In article >,
Alex Rodriguez > wrote:
>In article .com>,
says...
>
>>I didn't think superchargers were running full time, but were brought
>>into use by a clutch.
>
>It is possible to do it that way, Mel Gibson's car in 'Mad Max' comes to mind,
>but most do not have a clutch. They are 'on' all the time.

Useless trivia: Robbie Coltrane (Haggrid in the Harry Potter films) is a
gearhead. He made a documentary out of buying a 1949 (?) Cadillac in
California and driving it across the US to the port of New York to ship it
to his home in the UK.

He also hosted an interesting miniseries called Robbie Coltrane's planes
and automobiles. Each episode looked at a breakthrough in motive systems.
The ones that I can recall are the gas turbine, the diesel engine, the
two stroke engine, the V8, and the supercharger.

The episode on the supercharger looked at automotive racing in the 1920's
and aeroengines of WWII. Both of these situations involved competition
between the british and the germans.

Daimler-Benz introduced supercharging to auto racing with an on-demand
system that only began supplying boost near WOT. A british racer (whose
name eludes me) got tired of being smoked for a couple of races and
convinced his sponsor to pay for mounting a paxton blower on his bentley.
The engine was not built up to take the strain of the boost, so he did
not finish the next race. Score one for british pride, anyways, as the
germans had to keep running their supercharger to keep up, and their engine
failed first, allowing a normally aspirated bentley to take the win.


People say that all a supercharger does is cram more fuel/air mixture
into the cylinder, but that is a very simplistic and inaccurate description,
as it leaves important stuff out. What it does it make the engine behave
like a larger displacement engine, with a higher ratio. So a 2 litre engine
with an 8:1 compression ration, and 7 psi of boost, will behave as a 3 litre
engine with a 12:1 compression ratio. Unfortunately, the similarity ends
at top dead center. The 2 litre supercharged engine only expands the air
charge by a factor of 8, not 12, so it is as if the exhaust valve of the
larger, higher compression engine opened after the piston had only gone two
thirds of the downward stroke. This explains why supercharging reduces the
efficiency of an engine, as it increases the power. Turbocharging is more
efficient than an accessory driven super charger, because it uses the power
that is expelled out of the exhaust (by effectively openning the valve before
BDC) to drive the compressor.