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Dual alternators

 
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dj_theis



Joined: 28 Aug 2017
Posts: 56
Location: Minnesota

PostPosted: Thu Jun 17, 2021 4:50 am    Post subject: Dual alternators Reply with quote

I have a somewhat academic question regarding design and operation of a dual alternator system. I’ve imagined, but never tested, that running two alternators that are feeding a single battery bus would be dominated by current from one of the alternators.
My reasoning is that regardless of how closely the charge systems are matched, small variations would result in a slightly higher output voltage on one of them. This would then result in an even lower output from the lower voltage output charge system, which would reduce its current contribution.

If the above assumptions are accurate, how would it change if the two alternators shared a single regulator? (I’m not advocating this arrangement)

Would the result change If the two alternators were PMA units, mechanically tied together and producing a sin wave “in phase.”

WRT the dual PMA arrangement, Bob had advocated a center tap to ground with a tie point between the two PMA stators. This arrangement (I believe) separates the current load exactly in half between the two sides.

One final question I’ve wondered about. How do twin engine planes with two alternators manage (my assumed) tendency for two alternators in parallel quickly relying on the larger voltage out unit for most of the current?

Dan Theis.


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PostPosted: Thu Jun 17, 2021 9:22 am    Post subject: Dual alternators Reply with quote

I've imagined, but never tested, that running two alternators that
are feeding a single battery bus would be dominated by current from
one of the alternators.

My reasoning is that regardless of how closely the charge systems
are matched, small variations would result in a slightly higher
output voltage on one of them. This would then result in an even
lower output from the lower voltage output charge system,
which would reduce its current contribution.

Yes, until system loads exceeded the capacity
of the alternator with the higher set-point. The
bus voltage sags allowing the lower set-point
alternator to wake up and begin to support the
load.

This is what happens with a B&C standby alternator
system. #2 alternator is set to some lower-than-nominal
regulation voltage. Should the #1 alternator quit,
voltage sags and #2 wakes up. Similarly, should the
#1 alternator suffer an open winding with subsequent
loss of capacity, the voltage might sag and again,
#2 steps in to take up the slack.

In both cases, #1 alternator is running maxed out
whether by reason of total system loads =or= loss
of capacity.

If the above assumptions are accurate, how would it change if
the two alternators shared a single regulator? (I’m not advocating
this arrangement)

Cessna 336/337 Models do just that. Beech Barons too.
There is a switch for bringing a second regulator into
service should the first one fail. This philosophy
does produce a system whereby the two alternator
ammeters track each other fairly closely . . . in
my not so humble opinion, a totally misguided practice
to keep pilot's 'happy'.

There is no practical NEED for balancing the
loads on two alternators assuming (1) either
alternator can carry 100% of ships loads
with (2) demonstrated adequacy of cooling.

This shade-tree 'paralleling' scheme creates
single points of failure for both alternators.
How this got past in-house DERs is a mystery
to me . . . how the FAA bought it is a . . .
well . . . I won't go there.

Would the result change If the two alternators were PMA units,
mechanically tied together and producing a sin wave 'in-phase.'

The engines cannot be synchronized that tightly.

One final question I've wondered about. How do twin engine planes
with two alternators manage (my assumed) tendency for two alternators
in parallel quickly relying on the larger voltage out unit for most
of the current?

They parallel fields on a single regulator.

I designed a true paralleling regulator proposal for Cessna on
the 303 Crusader program thinking that there was a HUGH after market
opportunity to put the system on new and fielded Barons and Skymasters.
I don't know how the Cessna West was wiring their airplanes.

In any case, Cessna East didn't bite on the idea so I
was never funded to pursue the design. Twin Generators on
earlier Cessna and Beech models WERE fitted with paralleling
regulators which DID NOT compromise independence of the
two system. Turbine twins have always enjoyed that design
philosophy.

