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nuckolls.bob(at)cox.net
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 Posted: Fri Oct 31, 2008 1:50 pm Post subject: Using relay for engine control . . . |
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| Quote: | Comments/Questions: I have your book, and I read the section on relays,
but I'm still not sure a decision I made long ago is the right one. I have
an automotive engine in a pusher, and for whatever reason the engine must
be shut off by removing power from the computer. Rather than depend upon a
switch and a long run of wire, I connected the computer directly to the
battery contactor, through the normally closed contacts of a relay. This
way, the switch and wire are only used to activate the relay, which shuts
down the engine. By the way, the engine can also be shut off by pulling
breakers and shutting off fuel if necessary. Questions: What are the
consequences of using the normally closed contacts for this purpose? I ask
because they are rated lower and probably held closed with a spring. How
about a latching relay instead? Or a more appropriate relay for the task
that a layman such as myself might not know about? Thanks for the help.
|
Is the computer the only device that needs
power to keep the engine running? How about fuel
pump?
It's been my recommendation for years that electrically
dependent engines run from the always-hot, battery-bus.
This means that if you have smoke in the cockpit, you
can shut off the alternator-battery-master switch and
kill everything electrical without having the engine
quit.
Since you already have a switch and some length of
wire in place to control a relay, there is little value
in adding the relay unless the current needed to run
the computer is too large to consider running through
the switch and its associated wiring. There is a reliability
factor for the switch an associated wire . . . which can
only get worse by adding the relay. However, if there
is a performance driver for adding the relay (you don't
want to run 20A the length of the airplane twice and
a 20A rated switch is a real pig), then using a smaller
switch, wire and a relay to control that pathway is a
rational decision. This is what battery and starter
contactors are about also.
Why the n.c. contacts? Modern relays are pretty robust
and have excellent service lives under the hood of
an automobile. If this pathway and its controls have
to be 100% golden for the engine to run, then perhaps
a normally open solid state relay is called for. Eric
Jones has one, we'll have one too pretty soon. How
much current are you talking about?
What does this computer control? What other electro-
whizzies need to be powered up to keep the engine
runnnig?
I will invite you to join us on the AeroElectric List
to continue this and similar discussions. It's useful to
share the information with as many folks as possible.
A further benefit can be realized with membership on
the list. There are lots of technically capable folks
on the list who can offer suggestions too. You can
join at . . .
http://www.matronics.com/subscribe/
I check the List several times a day and attempt to be as
responsive as time will allow. Further, there are 1800
or so subscribers that include many technically capable
teachers. This is the best venue for assisting you in
your studies.
Bob . . .
----------------------------------------)
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
----------------------------------------
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bakerocb
Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA
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| Posted: Sat Nov 01, 2008 5:50 am Post subject: Using relay for engine control . . . |
|
|
11/1/2008
Hello Layman, Bob Nuckolls wrote:
1) "It's been my recommendation for years that electrically dependent
engines run from the always-hot, battery-bus.This means that if you have
smoke in the cockpit, you can shut off the alternator-battery-master switch
and kill everything electrical without having the engine quit."
I'd like to add another reason to have the electrically dependent engine
(and its electrically dependent vital accessories) run from the always-hot,
battery-bus: I am personally aware of the failure of two different
manufacturer's master battery contactors that left the two different
aircraft without any electrical supply whatever downstream of the contactor.
2) "Modern relays are pretty robust and have excellent service lives under
the hood of
an automobile."
Neither of the master battery contactors mentioned in 1 above were "modern",
but both (one in a type certificated airplane and one in an amateur built
airplane) are in very common use.
'OC' Says: "The best investment we can make is the effort to gather and
understand knowledge."
PS: You wrote: "By the way, the engine can also be shut off by pulling
breakers and shutting off fuel if necessary."
Do the electrical circuit(s) running through these breaker(s) also represent
some single point(s)of unwanted engine shut down possibilities?
-------------------------------------------------------------------
Time: 02:50:02 PM PST US
From: "Robert L. Nuckolls, III" <nuckolls.bob(at)cox.net>
Subject: Using relay for engine control . . .
| Quote: | Comments/Questions: I have your book, and I read the section on relays,
but I'm still not sure a decision I made long ago is the right one. I have
an automotive engine in a pusher, and for whatever reason the engine must
be shut off by removing power from the computer. Rather than depend upon a
switch and a long run of wire, I connected the computer directly to the
battery contactor, through the normally closed contacts of a relay. This
way, the switch and wire are only used to activate the relay, which shuts
down the engine. By the way, the engine can also be shut off by pulling
breakers and shutting off fuel if necessary. Questions: What are the
consequences of using the normally closed contacts for this purpose? I ask
because they are rated lower and probably held closed with a spring. How
about a latching relay instead? Or a more appropriate relay for the task
that a layman such as myself might not know about? Thanks for the help.
|
Is the computer the only device that needs
power to keep the engine running? How about fuel
pump?
