lml@lancaironline.net Mailing List Archive

George says:

> OK… but that means that the bulk internal combustion gas temperatures are a lot higher.

> And that means that you get a LOT more BTU’s transferred into the cylinder head.

> And, as everybody with some diesel experience is aware - - they tend - - as a

> result to use rather large radiators.

> Because that heat has to be dissipated - - - some way.

There is an assumption in here that is worth questioning.

Heat transfer rates from the combustion gases to the piston and cylinder

do indeed increase with gas pressure and temperature *difference* between

the gas and surface. The solution to the problem is simple... increase the

temperature of the surface to match that of the gases.

How to do that?

Thermally insulate the surface.

That's the idea behind ceramic coatings for the piston crown, combustion chamber,

and valve faces. The insulator needs to be able to run hot on one side, and cool

enough on the other side to

1) not melt or weaken the metal

2) not burn or polymerize the oil

3) not transfer so much heat that the benefits of high compression are

lost to cooling drag.

If that is worked out, then engine BSFC improves, and cooling losses

are reduced.

Just as in a gas turbine, the cooling drag is minimized by allowing the

hot section to actually run hot. High temperature, high strength alloys, and

insulating ceramics are the key to an efficient aircraft diesel engine.

FWIW, Aluminum is just about the worst material to use in a diesel head:

low strength at high temperatures, high thermal conductivity, poor fatigue life, etc.

The argument that "...heat has to be dissipated - - - some way." really boils

down to "because we let the heat get into the aluminum, we need to get it

back out before the piston melts."