Martin Gonzales wrote:
>
> >Thanks for the info, Martin! Why is it that the supercharger works best
> with
> >low compression engines?
> >Chad
>
> >From what I understand, the supercharger will boost the compression of the
> engine and the lower the compression of the engine the higher boost you can
> get from the supercharger. Of course, the boost depends on the diameter of
> the pulley.
>
> The above is information is what I read in some books or a website.
> Perhaps,somebody else can confirm that information.
It's a bit more complicated than that... for normal running, compression
in a supercharged engine has to be lowered to decrease the chance of
detonation--where the mixture explodes from heat of compression, rather
than being burned, which is when the spark plug ignites the mixture and
the flame front progresses smoothly through the mixture.
The second reason is for power production. Power is determined in large
part by the amount of fuel-air mixture drawn into the cylinders. In a
normally-aspirated engine, peak torque occurs at peak volumetric
efficiency (this is the rpm where the most mixture is drawn into the
engine, and is determined by cam timing and lift, and intake and exhaust
lengths). In an efficient, normally aspirated engine, the peak
volumetric efficiency is somewhere around 1 (meaning that if the volume
of the cylinder is 30 cubic inches, 30 cubic inches of mixture (at
standard atmospheric pressure) will fill the cylinder at the peak torque
rpm).
A supercharger essentially stuffs more mixture into the cylinders by
increasing the pressure. At lower speeds, it may be producing only 1
psi, but that is still 5-6 psi over the partial vacuum without
supercharging. At high speeds, without any air control, boost can be
very high. By weight, 15 psi boost will charge the cylinder with more
than double the mixture possible without supercharging.
For a given stroke, a lower compression ratio also means a larger volume
in the cylinder when the piston is fully down, even though the swept
volume is still the same (therefore the displacement is the same). The
loss of compression is more than offset by the entering pressure of the
mixture, and the larger initial volume means more power.
Cheers.
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