Here is a sketch on a generator paralleling system
on a Beech Model 18 I think . . . but typical of
all dual generator twins of the era.

http://aeroelectric.com/Pictures/Schematics/Parallel_Aircraft_Generators.jpg

This design uses voltage drop in the generator's
compensation windings as a current shunt. Any difference
in those two voltages biases the voltage sense windings
to depress excitation to the generator with the higher
load while elevating excitation on the lighter loaded
machine . . . pretty slick.


Bob . . .

Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"


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PostPosted: Thu Jun 17, 2021 2:01 pm    Post subject: Dual alternators Reply with quote

Bob, in the Beech paralleling schematic below, what is the symbol in the reg comprising the 6 parallel lines with the arrow through it, in series with the shunt field?

Brian Phillips

On 18/06/2021 3:21 am, Robert L. Nuckolls, III wrote:

Quote:
I've imagined, but never tested, that running two alternators that
are feeding a single battery bus would be dominated by current from
one of the alternators.

My reasoning is that regardless of how closely the charge systems
are matched, small variations would result in a slightly higher
output voltage on one of them. This would then result in an even
lower output from the lower voltage output charge system,
which would reduce its current contribution.

Yes, until system loads exceeded the capacity
of the alternator with the higher set-point. The
bus voltage sags allowing the lower set-point
alternator to wake up and begin to support the
load.

This is what happens with a B&C standby alternator
system. #2 alternator is set to some lower-than-nominal
regulation voltage. Should the #1 alternator quit,
voltage sags and #2 wakes up. Similarly, should the
#1 alternator suffer an open winding with subsequent
loss of capacity, the voltage might sag and again,
#2 steps in to take up the slack.

In both cases, #1 alternator is running maxed out
whether by reason of total system loads =or= loss
of capacity.

If the above assumptions are accurate, how would it change if
the two alternators shared a single regulator? (I’m not advocating
this arrangement)

Cessna 336/337 Models do just that. Beech Barons too.
There is a switch for bringing a second regulator into
service should the first one fail. This philosophy
does produce a system whereby the two alternator
ammeters track each other fairly closely . . . in
my not so humble opinion, a totally misguided practice
to keep pilot's 'happy'.

There is no practical NEED for balancing the
loads on two alternators assuming (1) either
alternator can carry 100% of ships loads
with (2) demonstrated adequacy of cooling.

This shade-tree 'paralleling' scheme creates
single points of failure for both alternators.
How this got past in-house DERs is a mystery
to me . . . how the FAA bought it is a . . .
well . . . I won't go there.

Would the result change If the two alternators were PMA units,
mechanically tied together and producing a sin wave 'in-phase.'

The engines cannot be synchronized that tightly.

One final question I've wondered about. How do twin engine planes
with two alternators manage (my assumed) tendency for two alternators
in parallel quickly relying on the larger voltage out unit for most
of the current?

They parallel fields on a single regulator.

I designed a true paralleling regulator proposal for Cessna on
the 303 Crusader program thinking that there was a HUGH after market
opportunity to put the system on new and fielded Barons and Skymasters.
I don't know how the Cessna West was wiring their airplanes.

In any case, Cessna East didn't bite on the idea so I
was never funded to pursue the design. Twin Generators on
earlier Cessna and Beech models WERE fitted with paralleling
regulators which DID NOT compromise independence of the
two system. Turbine twins have always enjoyed that design
philosophy.

Here is a sketch on a generator paralleling system
on a Beech Model 18 I think . . . but typical of
all dual generator twins of the era.

http://aeroelectric.com/Pictures/Schematics/Parallel_Aircraft_Generators.jpg

This design uses voltage drop in the generator's
compensation windings as a current shunt. Any difference
in those two voltages biases the voltage sense windings
to depress excitation to the generator with the higher
load while elevating excitation on the lighter loaded
machine . . . pretty slick.


Bob . . .

Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"


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nuckolls.bob(at)aeroelect
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PostPosted: Thu Jun 17, 2021 5:34 pm    Post subject: Dual alternators Reply with quote

At 05:00 PM 6/17/2021, you wrote:
Quote:
Bob, in the Beech paralleling schematic below, what is the symbol in the reg comprising the 6 parallel lines with the arrow through it, in series with the shunt field?

Brian Phillips

That is a carbon pile, variable resistor. Simply a stack
of carbon disks whose series resistance is a function
of compression force. A very common, high current analog
for a rheostat. Here's a carbon pile regulator in the
flesh . . .

http://aeroelectric.com/Pictures/Regulators/Carbon_Pile_1a.jpg

No transistors, no modern plastics, no software.
One of many fine examples of engineering dexterity
on the part of those who did not have our box
of tinker-toys to work with.



Bob . . .

Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"


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gliderjohn(at)yahoo.com
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PostPosted: Thu Jun 17, 2021 5:58 pm    Post subject: Dual alternators Reply with quote

Also helped a generation of pilots learn the habit of checking voltmeters in their pre-liftoff gauges check. You only need to smell batteries boiling as you climb into a 200’ overcast over one time to learn.
On Thursday, June 17, 2021, 06:45:39 PM PDT, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com> wrote:




At 05:00 PM 6/17/2021, you wrote:
Quote:
Bob, in the Beech parallelingschematic below, what is the symbol in the reg comprising the 6 parallellines with the arrow through it, in series with the shunt field?

Brian Phillips

That is a carbon pile, variable resistor. Simply a stack
of carbon disks whose series resistance is a function
of compression force. A very common, high current analog
for a rheostat. Here's a carbon pile regulator in the
flesh . . .

http://aeroelectric.com/Pictures/Regulators/Carbon_Pile_1a.jpg

No transistors, no modern plastics, no software.
One of many fine examples of engineering dexterity
on the part of those who did not have our box
of tinker-toys to work with.

Bob . . .

Un impeachable logic: George Carlin asked, "If blackboxes
survive crashes, why don't they make the whole airplane
out of that stuff?"


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barp99(at)gmail.com
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PostPosted: Thu Jun 17, 2021 6:33 pm    Post subject: Dual alternators Reply with quote

On 18/06/2021 11:33 am, Robert L. Nuckolls, III wrote:

Quote:
At 05:00 PM 6/17/2021, you wrote:
Quote:
Bob, in the Beech paralleling schematic below, what is the symbol in the reg comprising the 6 parallel lines with the arrow through it, in series with the shunt field?

Brian Phillips

That is a carbon pile, variable resistor. Simply a stack
of carbon disks whose series resistance is a function
of compression force. A very common, high current analog
for a rheostat. Here's a carbon pile regulator in the
flesh . . .

http://aeroelectric.com/Pictures/Regulators/Carbon_Pile_1a.jpg

No transistors, no modern plastics, no software.
One of many fine examples of engineering dexterity
on the part of those who did not have our box
of tinker-toys to work with.



Bob . . .

Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"

Thanks Bob, now that is amazing engineering, zero PN junctions, must be near bulletproof.
Cheers,
Brian.


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Peter(at)sportingaero.com
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PostPosted: Fri Jun 18, 2021 11:49 am    Post subject: Dual alternators Reply with quote

In Diamond twins one regulator is the master and has the job of balancing the alternators.
I’m not actually sure how it achieves that, but works well most of the time.
The regulators are provided by the engine mfrs (Continental or Austro).
The G1000 instrumentation systems use hall effect sensors to detect the output of each alternator.
The regulators use a different sensor to do their job.
Can result in the G1000 saying the loads are unbalanced, which makes pilots unhappy – although voltage is always good.
I have spent a lot of time troubleshooting, reloading G1000 software and other stuff to cure what seems to be an imaginary problem.
Peter

From: owner-aeroelectric-list-server(at)matronics.com <owner-aeroelectric-list-server(at)matronics.com> On Behalf Of Robert L. Nuckolls, III
Sent: 17 June 2021 18:21
To: aeroelectric-list(at)matronics.com
Subject: Re: Dual alternators

I've imagined, but never tested, that running two alternators that
are feeding a single battery bus would be dominated by current from
one of the alternators.

