What/how does compression ratio affect an engine?

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

Thanks,

Michael

Well, one thing I know, is that with the lower compression ratio (ie 7:1
for the 150HP) is that the engine can run on auto fuel. That's why I'm
planning on a 150 HP in my RV-4. The higher compression engines take 100LL.

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

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

Thanks,

Michael




--

wrote in message
...
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

Thanks,

Michael

The extra $100 is probably because the 7:1 pistons are a special order item,
as opposed to the 8.5:1 pistons which are the standard.

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.

As to autogas vs 100LL, plenty of people running 8.5:1 compression engines
are running autofuel. A bigger issue with autofuel (at least in engines
with a compression rapto of 8.5:1 or less) is vapor lock, rather than
detonation.

Kyle Boatright
160hp (8.5:1 Lycoming) RV-6

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."

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)

"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

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.

On Dec 10, 11:06 am, 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.

Most autos use electric in-tank fuel pumps now. When the
fuel is pushed to the engine there's little vapor-lock risk. Aircraft
still often use pumps on the engines that pull the fuel, so that the
pressure on the fuel in the lines drops and the vapor pressure of the
fuel will cause vapor lock under the right conditions. Low-wing
airplanes that have the tanks in the wings will have boost pumps
somewhere low in the system, but when they're turned off after takeoff
the risk of vapor lock rises with autofuels.
Remember the old high-school science demonstration of water
boiling at room temperature when a bell jar is placed over a bowl of
it and the air sucked out of the jar? The lowered atmospheric pressure
lowers the boiling point of the water. Gasoline has a higher vapor
pressure than water, so lowering the pressure on it will make it give
off vapors quickly, and those vapors displace the fuel in the lines
and prevent the fuel flow. The pump will be quite happy to pump
vapors, but carburetors and fuel injectors don't deal with vapors very
well, and the engine gets hungry and goes on strike.
A pump that sucks the fuel to lift it from the tank lowers the
fuel pressure between the tank and pump. A pump that pushes it upward
from the tank avoids that.


Dan

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.

"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.