Super charged vs. standard engine?

In article >,
says...

>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.

That's all it does. It is a simple device.
----------------
Alex

Alex Rodriguez wrote:
> In article >,
> says...
>
>
>>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.
>
>
> That's all it does. It is a simple device.
> ----------------
> Alex
>
>
Let's talk about the air. Ratio-wise the fuel is a small fraction, and
the charger does not ordinarily affect the fuel distribution since it is
usually but not always added AFTER the charger.

A non-supercharged engine is technically called a normally aspirated
engine, from the medical term aspiration for breathing. Even if the
pressure in the cylinder dropped to absolute pressure of zero the
maximum pressure difference driving air into cylinder would be 14.7
pounds. At high rpm this is not enough pressure to fill cylinder. Even
if it did, the weight of the air would be the weight of that
displacement volume of air.

Supercharging compresses air to a higher pressure and density. The
higher pressure does a better job of filling the cylinder quickly. Even
more important, however, if we double the pressure of the air being
crammed into the cylinder (and if we intercool it to prevent too high a
temperature rise) the density of the charge in the cylinder will be doubled.

Now, the weight of fuel in the charge must be proportional to the wieght
of the air (NOT the volume). If we double the weight of air in the
cylinder, double the fuel weight and double the energy content is in the
charge to be ignited.

The reason for intercoolers is to prevent the air from getting too hot.
Since density is INVERSELY proportional to temperature (on the
absolute scale) an increase in temp reduces the density and hence weight
of the charge.

BTW, compressing air raises its temp, and it IS possible to raise the
air temp so high in a high pressure supercharger that the temp reaches
the ignition temp of the fuel/air mix- bad situation. This was a
problem with early P-38s in WW2.