yellowduckduo(at)gmail.co
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 Posted: Mon Jan 27, 2020 10:17 am Post subject: Z-14 implementation for electrically dependent engine |
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Krea Welcome. A good thought provoking question.
I'm confident that properly implemented diode feeding is more reliable
than any switch. Especially for a switch that is not operated in
fight. I'm cautious about diode fed power though unless there will be
real time failure indications. Finding the failed issue on the next
start up is fine however I want to know about such failures well before
I decide where to land.
I believe that the risk from an in flight switch failure is minuscule
compared to the risk of several other failures not least of which is the
risk of aircraft handling after a power loss. So from an emergency
handling perspective diodes are attractive but there will still be
switches in the circuit. I think the dilemma is whether to wire both
busses through a dual throw switch with no diodes or two single throw
switches and diodes for each bank of 3 injectors. I'd favor the latter
choice or consider the comment in my last paragraph below. I avoided
dual throw switches and diodes myself but I had other options. I assume
these are battery busses that are at very low risk of going dark
simultaneously. It sounds like you understand the need to avoid cross
connecting two busses through a switch that can't handle it (especially
if one buss is dead). Sooner or later there will be switches
mispositioned.
Ecu's generally adjust injector timing for varying system ecu voltage so
a diode in the injector supply might very slightly lean the engine if it
was previously tuned without the diode.
I don't have SDS ecu's but I chose to power all my operating injectors
from the same buss at any given time to make switching everything one
quick action while concentrating on flying the airplane. Unless I was
confident that I could maintain flight on half my injectors I would do
the same with your system. I can't maintain flight very long with half
my injectors out but I still didn't want procedures such as:
A. try switching half the injectors - is it better or worse.
B. If still abnormal try switching the other two injectors - is it
better or worse.
C...
If one of my ecu's is misbehaving in any way I want the other ecu to run
everything rather than half and half.
Ken
On 27/01/2020 7:08 AM, Krea Ellis wrote:
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First post. Thanks for considering my dilemma.
Installing dual ECU SDS fuel injection and ignition on a 6 cylinder Lycoming on an RV-10.
Design goal was for independent busses for each “channel” of SDS. Easy to achieve for fuel pumps, ECU’s and coil packs. Not easy for fuel injector power. In normal mode, each ECU only controls half (three) fuel injectors. Injectors are ground triggered by the ECU, power to all injectors comes from the airframe bus(ses), not the ECU’s.
Was trying to avoid have power for injectors going through a single switch. Other option is a diode fed/protected “injector bus” but this creates failure modes as well.
So is power through an appropriately rated Honeywell TL switch more reliable/redundant than the diode fed bus?
Any other suggestions as to how to provide redundant power to all 6 fuel injectors?
Thanks much!
Krea Ellis
Sent from my iPad
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