What are superchargers?

Popularised for motorcars in post-war North America, a supercharger is a type of air compressor that is fitted to extract greater horsepower, or efficiency, from an engine. The more air that is present within the combustion chambers, the more oxygen is available to be burnt. Logically, therefore, if extra fuel is added, greater power outputs will result.

Yet, you will encounter different types. While some electric versions exist, most superchargers are driven mechanically by the engine. While a forthcoming blog will look at the practical issues covering superchargers, this one examines the theoretical aspects. This may help you, should you be looking to buy a supercharged car, seek to repair one, or even modify an engine to accept forced induction.

What are the different types of superchargers?

You will find three main types of superchargers. The differences depend on how the air is moved from the atmosphere and into the engine intake.

The Roots supercharger

The Roots unit is the most common and oldest type of supercharger. It works using counter-rotating meshed rotors to pump air into the engine. While being the least expensive supercharger to manufacture, it is relatively inefficient, due to the high amount of waste heat it produces. This is counterproductive, because the resultant temperatures reduce the air's density, curtailing the engine's power output. Even an air-to-air intercooler may not be able to cool the air down again.

Despite this, the Eaton Group of North America has refined the Roots supercharger and car manufacturers have adopted them. One of the main changes was a subtle helical twist made to the internal rotors. This type of supercharger is called the 'Modified Roots'.

You can find these superchargers on a variety of cars, from small to large. For instance, the fourth generation of the Eaton Group’s smallest supercharger, the M45, was fitted by BMW to the pre-2006 MINI Cooper S. Some versions of this car were fitted with the 5th generation M45, the rotors of which were treated with an Abradable Powder Coat (APC) instead of an epoxy layer.

This change was to reduce the internal tolerances to boost efficiency. Yet, Eaton's TVS (Twin Vortices Series) supercharger is used on larger vehicles. Perhaps the most common examples on UK roads are 5.0-litre versions of various Jaguars and Range Rover models. Despite the TVS series being more advanced and refined, it is still Roots supercharger at heart.

The Twin-Screw Supercharger

Taking advantage of an expired German patent from 1878, the twin-screw supercharger was developed first in the mid-20th century by Alf Lysholm, a Swedish engineer. Instead of using rotors, a set of worm gears mesh together. This draws air into the supercharger and, as the taper within the gears reduces in size, the air pockets shrink. This compresses the air, prior to it being expelled from the supercharger and into the engine intake.

Unlike the Roots alternative, the inlet and outlet sides of a twin-screw unit are airtight. Not only does this result in less wasteful heat being produced but twin-screws also require less power to operate, compared to Roots superchargers. Yet, this efficiency comes at a cost. A twin-screw supercharger is significantly more expensive to manufacture, which may explain why it tends not to be offered as original equipment by car manufacturers, although you may find it in specialist aftermarket applications, or conversion kits.

Centrifugal superchargers (and turbochargers)

While centrifugal superchargers have featured in automotive applications, they are relatively rare. Technically, the far more common turbocharger is a type of compressor but with a crucial difference.

While a turbocharger performs the same task, by forcing air into an engine instead of allowing it to be sucked in, it works very differently.

Instead of being driven by the engine, the turbocharger works centrifugally, by having a rotating vane powered by exhaust gases that exit the engine. By using this energy that would have been wasted otherwise, turbochargers are very efficient. They are also relatively inexpensive, which explains why they dominate the market, compared to engine-driven superchargers. Even so, you can find twin-charged cars, which are fitted with a supercharger for low engine speeds and turbochargers for higher RPMs.

Even so, turbochargers struggle to perform at low engine speeds. Therefore, modern developments, including variable-vane and geometry technology, have been introduced to solve these failings. Turbocharger operating temperatures tend to be extremely high, which places extra demands on the car's cooling and lubrication systems, as well as the unit's internal bearings. For these reasons, check out this blog for advice on turbochargers.

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