My reasoning is that regardless of how closely the charge systems
are matched, small variations would result in a slightly higher
output voltage on one of them. This would then result in an even
lower output from the lower voltage output charge system,
which would reduce its current contribution.

Yes, until system loads exceeded the capacity
of the alternator with the higher set-point. The
bus voltage sags allowing the lower set-point
alternator to wake up and begin to support the
load.

This is what happens with a B&C standby alternator
system. #2 alternator is set to some lower-than-nominal
regulation voltage. Should the #1 alternator quit,
voltage sags and #2 wakes up. Similarly, should the
#1 alternator suffer an open winding with subsequent
loss of capacity, the voltage might sag and again,
#2 steps in to take up the slack.

In both cases, #1 alternator is running maxed out
whether by reason of total system loads =or= loss
of capacity.

If the above assumptions are accurate, how would it change if
the two alternators shared a single regulator? (I’m not advocating
this arrangement)

Cessna 336/337 Models do just that. Beech Barons too.
There is a switch for bringing a second regulator into
service should the first one fail. This philosophy
does produce a system whereby the two alternator
ammeters track each other fairly closely . . . in
my not so humble opinion, a totally misguided practice
to keep pilot's 'happy'.

There is no practical NEED for balancing the
loads on two alternators assuming (1) either
alternator can carry 100% of ships loads
with (2) demonstrated adequacy of cooling.

This shade-tree 'paralleling' scheme creates
single points of failure for both alternators.
How this got past in-house DERs is a mystery
to me . . . how the FAA bought it is a . . .
well . . . I won't go there.

Would the result change If the two alternators were PMA units,
mechanically tied together and producing a sin wave 'in-phase.'

The engines cannot be synchronized that tightly.

One final question I've wondered about. How do twin engine planes
with two alternators manage (my assumed) tendency for two alternators
in parallel quickly relying on the larger voltage out unit for most
of the current?

They parallel fields on a single regulator.

I designed a true paralleling regulator proposal for Cessna on
the 303 Crusader program thinking that there was a HUGH after market
opportunity to put the system on new and fielded Barons and Skymasters.
I don't know how the Cessna West was wiring their airplanes.

In any case, Cessna East didn't bite on the idea so I
was never funded to pursue the design. Twin Generators on
earlier Cessna and Beech models WERE fitted with paralleling
regulators which DID NOT compromise independence of the
two system. Turbine twins have always enjoyed that design
philosophy.

Here is a sketch on a generator paralleling system
on a Beech Model 18 I think . . . but typical of
all dual generator twins of the era.

http://aeroelectric.com/Pictures/Schematics/Parallel_Aircraft_Generators.jpg

This design uses voltage drop in the generator's
compensation windings as a current shunt. Any difference
in those two voltages biases the voltage sense windings
to depress excitation to the generator with the higher
load while elevating excitation on the lighter loaded
machine . . . pretty slick.

Bob . . .

Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"


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PostPosted: Fri Jun 18, 2021 12:27 pm    Post subject: Dual alternators Reply with quote

Quote:

Thanks Bob, now that is amazing engineering, zero PN junctions, must be near bulletproof.
Cheers,
Brian.

Pretty close. If memory serves, some versions of
these regulators came with field remover/refurbish/
replace manuals. They're stone simple in their
physics of operation and very repairable as long
as they were not abused.



Bob . . .

Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"


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Kellym



Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ

PostPosted: Fri Jun 18, 2021 1:12 pm    Post subject: Dual alternators Reply with quote

I was under the impression that the Diamond twins either had Austro
(originally Thielert) diesel engines or Lycoming avgas engines. Where
did Continental come in?

