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

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

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