data point before fuel cut. An impressive number to be sure, but completely unusable. The Bosch Automotive Handbook, 4th Edition, states on page 380 that centrifugal compressors "are not suitable" for vehicle engines. This is qualified on page 424, where it is stated that "a transmission unit must be included to vary the rotational speeds if the pressure is to be maintained at a reasonably constant level over a wide range of flow volumes (ie. engine speed)." The accompanying diagram suggests a continuously variable belt-drive transmission."
NOTE: The Mc Cullogh centrifugal of the 50s worked that way.
I found further agreement at:
(BTW,his website has also a very comprehensive and interesting document concerning the history and tech development of the Jaguar V12)
Excerpt:
"The delivery is proportional to the square of the speed of rotation of the impeller, which is fine for an aircraft engine designed to run at a set speed, with the throttle controlled barometrically, but not so useful for a motor vehicle power unit with the usual wide range of operating speeds. The impeller must rotate at very high speed to do anything useful and therefore has a great deal of inertia which can subject a gear or belt drive to very high loads with changes of engine speed. Indeed aircraft engine supercharger drives always incorporated some sort of cushioning device to absorb these loads. A turbocharger gets round these problems by driving the impeller via an exhaust turbine to achieve the necessary high speed of rotation, simply dumping excess exhaust through a waste-gate when the required amount of supercharge is achieved, thus being able to function over an acceptably wide range of engine speeds. Inertia of the impeller and turbine give rise to what is known as turbo-lag which can never be entirely eradicated even though there are ways of making it far less noticeable. Obviously, a centrifugal supercharger driven from the engine will normally only provide useful boost at high engine speeds. For it to make any contribution to mid-range torque, a widely variable ratio drive is needed or the wasteful alternative of dumping excess charge at high speed. One might reasonably conclude that the centrifugal device, despite its appealing compactness and simplicity, is therefore not very promising as a potential supercharger for a motor car. Anyone who doubts this should remind themselves about the V16 BRM engine of the 1950s with its almost uncontrollable power delivery."
I'm not familiar with the big improvement helixed impellers are suposed to provide. If you have any data, I'll be glad.
Neil, I checked from several sources the data for automotive size compressors max. adiabatic efficiency:
Roots (Eaton): 50%
Lysholm : 65%
Centrifugal : 77%
Quote from:
"There are many on the list who _don't_ know that the Lysholms are NOT quite
as efficient as a well selected centrifugal. Lot's of folks seem to think
that because the Lysholms have been described as "much more efficient"
(than Roots blowers) that they must also be much more efficient than good
centrifugals! The numbers you quoted below would have been useful toward
the education of a lot of folks.
Later--
Greg
At 9:35 PM 10/14/01, Stephen Andersen wrote:
>Greg,
>
>I "respectfully" have copies of the Lysholm compressor maps
>in my briefcase, and have done the requisite efficiency
>calculations. While I agree that the lysholm AE is not that
>of a centrifugal compressor, they are still pretty darn
>good (range of 58-64% for my car), compared to 30-50% for
>a new style (Eaton) Roots."
I agree that a volumetric blower is easily and nicely packaged in the V of a car's V engine. The MB AMG 55 has impressive performances and quite a good efficiency with its declutchable Lysholm (yes it's a Lysholm, not a Roots!) compressor blowing at 0.8 bars into an intercooler. The S 55 AMG's got the same performances as the S 600 V12 and has a better fuel economy.
What to chose, then? It all depend what you want, what kind of engine you have, what space and money limitations you have.
For the best torque, power, efficiency, transient response: turbocompounding with 2-stage centrifugal compressor and 2-stage axial turbine and CVT.
For tremendous power, torque and good efficiency: turbocharging.
For transient response, power, torque, efficiency: declutchable Lysholm.
For transient response, some more power and torque: Roots, preferably with a by-pass valve.
For marine power (no torque back-up needed): centrifugal.
For aircraft altitude power: turbocharging or combined centrifugal and turbo.
BTW, a naturaly aspirated engine looses about 12,5 % of its power by 1000 m of elevation. A supercharger one looses some, but less. A turbocharged engine losses none or almost none till the turbocharger rotor and/or wheel blow up.
To answer Doglegracing original question: did we really answer? I think the response was in the first and second answers from Blacksmith and Speirera ļ.
Cheers
Aorangi
