What/how does compression ratio affect an engine?

Mine is in an experimental as well, so I'll see if I can find more info
on getting or making some sort of mixture control...thanks for the info!

Scott


wrote:

On Dec 12, 5:03 am, Scott wrote:

OK, so us poor bastages with A-65s with no mixture control might be
screwed anyways Anybody have any ideas on how to get a mixture
control onto an A-65? If I recall, my carb is an NAS3A1 Stromberg. I
THINK there might be a plate on top of the carb where a mixture control
may have been present at one time???


That's it. Finding the bits and pieces might be a hassle. I
made mine, since it's a homebuilt, and it works OK. It's a back-
suction type of mixture control, where the lever rotates a valve under
that cover. There's a hole into the float bowl, another into the dead
airspace behind the venturi, and another into the venturi itself. The
area behind the venturi is at roughly atmospheric pressure, and the
venturi, of course, is much lower than that. The mixture control
valve, when rotated toward "Lean," gradually shuts off that port to
the dead airspace, allowing the venturi port to start sucking against
the fuel in the bowl. That decreases the differential pressures at the
fuel nozzle and flow decreases.
It won't work as an idle cutoff. There's so little flow through
the venturi at idle that nothing happens when you pull the mixture all
the way out. Carbs with idle cutoff are using a valve inside the carb
that varies the flow of the fuel itself from the main jet in the
bottom of the bowl, so that if it's pulled full lean, all flow,
including idle fuel, stops.

Dan

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

No extra heat in high Octane. Same hydrocarbon structure is both. Same
BTUs. Only difference is the speed of combustion is controlled so it is
slower in the higher octane to prevent pre-ignition.
--
Cy Galley - Chair,
AirVenture Emergency Aircraft Repair
A 46 Year Service Project of Chapter 75
EAA Safety Programs Editor - TC
New address -
EAA Sport Pilot


"Scott" wrote in message
...
One thing I think "might" be a concern is that burning 100LL (can't get 80
octane avgas these days) in an engine built for 80 octane is the extra
heat. I think valves are most likely to be affected by burning the 100LL
instead of 80. Might be all wet on this, but that's what I've heard and
I'm NOT an engine mechanic...just a user You might try digging in the
Lycoming site and see what they say about 80 vs 100LL...

Scott


wrote:
OK, not cool to reply to my own post, I know. But I just found this
great resource which basically answers all my questions:

http://www.lycoming.textron.com/supp.../key-reprints/

"Lycoming provides helpful information in various publications,
including Lycoming Flyer Key Reprints. Lycoming's Key
Reprints is our effort to continually share our best practices,
key lessons and engines systems knowledge to empower
our customers."

Lower compression = lower chance of preignition = lower octane
required (ie. auto fuel)!
Yes, that would make a difference to the pocket book!

The issue of using auto fuel is addressed in this series also:

"Auto fuel is now being used as a substitute for Grade 80
aviation gasoline under STCs issued by the FAA. Most
major oil companies and engine manufacturers continue
to recommend that aircraft piston engines be operated
only on aviation gasoline. Deterioration of engine and fuel
system parts have been reported in aircraft using auto
fuel. Operators should consider the added risk of using
auto fuel in aircraft. Remember -- a pilot can't pull over
to the side of the road when fuel creates a problem with
the engine."



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

On Thu, 13 Dec 2007 04:00:58 GMT, "Cy Galley"
wrote:

No extra heat in high Octane. Same hydrocarbon structure is both. Same
BTUs. Only difference is the speed of combustion is controlled so it is
slower in the higher octane to prevent pre-ignition.


WAY off base, Cy.
On ALL counts, except the no extra heat and same BTU's.

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

On Dec 12, 9:08 pm, clare at snyder.on.ca wrote:
On Thu, 13 Dec 2007 04:00:58 GMT, "Cy Galley"
wrote:

No extra heat in high Octane. Same hydrocarbon structure is both. Same
BTUs. Only difference is the speed of combustion is controlled so it is
slower in the higher octane to prevent pre-ignition.

WAY off base, Cy.
On ALL counts, except the no extra heat and same BTU's.

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

Higher-octane rated fuels have about the same flame front
speed as lower-octane fuels, at around 100 feet per second. It's their
resistance to detonation, which involves flame fronts speeds of 5000
feet per second or more, that makes them necessary in high-compression
engines. As the combustion process begins in the cylinder, the burning
raises the pressure through the entire air/fuel mix, and low-octane
rated fuels will break down under that increasing pressure and
spontaneously combine with the oxygen, igniting almost all at once
instead of *waiting* (key word) for the flame to set them off in a
controlled chain reaction.
Detonation therefore requires time to develop. Low RPM with
high MP is pro-detonation. Lean mixtures burn more slowly, so they're
pro-detonation. Big cylinders take more time for the flame front to
cross, so they suffer more from detonation.
Preignition is something else. It's ignition of the mix
during compression by some hot spot in the cylinder, perhaps a glowing
bit of carbon in the head or on the piston. The burn begins early,
before the sparkplug was scheduled to fire, and gets way ahead of
itself due to the still-increasing compression as well as the
combustion, and detonation might happen. The damage is about the same.
Engines that aren't leaned properly will develop more carbon to cause
preignition. Too much lead fouling can do it. An oil-burner will
carbon up, too.
Octane ratings are different than octane content. In the old
way, fuel under test was fed to a variable-compression test engine and
the compression was raised until detonation began to occur. Then that
fuel was shut off and a mix of octane and heptane was introduced,
engine still running, and the octane/heptane ratio was varied until
the detonation point was established. An 80-octane fuel had the same
detonation characteristics as a mix of 80% octane and 20% heptane,
hence the rating. 80/87 reflected the detonation resistance at lean
and rich settings.
Fuels with ratings above 100 obviously have to be rated using
some other method, since we can't have a fuel consisting of, say, 115%
octane.

Dan

On Thu, 13 Dec 2007 04:00:58 GMT, "Cy Galley"
wrote:

No extra heat in high Octane. Same hydrocarbon structure is both. Same
BTUs. Only difference is the speed of combustion is controlled so it is
slower in the higher octane to prevent pre-ignition.



Cy

I'm on your side. See my prior post.

Lot of people are using different words to describe the same thing
which may be part of the problem.

Big John