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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