Saturday, November 25, 2006

AV - Aluminum Jugs

The use of aluminum barrels in VW engines converted for flight is a nice example of how to separate technologically naive homebuilders from their money.

On average, the cylinder barrels of an air cooled engine contribute only 17% of the waste-heat budget. This figure is true for everything from a Weedeater to a P&W R-2800, the 'on average' waffle factor reflecting how the barrels interface with the heads for the purpose of heat-transfer. In a mono-bloc casting, where the head and barrel are intergral, the figure can be as high as 25%. In the jugs of a real aircraft engine, where the barrel is thread and heat-shrunk to the head, the 17% figure is dead-on. For a compression interface, where the barrel is simply squeezed against the head -- as is the case with the VW -- the figure is actually lower. But since the factory manuals for Lycoming, Pratt-Whitney and so forth cite 17%, so do I since it allows people to look it up for themselves.

Remember the earlier thread about the sealing surface between the head and the cylinder? That's why VW (and Porsche) jugs see so little heat. There is certainly plenty of heat in the heads but there simply isn't enough surface-area between the heads and the jugs to allow an appreciable amount of heat to flow from the heads into the barrels.

So why did Porsche use them?

Because it was running out of cooling air.

To produce more power Porsche had to get more air to the HEADS but they'd already hit the limit on the size of the blower and shrouding they could fit in that particular chassie. Their solution was to send LESS air to the barrels so they could send MORE air to the heads. But to keep the barrels from overheating they had to go to a material having a higher rate of thermal transfer... such as aluminum.


If you can afford to spend $1800 to save 9 pounds there's no reason not to use nickies (say hello to Shad when you order them :-) ...but in a flying Volkswagen that is their ONLY benefit. If someone tries to sell you a set because they'll automatically make your engine "run cooler," just smile and walk on to the next booth.


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