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  #16  
Old 04-18-2007, 03:35 PM
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i will be using all of mine at full power at 25000 feet
Hmmmmmm......should be able to make any field within 100 miles in the event of an engine out.
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  #17  
Old 04-18-2007, 03:38 PM
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still don't get it. i thought the P/R was the ratio between ambient air pressure and air pressure "in the engine". so at 10.000 ft 2:1 meaning 1013 mbar in the engine (as there is at MSL) and 500 mbar at 10.000.

if this is correct, where's the problem ? as you are pumping in the 100 gals of MSL air (so to say 200 gals of 10.000 ft air), the same amount of volume will exit the engine again, thus providing enough force to drive the turbo at 10.000 just as it does and MSL.

or is the P/R related from one side of the turbo to the other (turbine to compressor) ?
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  #18  
Old 04-18-2007, 03:47 PM
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or is the P/R related from one side of the turbo to the other (turbine to compressor) ?
yes - thats it

the turbo is a pump that is powered by exhaust gasses being pumped out, only so much power available the the turbo can use.

As an example, I have been told that exhaust augmentation does not give much effect as the exhaust that is leaving the engine, "post turbo" has given all its energy to the turbo and does not have any left to pull air through for cooling.
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  #19  
Old 04-18-2007, 03:49 PM
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ah ok, this lightens up things. so a P/R of 2:1 means that for every 1 exaust gas going out, 2 fresh gasses are sucked in ?
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  #20  
Old 04-18-2007, 07:52 PM
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nope, it means that on one side of the compressor, the pressure is x and on the other side, once it is compressed, 2x

on the exhaust side, i gots no idea
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  #21  
Old 04-19-2007, 02:50 AM
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ok so it is as i suspected in the first place.

P/R is compressor intake pressure compared to compressor outlet pressure.
the turbine pressure has nothing to do with it except provide power.
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  #22  
Old 04-19-2007, 08:31 AM
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2 rotor engine:
291 Lbs / 190 HP (Naturally Aspirated Rotary)
328 Lbs / 230 HP (Turbocharged Rotary)
Those numbers are about right for a stock 2nd gen engine. According to leon promet, go to a 3rd gen with a little "die grinder therapy" on the ports and high compression rotors and you'll get about 40 more horses. Of course the rpm and boost make a big difference. Peak power is at about 6300 - 6500 rpm.

Quote:
I have been told that exhaust augmentation does not give much effect as the exhaust that is leaving the engine, "post turbo" has given all its energy to the turbo and does not have any left to pull air through for cooling.
Whoever told you this is wrong. I'm convinced that my (rudimetary) exhaust augmentation system is an important ingredient in my cooling system. Remember that a lot of exhaust is bypassing the turbo via the wastegate. There's plenty of energy left.

PR ratio choice for your T04 is an important issue. Based on RV's advice I chose 0.96. The Girrrls went with higher - 1.15, I think. This will limit me to about 30 map @ 18000' while the girrls will be able to normalize higher, but it gives me more power at SL.
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  #23  
Old 04-19-2007, 12:58 PM
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Whoever told you this is wrong. I'm convinced that my (rudimentary) exhaust augmentation system is an important ingredient in my cooling system. Remember that a lot of exhaust is bypassing the turbo via the wastegate. There's plenty of energy left.
Nah - not wrong, just my design parameters are different than yours. Mine is to use all power for the turbo, leaving nearly none for augmentation at altitude.

What you say about your turbo and the girls does not add up, ifin it can produce more boost at altitude, if can also do it at SL
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  #24  
Old 04-19-2007, 01:07 PM
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John, is would be real interesting to see a reading of the airflow through your augmenters, eracer is real knowledgeable on how to do that.
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  #25  
Old 04-19-2007, 05:24 PM
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The turbine A/R ratio controls the exhaust gas speed at the tip of the turbine wheel. The smaller the number, the higher the speed and the higher potential to recover more energy from the exhaust.

John's .96 ratio would theoretically allow him to achieve higher boost at any given altitude than CGs 1.15 before the wastegate was fully closed. It will also give a higher critical altitude and allow more latitude when using a variable pitch prop in power settings. To cruise most efficiently, we usually want to use lower rpms and higher manifold pressures. This cuts frictional losses in the engine and recovers more work from the compressor during the intake stroke. The larger A/R ratio may not allow you to cruise at say 18,000 feet at 5000 rpm and 35 inches because even with the wastegate fully closed, there is insufficient energy in the exhaust stream at the turbine to spin the compressor at the required speed.

For wastegated engines operating over a range of power settings and rpms, there is little positive effect in using larger A/R ratios. We see little or no difference in ultimate hp between two A/Rs when an external wastegate is used. With integral wastegates, there are measurable differences.

There is still plenty of potential energy in the exhaust stream after exiting the turbine housing for augmentation purposes. Remember that the exhaust stream has almost 50% more potential energy than we obtain at the crank or eccentric shaft. We generally see a 150-200F EGT drop across the turbine. This is indicative of the work being performed. Exit velocity is mainly a function of exhaust mass flow and pipe cross sectional area.

Last edited by rv6ejguy : 04-19-2007 at 05:39 PM.
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  #26  
Old 04-20-2007, 02:25 AM
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ok so, once again, for the stupid me

PR is the relation between air sucked in and air pressed into the engine. This gives me the maximum manifold pressure, right ? so lets say I have a 2:1 ratio, this means i can achieve a maximum of (29.92*2) 59.84 InHg, right ? This is independant of altitude, as long as the MASS FLOW through the turbo is high enough. The Waste Gate will let the excess pressure pass right into the exhaust manifold.

