Quote:
Originally Posted by Old Mechanic
Lets assume that throttle restriction is energy costly (it is), when an engine uses a throttle plate to control air flow into the engine. I guess the question is can you extract enough energy from the restriction to make it worthwhile?
I never really thought it was that much of an energy loss since the most pressure differential you can have is 14.7 PSI (give or take depending on atmospheric pressure).
Would any system be sufficient to do the job of the alternator?
regards
Mech
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Intake throttling losses are less than 5% of output power under most conditions. And there isn't any energy to be gotten at full throttle, and almost nuttin at idle. So you're kinda stuck trying to get a small fraction of your part throttle at cruise. Maybe your car cruises at 65 mph using 20 hp. So you have about 1 hp to spin the alternator. That would be fine for fair weather.
I remember seeing a patent application for a turbine throttle with variable vanes and a clutch which revved up a flywheel. Moving the vanes acted as the throttle. At cruise you store energy in the flywheel. When you need 100% power the vanes "reverse" and the flywheel powers the turbine. The math is right on the edge of working.
Also looking at jakobnev's graph. Well I think he's missed something. There's hardly any air flow at idle - "full vacuum" so there can't be much power. Not 1kw.
In fact you can try back of the envelope figurin' The power lost to pumping the air into an engine is W=dP * Q, for me, using conventional dimensions, that's horsepower = pressure drop in psi multiplied by flow in cubic feet per second. Well, there's a bunch of conversions in there. Assuming the engine is in the 20% efficiency range and putting out 1 horsepower at idle, that works out to about .66 lb of gasoline per hour. The engine will use 129 cubic feet of air to burn that gasoline. Or 2.2 cu ft per minute, which is about 3700 cu in per minute. The throttle is nearly shut, so the pressure drop might be .95 bar - 14 psi. 14 psi * 3700 cu in per minute is 52000 in - lb per minute or 50 ft-lb per sec. or almost 0.1 hp. about 75 watts.
-mort