[Diy_efi] fuel injection for an airplane engine

[Garfield Willis]

On Mon, 27 May 2002 21:19:18 +0100, "William Shurvinton"
<shurvinton at orange.net> wrote:

>I have been resisting on this topic, mainly due to a lack of knowledge =
about
>what is needed for a 'safe' aircraft, but here goes, feet first

Suggestion: if in an area you feel hesitant to stick your feet first in,
ASK a question. Hell, ask alot of questions both feet first. For
example, noting that Brian mentioned a backup IGN system AND a backup
fuel system (did you miss that too?), you might ask him what he
considered in his FMA, and how it was he chose the method he did for
redundancy. Like Yogi Berra used to say, "You can observe alot just by
watching". Or in this case, asking. BTW, I happen to know there are at
least half-dozen XA guys subscribed to diy_efi, perhaps more.

>The issue is being able to do a thorough FMEA on the system and to =
gather
>requirements for what is needed.

Yep, and anyone worth their salt in XA has that acronym impailed into
their psyche, not just on the propulsion system, but on everything.
Something automotive guys don't often have to consider, until the cup
money becomes painfully important.

>For example, if you have 4
>cylinders then 2 ECUs can be used, each driving 2 cylinders, so if one =
fails
>you are still in the air. Dual pumps, dual sensors etc, are all possible
>depending on what is needed.

Here's where an example of asking would have paid off. Usually power
from just 2 jugs, coupled with the pumping loss of the other two, WON'T
even keep you at altitude; it may extend your glidepath and give you
more time to divert to an alternate (read, nearer) destination, but
that's it. Usually you need somewhere around 50% of max power just for
straightNlevel flight. And losing 2 outta 4 jugs doesn't leave you with
anywhere near close enough to 50%. That's one reason why redundancy in
aircraft doesn't usually try to just keep fractional power levels, but
attempts to insure you can keep full power. Even if it means being STUCK
at full power/WOT until you're near your backup landing point (BTW,
pilots and especially eXperimental airplane pilots are ALWAYS thinking,
"if I lost power right now, where would I divert to". It's just part of
the training.)

>Where is gets interesting is the requirements. Aircraft, if i have it
>correctly have about 3 operating modes
>
>1. Idle on the ground
>2. Take off and climb (max welly)
>3. Cruise (max BSFC)
>
>All of which in a fairly narrow operating RPM range. A stock ECU is well=
 OTT
>for this. The simpler the better. Where it gets hazy for me is the =
mixture
>knob which most aviation FI systems still seem to have. Is this because
>pilots don't trust ECUs or is it because more inputs are needed than the=
 ECU
>is aware of to make these decisions. Possibly a bit of both.

Nope, it has to do with the history of where FI in aviation came from,
the difficulty in certain cases of using closed-loop w/O2 sensors where
the fuel has considerable lead (100LL aviation fuel isn't really "low
lead", just that it was lower than it's predecessor fuel variety). There
are presently flying in the XA fleet, both approaches; one where a
manual mixture is incorporated into the ECU, or the other, where O2
sensors *are* used and considered consumables).

>Anyway, if having a mixture control is considered a good thing it would
>point to a very simple ECU design, not dissimilar to the old Bosch
>LE-jetronic with minimum HW to handle the 3 operating modes. From what I
>know of the GM ECMs you could strip that down to a minimum map, but =
without
>exorcising all the redundant code it might be iffy. Ideally you want
>something based on one of the newer MCU with a TPU chip (or an external =
chip
>doing the same) such that once running steady state you could put a nail
>through the CPU and it would keep going until told otherwise.

Many people consider the old Bosch systems, until weight and other
factors go into the balance. Or use a conventional automotive targeted
ECU, with a fail-soft backup, like "5th injector". Or use two
MegaSquirts coded for the app, and ability to switch between them, OR
any number of even simpler schemes. Lots of ways to skinDcat.

The point is just because someone is using an automotive targeted ECU,
whether OEM or aftermarket, in his aviation scheme, doesn't imply
there's been no FEEMAH (as the acronym FMEA is pronounced in the
industry). And if he's already disclosed a backup scheme, you can
reasonably assume he's aware of the issues. At least you should give the
guy the benefit of the doubt.

Yes, you're right it IS a fascinating sidebar to the wider topic of diy
EFI in general. Airplanes, boats, cars, etc. they all have their cool
sidebars. Like Yogi Berra used to say, "You can observe alot just by
watching". Oh, I said that already. Well, bears repeating.

Gar

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