Potential flow in engine valves Page: 19 of 29
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N.A.C.A. Technical Memorandum No. 343
there arises the important questions as to which kind of flow
occurs in a given instance, whether there are circumstances un-
der which two different kinds of flow can occur, etc. For small
overlappings, the forces for case I show a slight decrease, but
then a gradual increase with the valve lift, For greater over-
lappings, the forces increase continuously, though very slowly.
Cases IIa and IIb (Figs. 21-22) exhibit great similarities. The
forces decrease very rapidly, reach a minimum at n = 1 (about)
and then increase rapidly. It is now necessary to consider
the equilibrium of a valve head actuated by a spring. The
power curve of the spring is always an ascending line. Equilib-
rium occurs at the point where the power curve of the spring
intersects the pressure curve. There are two possible cases.
Either the power of the spring decreases 'faster than the pres-
sure or vice versa. Only the former case is stable, since the
disturbance of the equilibrium there generates forces which
tend to restore the original condition (Fig. 24). In this
sense, it is easily recognized that the flows IIa and IIb are
stable under all conditions down to the minimum, beyond which no
definite statement can, however, be made. The flat coureesof
the forces in case. I (Fig. 23) demonstrates that the flow is to
be considered stable throughout. It is easily understood that
only the flows II can occur at small valve lifts, because of the
otherwise very great velocities. Other conditions will, how-
ever, arise, if the valve head is held rigidly and given no
freedom of motion, an arrangement on which were based the exper-
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Eck, Bruno. Potential flow in engine valves, report, December 1, 1925; (digital.library.unt.edu/ark:/67531/metadc65148/m1/19/: accessed January 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.