Supercharging isn’t easy.
By GA Projects
Two words immediately come to mind when discussing the difficulties of supercharging.
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 a large empty vessel with a fan mounted in its mouth. In order to increase the mass of air contained in our vessel we must pressurise it. As fans generally struggle to produce pressure no more air can be pushed into the vessel. Pressure is required to force extra air in which requires a pump of some type rather than a fan. Superchargers are an air pump that effectively overfill an engines cylinders by pressurising them. It is this extra air mass that is required to facilitate more combustion and hence more power.
Role of a diffuser or stator in superchargers.
To pressurise a volume of gas there has to be some kind of resistance, ie pressure can only occur when there is something to push against. In the case of a closed vessel (e.g. a cylinder of compressed air) gas i.e. air pushes up against its side walls. Pressure can then be increased by any of the following methods; reducing the vessels volume, increasing its temperature, increasing the mass of gas contained or any combination of the above. In a moving column of air (as is the case with an engine induction pipe or inlet manifold) there has to be some form of restriction for pressure to build up. On one side there is the engine itself forming a barrier but at the other end the supercharger has to contain as well as generate pressure. As a consequence of their design most mechanical superchargers form a natural one way valve allowing pressure to be maintained downstream from the charger. Impellor type superchargers (eg turbochargers, centrifugal/radial and axial chargers) don't have this advantage and rely on a diffuser for radial superchargers or a stator for axial superchargers to generate and maintain gas pressure. These are a fixed set of vanes that are mounted very close to the rotating supercharger impellor causing a pressure wave to build up at the leading edge of each fixed vane. Turbochargers often use a carefully designed lip around their periphery that functions in the same way as a diffuser. Radial (aka centrifugal) superchargers can maintain small amounts of pressure without a diffuser but are hopelessly inefficient in this configuration. Without a diffuser (or stator as the case may be) impellor type superchargers would struggle to generate pressure and in most cases probably shouldn't be called a supercharger at all.
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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