What/how does compression ratio affect an engine?

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

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

"Matt Whiting" wrote in message
...
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.

You're correct. My post was hastily composed and clearly didn't pass peer
review. ;-).

KB


Matt

"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?

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?

Maybe, maybe not. If the stress in steel remains below a certain
threshold, the fatigue life is essentially infinite. I've never heard
of a life limited con-rod so I suspect the stress levels are
sufficiently low that fatigue isn't an issue. In any event, no, I don't
think fatigue is considered to be wear by the standard definition.

Fatigue is a mode of deterioration, as is rust, but I don't think either
is considered to be wear per se.

Matt

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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