Alternator/starter combo?

Factual Questions
1

An alternator creates electricity from the engines kinetic energy, while a starter motor creates kinetic energy from electricity.

I know its possible for the same mechanism to be able to switch between motor and generator mode (Like in the new Honda Hibrid car).

Is it possible to put such a device on my car…one device would take the place of the two already there.

2

it has been done but it is not very efficient. You end up with a device that does both functions badly.

3
      • It is possible, but not advisable.
  • The brushes for a generator are phased differently than for a motor. If you construct two sets of brushes connecting to the same coils (which has been tried), you get electrical shortage across either set of brushed when you “use” the other, and the thing doesn’t work very well for either use. By adjusting the phases of both brushes, you can sort of choose which it does worse at. If you construct the brushes so far out of phase that they don’t short each other, it doesn’t work well either, because all the brushes’ phase ends up very short.
  • The other way would be to build an alt/starter with one set of movable brushes. I haven’t ever seen one, but I’d bet somebody has done it, and it doesn’t work as well for some other reason, such as the one below.
  • Also on modern cars the mechanical setup of alternators and starters is very different: the alternator usually spins 3-5 times for each engine revolution, where the starter spins 10-14 times per revolution, when it is engaged. A starter that could spin the engine using only a 5:3 reduction would be HUGE, much larger than a regular starter or alternator is now. All in all, it’s cheaper and easier to just use two “motors” than one. - MC
4

In the mid 80’s, I had a Simplicity garden tractor with a Briggs & Stratton engine of about 10 HP. It had a starter/generator in one unit. It was connected to the engine via a V-belt. I’ll say it worked OK, but did not have to supply a very large electrical load.

5

My Economy garden tractor has a starter/generator. The starting circuit is slightly different from the charging circuit, but still runs on the same set of brushes. It has worked well in my tractor since new, in 1957. It has had many batteries and one new engine, but the starter/generator has stayed the same. But as previous posters point out, this is a small (12hp) tractor, and doesn’t require a lot of juice to function. Could one be designed to do both jobs? Yes, certainly, but remember the starter in your car spins like a mutha in those few seconds- the mechanical advantage given it by spinning at the rim of the flywheel is dramatically different from attaching it via pulleys to the crank. Those type starters tended to suck batteries dry very quickly.

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