Originally Posted by
qazimoto
The surface on the baffled piston on the Polish engine shown above clearly demonstrates that it hasn't been anywhere near red heat. I'd guess an absolute max of about 250 degrees C.
The table below represents the surface temperature of a steel object against surface colour. Normally you start with a shiny surface and it's colour changes as the surface temperature changes. The colour charts have been used by Blacksmiths for hundreds if not thousands of years for heat treating steel. This is what makes it a such a useful material generally, being able to adjust it's mechanical properties by using heat. Pistons are normally made of cast iron. Different to steel but similar in that both are mostly Ferrous, made of mostly elemental Iron, Fe. In simple terms the difference is in the Carbon content. Cast Iron has more of it than Steel. The surface colour will be similar for both materials. There's also the difference between temperature and heat to be taken into account. Never the less clearly the surface hasn't gone beyond 250 degrees C. As you get into the red another issue is created. That of a scale on the surface. Derek's piston hasn't been anywhere near this temperature.
Surface temperature versus colour for a Steel object.
Derek's picture of the piston and contra of the Polish baffled diesel.