It's been my recommendation for years that electrically
dependent engines run from the always-hot, battery-bus.
This means that if you have smoke in the cockpit, you
can shut off the alternator-battery-master switch and
kill everything electrical without having the engine
quit.
Since you already have a switch and some length of
wire in place to control a relay, there is little value
in adding the relay unless the current needed to run
the computer is too large to consider running through
the switch and its associated wiring. There is a reliability
factor for the switch an associated wire . . . which can
only get worse by adding the relay. However, if there
is a performance driver for adding the relay (you don't
want to run 20A the length of the airplane twice and
a 20A rated switch is a real pig), then using a smaller
switch, wire and a relay to control that pathway is a
rational decision. This is what battery and starter
contactors are about also.
Why the n.c. contacts? Modern relays are pretty robust
and have excellent service lives under the hood of
an automobile. If this pathway and its controls have
to be 100% golden for the engine to run, then perhaps
a normally open solid state relay is called for. Eric
Jones has one, we'll have one too pretty soon. How
much current are you talking about?
What does this computer control? What other electro-
whizzies need to be powered up to keep the engine
runnnig?
I will invite you to join us on the AeroElectric List
to continue this and similar discussions. It's useful to
share the information with as many folks as possible.
A further benefit can be realized with membership on
the list. There are lots of technically capable folks
on the list who can offer suggestions too. You can
join at . . .
http://www.matronics.com/subscribe/
I check the List several times a day and attempt to be as
responsive as time will allow. Further, there are 1800
or so subscribers that include many technically capable
teachers. This is the best venue for assisting you in
your studies.
Bob . . .
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
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nuckolls.bob(at)cox.net
Guest
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| Posted: Sat Nov 01, 2008 6:37 am Post subject: Using relay for engine control . . . |
|
|
At 09:45 AM 11/1/2008 -0400, you wrote:
| Quote: |
11/1/2008
Hello Layman, Bob Nuckolls wrote:
1) "It's been my recommendation for years that electrically dependent
engines run from the always-hot, battery-bus.This means that if you have
smoke in the cockpit, you can shut off the alternator-battery-master
switch and kill everything electrical without having the engine quit."
I'd like to add another reason to have the electrically dependent engine
(and its electrically dependent vital accessories) run from the
always-hot, battery-bus: I am personally aware of the failure of two
different manufacturer's master battery contactors that left the two
different aircraft without any electrical supply whatever downstream of
the contactor.
|
Bet they didn't have a dual feed e-bus either.
| Quote: | 2) "Modern relays are pretty robust and have excellent service lives under
the hood of
an automobile."
Neither of the master battery contactors mentioned in 1 above were
"modern", but both (one in a type certificated airplane and one in an
amateur built airplane) are in very common use.
|
It matters not whether one paid $25 for a contactor . . . or $2500
there no guarantees that the contactor or combination of wires, terminals,
switches, etc that control them will not fail to perform as intended
at some point in time. Folks spend tens of $thousands$ on engines and
they run as advertised MOST of the time . . . but nothing is guaranteed
infallible.
This is why the prudent builder/owner/operator of any machine upon
which life and limb depend plans for and designs in support for the
most critical failures . . . or procures, designs, tests and maintains
what is believed to be a SYSTEM failure rate of less than one per
million flight hours.
The gentleman who started this thread appears to be planning
on an automotive conversion that like propellers and wing struts,
offers no options for graceful recovery from failure other than to
bail out . . . or perhaps fly day-vmc only over terrain likely
to offer high probability of survivable dead-stick arrivals.
The best he can do is minimize parts count and use parts
that are lightly stressed . . . and in this case, he might
want to consider a Honeywell/Microswitch toggle for computer
control . . . but there is still the terminal that's poorly
crimped, screw that is over or under torqued, etc. But when
all is said and done, his selection of hardware has increased
the numbers of single-failure items that will force an
uncomfortable if not dangerous termination of flight.
The ingredients that go into the final recipe for success
have to be gauged against his skills, operational plans for
the aircraft and amount of risk he's willing to accept. The
best we can do for him here is help drive the risk numbers
in the right direction.
Bob . . .
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