Text 15493, 53 rader
Skriven 2005-11-06 20:34:10 av Raymond Yates (1:3613/48)
     Kommentar till en text av Michiel van der Vlist (2:280/5555)
Ärende: Fog lights
==================
 MV> Hello Raymond,

 MV>>> It may not make much difference if you have a Hummer, but for
 MV>>> the fuel efficient moderate sized European cars, 100 Watts of
 MV>>> electricity make a substantial difference to the fuel
 MV>>> consumption. Let the engine run stationary and then turn on the
 MV>>> headlights. You can *hear* the engines's rpm drop.

 >> That's really interesting. does this suggect that the alternators
 >> are underpowered, or that the batteries are less than is required?

 MV> On the contrary. If the battery is full and the alternator can deliver
 MV> sufficient output to power the headlights, the power will indeed come
 MV> from the generator and not fro the battery when the lights are
 MV> switched on.

 MV> But power does not come out of nothing. The increased load on the
 MV> generateor means that it takes more power to turn it around and that
 MV> extra load has to come from the engine. And so unless the engine has
 MV> some provision to keep the rpm constant under varying loads, the rpm
 MV> will drop when the generator demands more power from the engine.

 >> The reason I ask this is I have a Jeep, which has four standrd
 >> lights @55 watts each. I can turn on all four lights, and then
 >> hold the switch so that the high beams also come on which is a
 >> total of six, and the ammeter neither mores, nor does
 >> the engine tachometer register any change..

 MV> Then either your Jeep has some device to keep the rpm constant under
 MV> varying loads, or the engine is so powerful that it does not feel the
 MV> extra load.

 MV> My Volvo 345 has a 1400 cc engine. The extra 100 Watts from the
 MV> headlight cause a noticible drop in rpm when I switch on the
 MV> headlights. (2 x 55 Watts)

I think there's a combination of two thngs then. AFAICT the jeep is wired so
it draws fromthe battery and then the alternator circuit replaces what it
draws. This is based on the fact that one way one could test to see if the
alternator was working was to disconnect the battery and see if the engine
would still run. You can't do that anymore with the new cars.

Second the Jeep has a 4000 cc engine, and idles at ~1000 rpm.

 >> So, this makes me curious.

 MV> Mount a generator on a bike with a suitable gear box so that when
 MV> pedaling it runs at normal speed. Now connect a 100 Watt lamp. You
 MV> will certainly feel it in your legs...

Been there, or close, and done that..
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 * Origin: Ray's Rocket Shop - Out to Launch (1:3613/48)