The reason I am so grumpy about this subject as I have
been battling with people for many years about the intake
design. I had a brief friendly discussion with Bill Eslick at
OSH this year on this subject. Bill has 400 hours on his
engine and he has changed the intake design at least once.
He seemed resigned to having a 150 mph airplane.
I know he is interested in going fast as he entered the
Dayton to OSH race last year. He did not do well. Three hours
and eighteen minutes or 142 mph. Most RVs are averaging around 200
mph in this race and Dave Anders did 244 mph in his RV4
this year. Bill's current intake is a long dynamic chamber which
I stated over and over again a few years ago is not
the right thing to do. Even Tracy abandoned it after he convinced
a lot of people it WAS the way to go. Chuck Dunlap's airplane
had something similar with equally disappointing results.
Mistral spent a lot of dyno time and money on the side port intake
and they got only 190 HP at 6000 rpm or so.
People think the RX8 engine is a magic 240 HP engine all
by itself regardless of the intake. It WILL generate 240
HP at 8500 RPM with the right intake system. That is 28 HP
per 1000 RPM. So 7500 is 212 HP and 6500 is 184 HP.
No getting around it. Putting it another way the max torque
on the side port engine is about 160 foot pounds maximum no matter
what you do. It is, after all, only a 160 cubic inch engine.
The p-port on the other hand is an easy 200 HP no matter
how you mess up the intake system. 200 foot pounds of torque
is no problem. Moving the torque peak up by shorting the
length of the runners results in a lot more HP. 300 HP at
9500 RPM with 2 inch diameter runners and a slide throttle
is no problem at all. We demonstrated 240 HP at 6500 RPM
on the dyno using about 2 inch runners and a carb no less.
The other major factor is the skin friction drag inside
the runner tubes. Four, 1.5 inch diameter runners, have four
times the surface area and skin friction drag as two 2 inch diameter
runners if my arithmetic is right. Probably more as the
air velocity could be higher. This increases the pumping losses.
The high power RX8 six port/runner engine is even more of a
nightmare. Mazda had to do it to get more torque at 2000 and
3000 RPM for car use. The four port/runner RX8 engine is worse
as the breathing is becoming limited over about 6000 to 7000 RPM.
A p-port intake system is easier to build as there is no flange
bolted to the side of the block. Just use the slide throttle
design on the web site.
Ironically there is no BSFC advantage with the side port.
Probably on the contrary as the pumping losses are higher
with the side ports and multiple runners.
Don't get hung up on low RPM. Forget everything you know about
piston engine RPM. The rotary is a different animal. It is
more akin to an electric motor than a piston engine.
There are no parts that jerk back and fourth, fatigue and
crack. Forget the 2.17:1 gear box. It is ancient history.
Go for the 2.8:1 gear box or even 3:1. Also go for a constant
speed prop if you want a lot of MPG.
I'll be talking more about props soon.
Paul Lamar ...No rotor no motor.
Paul,
In your defense of the P-port.
As a P-port convert I recently went over two years and many folders of
print
outs and notes. It was obvious to me my early interest for the Renesis
engine was based on information pertaining to automobile usage and the
'newest is best' mindset. For those of us after the big horses (over
240Hp)
in a small package, the P-port manifold is the least complicated and
lightest manner to get the required volume of air in the engine. This is a
fact not opinion. Mazda engineers have told me as much repeatable. Final
sunk in I guess.
I found it useful to make long lists evaluating my needs, the possible
engines configurations, PSRU ratios, and propellers to accomplish the job.
Excuse the evangelizing. New converts are usually the most enthusiastic:)
DougFir
Took long enough Doug!
George (down under)
For the new guys here is the Guru racing beautiful p-port
pictures again.
I don't know how much HP this beast has :)
Paul Lamar
Paul, what is the purpose of having two injectors per port?
Regards,
Gordon.
One to get enough fuel for high power. And two to act as a back
up. Tracy's computer stages them. One is used up to 15 inches MAP
and then both kick in. This gives more resolution at low
power.
Paul Lamar ...No rotor no motor.
Paul,
I want to boost my engine from 8K feet with the supercharger sized for
normalized cruise power at 15K. I am wondering if I need to include
adjustable length intakes for this engine. It would seem that a system that
is under boost from a supercharger or turbocharger wouldn't derive the
benefits from a tuned intake manifold. Perhaps for the dyno test engine when
I start with N/A mode it would be useful to have.
Another thought on intake length tuning. As I understand it the proper
length of intake manifold creates a resonate within the pipe depending upon
the opening size of the throttle plate and size of pipes. Other variables
like temperature and intake manifold pressure also affect the best design
point length. I guess without a system that can be tuned in flight our
'best lengths' at at sea level will be off design point at 8K+. By how much?
is the question. Lots of speculation now.
This begs the following thought and I know you have said it more than once
and it with every new flying rotary airplane it the following appears to be
true.
If the intake system is designed with large low resistance pipes, and uses
a large low resistance air filter, (see R.B) and absolutely positively is
attached to a dedicated intake plenum picking up outside cooler rammed air,
then the engine essentially operates under boost. Sure it may only be 1.5
to 2 psi of rammed air but that should make this engines perform a lot
better and run cooler as well. Go faster as well. Then the question 'How
long is your (intake) pipe?' is less important:)
DougFir
Mazda does not agree with you. All stock turbo Mazda engines have had tuned
intakes. However at low boost they perhaps were counting on
that to improve the 3000 RPM torque (dynamic chamber) and lower the BSFC.
The pipe length should be about the same as a NA engine. Namely around 24 to 28 inches.
The HP curve with a fixed length pipe is surprisingly flat relative to the
needs of an aircraft engine that takes off at 7500 and cruises at 5500 to 6000 RPM.
See the carb P-port dyno curve attached.
The last curve perhaps gives us a clue why the cut off stock
RX8 manifold HP is dropping off so much at 7200 RPM as Ian
noted. BMEP is about the same thing as torque. This is not
HP of course. You need to calculate the HP from this curve
after converting BMEP to torque.
Paul Lamar ...No rotor no motor.
The Rotary Engine NewsLetter. Powered by Linux.
ACRE NL web site.
http://www.rotaryeng.net
Copyright 1998-2006 All world wide rights reserved.
