subaru diesel

"Capt. Geoffrey Thorpe" The Sea Hawk At Wow Way D0t C0m wrote in message
news:N6adnXfOAtwpgHXanZ2dnUVZ_sWdnZ2d@wideopenwest .com...
"Dale Scroggins" wrote in message
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1. Most diesels wear more slowly than gas engines. The fuel is a better
lubricant than gasoline is, and the combustion products are a bit more
benign.

Except, of course, for the particulates. Particulates are hell on
bearings, and lungs, and...
NOx tends to run higher and isn't usually clasified as "benign"

The specific comparison I had in mind was a current-production piston
aircraft engine burning 100LL, compared to a current-production automotive
diesel engine. Given a choice of the lead compounds that collect within and
are emitted by the gasser, and the low levels of particulates deposited and
emitted by a current-production automotive diesel, I'd prefer the
particulates.

As to oxides of nitrogen, given that current production gas aviation engines
run with advanced ignition timing, have no EGR, and operate at elevated EGT
most of the time, I doubt a current production auto diesel would produce
more NOx, and would likely produce less. I'm open to numbers, if you have
them.

Again, my comparison was between current diesels and current gas aircraft
engines, not current automotive gas engines.


5. Automatic mixture control.

"mixture control is irrelevent" would be a more accurate statement.

For a turbodiesel, "automatic mixture control" is a more accurate statement.
Mixture control is quite relevent, and automatically managed.


7. Lower fire risks.

Not really.

Hmm. So the dozens of induction fires (some of which caused substantial
damage) that I've witnessed over the years are just as likely with a diesel
engine? And all the extra precautions we followed over the years when
fueling and defueling avgas (or mogas), compared to jetA, were a waste of
time?


10. Props are more efficient at lower RPMs, usually. Diesels have
torque to turn props slow in cruise, like a turboprop (1800-1900 RPM),
giving maybe another 5% efficiency gain, and a quieter cabin to boot.

Depends on how the engine was designed, not how the fuel is ignited.

Hmm again. So the burn characteristics of the fuel, the ability to inject
additional fuel after initiation of combustion, and the surplus of available
oxygen have no impact on engine torque curves?



Given time, I can think of more. Please note that gas engines were
abandoned decades, nay, scores of years ago in other transportation
sectors. It will be the same in aviation if the weight difference shrinks
enough.


Well, there's the problem, isn't it. Making a light weight diesel that
will last.

Note: I'm not suggesting that diesels are a bad idea - they do have there
advantages. But, let's not over sell...

Wasn't the prospect of a relatively light weight diesel available to
homebuilders the point of this thread? No one knows if Subaru's diesel will
match the durability of a current-production avgas engine. However, newer
diesel injection methods have helped reduce internal engine forces, so
durable, light diesel engines are on the horizon.

What do you see as the advantages of diesel aircraft engines?

DS

"Dale Scroggins" wrote in message
...

"Capt. Geoffrey Thorpe" The Sea Hawk At Wow Way D0t C0m wrote in message
news:N6adnXfOAtwpgHXanZ2dnUVZ_sWdnZ2d@wideopenwest .com...
...
Hmm. So the dozens of induction fires (some of which caused substantial
damage) that I've witnessed over the years are just as likely with a
diesel engine?

Probably not, since there is no fuel in the induction system.


10. Props are more efficient at lower RPMs, usually. Diesels have
torque to turn props slow in cruise, like a turboprop (1800-1900 RPM),
giving maybe another 5% efficiency gain, and a quieter cabin to boot.

Depends on how the engine was designed, not how the fuel is ignited.

Hmm again. So the burn characteristics of the fuel, the ability to inject
additional fuel after initiation of combustion, and the surplus of
available oxygen have no impact on engine torque curves?


On the contrary. The inabilty to mix the fuel with the air is the reason
that typical diesel torque curves flatten out at low speeds - a gas engine
can better use the air and generates more torque at the higher speeds. So
the TENDANCY is to select bore/stroke ratio's and displacements that favor
lower speeds in diesels. But there are lots of exceptions.

