Advantages and Disadvantages
By GA Projects
Theoretically speaking electrically powered superchargers should have enormous advantages over conventional types. They can deliver boost at any engine speed or load the designer wants, can be fitted wherever there is space, do not suffer from excessive heat build up and are mechanically efficient (low rotational mass and efficient motive power). Or at least that is how the theory goes, in practice electric supercharger designers have some enormous obstacles to overcome.
Mechanically driven superchargers typically require 10 to 15 kW (13 to 20 HP) to drive them. That would require an electric motor 10 times more powerful than a starter motor. Can you imagine the wiring and charging circuit required to service a motor that big? It has been done with the Thomas Knight system pictured left. Although an impressive engineering feat (using three electric motors) and almost certainly the most powerful electric supercharger available it is much more costly than others in this category and is technically challenging to install.
There are a number of alternatives many of which are based around centrifugal ventilation blowers or ducted fans as used in marine bilge fans and model aircraft. These fans generally lack a diffuser ring in the centrifugal blower or a stator in the ducted fan without which very little in the way of pressure can be generated. The principle difference between a fan and a supercharger is its ability to generate pressure.
Technically speaking superchargers using ducted fans and their like are axial compressors, they drive air along the line of their axis to generate flow momentum. Another type of compressor is the radial fan type. Turbocharger impellors (fans) are radial compressors, they draw air in along their axis then blast that air out radially (sideways) to achieve compression. Radial compressor characteristics are almost the opposite of axial compressors, they achieve pressure relatively easily but have a low volumetric efficiency. Turbo chargers have to rotate at fantastic speeds to be functional (typically 200,000 RPM or faster).
The GA Projects electric supercharger uses an axial impellor and a barrel type fan with diffuser to form a very efficient compromise between axial and radial compressors.
Electric superchargers have the advantage of a very lightweight (usually plastic) impellor unlike the massive compression elements of a mechanical compressor or the metal rotor of a turbocharger. Comparatively little energy is required to power an electric supercharger for any given amount of boost making them very efficient.
Electrically powered superchargers are unlikely to achieve the massive amounts of boost supplied by conventional types but have a viable application as a low cost intermediate performance enhancement. Their real advantage is that boost can be supplied at low engine speeds increasing power and torque in this range thus making the engine more flexible and responsive.
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