The Lysholm or Twin-Screw Supercharger

Stillen Twin-Screw Supercharger

A twin-screw supercharger for a Nissan 350Z

The twin-screw supercharger is quite similar to the Roots-type supercharger but has a few significant differences. The twin-screw supercharger was first patented by Heinrich Krigar in Germany in 1878 when it was developed as an air pumping compressor for industrial use. However, lack of precision engineering prevented the further development of the supercharger. With improvements in engineering precision, a Swedish engineer named Alf Lysholm was able to further developed the twin-screw supercharger in the 1930's for gas and steam turbine use. Lysholm developed the profile of the rotor lobes and testing various rotor lobe combinations and is credited with developing the modern twin-crew supercharger to the extent that the twin-screw supercharger is also referred to as the Lysholm supercharger.

Like the Roots-type supercharger, the twin-screw supercharger falls in the category of positive-displacement superchargers. It consists of counter-rotating rotor lobes housed in an aluminum casing, but the twin-screw supercharger differs in that it is not just an air blower but an air compressor that builds boost pressure internally. This internal compression is brought about by the profile and shape of the counter-rotating lobes. The two lobes do not overlap completely, leaving a small air pocket between them, which become gradually smaller as the air pocket moves through the supercharger and increases the air pressure. The lobes in twin-screw superchargers must thus be manufactured with high precision to ensure that internal leakage does not occur as internal compression increases.

The Pros and Cons of the Twin-Screw Supercharger

As it is based on the same principles as the Roots-type supercharger, and because it is a positive-displacement supercharger, the twin-screw or Lysholm supercharger has many of the benefits of Roots-type superchargers but few of its disadvantages. As a positive-displacement supercharger it has excellent boost at low RPM but it is more efficient than Roots-type superchargers as its rotor lobes are manufactured to greater precision that does not allow for internal leakage. However, this and the complexity of its rotor design make the twin-screw supercharger much more expensive to produce.

Also, because the twin-screw supercharger develops compression internally, it has a much better thermal efficiency of 70-80% compared to the 50-60% of the Roots supercharger. The improved thermal efficiency makes the twin-screw supercharger ideal for applications that require medium to high boost with good boost starting from low engine RPMs. However, the stronger construction is required to withstand internal compression, increasing the cost of manufacture.

Furthermore, because the twin-screw supercharger is a positive-displacement supercharger with internal compression, it produces more noise than the Roots supercharger that has external compression. Due to the internal compression, the air surges out as it leaves the supercharger, which causes an increase in noise levels. This, however, has not stopped car manufacturers from implementing twin-screw superchargers on high performance cars, such as the Mercedes-Benz SLR McLaren and the Koenigsegg CC8S.

Throttle Placement

As with the Roots-type supercharger, you must install a bypass valve and relocate the throttle body ahead of the supercharger's inlet port when implementing a twin-screw supercharger. If you don't move the throttle body, the pressure between the supercharger and the throttle plate will build up on idle, or when decelerating or changing gear. When the pressure between the supercharger and the closed throttle exceeds the boost pressure produced by the supercharger, the air will be forced back through the supercharger. This will cause the supercharger to cease as air can only move in one direction through a positive-displacement supercharger. As a result, the drive belt could be destroyed and the throttle plate could buckle and get jammed in the throttle bore as the pressure will not be released. However, relocating the throttle body will result in poor throttle response but this is unavoidable.