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  #46  
Old 01-05-2005, 10:28 PM
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mplafleur mplafleur is offline
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Quote:
Originally Posted by John Slade
OK, guys. Slow down a bit here.

I think VE means Volumetric Efficiency ...... but what does THAT mean, and why does it matter?
The theoretical maximum amount of air that your cylinder bore (also called the displacement) is called the swept volume. The amount of air actually ingested into the bore compared to the swept volume is the volumetric efficiency. Most 4-stroke engines are more than 90% efficient. I believe that at peak power is where the engine's peak VE is.

A highly tuned induction system can produce manifold pressure pulsations (at some engine speeds) which are higher than ambient pressure and which occur when the intake valve is open. Oversized ports, if they are too large, will reduce the inertia supercharge effect, which rams the air in just as the intake valve is closing. Inertia supercharge helps provide high volumetric efficiency.

Quoting from the superflow flow bench manual:

"Inertial Supercharging Effect:

When the intake valve starts to close, the fast moving air column
tries to keep ramming itself into the cylinder. If the inlet
valve is closed at just the right instant, the extra charge will be trapped
in the cylinder (called inertial supercharging). Volumetric efficiencies
up to 130 percent can be obtained......."

They then go on to define the inertia-supercharge index Z, which
is an empirical value which is a measure of the strength of the
inertia supercharge. To compute this:

1) Find Cv = (valve flow (total) for one cylinder filling event at given test pressure)
---------------------------------------------------
(Maximum flow possible at given test pressure)

Cv is intake system flow rating (normalized)

2) Find average inlet valve area:

A = Cv * (valve area in sq. inches)

3) Compute Z:

Z=((RPM/126000)) * sqrt((CID * Inlet Length)/(A))

Z will usually be between 0.9 and 1.2

An engine with 50% VE has essentially only half the displacement.
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  #47  
Old 01-06-2005, 01:33 AM
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John Slade John Slade is offline
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Thanks, Mike.

I'm almost sorry I asked.

So.... with a turbo running at a pressure ratio of 2.3 I have 230% VE - right?
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  #48  
Old 01-06-2005, 09:19 AM
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Mike,
Surge was explained yesterday this way:

For the others reading this post, low compressor eficiencies result in higher compressor discharge temps. Running the compressor near, on or to the left of the surge line is to be avoided. Surge is essentially a condition of stalled or reversed flow through the compressor and the engine will not run properly if this is encountered. The characteristic sound you hear on turbocharged engines when closing the throttle abruptly is surge and the air is actually flowing backwards through the blades making a chirping sound. This can damage the compressor at high boost if it is severe enough. Some lay people attribute this sound to the wastegate opening (incorrect) as the wastegate is closed when the throttle is closed. Some auto engines fitted with blowoff valves make a different sound, more of a whoosh or hiss as the throttle is closed. These valves are fitted to eliminate surge.
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  #49  
Old 01-06-2005, 10:30 AM
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Quote:
Originally Posted by rv6ejguy
Just sharing my real world experience with people, like anything else you read, you can discount it if you wish.

Hell, I could be wrong too! If so, I appologize to everyone for wasting their time. The Garrett engineers are certainly the real experts since they develop the stuff. If they tell you a T61 will be a good match, I'd listen to them too if they show you the calcs.

The airflow assumptions typically made by many people matching turbos to modern auto engines are just that- assumptions. The often used 80% VE is way off for any modern 4 valve or rotary engines for instance and more suited to '70's- 80s era V8s with lame camshafts and poor cylinder heads. It is easy to verify actual airflow on a dyno or calculate it from hp obtained on a dyno. A Stock Subaru EG33 at power peak is 114% for instance and over 120% at torque peak. Using 80% like is recommended by Turbonetics for matching this engine would result in the mass flow numbers to be off by up to 50%! The whole calculation is invalid at that point. After making hundreds of dyno pulls over the years on my own dyno and other's I can say this for a fact.
OK, Gotcha - my current set up from 1969 is probably the one you describe, but i plan on making a modern intake manifold and exhaust, sooooo it looks like i better get everything running before i pick a turbo, can do.

I know the calcs that were used by garrett and have basically matched them, but they hopefully go out the window once the new parts are in place, i better measure afterwards.

Hey, you are not wasting our time, we are just trying to catch up and understand what is going on and what you are saying, you have a 25 year lead on us

I might suggest that you put an announcement in the for sale - commercial services for those of us that need more specific help, that is what it is there for.
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  #50  
Old 01-06-2005, 04:40 PM
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Quote:
Originally Posted by interestedbutconfused
Mike,
Surge was explained yesterday this way:

Surge is essentially a condition of stalled or reversed flow through the compressor and the engine will not run properly if this is encountered.
I guess I was wondering HOW this happens.

If you are running wonderfully in the sweet spot of the map, what operating conditions put you near the surge line and how does this happen? I can see by the map that it is a high pressure ratio and low air flow.

