Invisible wrote:
> The starter motor is designed to convert electric to kinetic. The 
> alternator is designed to convert kinetic to electric. I'm musing over 
> whether either device can do the *opposite* job. :-P

If you essentially disengage the safeties and get them hooked up right, 
yes. But since the starter motor is designed to start a car that isn't 
running, there are (for example) no mechanisms to let you mesh the gears 
when the motor is already running. Try turning the key when the car is 
already running and you'll see what I mean.

Similarly, the generator (if you have one) really only kicks in when 
you're already going more than about 20MPH, and the alternator does 
funky things to keep drawing power even at low RPMs without stalling you 
out.

> doesn't repeatedly starting up a petrol engine waste fuel anyway?]

Depends on whether it's carburated or fuel-injection. With fuel 
injection, there's no "ramp up" to get the fuel from the tank to the 
engine, no already-carburated fuel sitting in pipes and then going to 
waste or evaporating, etc.  It used to be starting a car used 90 to 300 
minutes worth of idle-speed fuel. Nowadays, it uses maybe 2 seconds 
worth, or less with cars designed to turn on and off like the hybrids.

 > I've always wanted to know... An engine is a mechanical device. So why
 > does temperature have any effect on anything?

It's a mechanical device that derives power from the difference in 
temperature between the inside and the outside.

In the same way, a balloon zings around the room when you blow it up and 
let it go, but falls to the ground when it runs out of air.

-- 
Darren New / San Diego, CA, USA (PST)
  Helpful housekeeping hints:
   Check your feather pillows for holes
    before putting them in the washing machine.

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