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Good points to discuss, Dave.
Actually, so long as there are
pulsations in the intake manifold you can have the "Standoff" or fuel mist at
the entrance to the intake. Anywhere there is overlap between intake and
exhaust you have the potential for the standoff. To me this
phenomena appears meet the definition of a "standing wave"-
but, regardless of what you wish to call it. The problem is
real.
While it is true there is technically no "valve"
overlap in a rotary (after all - it has no valves) - there is
the equivalent with "Port overlap". The rotary
does has overlap between the exhaust and intake cycle and the phenomena of
powerful pulses being generated in the intake manifold by the exhaust residue
when the intake port opens. So we do have the necessary conditions in the
rotary's intake.
Now this applies to the older 12A and 13B engines,
by moving the exhaust port to the side housing, the newer Renesis engine
eliminated port over lap between intake and exhaust. So the potential for
"Fuel Stand-off" should be non-existent or certainly
lessened.
While "Fuel Spitback" was first noted with
carbuerated systems the phenomena is really not directly caused by carburetors
- it is a matter of pulses in the intake which are of sufficient magnitude
and frequency to resonate with the intake length to force fuel particles to flow
out of the intake. Such a pulse (Finite Amplitude Waves) can produce in
the range of 12 psi localized pressure as the wave front moves down the
intake. It can and does force air and fuel molecules in its direction of
travel which can be directly opposite to the macro air flow into the
intake.
There are standing wave patterns in practically all
intakes, however, most of the time they are not at the entrance to the intake
and you never see them and the fuel does not exit the intake. The location
of these patterns (stand wave nodes) vary depending on any number of
conditions but primarily their frequency - which is primarily related to
engine rpm.
You are correct - in that Fuel injected
systems normally do not have much problem because the
fuel injection generally occurs far down stream from the intake opening and near
the combustion chamber port. However, if the injectors are near the intake
opening (similar to a carburetor) as they are on the TWM throttle bodies (I
know I first flew with a TWM throttle body with 4 injectors) then you have a
greater probability of encountering the phenomena. Shorter intake tubes
appear to be more susceptible to this problem than longer ones.
If you have the intake enclosed in any sort of
Plenum then you may not notice the effect and since the fuel is constrained
inside the plenum where it is eventually "sucked" back into the intake and
it probably poses little if any problem. But, unconstrained (as pointed
out in several of the examples) and Dave's description of the fuel smell, the
fuel can be dispersed through-out the engine bay - definitely not a good
thing.
On the web "Fuel Spitback" is also used to refer to
the "slosh" or "splash-back" when refueling a tank which apparently is caused by
fuel flow pressurizing the tank until it "spits" back out the fuel tank
opening. But, the term is also used to refer to the phenomena we are
discussing.
So whether you call it a "standing wave", "Fuel
Standoff", "Fuel Spitback" or something else, the conditions necessary for it do
exist with the rotary as well as any engine with intake/exhaust overlap.
Porting of the intake and exhaust even acerbates the overlap and increases the
potential for this phenomena as do having the injectors near the manifold
inlet. It appears that an enclosure (Plenum) is a common means of dealing
with the problem.
You can decide as to whether you believe it applies
to this phenomena
Best Regards
Ed Anderson
----- Original Message -----
Sent: Thursday, October 28, 2004 9:18
AM
Subject: [FlyRotary] Re: Fuel
Standoff?
Ed,
What you are describing as a
"standing wave" appears to be a separate and different matter than "fuel
spitback" as described below.
My understanding of spitback as
described below is both intake and exhaust ports being open at once and
communicating, allowing high pressure exhaust to outflow through the intake
slightly before being sucked in by the intake stroke, and this "intake
outflow" causing the spray of fuel.
The spitback phenom, as
described in recips with valves, seems improbable in rotaries as long as there
is not overlap to the porting that allows the same chamber to be open to
exhaust AND intake. Likewise, the spitback prob appears to be a carb issue,
not likely to be reproduced with fuel injection, if the injection of fuel is
timed to occur optimally during the intake portion of the combustion
cycle.
As for a standing wave phenom, that would be interesting
if that comes into play here... I am all ears with regards to finding out what
is causing Dave L's probs. I hope he can resolve it to his wife's satisfaction
such that she will fly with him again in the RV.
Thanks for
passing on the info, regardless of how immediately pertinent it is... makes me
think and figure things out.
