Re: M3 Compression Ratio. Velocity pressure to static?

[Otter <kdsebe0@xxxxxxxxxxx>]

>If the highest measured pressure at a cylinder is 13.75 bar, and the
>intake pressure is atmospheric or 1 bar (actually, I remember that about
>1.012 bar is 760 mm Mercury or 1 atm), should not the compression ratio be
>about 13.75:1? 

Nope, that would be a pressure ratio.  The compression ratio is determined
by the volume at BDC and TDC.  Compression ratio is really mainly used to
determine thermodynamic diagrams of an engine.

>Perhaps the flow rate of the intake air, etc., leads to some 'charging'
>effect since some velocity pressure is converted to static pressure
>(Bernoulli Principle) before the valves are closed so the initial charge
>is not at atmospheric.  

Ummm... unless I missed something out, Bernoulli's Principle relates to the
pressure that a moving fluid exerts on it's container.  I think that
velocity pressure is a misleading term, as what really happens is that more
air molecules are crammed into a given volume than would normally exist at
STP.  This occurs because the relative velocity of the particle is so high
that the pressure exerted by the otter gas is not sufficient to
repel/reverse the molecules flow direction.

And remember that the "initial charge" is constantly changing based on
engine speed, timing advance, and variable valve timing (if equipped).

>This would explain the advantages of the ram
>air/scoop thing that people keep mentioning. 

I dunno if ram air will truely be effective until a chip takes advantage
over the gain in air pressure.  Once the ECU switches into open loop mode,
any additional air delivered to the engine serves to lean out the mixture
which changes what parameters the engine is supposed to operate in.  Maybe
this is why ram air hasn't been too successful in "at home" applications.

If you want a more mathematical approach to the above situations, write me,
as I feel that most people here could really care less about
Temperature-Entropy and Pressure-Temperature graphs.

Sorry about the confusion yesterday about the SI board, Neil!

Kenn(y)(eth) Sebesta

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