volumetric efficiency equations

[orlin steven jared]

> >However the book by Taylor &
> >Taylor from MIT Press does have some equations that indicate how various
> >changes effect volumetric efficiency.  An example is the equation that shows
> >how volumetric efficiency is affected by the ratio of Intake to Exhaust
> >manifold pressure.  For some applications that level of sophistication is 
> >sufficient.
> 
>  Could you give me a cite for said book, and also whoudl you be kind enough
>  to summarize (or just dump) that formula ... I'm VERY curious as to the 
>  effect of Exhaust backpressure on Ve ...
> 

The book is called _The internal combustion engine in theory and practice_
It's by Charles Fayette Taylor, and the one you want for these formulas
is Volume 1.

There are several formulas for volumetric efficiency he gives.

The one that includes the pressure ratio is the following:

Ev = 0.285 + (r - Pe/Pi)/(1.4*(r - 1))

Don't expect super accurate answers from this formula, because he
'derived' it from experimental results for engines with small
valve overlap and pressure ratios greater than 0.5
I would suggest reading the book.  And if your engine is similar
to what he tested, then your results will be better.   He also
includes graphs of Ev and every possible engine parameter.

Volumetric efficiency defined, for an ideal fuel-air cycle is:

Ev = Mi/(V1 - V2)pi

where:

Mi is the mass of fresh mixture supplied and pi is the density of
this mixture at the pressure and temp.   V1-V2 is the total displacement
of the engine.


Evolved into formulas you can use directly for engines:

Ev = 2*mass flow rate of fresh mixture / (rpm * total displacement * inlet dens)


You can also express this formula on the basis of dry air, and
make it even more user friendly:


Ev = 4*mass fl rate of dry air/(dens air * total piston area * mean piston spd)

Where mean piston speed = 2 * stroke * rpm

This is easier to use because you can calculate the density of dry air
with good accuracy for your given temp/press.  And the only thing you
need is the mass flow rate of the dry air.

To the guy who doesn't understand how you can get an Ev over 1,
consider a forced induction situation, or a tuned supercharge effect
from the intake design.   Also consider that the strict definition
of Ev only accounts for the mass of the mixture in the displacement
volume -NOT including the combution chamber volume. 


Even so, unless you have an extremely large combustion chamber, or
a forced induction, you should not rule out numerical inaccuracy in
your calculations.   But for the record books, you can have volumetric
efficiencies over 1.



Steve