What/how does compression ratio affect an engine?

Oh, not this old wive's tale again. THERE IS NO ADDITIONAL HEAT GENERATED
BY A HIGHER OCTANE FUEL.

Jim

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

"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.

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)

See my other post and clue me in to why they sell 100 octane valves for
A-65s and C-85s. Are they a waste of money?

Scott


RST Engineering wrote:

Oh, not this old wive's tale again. THERE IS NO ADDITIONAL HEAT GENERATED
BY A HIGHER OCTANE FUEL.

Jim


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

On Dec 10, 4:22 pm, Scott wrote:
See my other post and clue me in to why they sell 100 octane valves for
A-65s and C-85s. Are they a waste of money?

See http://www.fsv2000.at/woche/2001_10/conti_sb_m77_3.pdf for a
TCM Service Bulletin about alternate valves for those engines. They
say that erosion problems have been encountered with the high lead
content of 100LL. It's not because the fuel burns hotter; it's a
corrosion issue. If the fuel burned hotter, I would see both higher
CHTs and more power out of my A-65. But I don't. You need to do some
research on Octane Ratings and Detonation. Google those.

Dan

"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.

Let me reccomend a book. It's very old but packed with aero engine
knowledge. It's historically significant and a good read. Read it and
you'll know more about airplane piston engines than you could imagine there
was to know.

"The High Speed Internal Combustion Engine" by Sir Harry Ricardo.
See: http://www.ricardo.com/ricardoStore/...t.asp?P_ID=149 (1968)

(In this case, "High Speed" refers to MPH and not neccessarilly RPM.)

Sir Harry was a British mechanical engineer whose most significant work was
between WW1 and WW2. He championed the sleeve valve engines that were
developed into the Bristol Centurion but his most historic work was on the
Rolls Royce Merlin. He also worked on the last generation of high power
piston engines like the Napier Nomad and consulted with all the US engine
manufacturers. He is a towering figure in the history of airplane piston
engine development.

His work with high octane fuels, supercharging and high compression ratios
is has been acknowledged as a major contribution to the Allied victory in
WW2.

Bill Daniels

wrote:
Hi,

So I was fantasizing the other day (as I do quite a bit) about my
latest dream-plane-to-build: an RV-9A. (Note that this is fantasy in
the extreme as I'm not a pilot yet AND I have no money!)

I was thinking of how to power this plane. A Jet-A burning diesel
would be great but that's another story. I spotted an ad for
Superior's XP-series engines in Kitplanes. The website has a great
"build your own engine" feature where you get to change all the bits
and customize the engine.

One of the things you have to choose is compression ratio: 7:1
(150hp), 8.5:1 (160hp), or 9:1 (165hp).
Going for the 7:1 option (from the default 8.5:1) adds $100 to the
price! I'm assuming this is a supply/demand issue.

So my question (finally) is: what is the effect of a higher or lower
compression ratio? I believe TBO for all three engines is still 2000
hours.

- Is there a difference in wear?
- Would maximum power be produced at the same RPM for all three
engines? In other words is there a relationship like (power) =
(compression ratio) x (RPM) such that these engines all operate at the
same RPM? In which case wear would be the same...(?)

For the RV-9A 150hp would be fine. I guess I'm trying to understand
what benefit is to be had by spending the extra $100 to go for the
lower compression pistons. All in my fantasy

I'm not sure, but I don't think the difference in compression ratios
will have a significant affect on wear or where the power peaks. The
main difference is that lower compression engines are more detonation
resistant and thus you have more margin to run low octane auto gas.

Matt

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.

Matt

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

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