BTW:

I looked at the T-61 and mapped all the operating points from my spreadsheet. They all pass through the surge line and none even make it into the sweet spot. However, the fit on the Super T04E-40 is MUCH nicer. None go through the surge limit. But you have a problem running at a MAP of 30 inches and 9.6 inches. You're probably good from 11 inches to 27 inches.
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Last edited by mplafleur : 01-06-2005 at 06:15 PM. Reason: Wrong turbo map noted
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  #51  
Old 01-14-2005, 05:00 PM
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Default Compressor surge

Easiest to understand surge perhaps from a quickly closing throttle- while under boost condition. Compressor is spinning at 100K rpm pumping lots of air under pressure. Close the throttle plate, where is all that air going to go in that instant? Can go into the engine any more. Gotta go back out through compressor to low pressure at the inlet.

Operating near surge is like partially closing the throttle, the air delivered by the compresor starts to pile up and cannot be pumped through and you get flow reversals. Same thing happens in gas turbine engines with centrifugal or axial compressors sometimes when inlet vs. outlet flow is unbalanced for some reason. You get compresor stall which sounds more like exploding ordanace and flames as smoke out the front. Bad and scary if you have ever seen or had this happen to you.
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  #52  
Old 01-16-2005, 01:09 AM
eracer113 eracer113 is offline
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This is another reason I prefer superchargers over turbos. there is no chance of over reving the supercharger at altitude. It is a known fact that you can develope more HP from a turbo in comparison to a supercharger, but I personally like a supercharger better. A turbo will do better at 18000 but not too many fly that high including myself. I can not ask any better than + 300 MPH at 15000.

E Racer 113
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  #53  
Old 01-16-2005, 03:14 AM
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David Staten David Staten is offline
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Default Umm...

Quote:
Originally Posted by MarbleTurtle

So... fewer molecules to transfer heat, higher speeds that increase friction.
Umm.. friction is going to vary with the density of the air... Think along the lines of IAS with regards to friction issues.. up high, you will have a higher TAS but maybe the same IAS as down low. Drag/friction will vary according to what the airframe "sees" and "feels" (IAS)

Quote:
Originally Posted by MarbleTurtle
But if the MAF readings are the same at low and high altitudes, how can the mass of air flow be different?
I think that some bad info was inadverdently introduced in another post. Mass air flow readings (pounds per hr or whatever unit you use) that are the same mean that the flow is the same. Some round approximate numbers (which may confict with other posts) lets say you are at 25,000 ft. Up there you are at approx half of sea level pressure. A pound of air up there weighs the same as a pound of air on the ground. The volume's occupied by these masses vary directly (Ideal Gas law is PV=nRT). If everything else is held constant, pressure and volume vary inversely. Its a law of nature.

A good indicator of what "mass" the aircraft is flowing (through an intake, duct, whatever) is what a pitot tube would measure in there. Again.. think IAS (what the aircraft "feels".. the number of molecules hitting the diaphragm in the pitot indicator.. etc). TAS is not a good indicator of mass air flow. TAS is a value that has been corrected for temp and pressure..

So again.. at 25K feet (assume otherwise standard conditions), you may have a true airspeed of 300 (whatever unit...it dont matter).. your indicated airspeed is going to be 200. Your control surfaces are going to feel the same as if you were going 200 IAS at sea level. Your cooling airflow will be the same as if you were going 200 IAS at sea level. You will be moving the same mass of air (pounds per hr, drams per fortnight, again.. your choice) if all other things remain equal.

Engines, however are a different beast.. a normally aspirated engine is a volumetric displacement air pump. It see's volume per time.. at altitude that air is less dense, so you flow less mass (but the same CFM) through the engine unless you have a charged intake system. But if you can GET the same mass flow through the engine (by boost, etc) you can get the same power levels as at sea level.

Dave
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  #54  
Old 01-16-2005, 11:44 AM
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mplafleur mplafleur is offline
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Good examples Dave.

The MAF sensor measures MASS, not volume. So it also measure the number of molecules hitting that hot waire just as the IAS measures the "number of molecules hitting the diaphragm in the pitot indicator".

My example of comparing the two does not have the MAF sensor connected to the engine, but hanging out in the airstream. What the engine consumes is in no relation to IAS.

The original question though, is the amount of air available for cooling the same at different altitudes at the same IAS?
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  #55  
Old 01-16-2005, 12:11 PM
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Dave Moore Dave Moore is offline
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Default Please excuse my newbieness...

