Nikasil is not a liner material. It is a coating that is normally applied to aluminium, but can be applied to iron also, so the significant influence on heat transfer is the material to which the Nikasil is applied.
Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &
Yes, Pat, that is what I had suspected. The Nikasil is not thick enough to have a significant effect on heat transfer. The comparison I am interested in is aluminum air cooled cylinders--Nikasil coated aluminum bore vs iron sleeve pressed into aluminum cylinder.
I have absolutely no data, But I would stand naked on the town hall steps if the Nikasil aluminium did not transfer more heat than the aluminium iron combo.
Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &
Nikasil can be applied to iron or aluminum bores. It is used on iron bores because it gives better wear, reduced friction, and improved performance under marginal lubrication conditions. With aluminum bores a more precise fit can be achieved with an aluminum piston, due to similar CTE's.
Whether the bore is air-cooled or liquid-cooled, the thermal conduction across the liner wall thickness is probably more efficient in a monolithic aluminum structure, as opposed to an iron/aluminum composite structure.
However, I'm not brave enough to make unqualified statements backed by a promise to pose naked on the the steps of city hall like patprimmer does. There may be some extreme combination of engine operating pressures/temperatures and structural requirements that works better with the iron/aluminum combination.
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.
Back in the air cooled Buell Racing days we always changed production cylinders (Iron lined with cast aluminum fins) to aluminium with Nikasil coating (just cast without the liner). Seemed to help cylinder cooling quite a bit even with the same fin aera between the two designs. Pistons had large skirts so they seemed to benifit a little too. The main cooling issue with these engines was always the head and keeping it from melting and the cylinders had little to no effect on that. Biggest advantage to the Nikasil for our aplication was a huge weight reduction, second biggest advantage was the complete lack of cylinder wear. The cooling was nice but we would have changed just for weigh savings alone.
Both types of cylinders suffered some heat distortion as well as distortion form torquing the head down. "Torque Plates" were required on both designs when honing. The total power loss from cold to hot was pretty much the same, in other words both lost equal power once hot. The design life for the engine was 200 miles so we never had enough compression and couldn't turbo per the rules to really push the combustion pressure too high. We never had any Nakasil cylinder failures.
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