SUPERCHARGER TYPES & HOW THEY WORK

There are three types of supercharger: Roots, Screw and Centrifugal. The differences depend on how the air is moved from the atmosphere to the engine’s inlet manifold. The Roots unit is not only the oldest type, relying upon counterrotating meshed rotors to pump air into the engine, but is the most likely design that you will encounter, which is why it is the focus of this feature.

Patented initially by Francis Roots in the mid-1800s, Gottlieb Daimler exploited the Roots for an automotive application 40 years later. Despite being the least costly type of supercharger to manufacture, it remains only relatively efficient, because heat is the main by-product. The increased temperature reduces the air’s density, meaning that less power is extracted from the engine, and, even if an air-to-air intercooler is fitted to the car, it cannot remove all of the excess heat. Furthermore, the supercharger’s meshing rotors are not fully air-tight, creating a passage between the intake and outlet tracts, thus heat from the compressed air on the exhaust side of the unit can contaminate cooler air in the inlet, denigrating efficiency further.

Despite its shortcomings, the Eaton Group of North America has developed the Roots supercharger since 1949 and has turned it into a reliable and cost-effective unit for mass-produced applications. One of the major developments involved engineering a helical twist to the internal rotors, giving rise to the ‘Modified Roots’ name. The fourthgeneration of Eaton’s smallest supercharger, the M45, was fitted by BMW to the pre-2006 MINI Cooper S, for example, although later iterations of the pre-2006 Cooper S were fitted with the fifth-generation M45, the rotors of which were treated with an Abradable Powder Coat (APC) instead of an epoxy layer. This reduces the aforementioned internal tolerances to further increase efficiency. Eaton’s TVS (Twin Vortices Series) supercharger is used for other applications, such as the 5.0-litre Jaguars and Range Rovers, which, despite offering more refinement, is still a Roots supercharger in its operating principle and retains its intrinsic limitations.

To overcome the inbuilt disadvantages of the Roots supercharger, the Twin Screw compressor was developed in the mid-20th Century by Swedish engineer Alf Lysholm, using an expired German patent that dated back to 1878. Resembling a set of worm gears meshing together, air is drawn from the inlet side and, as the taper within the rotors reduces in size helically, the air pockets shrink, which results in compression taking place within the supercharger, prior to the air being expelled. The inlet and outlet sides of the twin-screw unit are also airtight, unlike the Roots alternative. Compared to pumping the air, the internal compression of the Twin Screw supercharger not only produces less heat, making it more efficient, but also requires less engine power to drive it in the first place. However, the Twin Screw unit is significantly more expensive to manufacture, which may explain why it tends not to be offered as original equipment, although you might find it in specialist aftermarket applications or conversions.

The third type of supercharger, Centrifugal, is the well-known turbocharger that exploits spent exhaust gas flow energy. Unlike mechanically-driven superchargers, turbochargers produce little boost at low engine speeds traditionally but higher boost at increased engine revs. Modern complications, including variable-vane and geometry technology, have solved these failings – almost. The impellers on turbochargers are subject to high rotational speeds and, because they are powered by hot exhaust gases, operating temperatures tend to be extremely high, which places extra demand on the car’s cooling and lubrication systems, as well as the unit’s internal bearings.

Turbochargers were looked at in detail in the September 2015 issue of Car Mechanics.