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I don't think so Eric, but I could be wrong - I
have studie a bit about the turbo and have turbocharged cars, but I do not
consider my self turbo expert.
But, Let me present my case about the BOV
(Blow Off Valve)
First we know that the engine will pump
essentially the same cubic feet
per minute of air regardless of the
boost. The way we increase the air mass
flow is to use the compressor
to increase the density of the air in that cubic
foot of air. So a two
rotor at 6000 rpm displaces 277 CFM (assuming
100%Ve) - no more, no less
regardless of boost. Now the density of that air will
change depending on the amount of
boost.
Wait, I know someone will say "Ed - at 1.5
pressure ratio (7.5 psi boost at sea level) you actually
have approx 1.5*277
= 415 CFM entering the engine". Not true, my friend - you only
have 277
CFM entering the engine (at 6000rpm).
However, You DO have 415 CFM entering the
inlet to the
compessor at 0.076 lbm/Cubic foot. But when it leaves the
compressor to
feed the engine the flow is reduced to 277 CFM(more or less I
am not taking losses into account here) at 6000 rpm, but at a
higher air
density (approx 1.5* 076lbm/Cubic foot) that is how you get more air into the
engine. The engine is a positive displacement pump and will pump
essentially the same VOLUME each revolution whether at idle or 6000 rpm.
What makes the difference is the density/pressure of the air that it is
displacing.
The way in which the turbo charger increases
pressure is not by stuffing more air into the intake manifold like our
positive displacement pump might. The blades of the compressor
actually speed up the air molecules by grabing them and whipping the around
and out the compressor housing exit at a higher velocity. This is
what any centrifugal compresser does. Recall the properties of a
diffuser? Right! The turbo charger is actually designed with a
diffuser nozzer which of course slows the velocity of an airflow and increases
the pressure. That is exactly what happens in the nozzle of the compressor
housing.
You increase its density and pressure of the air but not
its displacement amount.
The displacement amount stays at 277 CFM you have
simply increased the air
density by some factor say by a pressure ratio of
1.5 (the density won't increased linearly propotional to pressure
because of PV=nTK law, compressor efficiency, heating, etc will reduce that
amount a bit), but lets use it as an example to simply things a
bit.
Now it is convient to simply consider the
engine to have increased its displacement so that it can pump 415 CFM as the
Horse power results are the same. This concept is in fact use so
frequently it is called Equivalent Volumetric Efficiency.
But what is a convient concept to make it easier to
talk about does not mean it is technically was is actually happening. keep those
things in mind.
Does the BOV Increase compressor mass
flow??
So turning to the compressor map
and the question of the Blow Off or Pop Off value
between the
compressor and the engine. Eric, your statement was you believed that
the BOV might actually shift (increase) the air mass to the right on the
compressor chart (meaning more air mass flow). Well lets take a look at
that hypothesis.
Since the engine (it IS a positive
displacement pump) still pushing through 277 CFM at one instance with a pressure
ratio of 1.5, the next instance the BOV releases and you have the 277 cfm
at say a 1.2 pressure ratio. So now instead of the 31 lbm/min air mass
flow we had at 1.5 pressure ratio, we now have approx 1.2*277*.076 = 25
lbm/min (still at 277 CFM at 6000 rpm). This is an approx 20% decrease in
the airmass flow. Just the opposite of what you might think. So the
operating point shifts to the left on the compressor chart when the BOV
opens. This makes sense when you think about it. The way we get the
turbo to decrease it boost is to make the engine stop producing as much exhaust
mass flow. We do that by reducing the air mass flow into the
engine.
So the BOV opening actually decreases your mass flow by reducing the
pressure ratio from 1.5 to 1.2. But that is not in itself something bad, it
depends on how far it goes under what conditions. Its what happens between
the time the BOV opens and the pressure ratio stabilizes at the new level that
it appears bad things could happen.
By the way, There is a role for the BOV I
will mention later - I just don't think its in boost control of aircraft
turbo.
This next part talks about the effect the BOV
actually has on the compressor when it has just opened. Its a bit long ,so
just wanted to warn you.
Here's the situation. The compressor wheel is
spinning to compress the air in the intake manifold driven by the exhaust energy
driving the turbine wheel. On one side of the compressor wheel we have pressure
slightly below atmospheric as the air is drawn into the turbocharger. On the
other side of the compressor we have air pressurized to perhaps 1.5 atmospheres.
The compressor wheel is constantly fighting the battle to keep the compressed
air on the manifold side of the wheel as that air tries to move back
to the lower air pressure region on the inlet side of the turbocharger.
This resistance takes work and produces a load on the compressor wheel and
entire rotating assembly.
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.
So until the exhaust gas mass flow has reduced in
response to the BOV opening, the turbine wheel still has the same exhaust energy
whipping it around – but now at a instance in time, it happens that the
compressor wheel is seeing a lesser load than at boost. A lesser load due to the
reduced back pressure because the manifold pressure has just decreased. The
lesser backpressure combined with (as yet) no reduction in the exhaust gas mass
flow over the turbine wheel causes what to happened?
The combination causes the rotating assembly to
instantly (well, very, very quickly) increase in rotation speed. So
the path of operation on the compressor map might look like that in
the attachment.
Now as soon as the reduction in manifold pressure due
to the BOV has affected the exhaust gas output (by reducing it), then the
rotating assembly will again slow down until equilibrium is attained between the
loads on the compressor wheel and the energy driving the turbine wheel. The
effects are over with in a few fractions of a second. to perhaps a second
depending on the BOV and a number of different parameters. However, during that
time the compressor under lessor load could increased its speed dramatically.
