What/how does compression ratio affect an engine?

"Capt. Geoffrey Thorpe" The Sea Hawk at wow way d0t com wrote in message
news:KP6dnV86MLVlRcDanZ2dnUVZ_vmlnZ2d@wideopenwest .com...
"GeorgeB" wrote in message
...
On Mon, 10 Dec 2007 09:23:52 -0800, Richard Riley
wrote:

If you're flying a homebuilt you can burn whatever you want - but the
alcohol restriction wasn't put there at random, it increases vapor
lock problems dramatically,

How do the planes which do fly on ethanol handle that problem,
pressurized tanks?

The vapor pressure of ethanol alone (or gasoline alone) is less than a
gasoline ethanol mix. The maximum vapor pressure comes from about 10% to
20% ethanol and 80% to 90% gasoline. I don't recall why - just what is.


and is incompatable with many of the
materials commonly used in aircraft fuel systems.

The sealant sloshed in the tanks is one, I think ...

Do automobiles with flex-fuel capability do anything to minimize the
vapor lock issues? I'm sure the materials were selected to be ok.

They can run higher fuel pressures and/or increase the injector pulsewidth
as a function of measured or inferred fuel rail temperature. Another
helpful option is to have a system that returns excess fuel back to the
tank which tends to purge out any vapor bubbles.

--
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.

I'm not sure, and also not qualified; but enough others are weighing in...

So, IIRC, the real issue with the STC is whether the fuel in question can be
reasonably asserted to conform to the same ASTM specification as the fuel
which was used for the test program for certification of the STC.

In the case of automobiles, the decision was made by governmental edict to
simply treat E10 the same as "pure" gasoline for automotive purposes.
However, that was not made applicable to certified aircraft engines; so all
of the testing would need to be done again to obtain a new STC. Speaking
only for myself, I would be reluctant to invest much effort or funding in
such a venture because the formulation of the fuel could be a moving
target--for example, next year mogas could magically become E15.

As to whether any, or all, of the other concerns are valid, I really don't
know. But, I do recall reading that the original specification regarding
vapor presure, which was indeed written to minimize vapor lock, may have
been written in error--shortly after WWI!

So, everyone else's guess is probably at least as good as mine.

Peter
Just my $0.02

"Matt Whiting" wrote in message
...
Kyle Boatright wrote:

Max RPM is the same for all 3 engines, and the only wear difference
should be on the connecting rods. Apparently the difference isn't enough
to change the TBO, which is a theoretical figure anyway.

Connecting rods don't wear. Their bearings wear, but the con-rods don't
wear. I doubt the slight difference in force on the connecting rod and
crank bearings is enough to cause a measurable difference in wear.

You're correct. My post was hastily composed and clearly didn't pass peer
review. ;-).

KB


Matt

FOUL!! 100LL has 4 times the lead content than the old 80 avgas and a
LOT more than unleaded auto! Therefore, the 100 octane valves should be
supper lubricated! But, in fact, the 80 octane valve tend to stick if
much 100LL is run through them...

So, somebody answer my question...what is special about 100 octane
valves? The ones I put in my A-65 were Stellite. What does Stellite
do? What about sodium filled valves? What's their claim to fame??

Scott


Matt Whiting wrote:
Scott wrote:

Like I said, MIGHT OK, if there is no difference in heat, what are
the 100 octane valves in my A-65 for? Or more generally, why do they
sell 100 octane valves for A-65s and C-85s, etc. that were certified
on 80 octane??? Like I said, I use them, I don't wrench on 'em


Because valves that were designed to be lubricated by the lead in leaded
fuel may not last long when using fuel with no or less lead.

Matt

--
Scott
http://corbenflyer.tripod.com/
Gotta Fly or Gonna Die
Building RV-4 (Super Slow Build Version)

"Matt Whiting" wrote in message ...

Connecting rods don't wear. Their bearings wear, but the con-rods don't wear. I doubt the slight difference in force
on the connecting rod and crank bearings is enough to cause a measurable difference in wear.

Matt

They do experience fatigue cycles. Is that wear?

"Scott" == Scott writes:

Scott Like I said, MIGHT OK, if there is no difference in
Scott heat, what are the 100 octane valves in my A-65 for? Or
Scott more generally, why do they sell 100 octane valves for
Scott A-65s and C-85s, etc. that were certified on 80 octane???
Scott Like I said, I use them, I don't wrench on 'em

I'm guessing--and this is just a guess--that their design differences
are solely to handle the considerable amount of lead still in 100
"low-lead" fuel. I use TCP in my C-90 to deal with the lead.
--
Happiness is an imaginary condition, formerly attributed by the living
to the dead, now usually attributed by adults to children, and by
children to adults.
~ Thomas Szasz

That depends on the BTU content of the fuel. The higher octane will allow
higher compression and tolerate heat better before knocking. Part of that
heat is the release of BTU energy. When I used to roadrace motorcycles long
ago in a mis-spent youth, I used a fuel made by ERC in Hayward. CA. The air
cooled bikes used stuff around 106 octane with about 18K BTU's. The water
cooled stuff could tolerate more BTU as the cooling system removed heat
before it became destructive. The fuel used for drag racing was rated at 22K
BTU. The engines would tolerate this heat release as it was for very short
times with an engine that was not fully warmed up. If this fuel was used in
an endurance event with one of our air-cooled engines, it would result in
holes in the pistons and seizures.

Now, what would 22K BTU stuff do in an 7 1/2 to 1 aircraft engine? Probably
nothing as the lower compression would limit the amount of work actually
being done. But it will still burn hotter than a fuel with a lower BTU
content. Might have to richen the mixture a bit if CHT or EGT were being
monitored, but knock and/or detonation won't be a problem.

