The centrifugal supercharger
Mention superchargers and I’m sure most people will think of those huge positive-displacement air pumps sitting in the centre of the vee in a V8 engine. Driven directly off the nose of the crankshaft using a vee or toothed belt drive, and poking out through the bonnet or hood, just the sight of one tingles the spine, a little in anticipation of the engine performance. Delivering huge amounts of engine torque at impressively low engine speeds may be great, but in the modern world where thermal efficiency and under-bonnet packaging count in many people’s eyes for more, there is another way – the centrifugal supercharger.
A sort of cross between the traditional blower and a turbocharger, the unit is effectively a mechanically driven turbocharger compressor wheel driven directly from the crankshaft, and instead of generating the engine boost by forcing too much air into the engine intake ports (therefore raising the back-pressure in the manifold) – as in the case of, say, the Roots blower – the centrifugal compressor actually compresses the intake charge as it passes through. Drawing air into and through the centre of the spinning impeller, the centrifugal compressor accelerates the air radially, thereby increasing its energy, before entering the diffuser which, by slowing it down again, increases its pressure.
In terms of compression efficiency of the air, the centrifugal compressor is therefore likely to be far more efficient than other types of superchargers, and values of adiabatic efficiency of more than 80% can easily be realised. More important though, efficient compression minimises the increase in temperature of the engine intake charge, and with low charge temperatures the chances of combustion detonation is reduced. And if there is one thing that supercharged spark-ignition engines like, it is lots of cool, dense incoming air.
The problem with the centrifugal supercharger, however, is that the air flowing through it is proportional to the square of the rotational speed of the impeller. Compare that with the air flowing through an internal combustion engine, which is proportional only to the speed, and immediately you can see there is a mismatch. This is never a problem when centrifugal compressors are powered by an inward flow radial turbine, as in the case of a normal turbocharger. Here, the mass flow of exhaust gas powering the turbine effectively balances out the work done by the compressor and the system, if designed well, will find its own equilibrium. But when this compressor is mechanically driven from the crankshaft, compromises have to be made.
If the unit is geared to supply sensible boost pressures at low to medium speeds, at higher speeds excessive intake pressures will be generated. To combat this, some kind of blow-off valve limiting the maximum boost pressure could be provided, but this is highly inefficient and inevitably noisy too. On larger installations, where weight and or size are not an issue, some kind of constantly variable transmission could be incorporated but at this level of complexity, weight might also be an issue.
The most sensible way therefore is to gear the compressor wheel speed to produce maximum boost at maximum engine speed. That should produce knock-free motoring when pressing on hard but little or no boost effort when it simply isn’t required, as for instance when cruising on the street. Furthermore, since there will be little or no boost at typical road driving speeds, when used as daily transport, fuel economy and tailpipe emissions should not increase beyond legal requirements.
Not perhaps the best supercharging solution for competition purposes but for applications where compactness of design and thermal efficiency are important – for example on street machines – the mechanically driven centrifugal blower could be a wise choice.
Fig. 1 - The V16 BRM used a compact two-stage centrifugal supercharger
Written by John Coxon
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