What/how does compression ratio affect an engine?

GeorgeB wrote:
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?

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.

Ole Jerry said: most of the automobiles in the last 20 yrs or so have
the GAS PUMP inside the GAS Tank. Vapor lock is usually induced by
Sucking on the fuel at some point in the fuel system.

"Jerry Wass" wrote in message
. net...
GeorgeB wrote:
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?

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.

Ole Jerry said: most of the automobiles in the last 20 yrs or so have the
GAS PUMP inside the GAS Tank. Vapor lock is usually induced by Sucking on
the fuel at some point in the fuel system.

Maybe off topic but this involves fuel pumps in tanks. They fail way too
often and when they do, you aren't going to fix it on the road side. After
five failures at $800 a pop, I removed the pump from the tank and put an
aftermarket pump in the fuel line where I can replace it with a screw driver
for $50. No vapor locks yet.

On Mon, 10 Dec 2007 14:32:55 -0800, "RST Engineering"
wrote:

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

Jim
And, by default, high octane gasoline is NOT slower burning. SOME high
octane fuels MAY burn slower, but others WILL burn faster. The
air/fuel ratio has a more predictable effect on burn speed.

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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.


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On Mon, 10 Dec 2007 19:25:31 -0500, "Capt. Geoffrey Thorpe" The Sea
Hawk at wow way d0t com wrote:

"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.

Not only purges bubbles but also cools the lines. The pump moves
multiple quantities of fuel compared to what the engine actually
consumes (on some vehicles as much as TEN TIMES.- but most closer to
3)

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On Tue, 11 Dec 2007 01:23:01 GMT, Matt Whiting
wrote:

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
And that is only true to a point.
A case in point.
The Corvair engine with the low compression ratio "smog" or "open"
heads is significantly MORE detonation prone than the non smog or
"closed chamber" high compression heads. Also, lowering the
compression ratio of the high compression engine by using thicker head
sealing rings (gaskets) actually CAUSES detonation, rather than
reducing it. The difference in quench and squish is AT LEAST as
significant as compression ratio as far as detonation is concerned.

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On Tue, 11 Dec 2007 01:27:52 GMT, 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
Good arguement EXCEPT 100LL has MORE lead than the early 80 octane. It
is only low in lead compared to the earlier 100/115 or whatever the
"real beetle juice" was rated at.

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On Tue, 11 Dec 2007 01:55:38 +0000, 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

The 100 octane valves will have different valve stem design (and
likely clearances) and will (from what I've read on the subject) be
designed to run HOTTER to keep the TEL from building up on the tulip
portion and the neck of the valve. This is what causes the "morning
sickness" referred to as hanging valves - otherwise known as "lead
poisoning". Aggressive leaning at less than peak power will also help
reduce this lead build-up.

Stellite can stand a LOT more heat. Sodium cooled valves have liquid
sodium inside the stem and head to act like a "heat pipe" and suck the
heat down the stem to transfer the heat more efficiently to the head.
This would cause the stem area to run hotter, possibly explaining
their use in 100ll conversion valves?

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


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On Tue, 11 Dec 2007 01:57:26 GMT, "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?

Fatique cycles in a properly designed rod are almost immaterial. There
is virtually NO bending motion, In compression steel is almost totally
unaffected, and the rods are generally designed to take the tension
loading from high RPM - which is not directly affected by higher
compression. High compression engines that are run at extreme speeds
DO need to worry about fatigue cycles. "tractor engines" as a rule
have no such worries. (unless you are talking "pulling tractors")

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On Tue, 11 Dec 2007 03:40:42 +0000, Scott
wrote:

OK, this is where I probably made my wrong assumption (even though I
precluded my original post with the word "might" when referring to more
heat from 100 octane). So it's not heat produced strictly because of
higher octane, it's just that the mixture may still be burning as the
exhaust valves open on the exhaust stroke and "burns" the valves. So,
it may not burn hotter, but it seems to burn longer, thus heating the
valves more, so it's kind of semantics since the valve is still getting
hotter by burning 100LL rather than 80 (but for a different reason than
I originally posted and ****ed everyone off by regurgitating an old
wive's tale). Is this correct?

Scott

Not necessarily.
By definition high octane fuel does NOT burn slower than regular gas.
Burning FASTER would be more productive in reducing detonation.
Counterintuitive, but DEFINITELY true. The faster the burn, the less
heart absorbed by the "end gasses" in the chamber, and the less chance
they will "dissassociate" and destabilize - which IS what causes
detonation. Gasoline BURNS. Hydrogen radicals EXPLODE.
To prevent detonation you need to burn cooler, burn faster, or the
fuel needs tobe more resistant to thermal dissassosiation .

This is why detonation is GENERALLY only a problem at lower RPMs. You
can produce X HP at 2500 RPM and get detonation, and 1.5X at 3200 RPM,
for instance, with no detonation. - with the same or higher cyl
pressures at the higher speed.



Big John wrote:

On Tue, 11 Dec 2007 01:55:38 +0000, 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



Scott

High octane fuel burns slower in low compression engines. This lets
the flame front go out the exhaust valves and burn them if you run a
lot of hours.

We ran into this in the 'old' days when the fighters used 115-145
octane fuel. If we used that fuel in our cars we had to mix some oil
with it to prevent the burnt valves.Occasionally someone would burn a
tank full to clean their engine out (burn carbon off cylinders and
valves, etc)

This is probably the reason they put 100 octane valves in your low
compression engine so you could use 100 octane and not destroy the
valves?

Big John


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