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Supercharging isn’t easy.
By GA Projects
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Design Considerations
Two words immediately come to
mind when discussing the difficulties of supercharging.
1.
Pressure
2.
Volume
 Superchargers squeeze (pressurise)
air into the engine so that additional fuel can be combusted creating extra
power – see previous article for a more complete explanation. People
sometimes ask why can’t you just put a big fan in the engines inlet? Fans are good at accelerating a mass of air but in
general they cannot produce substantial amounts of  pressure. Without pressure the accelerated blast of air
will come to a grinding halt at the first sign of resistance. Sometimes
people get confused between high flow rate (i.e. the blast of air from a
large fan) and pressure. Imagine if you will filling an otherwise empty
container with air (we will ignore for the moment issues with vacuum and
pressure differential). Using a large fan would fill the container more
quickly but once full the flow of air would stop. As fans generally struggle
to produce pressure no more air can be pushed into our vessel. Pressure is
required to force extra air in which is exactly what superchargers do to
effectively overfill an engines cylinders. It is this extra air that is
required to facilitate more combustion and hence more power.
Not only do we need to pressurise air for efficient supercharging we have to pressurise lots of it, huge amounts in fact. Even without
supercharging an engine draws vast quantities of air in through its inlet so
to make an increase in the inducted amount of air an enormously efficient
pump is required. These pumps (superchargers) require a considerable amount
of power to drive them typically 10 to 15 kW (13  ½ to 20 HP)
for mechanically driven devices. Turbo superchargers (a.k.a. turbochargers or
turbo’s) overcome this problem to a small extent by
using the engines out rushing exhaust gasses to drive them. There is a common
misconception regarding turbo chargers whereby it is believed that they
impart no load on the host engine, the boost is free so to speak. This of
course is not true as the chargers turbine impedes exhaust gas flow which in
turn requires the engine to do work to overcome. Overall turbochargers are
more efficient than mechanically driven superchargers. There are shortfalls however, turbochargers are limited in the maximum amount
of boost achieved by the total amount of drive force obtainable from exhaust
gas flow. Heat is another problem to be dealt with as exhaust gasses are
extremely hot when they exit the engine. Turbochargers are fan like in their
design and have to spin at incredible speeds to be efficient. Achieving these
high rotational speeds takes time delaying engine response (often called
turbo lag).
 The pump (supercharger) also has to be variable in its
delivery. That is to say the engine only demands full boost when maximum
power is required. Conversely a supercharger must not restrict the flow of
air when not required to produce boost. This is especially difficult in the
case of mechanically driven superchargers as their rotational speed is fixed
to engine speed not to the engines load. For example, when driving down a
steep incline the engine may well be spinning quickly (high revs) but is not
required to produce large amounts of power. Mechanical superchargers cannot
be simply disengaged at idle or light load as air will not pass through most
designs when the rotors are not spinning thus choking the engine into a
stall.
 Superchargers are not just a challenge to design,
building them usually requires absolute precision machining and only the
finest quality materials will suffice. Considerable modifications to the
engine are usually required when fitting a supercharger. Hence superchargers
are costly to build, install and maintain ($10,000 is a common purchase
price).
A relatively unexplored area
of supercharging is the use of electric motors to drive a charger. This will
be discussed in the next article.
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