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Hi Dave,
Yes, the compressor map only knows its input
(not the engines input - lots of confusion out there on this point) Some
folk use the engine's CFM which is OK IF you adjusted for the boost as you
did. You appear to have the concept down.
However, I am a bit confused, I notice your have a
13B, yet you use 222CFM at 3000 rpm - which is about twice what you should be
getting out of a 13B, I get about 153 CFM. Wait! 153*1.6 = 244 CFM, Ok I
see where you are coming from. Now the crucial question is - does your
engine produce enough exhaust mass flow to drive the T06H turbine wheel
fast enough to produce 1.6 pressure ratio at sea level? If it does then
6000 rpm underload will likely produce much more than 1.6 pressure ratio.
What I am saying is if you could get 1.6 at 3000 rpm then you would
unquestionably need a waste gate to keep from blowing your engine at 6000 rpm
under load.
Also notice I keep referring to Load. If you
are not producing a load on the engine (like when you are at idle) then
your exhaust mass flow is very low and therefore so is your turbine speed and
boost (you may be getting some, but not enough to get your boost gauge much past
atmospheric) in none existent.
That is what makes it difficult to predict exactly
what is going to be the end condition.
Yes, you are correct, if you have to err then error
toward the side of less efficiency - that means you air will run hotter, but
surge can destroy a turbocharger and damage an engine if encountered, so I would
stay away from the left side.
Now one thing about the T06 - its not the best
match for the 13B in my opinion - not to say it can not be used. But, lets
say you only want to run 2- 3 psi of boost on an extended climb to keep temps
down (hypothetical case). That would gives a pressure ratio of (taking the
3 psi). of (14.7+3)/14.7 =.17.7/14.7 = 1.2 pressure ratio which is off the map
for the T06 as you can see.
The T04 is a better choice in my opinion since we
are unlikely to be running above 10 psi of boost - at least I would not in an
aircraft application.
Selection of the compressor wheel is 1/2 of the
challenge. Compressor maps and an understanding of your engine flow
capability and how to compare compressor maps takes care of that part. The
second half of the equation is getting the right turbine wheel and correct A/r
size for your Turbine housing. The is no cut and dry formula for
that although Corky Bell in his book "Maximum Performance" has some
general guide lines to get you in the ball part.
Even with our stock Mazda which is apparently
excellent on the second generation Turbo engines turns out not be the right
combination for aircraft. It was designed to produce lots of boost at low engine
rpms (read low exhaust mass flow) and the engineers decided that most folks
would not keep their foot to the floor for over a few seconds. Well, that
turbine housing was specifically designed for low rpm acceleration rush, high
engine rpms for long duration are simply pushing the stock turbo beyond its
design and intended limits as configured for the auto. An Aussi company is
making some modifications to the stock turbo which make it much better suited
for our aircraft needs in my opinion.
John Slade should be receiving his in a week or so
and hopefully we will see how it does.
In any case, Dave, you are on the right
track. I especially like the fact your are considering the effect of
"boost creep" caused by lessen ambient pressure at altitude - not everyone
considers that important factor. They may luck-out or not.. Failing
to do that can have you trying to use a turbo that is near or in its surge zone
under boost at altitude.
Hope this helps
Ed
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
----- Original Message -----
Sent: Sunday, August 22, 2004 11:39
AM
Subject: [FlyRotary] Re: Compressor
maps
Ed,
I am trying to work my way
through the compressor maps and formulae that I have stumbled across in
the past few days, and looking at your formula, I am able to make sense of
it.. and I am using a website with a calculator built in that reproduces your
values...but the value of 277 cfm.. thats how much air the engine is pumping
normally aspirated at wide open throttle.. right?
So plotting
this at 1.0 atm and 277 cfm wouldnt event be on the "map" at all.. If I am
understanding this at all.
Using the TD06H-20G map listed at http://cybrina.mine.nu/MR2_Docs/compressor_flow_maps.htm
(and attached to this mail)
if I wanted to maintain 23 PSIA (8.3 PSIG)
boost (48" map). which comes out to a presure ratio of 1.6..
. then I would
need to calculate the CFM required at 1.6 PR..
At 3000 rpm I am
getting 222 CFM and
At 6000 rpm I am getting 444 CFM... (1.6 PR),
which
falls in the map on that site....
To take it further.. if I
wanted to maintain that amount of boost (23 PSIA/8.3 PSIG) to say.. 12,000
feet..
Ambient pressure at 12k is a ballpark value 18"/9psi.. the pressure
ratio there is 2.5...
3000 rpm, PR 2.5, comes out to 347 CFM.. appears
to fall just left of where I'm told the "surge line" is
6000 rpm, PR 2.5
equals 697 CFM and falls to the right of the 65% efficiency island..
If
I am interpreting this correctly that means that this turbo would be able to
give the desired performance but would NOT be able to maintain 23" absolute
boost to 12k feet....
5000 rpm at 2.5 PR gives 578 CFM and DOES fall
within the map though...
I am throwing all this out there to see if I
am grasping the concept clearly or not.. and to see what I need to adjust
conceptually..
I am also guessing that the .55 value listed below by
Mike, is BSFC.. correct?
I am trying to comprehend the
"volumetric efficiency" concept as it relates to pressurized/charged intakes..
are we assuming a value of 1.0/100% or is this not realistic? What about
porting... will that improve the VE? (I'm wanting to say YES) Iis it realistic
or even possible to have a VE > 1.0/100%?
The reason for all
the head scratching on my end is that I have several turbo 13b cores.. but
none of them have turbo's attached.. so I get to pick what I want on there..
but I want to make sure I have a grasp of the concept before I start turbo
shopping..
I must admit that this listserv group has been a
literal gold mine of information. I have exposed to a truckload of data in the
past 2 weeks and am just trying to organize it at this
point.
Dave Staten
2nd Gen 13B rebuild in progress..
Ed
Anderson wrote:
Mike makes a good point. While the official displacement of the 13B is
1.308 liters (actually 1308 cc), for airflow/power purposes it acts the same
as a 4 cylinder engine of 40 CID (each cylinder) or 160 CID or 2.6 Liters.
CFM = (4*40)RPM/(1728*2) so for 6000 rpm, CFM = 160*6000/(1728*2) = 277.77
CFM at 100 Ve
Some compressor maps use CFM on the X axis and some use mass flow usually
Lbs/min. My spreadsheet automatically calculates the lbs/min of air flow
for every rpm and power situation. Or its easy enough to calculate.
Just multiply your air flow volume in CFM by 0.076 for sea level density.
So in this case 277.77*0.76 = 21.1052 lbm/min
Ed
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
----- Original Message -----
From: "Mike Robert" <pmrobert@bellsouth.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Wednesday, August 18, 2004 11:14 AM
Subject: [FlyRotary] Re: Compressor maps
John Slade wrote:
John, here is a URL to a pretty good discussion on reading compressor
maps with examples
http://cybrina.mine.nu/MR2_Docs/compressor_flow_maps.htm
That's great, Ed. And you did it in one sentence. :)
Do you happen to know the engine capacity (in litres) and the
volumetric efficiency of an REW engine?
Regards,
John
John, the auto people use 2.6 litres and .55 for those turbo map calcs.
HTH, Mike
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