In reading these posts over the past few weeks, I had no idea that the MAF sensor in the examples was a hot-wire type, as opposed to the vane type. I did a lot of research on these to try to find a way to get more air into my car's engine as part of a turbo upgrade/engine swap. See my webpage for more details - sorry, not aviation related.
But the way the hot-wire MAF sensor works is to measure the cooling effect of the air moving past it by measuring and regulating the amount of power it takes to keep the wire at a specific temperature. If this is the same setup that I think it is, then it would be a good indicator of cooling effects at altitude in general, especially if the measurements are taken in the airstream and not as part of the engine's intake system. The only problem I can see is that in automotive applications (I have no idea how they're setup for aviation engines! ) part of the "equation" in the cooling effect (and probably parts of other equations as far as the intake system is concerned) is the ambient temperature, hence the 2 or 3 wires, depending on model, that are found poking up into the intake.
I guess the only way to REALLY find out is to hook some up on a test-bed and see how they all read. Get a pitot, a vane type and a few hot-wire type MAF sensors out of a junkyard up there and find out what they tell us! John, as our resident self-admitted "test pilot" are you up for it? If it doesn't violate your phase 1 restrictions, that is...
Again, I know automotive applications, so please excuse my aviation newbieness!
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  #56  
Old 01-16-2005, 12:31 PM
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Default Mass flow

Mass flow at 25,000 feet would be closer to 1/3 that of SL at same TAS. You won't be able to indicate anywhere near the same AS at 25,000 as at SL even with turbocharging. So in a nutshell, you will have far less mass flow available for cooling up there as at SL.

If we take our 1 square foot area duct at SL standard day at 120mph TAS, we'd see a theoretical mass flow of 808 lbs./min. At 25,000 feet, assuming the same temperature, same TAS, you'd only see 300 lbs./min. Granted temp will be less and TAS will be more under real conditions. Count on less than 50% of available mass flow with standard lapse rates and TAS increase under those real conditions.

In extensive testing, we see intercooler discharge temps rise at the same pressure ratio with increasing altitude. Same reason cowl flaps must be opened on some aircraft at high altitude to maintain CHTs in the green. Same reason better intercoolers and radiators had to be developed for the P38 later in the war to make it right.

The old supercharger vs. turbo debate will rage forever I supspect. Compressor type should be specified in these comparisons. None of the Roots types compressors have anywhere near the adiabatic efficiency of modern turbocharger centrifugal compressors hence their discharge temps will always be higher at a given PR. Even most of the available centrifugal supercharger compressors are somewhat inferior to the latest Garrett offerings.

Turbo overspeed is not a consideration at any reasonable PRs to be used in light aircraft below 25,000 feet when the turbo is properly matched. Compressor speed is far easier to control on a turbo via the wastegate than on a belt driven supercharger. Additionally, turbine efficiency improves with altitude as Delta P across the turbine increases.

There were only a couple of supercharged production certified light aircraft engines offered post war. I suspect that the engineers involved in designing the blown installations for all the turbo engines offered, weighed and discarded the supercharger possibility along the way for various reasons.

The point put forth below is essentially true, modern superchargers work just fine at moderate altitudes and pressure ratios and so do turbochargers. Use what you like.
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  #57  
Old 01-16-2005, 01:51 PM
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Here I am trying to get some rest from the bloody meteorology crap I have to learn and then I stumble on stupid words like "adiabatic"...

Well, I guess it's a sign to go back looking at föhn-winds or chinooks as most of you guys would call them
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  #58  
Old 01-16-2005, 02:11 PM
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Quote:
Originally Posted by Control
Here I am trying to get some rest from the bloody meteorology crap I have to learn and then I stumble on stupid words like "adiabatic"...

Well, I guess it's a sign to go back looking at föhn-winds or chinooks as most of you guys would call them
Mr. Control.
now why does a perfectly good Greek word like "adiabatic" sound stupid to you?
Isn't it nice though, when almost all science is based on Greek words?
Now I only wish you would all start using metric and I would be perfectly happy
Oh well, back to the plans ordering process.
Kumaros
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PS For what it's worth, I wholeheartedly hated heat theory too.
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  #59  
Old 01-16-2005, 08:00 PM
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The problem with saying that cooling at all IAS speeds will be the same if we do not count temperature is that dynamic pressure builds at the square of speed. If you can go twice as fast with the same energy you will probably have one forth the air to push through. If you where able to somehow saturate the air with heat just as well at all speeds than you would have half the cooling if you went twice as fast with one forth the air.

But I am no expert, and could be wrong.
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  #60  
Old 01-17-2005, 12:25 AM
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Default Mass flow

The problem is of course that there is no piston powered aircraft that will indicate at 25,000 what is does at SL. Despite somewhat higher TAS and lower temps at altitude, you just won't have the same cooling capability up there.

Worse scenerio mass flow from a cooling standpoint is high altitude, max climb rate speed. These effects can be offset somewhat by maintaining a bit higher IAS is you have a good power to weight ratio, trading some climb rate for increased mass flow. I use this on the RV6A on hot days, doing more of a cruise climb at 100 knots instead of 80-85. Helps the intercooler discharge temps a bit also.

Once you are in cruise, most aircraft will have an excess cooling capacity, at least at moderate altitudes. Here, it would be beneficial to have a cowl flap to close up to cut the drag a bit. WW2 aircraft all had these for good reason and these designers forgot more than any of us will ever know about this whole subject.
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