Rotating Injectors Among Cylinders

Something to be aware of here... it was the "Coke bottle test" that got
George into all the GAMIjectors to begin with. He didn't like the way
the engine ran. So, although of questionable legality, played with the
"Coke bottle test" until he got all the injectors to put out the same
amount of fuel. ... And the engine ran WORSE. He figured he'd spend
the rest of the day figuring out why... and a couple of years later, he
had! G

What you want is the fuel/air mixture to be the same for ALL cylinders.
Ideally this would equate to the same amount of fuel in each, but in
the real world it does not. When set up right, since all cylinders
have the same FA mix, they will all peak at the same time. Differences
in probe positions, and differences in cooling air flow, will generate
different measured EGT's (as well as condition of plugs, mags, etc.).

With standard injectors just rotating them should be okay (and legal)
-- all you are doing is using the really terrible QC from the
manufacturer to your benefit. Traditionally the Coke bottle test
involves also ordering sets of different orfice dimensions form the
mfr. and adding them into the mix. This messes up the measured fuel
flow readings and required recalibration of the fuel metering system.
[Hence the reason the FAA frowns on it.]

Frankly, 110 max difference in EGT isn't meaningful. As someone else
suggested, look at the spread in FF vs. peak temp for each cylinder.
THAT will tell you if you have a problem or not.

The difference is 110 degrees between the coldest and hotest cylinder.
A colleague of mine says that is a bit high for a fuel-injected system.
Is that right?

That's not right. It's not even wrong.

EGT is not of much value as an absolute measurement. That's what you
have TIT for. The sort of difference you have can easily be explained
by a slight variation of probe installation - if the #1 probe is a
little farther from the exhaust port than the #5, that can explain all
the difference.

The important question is this - do all the EGT's peak at the same fuel
flow? The thing to do is make incremental adjustments with the
mixture, and at each setting record fuel flow (0.1 gph increments are
good) and EGT on each jug. Then you generate a curve of EGT vs fuel
flow for each of the jugs. As long as they all peak together (meaning
at the same fuel flow), absolute differences mean little.

If they don't peak together, you can try swapping injectors around.
It's not strictly legal - but it's not actually illegal either. There
is no requirement to keep track of which injector goes to which jug
when they're cleaned, since in theory they are all the same diameter.
In practice the published tolerances are ridiculously wide, and half of
them don't meet spec anyway, but let's not confuse the issue with the
facts. If you "accidentally" got them reversed next time they were
cleaned, well, these things happen.

That used to happen a lot on my engines, until they strated running LOP
smoothly. Ever since then, I've kept track of which injector goes to
which jug. No rule against that either, you know.

Michael

"Michael" wrote in message
ups.com...
If they don't peak together, you can try swapping injectors around.
It's not strictly legal - but it's not actually illegal either. There
is no requirement to keep track of which injector goes to which jug
when they're cleaned, since in theory they are all the same diameter.
In practice the published tolerances are ridiculously wide, and half of
them don't meet spec anyway, but let's not confuse the issue with the
facts. If you "accidentally" got them reversed next time they were
cleaned, well, these things happen.

That used to happen a lot on my engines, until they strated running LOP
smoothly. Ever since then, I've kept track of which injector goes to
which jug. No rule against that either, you know.

Let me understand this: you achieved smooth LOP operation by playing
"musical injectors"?

"jmk" wrote in message
oups.com...
Something to be aware of here... it was the "Coke bottle test" that got
George into all the GAMIjectors to begin with. He didn't like the way
the engine ran. So, although of questionable legality, played with the
"Coke bottle test" until he got all the injectors to put out the same
amount of fuel. ... And the engine ran WORSE. He figured he'd spend
the rest of the day figuring out why... and a couple of years later, he
had! G

Do you have a source for this?


