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Old 07-14-2008, 02:14 PM   #29 (permalink)
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Originally Posted by Frank Lee View Post
I don't recall where, but someone once provided a link to an engineering text that mentioned 1000-1200 ft/mn as optimal piston speed for economy.

My question is, is that true and why is it?

Why wouldn't, say, 700-840 ft/mn be even better?
Sorry everybody,I took a couple days to dig through my rat's nest.Here's some stuff I culled out.The only statement citing an optimum piston speed range is from "Internal Combustion Engines",an old textbook,by Edward F. Obert.Piston speed wasn't even listed in the index,but after forraging,I found this citing:-------------------------- "...large and small diesel engines(designed to develop 125-200 bmep),attain their minimum specific fuel consumption in the range of 1,200-1,500 ft/min mean piston speeds."------------------------------------- A large-bore 100-rpm,700 hp/cylinder diesel engine for direct propeller drive was given with the lowest BSFC,@ 0.32 lb/hp-hr.------------------------------------ Friction seems to be the overriding factor for maximum BSFC.----------------------------------------- for spark-ignited gasoline engines,best BSFC occurs at point of maximum charge loading efficiency,which is basically where torque peaks,which is at about 80% load.At light load,or full load,efficiency suffers.--------------------------------------- BSFC maps showed minimums of about 0.45 lbs ( 275 grams ) per hp-hr.(0.746 kWh).-------------------------------- The Orbital Engine Co. 2-stroke is rated at 7-12% better BSFC,but they can't seem to clean up the emissions enough to use it.------------------------------------------- SAE said that an"adiabatic compound engine" operating at 1,300-1,500 degrees F,with no cooling system,and all waste energy into exhaust,could be harvested with compound turbos doing double duty,supercharging and doing shaft work with a BSFC 0.29 lb/hp-hr.------------------------------------ I guess that means a ceramic engine we've been hearing about for 20-years or so.
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