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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