On 6/18/2021 12:48 PM, Peter Pengilly wrote:
Quote:
In Diamond twins one regulator is the master and has the job of
balancing the alternators.

I’m not actually sure how it achieves that, but works well most of the time.

The regulators are provided by the engine mfrs (Continental or Austro).


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Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
KCHD
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PostPosted: Fri Jun 18, 2021 8:34 pm    Post subject: Dual alternators Reply with quote

At 02:48 PM 6/18/2021, you wrote:

Quote:
In Diamond twins one regulator is the master and has the job of balancing the alternators.

I’m not actually sure how it achieves that, but works well most of the time.

Is there a .pdf of the alternator wiring you could
share? Sounds like an interesting system I'd like
to understand.



Bob . . .

Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"


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PostPosted: Sat Jun 19, 2021 4:06 am    Post subject: Dual alternators Reply with quote

The whole AMM is downloadable and is in ATA100 format.

DA42 with Continental engines http://support.diamond-air.at/fileadmin/uploads/files/after_sales_support/DA42_Twin_Star/Airplane_Maintenance_Manual/Basic_Manual/70201-DA42-AMM-r4.pdf
See page 24-30-00 (pdf p 567) and on. Wiring diagrams Ch 92, p2077 & on
DA42 with Austro engines http://support.diamond-air.at/fileadmin/uploads/files/after_sales_support/DA42_New_Generation/Airplane_Maintenance_Manual/Basic_Manual/70215-DA42-NG-AMM-r4.pdf
See page 613
DA62 (Austro) http://support.diamond-air.at/fileadmin/uploads/files/after_sales_support/DA62/Airplane_Maintenance_Manual/Basic_Manual/70225-r1-complete.pdf
pdf p 525
None of these is very specific. I will look in the engine manuals next week.

From: owner-aeroelectric-list-server(at)matronics.com <owner-aeroelectric-list-server(at)matronics.com> On Behalf Of Robert L. Nuckolls, III
Sent: 18 June 2021 21:28
To: aeroelectric-list(at)matronics.com
Subject: RE: Dual alternators

At 02:48 PM 6/18/2021, you wrote:
Quote:

In Diamond twins one regulator is the master and has the job of balancing the alternators.

I’m not actually sure how it achieves that, but works well most of the time.


Is there a .pdf of the alternator wiring you could
share? Sounds like an interesting system I'd like
to understand.


Bob . . .

Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"


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PostPosted: Sat Jun 19, 2021 9:02 am    Post subject: Dual alternators Reply with quote

At 07:05 AM 6/19/2021, you wrote:

Quote:
The whole AMM is downloadable and is in ATA100 format.



snip

Quote:
None of these is very specific. I will look in the engine manuals next week.

You got that right! It's been a long time since
I did an ATA100 manual. I was still at Electro-Mech
so this would have been about 1980. I got 'dinged'
for 'too complex', Symptom/Cause/Fix trouble shooting
charts. I can see that the authors of these manuals
were not at risk for similar debasement!

Found a schematic for the left alternator where
the regulator appears to be a paralleling regulator;
data from sensors on both alternators is supplied
to the regulator. Didn't find anything on how the
right alternator differed.

Man! That's a really 'busy' electrical system.
It would be interesting to see the wiring diagrams.



Bob . . .

Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"


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peter(at)sportingaero.com
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PostPosted: Sat Jun 19, 2021 12:39 pm    Post subject: Dual alternators Reply with quote

I think the right side regulator is the same box but the current sensor is driven from the LH box, will confirm on Monday.
Wiring diagrams are in Ch92 at the end of the book.
Peter

From: owner-aeroelectric-list-server(at)matronics.com <owner-aeroelectric-list-server(at)matronics.com> On Behalf Of Robert L. Nuckolls, III
Sent: 19 June 2021 18:02
To: aeroelectric-list(at)matronics.com
Subject: RE: Dual alternators

At 07:05 AM 6/19/2021, you wrote:
Quote:

The whole AMM is downloadable and is in ATA100 format.



snip
Quote:

None of these is very specific. I will look in the engine manuals next week.


