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 Tue, 11 Dec 2007 18:08:03 -0700, "Bill Daniels"
bildan@comcast-dot-net wrote:


"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.

Strange. I have 362000km on my current vehicle (pump in tank - still
original and 14 years old) and had 240,000 on each of my last two
vehicles - 14 and 18 years old - all on the original in-tank pumps.

I NEVER got that kind of mileage out of a mechanical pump, and frame
mounted electric pumps virtually ALL rusted out in less than 10 years,
requiring replacement. This is why, to a large degree, most
manufacturers went to in-tank pumps a long time ago.
If you don't change fuel filters, you WILL burn out pumps. If you
always run on the bottom 1/4 tank you will LIKELY burn out pumps - but
the top 1/4 doesn't cost any more to keep full than the bottom 1/4.

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

Compression ratio affects the efficiency of an engine, and something
called the 'brake specific fuel consumption' which is a measure of how
much fuel the engine must burn to produce some given amount of
horsepower... or something like that.

A higher compression ratio engine is more efficient in turning fuel
into useful work. But there are trade-offs in that a higher-
compression ratio is harder on the engine in terms of wear and tear,
and high compression needs higher octane fuel to prevent detonation
from happening inside the cylinders.

I have learned from the old-timers around my airport, that the 8.5:1
compression ratio 160hp version of the O-320 seems to be the sweet
spot, in that it can burn auto-fuel as long as you use premium
unleaded, 91 octane, and make sure you have big enough fuel lines that
are well-insulated from any heat source, and use a bigger, stronger
fuel pump system to prevent vapor-lock from happening. A 160hp O-320
that is throttled back to produce 150 hp will use less fuel per hour
than a 150hp low compression version of the same engine running at the
full 150hp power setting. The 160hp will also have cooler running
cylinder temps at that power setting too.

9:1 compression ratio in the otherwise same exact hypothetical O-320
will need 100LL to keep from suffering detonation, and will probably
suffer some anyway, and might be a maintenance headache, but it sure
will run strong when it's fresh.... for a little while anyway.

I had a buddy who built a Glasair III and had 10.5:1 pistons in his
IO-540. It sure was fast and strong, but he put a new set of pistons
in it about every 18 months. The old ones always came out with
multiple cracked rings, cracked ring grooves/lands, and sometimes
small holes melted between the dome and the 1st compression ring
groove. I think he must have had to work on that plane 20 hours of
repairs for each hour he flew it.

The 8.5:1 160hp O-320 seems to be the best proven choice for power
and longevity. I'd recommend that.

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

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.

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