ANNUAL, 2005 Redux...

: In any case, there are exactly two things "wrong" with aircraft engines. The
: spark timing is fixed and the mixture control is in the hands of the pilot.
: And you can't do anything about the first problem until you fix the second.

The first is a compromise, but generally works really well for aircraft
engines... they *do* run at a constant RPM all the time. The second isn't that big of
a deal if the pilot is properly trained to use the mixture knob. Trouble is most
aren't because of the great degree of OWT and misinformation out there on the subject.

I would argue that the #1 biggest problem with aircraft engines is that they
are still air-cooled. Removing the tremendous thermal stresses of having 450 degree
CHT's make most of the "routine" aircraft engine problems go away. Stuck valves,
cracked exhaust flanges and cylinder heads, ridiculous octane requirements (100 for
8.5:1?) due to the heat and low RPM, galled cylinders/pistons due to overheating,
shock-cooling, and cold-starts, etc.

Liquid cooling stabilizes everything, lets more power be made more efficiently
with greater reliability. It doesn't even have to add too much weight.

-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

wrote in message
...
: In any case, there are exactly two things "wrong" with aircraft engines.
The
: spark timing is fixed and the mixture control is in the hands of the
pilot.
: And you can't do anything about the first problem until you fix the
second.

The first is a compromise, but generally works really well for aircraft
engines... they *do* run at a constant RPM all the time. The second isn't
that big of
a deal if the pilot is properly trained to use the mixture knob. Trouble
is most
aren't because of the great degree of OWT and misinformation out there on
the subject.

Except that the optimal spark timing is a strong function of the air fuel
ratio. To get the full benifit of leaning, you need to change the spark. You
can't optimize the spark for the mixture if you don't know what the mixture
is.

I would argue that the #1 biggest problem with aircraft engines is that
they
are still air-cooled.

Liquid cooling has it's advantages.

Removing the tremendous thermal stresses of having 450 degree
CHT's make most of the "routine" aircraft engine problems go away. Stuck
valves,
cracked exhaust flanges and cylinder heads, ridiculous octane requirements
(100 for
8.5:1?)

Octane requirement is a stong function of spark timing...
Of course, large, open combustion chambers with lower charge motion tend to
require higher octane too.

due to the heat and low RPM, galled cylinders/pistons due to overheating,
shock-cooling, and cold-starts, etc.

Liquid cooling stabilizes everything, lets more power be made more
efficiently
with greater reliability. It doesn't even have to add too much weight.


Getting rid of points would be an improvement also. Solid state magneto's
are not hard to make.

--
Geoff
the sea hawk at wow way d0t com
remove spaces and make the obvious substitutions to reply by mail
Spell checking is left as an excercise for the reader.

Capt. Geoffrey Thorpe The Sea Hawk at wow way d0t com wrote:
: Except that the optimal spark timing is a strong function of the air fuel
: ratio. To get the full benifit of leaning, you need to change the spark. You
: can't optimize the spark for the mixture if you don't know what the mixture
: is.

Agreed. All I'm saying is that the for the aircraft application, the
operating regime (cruise) is a very small window of operating conditions. If it can
be optimized within that window, but made simultaneously safe to operate outside of
it, that'd be enough. I haven't thought it through completely, so they may be
mututally exclusive.

: Octane requirement is a stong function of spark timing...
: Of course, large, open combustion chambers with lower charge motion tend to
: require higher octane too.

Isn't it a stronger function of compression ratio and CHT, though? Timing
will help manipulate the time at which the peak pressure occurs, but the detonation
margin has very little to do with timing.

: Getting rid of points would be an improvement also. Solid state magneto's
: are not hard to make.

Agreed. Points suck. Making them lightning-robust might take some
thought, however...

All of these things (solid-state "points," variable timing intelligently
dependent on RPM and manifold pressure, etc) would be very difficult to accomplish
mechanically to aviation robustness standards. Computers like FADEC would make it
easier to implement, but still quite difficult to certify.

-Cory


--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************