One of the main benefits of using higher compression is the amount of power
available at part throttle. You can get more power out of a higher
compression engine at very low power settings than a lower compression
engine. The result of this is a lower fuel burn, unless the mixture is
richened to cool the cylinder. But cooling at part throttle shouldn't be an
issue all things being equal.

Dale Alexander


Eh? Where's this "extra heat" come from? There is no practical unit
energy difference between different octane fuels.
--


Exactly. "Octane" is, by definition, a measure of a fuel's resistance to
knock under specific conditions. "High Octane" fuel does not burn any
hotter, generate any more power, or improve your fuel economy (note: see
exception below). Higher octane fuel lets the engine designer use a
higher compression ratio, or more spark advance, etc. without triggering
knock. It is the compression / spark changes that result in more power,
etc.

As far as cars are concerned, for quite some time now, most vehicles have
had returnless systems where the pressure regulator and pump are in the
tank. The reason that fuel is not returned to the tank is that the fuel,
having made the trip to the fuel rail and back, has picked up heat. This
heat is released in the fuel tank, heating the fuel and increasing the fuel
tank pressure. This wrecks havoc with the evaporative emission systems
ability to detect leaks smaller than the current spec of 0.020".

Now with an aircraft, this shouldn't be a problem as there are now emission
systems on them...yet.

Dale Alexander


They can run higher fuel pressures and/or increase the injector pulsewidth
as a function of measured or inferred fuel rail temperature. Another
helpful option is to have a system that returns excess fuel back to the
tank which tends to purge out any vapor bubbles.

--
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.

Scott wrote:
FOUL!! 100LL has 4 times the lead content than the old 80 avgas and a
LOT more than unleaded auto! Therefore, the 100 octane valves should be
supper lubricated! But, in fact, the 80 octane valve tend to stick if
much 100LL is run through them...

So, somebody answer my question...what is special about 100 octane
valves? The ones I put in my A-65 were Stellite. What does Stellite
do? What about sodium filled valves? What's their claim to fame??

Scott


Matt Whiting wrote:
Scott wrote:

Like I said, MIGHT OK, if there is no difference in heat, what
are the 100 octane valves in my A-65 for? Or more generally, why do
they sell 100 octane valves for A-65s and C-85s, etc. that were
certified on 80 octane??? Like I said, I use them, I don't wrench on
'em


Because valves that were designed to be lubricated by the lead in
leaded fuel may not last long when using fuel with no or less lead.

Matt


No kidding. What's your point? I know that 100LL has a lot more lead
than 80. That doesn't change the correctness of my statement.

Matt

Blueskies wrote:
"Matt Whiting" wrote in message ...
Connecting rods don't wear. Their bearings wear, but the con-rods don't wear. I doubt the slight difference in force
on the connecting rod and crank bearings is enough to cause a measurable difference in wear.

Matt

They do experience fatigue cycles. Is that wear?

Maybe, maybe not. If the stress in steel remains below a certain
threshold, the fatigue life is essentially infinite. I've never heard
of a life limited con-rod so I suspect the stress levels are
sufficiently low that fatigue isn't an issue. In any event, no, I don't
think fatigue is considered to be wear by the standard definition.

Fatigue is a mode of deterioration, as is rust, but I don't think either
is considered to be wear per se.

Matt

Stellite is a very hard metal that does not transfer to the seating metal.
Valves and seats that are made of stellite do not wear at anywhere the rate
that older materials would. Use of hardened materials is one of the reasons
that contemporary engines (such as Toyota's) do not require a valve
clearance adjustment for the first ONE-EIGHTH of a MILLION MILES!!!

Elemental Sodium metal is liquid at temperatures that are common in a valve
stem. It is used in specially constructed valves that have hollow stems. As
the valve heats up, the Sodium becomes liquid. Now the trick is that the
hollow portion of the valve stem is not completely filled with Sodium. When
the valve opens,the Sodium will fill the end of the valve head (inside the
combustion chamber). Here it acts as a heat sink soaking up heat from the
exhaust gases as they exit the cylinder. When the valve moves to seat
itself, the heated Sodium moves to the valve stem area where the heat picked
up in the valve head area is transfered to the valve guide. This back and
forth transfer of heat helps keep the valve cool.

The downside of a Sodium filled valve is that the valve stem becomes quite
large. Because of this and the fact that better materials are now available,
they are not used on smaller bore engines as the increase in valve stem size
acts to reduce the amount of port area (in a critical area) needed to move
airflow in a high RPM engine. But in an inefficient aircraft engine where
heat is a larger concern that power, Sodium valves can be found.

Hope this helps,

Dale Alexander

"Scott" wrote in message
.. .
FOUL!! 100LL has 4 times the lead content than the old 80 avgas and a LOT
more than unleaded auto! Therefore, the 100 octane valves should be
supper lubricated! But, in fact, the 80 octane valve tend to stick if
much 100LL is run through them...

So, somebody answer my question...what is special about 100 octane valves?
The ones I put in my A-65 were Stellite. What does Stellite do? What
about sodium filled valves? What's their claim to fame??

Scott


Matt Whiting wrote:
Scott wrote:

Like I said, MIGHT OK, if there is no difference in heat, what are
the 100 octane valves in my A-65 for? Or more generally, why do they
sell 100 octane valves for A-65s and C-85s, etc. that were certified on
80 octane??? Like I said, I use them, I don't wrench on 'em


Because valves that were designed to be lubricated by the lead in leaded
fuel may not last long when using fuel with no or less lead.

Matt

--
Scott
http://corbenflyer.tripod.com/
Gotta Fly or Gonna Die
Building RV-4 (Super Slow Build Version)