Depending on a number of different parameters and operating conditions, this
could result in an over-speed and damage to the turbo. The smaller the
compressor wheel relative to the boost it is producing the worst the condition
becomes as its lesser inertia will cause it to spin up even faster than a turbo
with a large compressor (and more mass/inertia). In effect, the blades of
the compressor are stalled due to the pressure differential across the wheel and
like a stalled Prop can achieve a higher rpm without the load of an unstalled
blade(s).
There is a role for a BOV but I do not believe it is
for boost control on an aircraft installation. In an automobile when the
compressor is producing boost in the manifold and the throttle plate is suddenly
closed, there is a spike in the air pressure the compressor wheel sees. Just the
opposite of the condition just described. This spike in pressure tries to get
pass the compressor wheel and procedures a heavier load as the compressor wheel
fights to keep the pressurized air contained. This in turn slows down the
compressor wheel momentarily. So even if the throttle opens again immediately,
it takes a moment for the load to bleed off and compressor to spin back up. This
is normally a condition encountered in an automobile perhaps in some types of
racing scenarios as the throttle is opened and closed shifting gears, etc.. The
BOV in effect alleviates that problem by providing a path for the pressure spike
to escape in a momentary whoosh of an opening BOV.
So the BOV may (in my opinion) have a role in an
automobile, but I believe its uses as a boost controller in an aircraft may
actually lead to damage to the turbocharger.
Now decreasing boost by decreasing the exhaust mass
flow through the turbine housing by a wastegate of some time does not have any
of these potentially harmful side effects. This simply reduces the mass
flow by diverting some of it away from the turbine wheel which naturally slows
down thereby reducing the speed of the compressor wheel producing less
boost.
In any case, that is how I see the
situation. Different viewpoints are of course, welcomed.
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews,
NC
----- Original Message -----
From: <ericruttan@chartermi.net>
To:
"Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Friday, July 09, 2004 9:52 AM
Subject: [FlyRotary] Re:
Turbo post mortem
> Ed;
> Wouldn't an opening in the intake
like john had shift the air mass the
> compressor is putting out to the
right in the compressor map?
>
> I though John put the hole there to
AVOID over speeding, (John has since
> explained that was not his
INTENT). The idea being, relative to a
> compressor map, instead of
going up the map, increasing the pressure
ratio,
> it moves right,
increasing air mas, but keeping in a safe pressure ratio.
>
> Less
efficient, but no surge.
>
>
> Ed Anderson wrote:
> >
John, I think you may be correct about the overspeed.
> >
> >
That is one of the dangers with using a blow off valve as I mentioned
>
> before. The compressor wheel is already speeding huffing and
puffing
faster
> > at altitude than sea level to produce the same
amount of boost and
suddenly
> > you remove some of the
resistance it is working against on the intake
side
> > through a
blow off valve. Already revving at high speed because of the
> >
altitude with plenty of exhaust mass flow spinning the other end - the
>
> turbine the blow off valve suddenly reduces the pressure (and
therefore
> > resistance the compressor wheel sees) and with less load
on the
compressor
> > wheel the rotating assembly rapidly increases
in speed even more.
> >
> > A waste gate of course reduces the
exhaust mass flow and slows the
turbine
> > down, a blow off valve
(at least momentarily) simply reduces the boost
by
> > bleeding off
the air the compressor is striving to pressurize to
maintain
> > the
boost pressure. Yes, eventually the lack of boost will cause the
>
> exhaust flow to slow down - but not perhaps before overspeeding
the
rotating
> > assembly.
> >
> > So while blow
off valves may be OK for autos at sea level, I would
really
> >
hesitate to put one on an aircraft. That of course just my
personal
> > opinion.
> >
> > Ed
> >
>
> Ed Anderson
> > RV-6A N494BW Rotary Powered
> > Matthews,
NC
> > ----- Original Message -----
> > From: "John Slade"
<sladerj@bellsouth.net>
> >
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
> > Sent: Thursday, July 08, 2004 10:43 PM
> >
Subject: [FlyRotary] Turbo post mortem
> >
> >
>
>
> >>I took my (ex Rusty's) turbo apart this evening. The
bearings seem to be
> >
> > in
> >
>
>>fairly good shape and the shaft looks ok. It looks like the
compressor
> >
> > wheel
> >
> >>just
"came off the end" of the shaft, much like the other one did. My
>
>>uneducated guess would be that I overspeeded it.
>
>>
> >>By the way, I was showing 38 MAP at 11,500 ft with the
wastegate fully
> >
> > open.
> >
>
>>However, there's an open 1/2 inch air bleed on the intercooler (to
be
> >>closed off) and a blow off valve, so the turbo may have been
putting out
> >>much more than the MAP showed.
>
>>
> >>John Slade
> >>Rotary Cozy IV
>
>>
> >>
> >>
> >>
>
>>
> >>>> Homepage: http://www.flyrotary.com/
>
>>>> Archive: http://lancaironline.net/lists/flyrotary/List.html
> >>
> >
> >
> >
>
>>> Homepage: http://www.flyrotary.com/
> >>> Archive: http://lancaironline.net/lists/flyrotary/List.html
> >
> >
>
> >>
Homepage: http://www.flyrotary.com/
>
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