Cessna Anticipates AvGas Demise

On Sat, 06 Oct 2007 01:41:50 GMT, "Vaughn Simon"
wrote in
:


"Larry Dighera" wrote in message
news

I like the idea that there is more energy per pound in Jet A fuel than
AvGas,

Whoa! There is more energy in Jet A per GALLON. Depending on which weight
and BTU averages you are using, Jet A and Avgas come out pretty close on BTUs
per pound. Remember, Jet fuel is heavier than gasoline.

Vaughn


Okay. The point is, that there should be a commensurate increase in
range.

Roy Smith wrote:
In article ,
"Vaughn Simon" wrote:

"Larry Dighera" wrote in message
news
I like the idea that there is more energy per pound in Jet A fuel than
AvGas,
Whoa! There is more energy in Jet A per GALLON. Depending on which
weight
and BTU averages you are using, Jet A and Avgas come out pretty close on BTUs
per pound. Remember, Jet fuel is heavier than gasoline.

Which weighs more, a pound of Jet-A or a pound of Avgas?

It depends on how much you pay for it. As you need to subtract the
weight lost from your wallet from the fuel to get net weight per pound.
:-)

Matt

Larry Dighera wrote:
On Sat, 06 Oct 2007 01:41:50 GMT, "Vaughn Simon"
wrote in
:

"Larry Dighera" wrote in message
news
I like the idea that there is more energy per pound in Jet A fuel than
AvGas,
Whoa! There is more energy in Jet A per GALLON. Depending on which weight
and BTU averages you are using, Jet A and Avgas come out pretty close on BTUs
per pound. Remember, Jet fuel is heavier than gasoline.

Vaughn


Okay. The point is, that there should be a commensurate increase in
range.


That depends. Most diesel engines weigh more than similar power gas
engines. So, you may well have less fuel capacity to stay within weight
limits.

Matt

M wrote:
Another factor, which only has to do
with economy, is that we pay for fuel in volume (gallons), not
pounds. If Jet A and 100LL are both $4 per gallon, you get more
pounds of jet A with that $4. These two factor combined, diesel
powered Cessna would burn about 30% less $ worth of fuel per hour at
the same power output, assuming JetA and 100LL cost the same per
gallon. That's quite significant. These days it seems that Jet A is
consistently less per gallon than 100LL.


Its also true that the federal taxes on JETA is significantly higher
than AVGAS, thus the price you actually pay is not a lot less than
AVGAS.

wrote:

The lower fuel burn comes not from the higher energy of Jet-A, but
rather from the higher compression ratio which converts a higher
percentage of the energy to useful work. Diesel comes closer to an
ideal Carnot cycle than gasoline.


How much better fuel economy and efficiency would result
if modern internal combustion designs were made easily
available to the GA fleet (versus diesel engines) ?

These new diesel engines employ the latest of modern technology
in terms of materials and design, where our old Lycomings and
Continentals are basically 75 year old technology. Not a
fair (or even logical) comparison IMHO.

On Oct 6, 7:26 am, kontiki wrote:
wrote:

The lower fuel burn comes not from the higher energy of Jet-A, but
rather from the higher compression ratio which converts a higher
percentage of the energy to useful work. Diesel comes closer to an
ideal Carnot cycle than gasoline.

How much better fuel economy and efficiency would result
if modern internal combustion designs were made easily
available to the GA fleet (versus diesel engines) ?

These new diesel engines employ the latest of modern technology
in terms of materials and design, where our old Lycomings and
Continentals are basically 75 year old technology. Not a
fair (or even logical) comparison IMHO.

Turbo diesel auto engines get around 50mpg compared to equivalent
gasoline engines that get around 30mpg. That's apples to apples for
you. Diesel is more efficient. Higher compression buns more of the
fuel and captures more of the thermal expansion in the form of
mechanical piston energy. Basic physics.

On Sat, 06 Oct 2007 14:49:21 -0000, wrote in
.com:

Higher compression buns more of the
fuel and captures more of the thermal expansion in the form of
mechanical piston energy. Basic physics.

Diesel engines have a longer TBO than gasoline engines also, don't
they? Given the higher temperatures and pressures, that seems to fly
in the face of basic physics, but I suppose it's a result of lower RPM
operation.

On Oct 6, 6:26 am, kontiki wrote:


How much better fuel economy and efficiency would result
if modern internal combustion designs were made easily
available to the GA fleet (versus diesel engines) ?

These new diesel engines employ the latest of modern technology
in terms of materials and design, where our old Lycomings and
Continentals are basically 75 year old technology. Not a
fair (or even logical) comparison IMHO.


If you have seen the BSFC numbers of the latest and greatest
automobile gasoline engines, they're really not better than a
IO-520. Why? because it's a lot easier to optimize a gasoline
engine for fuel efficiency when it's operated at a narrow RPM range
with little power variation. Automobile engines achieve good fuel
efficiency through electronic engine control only because it's much
harder to gain good efficiency across a much wider RPM range and
throttle setting. From a thermo efficiency point of view, the
gasoline aircraft engines designed in the 50s are very good, if
they're leaned properly (which can be easily done with good fuel
injection and EGT instrumentation), I doubt they can get much better
in fuel efficiency regardless of what electronic you put on them.

Diesel engine has better BSFC because they have fundamentally better
thermo efficiency, not because they have fancy electronics.

Larry Dighera wrote:
On Sat, 06 Oct 2007 14:49:21 -0000, wrote in
.com:

Higher compression buns more of the
fuel and captures more of the thermal expansion in the form of
mechanical piston energy. Basic physics.

Diesel engines have a longer TBO than gasoline engines also, don't
they? Given the higher temperatures and pressures, that seems to fly
in the face of basic physics, but I suppose it's a result of lower RPM
operation.

Yes, if properly designed, no, if not (think GMs first auto diesels).
They tend to last longer for a couple of reasons:

1. Diesel engines have to be made much heavier to handle the higher
compression and this tends to contribute to longevity.

2. Diesel fuel is less of a solvent than is gasoline and tends to wash
the oil from the cylinders a little less.

The penalty you pay with diesels is they also tend to be a fair bit
heavier for the same power output.

Matt

On Oct 6, 11:24 am, Matt Whiting wrote:

The penalty you pay with diesels is they also tend to be a fair bit
heavier for the same power output.

The Thielert also has maintenance drawbacks. We checked
into getting some for our 172s, and to maintain them you need either
to take them to an approved repair facility, or go to Texas and take a
two-week (IIRC) course in their maintenance. And buy a bunch of
expensive tools. Any major repairs requires a removal of the engine
and sending it to the approved repair people. Can't take the head off,
for instance, and re-ring a piston or do the valves.
There's no TBO. They call it a TBR, where the whole engine is
replaced with a new one. I can't put my finger on the time but I think
it's around 2400 hours.

Dan