MikeHalloran,
That's a good point. But simply comparing dimensionally identical cylinders (one monolithic aluminum, one having an iron liner with finned aluminum sleeve, and one monolithic iron) does give a true picture of the situation. On one hand, aluminum has much better thermal conductivity than iron. But on the other hand, iron has a couple of things in its favor. Iron has a higher MoE than aluminum, it has better strength at elevated temperature, and iron has a much higher allowable operating temperature limit than aluminum (400+degF vs ~275degF). To get an honest comparison, we should compare each cylinder construction after it has been optimized for all operating variables.
Assuming an equally stiff cylinder structure, the iron liner wall can be much thinner than the aluminum liner wall. And if the lube conditions at the bore surface permit, the iron liner can be operated at higher temperatures than the aluminum liner. This would help thermal efficiency by reducing heat transfer from the combustion gas to the liner wall due to a smaller deltaT.
The big recip piston aircraft engines of the late '40s showed that air-cooled steel liners can be made to work quite well while achieving low weight and high reliability.
Interesting discussion.
Terry