The pistons, in direct contact with combustion, has no water circulating through it. Not all engines have oil jets to
circulate oil towards the underside of the pistons, so
how does the pistons in the vast majority of engines
keep from melting?
There are several factors at work here, but the
biggest one is simply…
The pistons are made of specially formulated steel
that is very tough and that has a very, very high melting point. The amount of heat produced by the engine’s
normal activities is nowhere near sufficient to melt
As Chas.E pointed out, the piston also contacts a
couple of other pieces of metal. The piston rings touch
the inside of the cylinder, so you get a little conduction
there. Also, the piston is attached to a rod that connects
it to the crankshaft. There’s obviously going to be some
conduction of heat going down that path too.
I would not think that incoming air would cool the
piston much. Air has a very low heat capacity (it can’t
absorb much heat), and any positive effect would be
totally negated when the spark plug fires and the mixture
burns. Similiarly, the piston surely radiates some heat,
but again there’s not much of a cooling effect there.
As an aside, though you would probably never see it
in a normal car engines, pistons CAN melt. Engines that
are turbocharged or are using Nitrous Oxide to enhance
performance produce a lot more heat than normal. Turn
up the turbochargers too much (too much boost) or spray
too much nitrous in there and the temperature soars. And
pistons can sometimes get holes melted through them.
(Though it’s usually some other component of the engine
that fails first in those cases.)