How It Works


Classic Car Weekly (UK) - - This Week - FUZZ TOWN­SHEND CCW’S MASTER ME­CHANIC

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’

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