Quote:
Originally Posted by TheEnemy
Thats MACH index as in speed of sound?
|
...yep.
...quoting from page 245 of Taylor & Taylor (Compressible Flow Through a Fixed Passage, in Chapter 10):
"
Inlet-Valve Mach Index. Some years ago (10.41) experiments were made under the author's direction to determine if some easily evaluated velocity could be used for
µ in Eq. 10-17. The velocity finally chosen was the following:
µ = (Ap·s)/(Ci·Ai)
where
Ap is piston area,
Ai is nominal inlet-valve opening area, and
Ci is the inlet-valve flow coefficient based on the nominal area
Ai. It will be noted that the resultant value of
µ is the velocity of an incompressible fluid through an opening of area
Ci·Ai into a cylinder of piston area
Ap with piston velocity
s. Using this nominal velocity, the ratio
µ/a in Eq. 10-17 can be written:
µ/a = (b/Di)^2 × [s/(a·Ci)] = Z
...where:
b = cylinder bore;
Di = inlet-valve diameter, the nominal inlet-valve area being taken as
(PI·Di^2)/4;
s =mean piston speed;
a = inlet sonic velocity;
Ci = inlet-valve average flow coefficient."
"...the variable,
µ/a, refers to the Mach index [
Z] through the controlling flow section, which is usually the inlet-valve opening."
"...Mach index (Z):
Z = (b/Di)^2 × [s/(a·Ci)]"
...and, lastly, from pages 248 and 249:
Page 248: "
Effect of Piston Speed. Where piston speed is the only variable, the effect on volumetric efficiency is through
Z, which is directly proportional to piston speed."
Page 249: "One important relation is observable...namely, that volumetric efficiency starts to fall off rapidly as
Z exceeds 0.5 to 0.6."
"...from the aforementioned considerations...the inlet-valve size and design should always be such that
Z will not exceed 0.6 within the operating regime of any engine."
...and, similar, but
higher order limitations (due to higher pressure and gas temperature) occur getting exhaust gas through/past the exhaust valve.
addendum: see posting #53 below