Withstanding Peak Temperatures

O. Sami Saydjari wrote:
: OK. I have a basic question. Why is running an engine at peak
: temperature (as in Rich-of-Peak operation) not good for the engine? In
: the scheme of things 50-100 degress cooler than peak temperature (where
: many folks recommend the engine be run) does not seem significantly
: lower in temperature than the peak temperature. It would seem if the
: engine parts can stand Peak-100 degrees, they would be able to stand
: peak. What am I missing? (Oh, and I have read Deakin's articles on
: AVWEB, and still have this question).

Sami, I think that Deakin is using TEMPERATURE, which can be determined by
cockpit instrumentation to describe TIME, which cannot be seen on cockpit
instruments. When running 50-100 ROP, the temperatures are well controlled.
The difficulty as I understand it is because the power pulse from the piston
occurs very shortly after the piston reached Top Dead Center (TDC).

What this means is that the force of the piston is being transferred to
the crankshaft at a very small angle - this can be demonstrated by holding
your arm out, elbow straight, and pushing on the wall: you can't make
your elbow bend. If you bend your elbow a little bit, and then push the
wall, it is quite easy to bend your elbow more. (In my example, your elbow
represents the connecting rod/crankshaft, and your hand the piston).

Running at LOP values, or very rich values, slows down the buring of the
gases above the piston, resulting in the push force being delivered when the
crankshaft has turned a couple more degrees (Deakin claims that 16 degrees
after TDC is optimum). This is easier for the engine to take, the bearings
are exposed to lower forces, etc. The use of TEMPERATURE to determine
TIME (crankshaft angle) is because there is no instrument to determine
crankshaft angle vs. combustion event timing.

Please note that this is a significant difference from auto engines! Car
engines have adjustable spark timing to overcome these difficulties, while
airplane magneto engines almost always have fixed timing (there is an
adjustable timing system for airplane engines).

I am not sure that I explained this as well as it is possible to do so,
but the basic point that I am trying to make is that the TEMPERATURE is not
in itself the critical issue, so long as you don't melt the cylinder heads.
Rather, TEMPERATURE is used to indirectly indicate another, immesurable,
parameter, crankshaft angle vs. combustion event timing.
--
Aaron Coolidge (N9376J)