The higher you go, the less is passed through the wastegate,up to a point at which the wastegate is closed to sill provide 100% boost pressure. This is "Critical altitude" ? ( i guess,as upwards of this alt you cannot maintian your desired boost pressure )

The PR ratio does not give any indication of how powerful the turbo is in the sense of how much air MASS can be moved, right ? a small and a large turbo can both give you a 2:1 ratio, the smaller one will just not be able to provide it as high up as the larger one, simply for the fact, that if you choose a well oversized turbo, on ground and low levels you will be sending alot of pressure out the wastegate, as there just is alot more air volume available. again, as you climb higher, the smaller turbo will find it's limits at lesser altutides that the bigger turbo.

So: can turbo's be measured in CuFt (Air) per minute (at full RPM) ?
you can calculate what your engine needs per minute, and then try and find a turbo that'll give you this number x2 and you're good up to 10.000 ft (half ambient pressure, so douple the volume of air(10.000) has to be sucked in to provide MSL power)

if you have a smart engine that measures air intake (as most modern car engines do) and just wan't a little boost for T/O, why not choose an electric turbocharger. this might give you an extra 10% HP just for take off (of course, NOT at altitude)
Dunno if these things work though ... just saw em on some page
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  #27  
Old 04-20-2007, 09:59 AM
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the waste gate lets exhaust pass by the turbo, not excess compressed air out, no need to compress it and then waste it. the only thing that would do what you describe, letting compressed air out, would be a "pop off" valve that protects against excess manifold pressure.

as far as adding 10% power just for take off, never heard of it being done cause the real benefit, in my eyes, it to have more power available at altitude.

If the N/A engine is underpowered at take off then it will probably be underpowered at all altitudes and need some help.

I am adding a slight amount of power at SL, the normal Manifold pressure would be about 27 or 28 inches of mercury, i will stay with my engines design amount of 31.5 , this gives my 360 cu inch engine 210 hp.
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  #28  
Old 04-20-2007, 10:03 AM
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Quote:
Originally Posted by rv6ejguy View Post
There is still plenty of potential energy in the exhaust stream after exiting the turbine housing for augmentation purposes. Remember that the exhaust stream has almost 50% more potential energy than we obtain at the crank or eccentric shaft. We generally see a 150-200F EGT drop across the turbine. This is indicative of the work being performed. Exit velocity is mainly a function of exhaust mass flow and pipe cross sectional area.
GREAT INFO, glad i mentioned the info i had found elsewhere. When i heard that info, I did not "like" it, so, contrary to most peoples opinion of me, I tend to believe info i do not like easier than info that i do like.

I gonna write a book on turboing, it will be easy - I'll just take all of rv6ejguy's posts and publish em - tanks again
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  #29  
Old 04-20-2007, 11:26 AM
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Quote:
Originally Posted by Leon View Post



The PR ratio does not give any indication of how powerful the turbo is in the sense of how much air MASS can be moved, right ? a small and a large turbo can both give you a 2:1 ratio, the smaller one will just not be able to provide it as high up as the larger one, simply for the fact, that if you choose a well oversized turbo, on ground and low levels you will be sending alot of pressure out the wastegate, as there just is alot more air volume available. again, as you climb higher, the smaller turbo will find it's limits at lesser altutides that the bigger turbo.

So: can turbo's be measured in CuFt (Air) per minute (at full RPM) ?
you can calculate what your engine needs per minute, and then try and find a turbo that'll give you this number x2 and you're good up to 10.000 ft (half ambient pressure, so douple the volume of air(10.000) has to be sucked in to provide MSL power)

if you have a smart engine that measures air intake (as most modern car engines do) and just wan't a little boost for T/O, why not choose an electric turbocharger. this might give you an extra 10% HP just for take off (of course, NOT at altitude)
Dunno if these things work though ... just saw em on some page
It's true that a smaller compressor will also make the same PR as a larger one in many cases but it will not flow the same amount of air. As you climb higher, the compressor has to pull in an increasing volume of air. When using a small compressor, this increases compressor discharge temperatures (CDT) and N1 speed as well. Finally at some point, the small compressor is at or above its N1 limit and CDT is perhaps exceeding 300-350F. Not good for pre-ignition margins or turbo longevity. Often a larger compressor will still be well matched at low boost near sea level. We also have to be concerned about high altitude compressor surge in many cases as well. These are all good reasons to run the math to match the compressor for the intended mission.

Yes, compressor maps are provided for matching purposes. These show compressor flow in either CFM or lbs./ min. Power is more dependent on density ratio than pressure ratio but we must first establish PR to find DR. There are many other factors involved than just hp vs. manifold pressure which is why it is almost impossible to predict hp on a turbocharged engine and indeed even come up with correction factors when dynoing them. Most engine dyno software apply invalid (typically standard SAE) correction factors which is why you see some crazy numbers these days.

Electric superchargers are essentially useless and demand very high current for very low PRs. A typical turbine may deliver 25-35 hp to the compressor, say 20,000+ watts or 1500 amps at 14 volts roughly. You can quickly see the limitation here.
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  #30  
Old 04-20-2007, 11:58 AM
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as far as adding 10% power just for take off, never heard of it being done cause the real benefit, in my eyes, it to have more power available at altitude.
I think you'll learn with experience that the real benefit of a turbo is shorter take-off and quicker climb. Despite your plans for high altitude travel I doubt that, in the real world, your Cozy will ever see the higher flight levels. I've found 10 - 12,000' ideal for cruising.
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