There's probably a legitimate reason why we have 2 different parts for doing 2 different jobs.

However an electric motor can also be used as a generator. If the design allows it, why isn't starter used as an alternator?

  • Really good question! My guess would be it having to do with one working really hard for short periods of time (the starter), and the other just having to work for a long time (the alternator). But I could be wrong!
    – Cullub
    Aug 1, 2019 at 20:10
  • I'm not brave enough to post an answer because these 2 car parts seem like they could round down to one, but they can't because they're actually very different. I would suggest reading more about how the ignition system activates a car starter, and then how an alternator actually works vs a dc permanent magnet motor. Thats the only way i could imagine it being one part, is some crazy dc permanent magnet motor that has high torque and wouldn't burn up with high RPMs, or maybe theres gears to modify the RPMs. Honestly it just sounds like a disaster.
    – Jeffyx
    Aug 1, 2019 at 20:43
  • @Jeffyx See my answer, these were combined as one unit...
    – Solar Mike
    Aug 1, 2019 at 21:02
  • thats back when cars were not much different than horses :)
    – Jeffyx
    Aug 1, 2019 at 21:09
  • 1
    @SolarMike Pretty sure that was only if a woman was driving. (Don't throw things - I didn't approve that ordinance.)
    – 3Dave
    Aug 3, 2019 at 21:03

3 Answers 3


They were combined many years ago, one was called the Dyna-start (made by Lucas aka the prince of darkness... :) ), other manufacturers were Bosch and Delco.

They fell out of popularity due to the fact that they would do both jobs but increasing demands of power both for the starting function and the generating function increased. More powerful engines needed better starters and more electrical items ie lights etc needed more power.

So, the functions became separated into the units we see now.

Here is a link to more info, as these units are still reconditioned.

  • I knew it someone in history must of tried this lol, pretty interesting that old 2 stroke engines kinda like this starter because they don't take damage when it spins backwards. +1
    – Jeffyx
    Aug 1, 2019 at 21:12
  • The Bullnose Morris had a Dynastart, but not a 2 stroke engine...
    – Solar Mike
    Aug 1, 2019 at 21:19
  • 1
    The reasoning seems kinda backwards. If we need more power (either as input or output) then it makes even more sense to merge the functionality into one device instead of lugging two even heavier separate devices around.
    – Michael
    Aug 2, 2019 at 8:44
  • @Michael if you want a more technical understanding, the link will give you that, but in short a winding designed to provide starting power will have to carry several hundred amps which will produce heat that has to be dealt with. That same winding will not make an efficient generator winding which will perform better with many turns of thinner material. So, my explanation is not backwards.... The total weight of two separate high efficiency devices is better used than a slightly lighter device that does neither job well...
    – Solar Mike
    Aug 2, 2019 at 8:48
  • Note that a dyna-start is a dynamo-starter combined, NOT an alternator-starter combined. A standard car alternator produces 3 phase AC, which is then transformed into DC by the regulator diodes. You can't turn an alternator into a motor by supplying it with electricity, the closest analogue would be a PMDC motor and controlling that opens a whole new can of worms.
    – Ralph
    Aug 2, 2019 at 10:08
However an electric motor can also be used as a generator. If the design allows it, why isn't starter used as an alternator?

You're not the only one to realize this.

Honda has integrated motor assist, which is an integrated starter-generator. It allows providing electric boost to the engine from a battery when such boost is required, and then charging the battery when an excess of power can be produced. These are used in Honda's hybrid cars, which are not as sophisticated as Toyota's hybrids.

Toyota has gone a bit further and there are two motor-generators (MGs) in Toyota hybrids. One of the MGs, MG2, is connected directly to the wheels. MG1, on the other hand, is connected to a planetary gearset that has the engine, MG1 and wheels+MG2 in its shafts. The MG1 and MG2 together are used to adjust the effective gear ratio of the electric CVT, and also to charge the battery, to provide electric boost, and to start the engine.

So, as summary, in hybrid cars, this actually makes sense and is the way things are done.

However, if you don't need the electric boost, you won't need much charging capacity then. But you do need the ability to start the engine. The starter actually has a heftier amount of work to do than the alternator, and this is the reason traditional cars have these two components separate. Starter is only for intermittent use and produces lots of power. Alternator provides only very little charging current for the battery and to power the car electrical systems, but it's for continuous use.


For what it's worth, in this new generation of GLE from Mercedes-Benz, they've incorporated what they call an Integrated Starter-Alternator which does exactly what you lay out.

They've managed to integrate this into their new line of I-6 engines and have been able to reduce weight on the engine significantly.

Engineering Explained talks about it a little bit here on his video on In-line 6 cylinders https://youtu.be/7PkX-NH6bE0

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