Dave Staten
Ed Anderson wrote:
Hi Dave,
Here is something to consider. With certain
lengths of intake combined with certain engine RPMs the pulses
generated by the rotors in the intake can actually cause a condition
referred to as "Standoff". This is where the pulses actually cause a
"Standing wave" of fuel particles ejected by the pulses at the
entrance to the air intake.
There is some information on the web about "Fuel Standoff" or "Fuel
spitback". Here are a few examples.
We noticed a phenomena, after a race
the engine cover was wet with fuel, the phenomena is called fuel standoff or
spitback, during a lecture in the States, Allen Lockheed(son of the founder
of Lockheed aerospace) alluded to the phenomena saying that this is caused
because the exhaust is out of tune and the pulses were pushing the fuel out
of the intake.
Posted a question a while back about my
1986 175 Merc spitting out gas from thr carbs. I'm in the process of
reviving this old boat so I've not fooled with it much since i posted last
(been working on other stuff). Some suggestions were bad reed valves, gummed
up reed valves, stuck floats or cranks seals. Well this evening I had her
running with the carb air box cover off. It's not spitting gas, its actually
blowing the gas out the throat of the carb. The middle and bottom carbs are
the worse. One side of the top carb is doing it a little. It is blowing so
much out, the gas pools in the throat and runs down the air box. I also
noticed once while cranking it over the middle and bottom carbs puff out
white smoke a time two like it was exhausting thru carb.
Subject: velocity stacks and
stand-off
While we were dynoing this 3/4 race engine
(280 degree duration cam) installing the velocity stacks in every case made
the fuel "stand-off" disappear. Without the velocity stacks there was a
cloud of fuel in front of the carb inlets that you could feel the wetness on
your hand up to over 1 ft away. The fuel seemed to be in constant motion
into and out of the carb throat. the fuel did not seem to "blow away" into
the dyno room. When the velocity stacks were installed, the cloud was no
longer there, and the engine made slightly more power from the point where
it came on the cam, ie. about 3200 rpm up to around 4500 rpm. Above that rpm
the power was always less with the stacks than without them.
There is however a mechanical downside
to the Weber 4BIDA, and that is that they are somewhat hard to tune. The
carburetor's fidgety nature has been known to produce a
potentially-fiery-phenomenon known as "Fuel standoff," which can transform a
race car into a smoldering pile of ash in short order.
.....is correct on both
valves being open at the same time (even if just a little), the longer
duration ,usually the more overlap. This is actually used to create a slight
intake "siphon". The longer the overlap and the more combustion gas will
enter the intake tract. In engines that have short intake tracts with
multiple carbs (like Webers) you will actually get a fuel "standoff" above
(or beside if they mount crossways)the carbs.
So, Don't know if that could be part of your problem or not, but
thought you might want to know. It would probably only happen at
certain rpms.
Ed Anderson
----- Original Message -----
Sent:
Thursday, October 28, 2004 12:40 AM
Subject:
[FlyRotary] There I was, half way to Vegas...
So as you know I have converted to a normally aspirated state.
I went flying to vegas this weekend, and actually won $300 net even after
my wife lost $100.
The engine ran as well as coulld be expected considering my sea level
WOT MAP is only 28". Max n.a. static is 46-4700 RPM. Plenty
enough to take off well over Van's recomended gross and fly at 130 KTAS
(using only 8 gal/hr). Better than a typical cessna - but less tan I
had with the turbo. Anyway,
The point of this e-mail is to discuss injector backflow... I
have always had a problem with a faint fuel smell in the cocktpit.
As part of the n.a. conversion I removed the plenum over the intake (see
pic.). Every flight since removing that plenum has included a rather
strong smell of fuel coming in theough the passenger air vent. This
smell is only evident after take off. It turns out that my TWM
secondaries, mounted on a downhill part of the intakes, freely leak fuel
down and out of the bellmouths even during WOT operations. This
leaked fuel then seeps out the cracks in my cowl and goes into the
passenger air vent. It also loostly covers everything on that side
of the engine, and the side of the fues around the pax air intake with
2-stroke oil. This was enouth to make me uncomfortable. The
wife, who is still reeling from the emergency landing after the turbo
broke, now states that she will only fly cessnas from now on - due to the
very disconcerting smell of fuel the whole trip - and I can't blamer
her.
While in vegas I tried to program the 'B' computer to use the
secondarys only above 27" MAP but I guess the adjustment did not take
because we still had the same condition on return. I have checked
and re-checked the fuel system far any evidence of a leak. It is
coming out the back end of the intake for sure.
Something to think about if you are installing the TWM injectors near
the air inlet end.
Dave
Leonard
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