..
Wasn't the prospect of a relatively light weight diesel available to
homebuilders the point of this thread? No one knows if Subaru's diesel
will match the durability of a current-production avgas engine. However,
newer diesel injection methods have helped reduce internal engine forces,
so durable, light diesel engines are on the horizon.

What do you see as the advantages of diesel aircraft engines?


Primary advantage is fuel consumption - higher compression, lower pumping
losses (minor benifit for aircraft) and reduced heat loss due to the
stratified nature of the combustion all help to improve efficiency.
Increased fuel density also gives a boost to "miles per gallon" - but for
aircraft it's about "miles per pound".

Primary disadvantage is power to weight. Diesels are typically smoke
limited - they can't use all the air, so the power is limited - that's why
turbos are so popular on diesels. The higher combustion pressures also
require a heavier block and a heavier bottom end which, well, being heavier,
adds weight.

Secondary advantage is no ignition system. Secondary disadvantage is a
somewhat fussy fuel system (and, with a common rail system, you are just as
reliant on electricity as an electronic fuel injected gas engine). Probably
less of a problem for aircraft use when compared to small marine
applications - do a search on "fuel polishing"...

FWIW - look for the press propaganda on Ford's "Eco-Boost" - turbo charged,
direct injection, spark ignited gas - looks like they are shooting to get
closer to diesel fuel economy without the cost (diesel aftertreatment is
REAL expensive at the lower emission standards). Improving power to weight
lets them downsize the engine, down size vehicle componants, etc.

Of course, one could say that direct injection gasoline has all the
disadvantages of both gasoline and diesel engines. ;-)

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

"Capt. Geoffrey Thorpe" The Sea Hawk At Wow Way D0t C0m wrote in message
...
"Dale Scroggins" wrote in message
...

"Capt. Geoffrey Thorpe" The Sea Hawk At Wow Way D0t C0m wrote in
message news:N6adnXfOAtwpgHXanZ2dnUVZ_sWdnZ2d@wideopenwest .com...
...
...
What do you see as the advantages of diesel aircraft engines?


Primary advantage is fuel consumption - higher compression, lower pumping
losses (minor benifit for aircraft) and reduced heat loss due to the
stratified nature of the combustion all help to improve efficiency.
Increased fuel density also gives a boost to "miles per gallon" - but for
aircraft it's about "miles per pound".

Oh, I forgot, they can run on Jet fuel if the injector pump will tolerate
it...



On the downside, diesel fuel is good for growing algae, dunno about jet
fuel - but given the number of hanger queens at the typical airport, I
assume we will find out when diesels make a dent in the aircraft market.







--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

"Capt. Geoffrey Thorpe" The Sea Hawk At Wow Way D0t C0m wrote in message
news:mLWdneP7Q9QVCXTanZ2dnUVZ_sqinZ2d@wideopenwest .com...

Primary advantage is fuel consumption - higher compression, lower pumping
losses (minor benifit for aircraft) and reduced heat loss due to the
stratified nature of the combustion all help to improve efficiency.
Increased fuel density also gives a boost to "miles per gallon" - but for
aircraft it's about "miles per pound".

Oh, I forgot, they can run on Jet fuel if the injector pump will tolerate
it...

AFAIK, the newly demanded ultra low sulphur diesel fuel is nasty stuff for
the pumps, because the hurriedly implemented refining process also strips
lubricants from the fuel. I have no idea whether this will be a continuing
problem or whether other processes maintain the lubricity of the fuel.


On the downside, diesel fuel is good for growing algae, dunno about jet
fuel - but given the number of hanger queens at the typical airport, I
assume we will find out when diesels make a dent in the aircraft market.


I don't know about the kerosene based jet fuel sold here in the U.S., but
some of the foreign stuff is (or at least was) a feeding ground for
micro-organisms. The Eastern Airlines L-1011 that went down in the Florida
Everglades some years ago was apparently full of the stuff after having been
previously refuelled in Israel.

BTW, I'm glad someone remembers what a hangar queen is. ;-))

Peter