What you want is the fuel/air mixture to be the same for ALL cylinders.
Ideally this would equate to the same amount of fuel in each, but in
the real world it does not. When set up right, since all cylinders
have the same FA mix, they will all peak at the same time. Differences
in probe positions, and differences in cooling air flow, will generate
different measured EGT's (as well as condition of plugs, mags, etc.).

http://www.avweb.com/news/reviews/182558-1.html (Mike Busch)
"The first theory advanced was that the front cylinders had to breathe their
induction air through a longer path than the rear cylinders and therefore
were getting less air, while the rear cylinders had a shorter induction path
and got more air. If the rear cylinders got more air but the same amount of
fuel, that would explain why they ran leaner and reached peak EGT first.
Conversely, if the front cylinders got less air but the same amount of fuel,
that would explain why they ran richer and reached peak EGT last.

It was an appealing theory. But it was wrong! In fact, all cylinders in
these engines breathe very nearly the same amount of air.

After investigating further and consulting with a very smart ex-TCM
engineer, Braly discovered the real reason why the mixture distribution was
skewed. The TCM fuel injection system is a continuous-flow system, which
means that each injector nozzle sprays fuel into the intake port of its
cylinder all the time...even when the intake valve is closed. During those
valve-closed periods, a certain amount of atomized fuel is sucked into the
induction manifold beneath the cylinders and is carried forward by the
airflow through the induction system.

What this means is that a bit of the fuel sprayed by the rear cylinders'
nozzles wind up in the middle and front cylinders. Likewise, the middle
cylinders "help" the front cylinders with a bit of their fuel. As a result,
the rear cylinders actually get a bit less fuel than what their injector
nozzles deliver, and the front cylinders actually get a bit more!"



With standard injectors just rotating them should be okay (and legal)
-- all you are doing is using the really terrible QC from the
manufacturer to your benefit. Traditionally the Coke bottle test
involves also ordering sets of different orfice dimensions form the
mfr. and adding them into the mix. This messes up the measured fuel
flow readings and required recalibration of the fuel metering system.
[Hence the reason the FAA frowns on it.]


"Once Braly understood the reason for the mixture maldistribution, the
solution was obvious: vary the orifices in the injector nozzles to
compensate for the "borrowing" of fuel between cylinders. The lean-running
rear cylinders need larger-orifice injectors that deliver a bit more fuel,
while the rich-running front cylinders need smaller-orifice injectors that
deliver a bit less.
Of course, George wasn't the first to come up with this idea. Knowlegable
A&Ps had been quietly playing "musical injectors" on their big-bore TCM
engines for years. I say "quietly" because the use of different-sized
injectors on a TCM engine wasn't exactly legal: the engine's type
certificate data sheet specifies that all injectors are to be the same size.
So this is the sort of thing that mechanics would usually do only on their
own airplanes, and it usually wouldn't show up in the logbooks or be spoken
of in public. Generally, this injector swapping was done on a hit-or-miss
basis without engineering discipline or instrumentation. Sometimes it
worked, sometimes not."


Frankly, 110 max difference in EGT isn't meaningful. As someone else
suggested, look at the spread in FF vs. peak temp for each cylinder.
THAT will tell you if you have a problem or not.

Don't think so...more like 25-50 max...

Matt,

Do you have a source for this?


Uhm, George Braly telling the story of his life.

--
Thomas Borchert (EDDH)

"Do you have a source for this"

Hey, it's what George told me. And a lawyer wouldn't lie, would he?
G

Frankly, 110 max difference in EGT isn't meaningful. As someone else
suggested, look at the spread in FF vs. peak temp for each cylinder.
THAT will tell you if you have a problem or not.

Don't think so...more like 25-50 max...

I assume what you mean is you feel the max EGT spread should be 25 - 50
degrees max? Heck, in an ideal world, it would be zero. But in
reality it may or may not happen. Even the CHT readings (which are a
lot more accurate) probably vary more than 50 degrees. I wish it
didn't... and George has done in-flight tests that show that much
variation just from one side of the cylinder to the other, on the same
cylinder. [Really wish Gami would do their cooling shroud for my
plane, like they do for the Bo's., but just not enough market for the
FAA hassles.]

