How It Works
Perhaps the first application of a supercharger in the automotive world was in 1900, when a Daimler car was equipped with a Roots-type unit.
The reason for the use of supercharging is straightforward. There is a ‘magic’ ratio of one part petrol to 14 parts of air for operational efficiency. To achieve more power, more fuel is necessary and so a greater volume of air is needed. There are a couple of ways to achieve this. One is to build a bigger engine. Another is to force a greater volume of air into the existing combustion chamber.
Superchargers, therefore, are rotor, screw or centrifugal air compressors and drive is via belt, gear or chain. But compressing air causes it to heat up and warm air is less dense than cool air and so, for optimum efficiency, an intercooler needs to be added to cool the compressed air charge before it enters the combustion chamber.
So, now we have air at above atmospheric (14.7lb/in2) pressure and it is cool, so denser. This means the amount of fuel can be increased, which gives a more powerful burn and greater power can be extracted from an engine of a given size.
Rotor-type chargers were the first to hit the scene, the method having originated as a means of ventilating mineshafts in the mid-19th century. Cloverleaf-like rotors draw air in and squeeze it between their lobes before expelling it via the compressed air exit. Simple, if relatively inefficient.
Next in line, in terms of efficiency, comes the screw-type charger. In effect, this is a helical meshed pair of rotors, squeezing air in much the same fashion as the rotor. However, owing to their distinctly conical nature, a larger amount of air can be induced at the wider inlet end, compressed and expelled at the narrow, tapered outlet end.
Finally, the most efficient charger is the centrifugal-type – in essence a mechanically-driven version of a turbocharger. Here, air is drawn in by an impeller, at its hub, rotating at speeds of up to 60,000rpm.
It then radiates outwards under the effect of centrifugal force and leaves the impeller at high speed, but low pressure, before being directed to the outlet via a set of stationary vanes, known as a diffuser. This slows the air molecules, increasing the pressure and giving a greater volume of air per square inch.
The big advantage of a supercharger over a turbocharger is lack of lag – the time taken for an exhaust gas-driven propeller to spin up to a speed fast enough to provide an impeller with compression power.
Thus, combining a supercharger and a turbocharger with an intercooler can provide a smooth boost of power across an engine’s entire rev range, and this can work effectively for any classic power plant, whether it’s installed in an Austin 7 or a Jaguar.
‘The supercharger’s big advantage over the turbocharger is the lack of lag’