What/how does compression ratio affect an engine?

"Scott" == Scott writes:

Scott One thing I think "might" be a concern is that burning
Scott 100LL (can't get 80 octane avgas these days) in an engine
Scott built for 80 octane is the extra heat.

Eh? Where's this "extra heat" come from? There is no practical unit
energy difference between different octane fuels.
--
"You, Mr. Wilkes, will die either of the pox or on the gallows."
-The Earl of Sandwich

"Bob Fry" wrote in message
...
"Scott" == Scott writes:

Scott One thing I think "might" be a concern is that burning
Scott 100LL (can't get 80 octane avgas these days) in an engine
Scott built for 80 octane is the extra heat.

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.

Exception: _Some_ automobiles have sensors that detect knock and will adapt
the spark advance to match the fuel properties which will then improve your
fuel economy.

--
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.

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.

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

Scott


Bob Fry wrote:
"Scott" == Scott writes:


Scott One thing I think "might" be a concern is that burning
Scott 100LL (can't get 80 octane avgas these days) in an engine
Scott built for 80 octane is the extra heat.

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

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

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" == 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

I do, and I do.

100 octane valves have a different seat angle and a smaller stem diameter
than 80 octane valves.

Lead is the big problem with 100 octane in a low compression engine. While
the gasoline itself and the combustion process makes not a whit of
difference in the temperature of combustion, the compression ratio does.
Gasoline in a high compression engine burns hotter, be it 80 or 100 octane.
Tetraethyl lead requires a relatively high temperature to be completely
vaporized and exhausted in the combustion process. A high compression
engine does this, while a low compression engine does not. Bromine is
introduced to the mixture in an attempt to "help" the lead "burn", but even
bromine doesn't do much in a low temperature environment.

The different seat angle is an attempt to "get the lead out" of unburned
tetraethyl lead. Even so, the low temperatures encountered in a low
compression engine lets the lead coagulate on the first cool surface it
finds. That happens to be the valve stem. Therefore, the valve stems are
ever so slightly reduced (.005" comes to mind) so that the lead can plate
out on the valve stem and still not cause the stem to stick on the guide so
often. "So often" is the operative term here. 100 octane will cause valve
sticking, but with the 100 octane valves, just not quite as soon.

Jim

--
"If you think you can, or think you can't, you're right."
--Henry Ford

"Scott" wrote in message
.. .

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

Scott

On Mon, 10 Dec 2007 23:17:23 +0000, 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

Scott

It has to do with the difference in lead content from what I have
gathered. 100LL has a LOT of lead, and an A65 will get "morning
sickness" quite early in it's life if run on 100LL with standard
valves without agressive leaning.


Bob Fry wrote:
"Scott" == Scott writes:


Scott One thing I think "might" be a concern is that burning
Scott 100LL (can't get 80 octane avgas these days) in an engine
Scott built for 80 octane is the extra heat.

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


--
Posted via a free Usenet account from http://www.teranews.com

Bob Fry wrote:
"Scott" == Scott writes:

Scott One thing I think "might" be a concern is that burning
Scott 100LL (can't get 80 octane avgas these days) in an engine
Scott built for 80 octane is the extra heat.

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

Some believe that higher octane gasoline burns more slowly and thus can
put more fire past the exhaust valve. However, most of my reading on
the subject suggests that this is a myth and there is no substantial
difference in burn rate as a function of octane.

Matt

"Matt Whiting" wrote in message
...
Bob Fry wrote:
"Scott" == Scott writes:

Scott One thing I think "might" be a concern is that burning
Scott 100LL (can't get 80 octane avgas these days) in an engine
Scott built for 80 octane is the extra heat. Eh? Where's this
"extra heat" come from? There is no practical unit
energy difference between different octane fuels.

Some believe that higher octane gasoline burns more slowly and thus can
put more fire past the exhaust valve. However, most of my reading on the
subject suggests that this is a myth and there is no substantial
difference in burn rate as a function of octane.

Matt

I have not had occasion to test this premise on an aircraft engine--since
only 100LL was available when and where I flew. Therefore, no comparison
was possible.

However, back in the "bad ol' days", running a "regular gas" automobile
engine on "hi-test" had much the same effect as running with the ignition
timing retarded relative to that called for in the manual--the engine ran
noticeably warmer. Therefore, I am fairly certain that Big John is correct
in his assessment (elsewhere in this thread).

There is also the oft' stated issue of lead fouling, and it is possible that
the 100LL valves have some greater immunity to that as well; although I am
neither an engineer nor a wrench turner and I really have no idea how that
might be accomplished.

Peter