DIY Electric Car Forums banner

Using alternator as starter engine

16362 Views 26 Replies 9 Participants Last post by  jared2009gregg

I'm building a hybrid vehicle, and for this I need to use (one of) the alternators attached to my internal combustion engine as a starter engine. My initial idea was to just use a starter motor, but since I can't use the retract function of this, it needs the starter engine to remain connected all the time, and (besides that this lower efficiency), it would also rip the starter engine apart due to the sheer force when running at full speed. So obviously I need to use one of the alternators as the starter motor.

The internal combustion engine I'm using is a 17 - 20 HP internal combustion engine. It runs at between 6000 - 7000 rpm (say 6500 rpm)
The most efficient rpm for the alternators (regular running, NOT starting !) is between 4000 rpm and 5500 rpm. So, I'll be using a 65/40 or 65/55 gearing in between the internal combustion engine and the alternators. I'll be using not gears, but a toothed belt for this, probably in a V-type setup.

The alternators are 2 Leece-Neville 8SC3009ZA's (well actually Prestolites, but these have the same specs as the Leece-Nevilles). They run at 24 Volt (output of the 2 is hence 48 V). Power curves are at

The batteries the alternators charge will probably be 4 batteries of 12 Volt (with 2,3Ah), in specific, these ones (as they're quickly rechargable):

I'll actually be using 2 sets of 4 batteries, so as to allow running the engine from 1 set, while charging the other set of batteries.

Anyway, the question I have is: won't this damage my alternators (as they're not intented to be run as engines, and I'll be starting (and stopping) the engine a lot), and is the power of the batteries (140 A @ 48 V) sufficient ? I assume so, from the power curve, but I still like to hear a second opinion, and hear that it is indeed possible to run these alternators as engines without damaging them.

If the above won't work, will some other batteries from cs-shop (in particular the 150A @ 12V, with 2,5Ah) be able to do it, and/or do I need still other batteries.

Note that the batteries have so little Ah mainly because I like to keep the vehicle very lightweight.
See less See more
1 - 2 of 27 Posts
I could be wrong but I would be absolutely amazed if an alternator would have enough torque to turn a stationary engine

There is absolutely no need for anything like that much torque in its normal operation

A starter motor uses hundreds of amps - an alternator is wired for tens of amps
And more importantly a starter motor is geared down massively - 40:1 at a guess

So at a first approximation an alternator will have about 1/100th the required torque
Hybrid systems using the alternator have a muckle great alternator - and don't drive it though a dinky wee belt!

As to how much torque it will take to start your motor - remember the old days of starter handles?
They would have a lever of maybe 9 inches and would take at least 100lbs force

So 75 Ft lbs = about 114 Nm

Your motor is smaller - but also higher compression
1 - 2 of 27 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.