Ive asked my neighborhood mechanic this and not recieved a satisfactory answer…
The engine has a cooling system to draw off heat created during the combustion process. The heads, containing the valvetrain, has water passeges, as does the block, containing the cylinder walls.
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?
Well, I might be off on this one, but I would imagine that the incoming fuel and air mixtue would carry off excess heat enough that the pistons themselves never reach the melting point.
Plus there is likely enough heat reduction by the rest of the engine to keep the temperature down. Remeber that when the ignition occurs, the piston is forced downwards, and the warmth can then transfer to the cylinder walls, which would be cooled by the cooling system. These two processes are all that I can think of.
It’s basic thermodynamics. What happens when you put a hot casserole in a refrigerator? The heat energy flows into the cold air. Same thing that happens when you put a hot piston in a cooled cylinder.
<i>
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?
</i>
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
them.
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.)
-Ben
Additional cooling is provided by the fuel prior to combustion process. Big bore Continental aircraft engines are metered to provide this extra fuel at full throttle to help keep the engine cool during the high power and low cooling (via air) present at takeoff. I am guessing that the fuel makes a significant change to the heat capacity of air.
Most all engines in production cars made since 1970 have aluminum pistons, not steel.
This part is true.
According to “Internal Combustion Engine Fundamentals” by Heywood, the piston rings are responsible for most of the heat transfer. I quote from page 701:
Also, he gives examples of heat fluxes in different regions measured for two specific cases - a diesel engine at 1500 rpm and no load, and the same engine at 3000 rpm and WOT. Here are his results:
Region Heat Flux, Heat Flux,
1500 rpm 3000 rpm
Plain top ring 33% 33%
Taper-faced 2nd ring 17% 14%
Oil ring 15% 18%
Ring lands 5% 8%
Pin boss zone 10% 11%
Crown internal zone 13% 8%
Skirt zone 7% 8%
The fuel does not change the heat capacity of the air significantly. But evaporation of fuel droplets can cool the combustion air.
In the aircraft reference given - the extra fuel lowers the Air/Fuel ratio, which results in richer combustion, which has a much lower peak flame temperature than the typical ideal, or lean cases.