Lean mixtures actually burn with less temperature, but because a lean
mixture also burns slower, the heat is held to the piston for a longer
period of time, hence "Melt Down."
"lean mixtures actually burn with less temperature " - compared to what,
an ideal perfect mix , yes, compared to a rich mix, not necessarily. in the
first case the combustion process is starved of fuel, in the latter the
process is starved of air, neither yields maximum thermal results. lets
take a fresh look at whats happening inside the combustion chamber with
burn rates. the fuel mixture starts burning from the time the spark plug
discharges and may burn through the time the piston pushes this burning mix
out the exhaust port. during this time the heat from combustion is
transferred to the engine, including the piston dome. now lets consider a
slow burning scenario first. the slow burning mix would ignite and burn in
the chamber, and with the slow burn comes slowly increasing heat, the more
fuel burned, the more heat liberated. so, consequently it takes a period of
time for the combustion chamber to reach maximum temperature since it takes
a period of time for all the fuel mix to get burned. in other words, the
cylinder does not reach max temp quickly. in a fast burning scenario, the
combustion chamber reaches max temp much more quickly since the burning
fuel mix reaches max temp much more quickly since the fuel mix burns much
more quickly. so in a fast burning scenario the cylinder is exposed to
maximum temp for a longer period of time, and therefore causes more heat to
be transferred to the engine, including the piston dome - i.e. the heat is
held to the piston dome for a longer period of time in a quick burn, not a
slow burn. another principle of household physics to observe is the ideal
gas law ( gas as in "gaseous" not "gasoline" ) which states that pressure
times volume divided by temperature is always constant. so in a fast
burning scenario the liquid fuel portion of the mix is convertd to gaseous
combustion products much more quickly,and with more gaseous combustion
products crammed into the combustion chamber, cylinder pressure is
increased and by the ideal gas law, if pressure increases, so must
temperature in a fixed volume ( yes , the volume of the combustion chamber
is always changing with crankshaft rotation, but it is comparitively the
same for the same engine weather it has a slow burning or fast burning
mix). so , pressure is held to the piston for a longer period of time also.
as pressure goes up, temp goes up, piston temp goes up, margin to melting
goes down.
regards-
doug@black radon
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