"JGalban via AviationKB.com" u32749@uwe wrote in message
news:7159afcd0261c@uwe...
Matt Barrow wrote:

TSIO-550C

That's what I figured. Turbocharged engines are a different kettle of
fish.

So was my last one; TNIO-550 (okay, turboNORMALIZED).

The main reason is the higher temperatures in the induction system lowers
the
detonation margin considerably.

Hmmm...

Deakins "Detonation Myths".

/quote
Now, somewhere about 20 to 25 degrees before the piston reaches top dead
center (TDC) of piston travel, the spark plug lights the fire. The flame
front starts spreading from each spark plug, slowly at first, then more
rapidly within the cylinder. This flame front plays an important role in all
of this. Ever stick your hand up close to a hot flame? Not in the flame,
just close? It gets hot fast. There is a LOT of infrared heat being given
off by that flame front. It travels at the speed of light. Maybe a few
million times (or so) faster than the flame front is traveling across the
cylinder. That infrared radiation heats up those little local pockets of
fuel and air.

Further, since the piston is rising rapidly in the cylinder, those little
remote local pockets of fuel and air are also experiencing a sudden rise in
pressure.

Still further, because the flame front is a combustion process, it, too, is
causing a further and much larger rise in pressure in the cylinder.

Hold that thought for a moment, while we mention the time scale for all
this.

During the combustion event, the speed of sound (at the higher bulk gas
temperatures) is such that a sound wave can bounce across the cylinder and
back in about 1/5000th of one second, or about 1/5th of a millisecond. This
is easy to instrument and measure. You see the evidence of this in the
little detonation shock waves bouncing back and forth past the pressure
transducer on the back side of the down slope of the combustion pressure
event in the graphics depicting the medium and heavy detonation.

The crankshaft is rotating about 45 times per second and that works out to
about 22 milliseconds for each crankshaft rotation, or about 16 degrees of
crankshaft rotation for each millisecond. So in the time it takes a shock
wave to travel back and forth across the inside of the cylinder, the
crankshaft has only moved about three degrees.

So, now that we have the time scale firmly in mind, we go back and summarize
what is going on:

1.. We have nice cool induction air and fuel entering a cylinder;

2.. The cylinder happens to have very hot walls. Those hot walls cause
some of that nice cool induction air to start to heat up. And it doesn't all
happen uniformly.

3.. Further, shortly after the sparks go off, we have a couple of flame
fronts, giving off lots of infrared heat, adding to the continuing heat load
being absorbed by some of those little remote pockets of fuel and air that
are waiting for the flame front to arrive and consume them;

4.. The unburned mixture is experiencing a very rapid increase in
pressure, because of two things: A) The piston is rising rapidly during the
compression stroke; and B) the flame front combustion products are creating
a huge increase in released energy and resulting bulk gas pressure, all of
which is neatly measured on the pressure traces you see in the accompanying
graphics.

5.. At least some of those little "local pockets" of unburned combustion
mixtures have exactly the right mixture of fuel and air to be just a
hair-trigger away from exploding.

6.. And . if the fuel is the wrong octane, or the spark advance was set
too soon, or the manifold pressure was too high, or the cylinder head
temperature was too high ... then one or more of those little "local
pockets" of unburned fuel do just that ... they "explode."
That is what we call "detonation".

Each explosion creates a shock wave that travels at the speed of sound
(remember, the speed of sound inside the cylinder, at somewhere near 4000
degrees, is very much faster than at a standard day!) and bounces off the
walls of the combustion chamber every 1/5th of a millisecond or so (giving
off a 5KHz "ping" that you will not hear in the cockpit). Each of those
explosions creates a very sharp rise in pressure and sets off a shock wave,
which vibrates back and forth across the cylinder. This shock wave can be
just the right amount of additional pressure to cause some other little
remote local pocket of fuel and air to, in turn, explode, adding to the
problem.

As detonation grows more serious, it will become audible, and this is the
pinging you'll hear from that old auto engine on the uphill grade. Remember,
you will NOT hear it on an aircraft engine.

Let's Talk Temperatures

We know that combustion temperatures are in the 3,000ºF to 4,000ºF range,
but TIT and EGT "only" run around 1,600ºF, and CHTs down around 400ºF. How
can this be? 4,000ºF is more than enough to melt steel, so how does the
interior lining of the cylinder survive? Why don't we see hotter
temperatures on our instruments? Why doesn't the aluminum piston melt down,
when aluminum melts at less than 1,000ºF?

There is a "thermal boundary layer," on the order of a millimeter thin or
so, that acts as a buffer to protect the steel cylinder walls and the
surface of the aluminum piston. Think of it as the thermal equivalent of the
aerodynamic boundary layer out on your wing. The metal and the molecules
right next to it will be at roughly the CHT reading or a bit higher, the
next layers will be hotter and hotter, until the layer next to the
combustion event will be at the combustion temperatures. That very thin
thermal boundary layer acts as a nice insulation barrier, limiting the rate
at which heat can be transferred from the bulk combustion gases into the
interior walls of the cylinder head, cylinder barrel, and piston.

The heat transfer is continuous, as the heat moves first through the
boundary layer, and then the cylinder wall and is finally carried away by
the cooling airflow around the fins on the cylinders. Each intake stroke
brings in a cool new charge, which starts the process all over again. There
is also a matter of time of exposure. The high-pressure part of the
combustion event takes up only about 40 degrees or so of crankshaft
rotation, and the very hottest part of that only about 20 degrees, so during
the other 700 degrees of crank rotation, cooler temperatures prevail. Many
pilots mistakenly focus on the temperature of the exhaust gas as measured by
their familiar EGT probes. EGT shows only a number that represents a
momentary flash of heat during a small portion of the combustion cycle (when
the exhaust valve opens and exhaust gas flows across the EGT probe), and a
rapidly dropping temperature at that.

This is NOT the major factor that determines how hot their exhaust valve is
during operation. The events that happen a few degrees of crankshaft
rotation earlier are much more significant because the temperatures are MUCH
hotter than the piddling little 'ol 1500ºF measured by the EGT probe.

Once detonation becomes serious enough, it disrupts the previously
well-organized thermal boundary layer and allows a greatly increased rate of
heat transfer from the very hot bulk combustion gases (up around 4,000F)
into the cylinder head and the piston. This last stage in the process is
what starts the damage, and drives the CHTs up.

/end