Well I have a rather unorthodox view of combustion developed as a
smog tech. One of my smog instructors said that the difference between
a rich mixture and a lean mixture is that the rich mixture burns faster
because fuel molecules closer together and a lean mixture burns slower.
So the way I look at it is a rich mixture will burn when the piston is
near Top dead center producing alot of pressure and heat where you can
do the best work for pushing down the piston.
Now a lean mixture will not burn fast enough to produce maximum
push and produces pressure when piston is further down and volume is
too great for maximum power.
So heat and pressure of rich mixture is converted more into
kinetic energy of piston, which lowers the temperature of the burning
mixture as the piston goes down. The heat of the lean mixture is not
converted to kinetic energy of piston so it retains more heat as piston
goes down and that is why exhaust is hotter and metal is hotter.
So with a lean mixture you have more waste heat.
Why does a diesel engine have more torque, because it pushes on
piston longer as long as injector is open while with a gas engine it
just pushes alittle while after top dead center.
So a rich mixture produces more instant heat and pressure energy
right where it is needed for maximum power while a lean mixture extends
the burning time and therefore pushes weaker and at past best time. As
you know from physics energy is converted from one form to another. So
if heat and pressure of burning gasoline turns into more kinetic energy
then it must lower its temperature or heat energy more than a lean
mixture. And you get more power from a rich mixture. Of course the
unburned gasoline that can't find any oxygen to combine with will
absorb some heat energy and also the heat released upon conversion to
carbon monoxide is not as much as released with conversion to carbon
dioxide (adequate oxygen)..