Let me understand this: you achieved smooth LOP operation by playing
"musical injectors"?

Well, no, that wouldn't be legal. But when my engine ran rough, I
suspected injector fouling and cleaned the injectors (my A&P
supervised, and the cleaning was properly logged). Since there was no
rule saying that each one had to be returned to the original cylinder
(they are, after all, identical) I didn't bother with that. Once the
engines started running smooth, I stopped cleaning the injectors so
often. Once a year is plenty. And I keep track of which one goes with
each cylinder. No rule against that, you know.

Michael

Since there was no rule saying that each one had to be returned to
the original cylinder
(they are, after all, identical) I didn't bother with that. ...And I
keep track of which one goes with
each cylinder. No rule against that, you know

Right. With stock injectors all the same size, there is no requirement
that they go back into the same cylinders, and as far as I know,
nothing that really stops you from deliberately moving them around.
Where the FAA has gotten concerned in the past is where they are
specifically sized (TCM, for example, makes three sizes of injectors
for the same engines). What a lot of people were doing was ordering
some of the "other sizes" and swapping them into the mix, in an attempt
to even out the fuel flow. Or worse (in the FAA's mind), simply using
an abrasive wire to open up the injectors themselves.

Where the FAA has gotten concerned in the past is where they are
specifically sized (TCM, for example, makes three sizes of injectors
for the same engines). What a lot of people were doing was ordering
some of the "other sizes" and swapping them into the mix, in an attempt
to even out the fuel flow. Or worse (in the FAA's mind), simply using
an abrasive wire to open up the injectors themselves.

Right. And to get consistent fuel-air distribution on the big
Continentals, you pretty much had to do that (or go to GAMI's). But I
have small-bore Lycomings. They have pretty good fuel-air distribution
to begin with, so the manufacturing tolerances on the injectors are
enough to get the job done. That's why GAMI's are pretty much a waste
of money on small-bore Lycomings. You can get the same effect by
swapping the stock injectors around. They're supposed to be identical,
but the manufacturing tolerances are such that they are not. (My
favorite tall tale is the one about Eli Whitney and the interchangeable
parts.) They make a lot of sense for big-bore Continentals.

Michael

Interesting thread. I was inspired to do a bit more analysis on the
data I have been collected. Here is what I found.

Cyl #5 peaks first at 12.5 gph fuel flow
Cyl #6 peaks second at 12.0 gph
Cyl #4 peaks third at 10.5 gph
Cyl #3 peaks fourth at 10 gph
Cyl #1 and #2 do not seem to reach peak even down to 9.5 gph

That seems like a pretty significant range. Does this suggest a
specific problem I should have my A&P look at?

My manual talks about 2 modes of operation, best-power at 100 deg ROP
and best-economy at peak. There is such a difference between cylinders,
it is hard to know where to set it.

For example, if I use cyl #5 to lean (at a given altitude and set of
conditions), when it is at peak, the other cylinders were at:

#1: 80 deg ROP
#2: 60 deg ROP
#3: 50 deg ROP
#4: 30 deg ROP
#5: 0 deg ROP
#6: 0 deg ROP

If I use #5 to set to best-power, 100 deg ROP, then the other cylinders
were as follows:

#1: 190 deg ROP
#2: 185 deg ROP
#3: 160 deg ROP
#4: 130 deg ROP
#5: 100 deg ROP
#6: 85 deg ROP

Neither of these seems very satisfying.



O. Sami Saydjari wrote:

My per-cyclinder avg EGTs are as follows:

#1: 1370
#2: 1390
#3: 1450
#4: 1450
#5: 1480
#6: 1430

The difference is 110 degrees between the coldest and hotest cylinder. A
colleague of mine says that is a bit high for a fuel-injected system.
Is that right?

He suggest rotating the injectors from the hotest cylinders to the
coldest ones to try to better balance them (so, for example, swapping #5
and #1). I am not sure why one would do that. Have others done that
with success?

-Sami
N2057M, Piper Turbo Arrow III