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Hi Bob,
Some good questions are being
raised and I am scratching my head over them. Its taking a bit of research
as I have no experience with BOVs. I believe the answer to your question
about the power surges is that there is hysteresis in the action of the BOV in
that it doesn't just open at say 1.5 and close again at 1.5. While it may
lift off at 1.5 the pressure would likely have to reduce to some lesser value
say 1.45, 1.475 before the spring would actually force the valve shut again
against the flow. Otherwise the valve would likely chatter as you
hypothesized. But I would imagine if the boost was not brought under
control by reducing the throttle or some other means that it might
well continue to open and shut or simply stay open depending on how effect it
has on the boost equation.
Eric also raised a question concerning my
conclusion about the effect of the BOV on air mass flow. I am working on a
couple of hypothesis to that question. It appears that there are two
possibilities. Either the BOV effect does not materially affect the air
mass flow directly by its action of venting of some of the boost air or it
does. How about that for nailing it down
{:>).
By directly I mean - does the amount of air
mass the BOV vents amount to an significant amount of the total air mass flowing
through the system. If it does not vent an appreciable amount of the total
mass flow then I do not believe its the diverting of air mass that causes the
boost to drop back to its set point, but the fact that a relative small amount
of air vented affects the pressure ratio causing it to decrease, causing the
density to decrease causing the mass flow to decrease.
I guess the problem I have with possibility that
the BOV diverts sufficient air mass to affect the boost is that the engine can
be though of as an 80 cid cup that is ladling 80 cid of air 100 times a second
at 6000 rpm. That is a lot of air being removed from the manifold, yet the
small compressor wheel has no problem keeping the manifold pressurized against
that type of outflow. So unless the BOV vents a considerably amount of air
mass I just don't see right now how that could materially affect the boost ratio
or the air mass flow. But, I will post my analysis for consideration
tomorrow as Its getting a bit late. I may see the light
tomorrow.{:>)
Its certain good to have folks jump in with such
questions.
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
----- Original Message -----
Sent: Friday, July 09, 2004 10:10
PM
Subject: [FlyRotary] Re: BOV more/less
air flow
So what happens when the BOV opens. The pressure on
the intake manifold side starts immediately venting to the atmosphere reducing
the pressure inside the manifold – this is of course what the BOV is designed
to do. The compressor wheel has been spinning at 90,000 rpm with 1.5 boost
pressure ratio. Now the boost pressure ratio may be 1.2 or less in a very
short duration of time (milliseconds). This immediately reduces the
backpressure on the compressor wheel caused by the manifold pressure being
less than it before the venting. . The engine has not yet reacted to this
change, as it is still combusting the boost density air it had ingested into
the combustion chamber before the BOV opened. Neither has the exhaust gas flow
been affected - yet.
Ed,
In the situation that you describe what would
prevent the BOV from alternately opening and closing causing noticeable power
surges?
IMHO
I would also think that the turbine that had the larger compressor would be the
one whose shaft would suffer from
the sudden attempt to accelerate and the
increase in vibration as the speed increases when the back pressure is
lost.
I would also like to take this opportunity to thank
you and all the others on this list for all the information that is freely
imparted.
Bob Perkinson
Hendersonville, TN.
RV9A
If nothing
changes
Nothing changes
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