You got that right! It's been a long time since
I did an ATA100 manual. I was still at Electro-Mech
so this would have been about 1980. I got 'dinged'
for 'too complex', Symptom/Cause/Fix trouble shooting
charts. I can see that the authors of these manuals
were not at risk for similar debasement!

Found a schematic for the left alternator where
the regulator appears to be a paralleling regulator;
data from sensors on both alternators is supplied
to the regulator. Didn't find anything on how the
right alternator differed.

Man! That's a really 'busy' electrical system.
It would be interesting to see the wiring diagrams.


Bob . . .

Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"


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PostPosted: Sat Jun 19, 2021 12:57 pm    Post subject: Dual alternators Reply with quote

Quote:

Man! That's a really 'busy' electrical system.
It would be interesting to see the wiring diagrams.

Found them. It will take a bit to sift out
the features of interest . . .


Bob . . .

Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"


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PostPosted: Sat Jun 19, 2021 1:17 pm    Post subject: Dual alternators Reply with quote

At 03:38 PM 6/19/2021, you wrote:

Quote:
I think the right side regulator is the same box but the current sensor is driven from the LH box, will confirm on Monday.

Wiring diagrams are in Ch92 at the end of the book.

Yup . . . found 'em. I think this paralleling system is
of the same genre' as that I proposed to Cessna for
the C303. Only one regulator is fitted with paralleling
smarts. The other is a plain vanilla stand-alone
device.

Stand alone regulator is set for 'nominal' bus
voltage, say 28.5V.

Second regulator watches current from each
alternator and adjusts its own output to
match it's partner with the stand-alone
system.

The paralleling circuit steering authority is limited
to about plus/minus 0.5V. Setup says adjust each
system independently for nominal set-point. Then
operate normally as a dual system.

Both systems retain their redundancy independence
with limited but adequate ability to run in
lockstep with each other.

I thought it would work pretty good . . . never
had a chance to explore it in working hardware.
Today we would enjoy the benefits of hall-effect
sensors for current monitoring . . . back then
I had to look at shunts with a rather energetic
hat-dance to resolve alternator current signals
from the bus common mode noise.


Bob . . .

Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"


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PostPosted: Sun Jun 20, 2021 3:09 am    Post subject: Dual alternators Reply with quote

Continental bought Thielert several yers ago…

Neal George
On Jun 18, 2021, at 4:11 PM, Kelly McMullen <kellym(at)aviating.com> wrote:



I was under the impression that the Diamond twins either had Austro (originally Thielert) diesel engines or Lycoming avgas engines. Where did Continental come in?


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Posts: 1705
Location: Sun Lakes AZ

PostPosted: Sun Jun 20, 2021 7:18 am    Post subject: Dual alternators Reply with quote

OK, so it is just the original diesel engine, not a TCM gas engine.

On 6/20/2021 4:08 AM, Neal George wrote:
Quote:


Continental bought Thielert several yers ago…

Neal George


On Jun 18, 2021, at 4:11 PM, Kelly McMullen <kellym(at)aviating.com> wrote:



I was under the impression that the Diamond twins either had Austro (originally Thielert) diesel engines or Lycoming avgas engines. Where did Continental come in?







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PostPosted: Sun Jun 20, 2021 12:35 pm    Post subject: Dual alternators Reply with quote

Continental (Thielert/Technify) and Austro are different companies and make different engines, although their main engines are both based on a Mercedes 2.0 litre.
Diamonds are now only available with Austro engines (except the DA50 which has 300hp Continental diesel), in the past they have been available with Continentals and with Lycomings - may still be sold in US/Canada with Lycs.

Peter

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