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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