Good Information. Check out the post on turbos.....


http://www.sportcompactcarweb.com/ar...h02_0701.shtml

Supercharger Overview
By Dan Barnes

Automotive superchargers fall into two categories: positive displacement and centrifugal. Centrifugal superchargers are basically like the compressor side of a turbocharger, driven by a belt from the crankshaft through some sort of overdrive device,either a second belt drive or gears. Air flows continually and is compressed inside the housing,giving centrifugal superchargers high efficiency, especially important at high boost pressures.
Centrifugal compressors, because of their
high-speed operation, can move a lot of air relative to their physical size, typically making them easier to package in an engine compartment. A small number of moving parts reduces manufacturing costs and enhances durability.

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.

Centrifugal superchargers have proven especially popular with muscle car owners, whose vehicles typically have healthy low speed torque, but need a boost at high rpm. Nevertheless, a wide variety of compact applications have found their way to market,
most based on the superchargers of just four
manufacturers: Z Engineering, Powerdyne, Paxton and Vortech, the latter claiming the lion's share of volume at present.

Positive displacement superchargers fill a chamber
of a fixed volume with air at atmospheric pressure, and move that air to the high pressure side. The mass flow rate of the air thus moved depends primarily on the volume of the initial chamber and the speed of operation, i.e., how many times that chamber is filled and its contents squeezed out the
other side, and is relatively independent of pressure ratio. Positive displacement compressors generally give a flat torque curve throughout the engine's operating range and good throttle response. The main drawback of positive displacement compressors is that the fill/discharge cycle is discrete, or more simply, lumpy: air comes in bursts, rather than smoothly and continuously as with a centrifugal compressor. Adding to the problem is the
fact that most positive displacement schemes
compress the air simply by shoving it into the intake manifold. As a chamber of not-yet-pressurized air is opened to a manifold of already-pressurized air, air first rushes from the manifold into the chamber, before being shoved back out into the manifold.
Thus, much of the air is pumped twice, and it goes past the edges of the supercharger exit three times.
The resulting turbulence heats the air, reducing compressor efficiency. As one might suspect, streamlining of compressor inlets and outlets becomes very important.

Perhaps the oldest positive displacement
compressor is the Roots type, invented in the 19th century to ventilate coal mine shafts. These were room-sized, with wooden rotors. Later, between the wars, GMC used Roots blowers on two-stroke diesel
truck engines, which were not self-aspirating. It was only a few years before the first hot rodder put one on
top of a V8 engine to go faster on the dry lakes. The big superchargers sold by B&M and Weiand, and used on Top Fuel drag cars, have their "roots" in these GMC units. Unfortunately, in addition to being
sized for 500 c.i. engines, they are very inefficient.

The most modern roots blowers, manufactured by Eaton, have made great strides in port design, reducing clearances between the air-pumping lobes and the camber walls, and an added twist--slightly helical rotors that smooth out the pulsating flow. The
result is that these modern Eaton Roots blowers are far more efficient than the old-school B&M and Weiand units. They are still, however, best suited to relatively low boost applications.

The Lysholm supercharger looks similar to a helical Roots blower, but with rotors that are far more twisted, and have a conical taper. The result is that these "screw-type" compressors have an internal compression ratio, making them more efficient in high-boot applications.

One disadvantage of this internal compression ratio is higher parasitic losses during
off-boost operation. A Roots blower can be bypassed with a simple butterfly valve at part throttle and the only drag will be the mecahnical movement of its gears and lobes (which accounts for only about 0.3 hp according to Eaton). A Lysholm compressor, on the other hand, will continue to compress air as long as it is being turned, even if a bypass valve allows that air to be recirculated. The only way to eliminate a Lysholm compressor's parasitic drag at cruise is to let it stop turning via a clutch. (Ironically, the Eaton Roots blower used by Mercedes-Benz uses a clutch instead of a bypass valve, despite this being
the more complicated solution. Mercedes does things its own way.) There are currently three companies building Lysholm-type compressors, all in Sweden. One is not imported to the U.S., another is sold here as the Whipplecharger, and the third, Autorotor, is integrated for automotive use by several tuners.
The G-lader is a very old French invention, but required the precision and efficiency of CNCmanufacturing techniques to be commercially viable. Volkswagen is the only company I am aware of to have used it extensively.
Sliding-vane compressors use an eccentrically-mounted cylinder to drive
centrally-mounted vanes inside a housing of decreasing radius, providing internal
compression. The Judson supercharger, popular in the '50s and '60s, was of the
sliding-vane type. It was applied to many diverse vehicles, from Corvairs to MGBs.
Obsolete, it has nevertheless achieved a small, all but invisible cult following among
vintage car enthusiasts, and a surprising level of service is available.

The Wankel rotary engine was originally conceived as a supercharger, a purpose for which it is better suited than may appear on first glance. The rotor moves at only 1/3 crankshaft speed, meaning it can operate at high speeds, while it fills a chamber once each crankshaft revolution, meaning it breathes like a four-stroke engine of twice the displacement. Furthermore, it provides internal compression for theoretically efficient
operation. The Wankel was used successfully, if not widely, as a supercharger before the
focus changed to using it as an engine, and any surviving examples are most likely to be
found either in a museum or slowly fading away in some German barn.

------------------
'01 Silver CL-Type S
!BEWARE THE SILVER SWORD!