Small(est) engine powered auto alternator

[Peter-]

What kind of car do you have?  Is it a 70's vintage or is it late 80's or 90's or newer with fuel injection and computer controls?

You imply idling that gas engine is nearly free and so is the wattage used by electrical accessories and such.  First, is you alternator so new as it is field controlled by your ecm?  Is the alternator driven by resistance from a load such as a bulb?  Or, has your alternator fully charged you battery and now at float stage?  Or, the more relevant the question, have you started your vehicle and swapped the battery for a very low charge state battery and then watched the tach?  A WATT is a WATT.  Once your engines mass has delivered in the way of energy, to the load, it's mass is no longer usable.  meaning for every action, there is an equal but opposite reaction.  That engine now has to deliver the power to sustain the new load.

If that is not a good enough explanation, put a vacuum gauge on your engine.  Put a load on that engine and watch the vacuum gauge when it starts towards zero, that is your throttle delivering more air and therefor fuel.   That car engine idling is very inefficiency, just to keep it idling takes a huge amount of energy. 

Seems you need to remember HP is a product of BTU.  Huge mass is great for getting a load moving or the initial start of huge energy requirements but only for very short burst but none can be more of a load than that available as energy. 

Point taken. Mine is a 1998 4-banger with ecm and etc. The flattest battery I ever had in it, barely cranked it enough to start. The tach didn't show any difference. However, I was picturing a different scenario during this discussion: mainly, something that is basically running all the time without deep discharge. Much lke a stationary power plant. I take it that this isn't the case. I do wonder if enough flywheel couldn't overcome even that though; because I've never seen a car alt stay above a few amps for more than like 5 minutes. They put a bunch of power right away and then taper right off as the battery tops up.

[bob g]

some of the higher output car alternator oem/manufactures warn against starting a car
without a fully recharged battery after replacing the alternator, reason being the thing gets too hot.

odd, how in reality car alternator are subjected to repeated starts on a very poor condition battery and seem to
survive, but

i know of one manufacture that won't honor the warrantee if the battery is not fully charged, which doesn't make sense
to me.

whatever the case, unless one makes provision to deeply derate a typical high output car alternator it likely will have
a thermal meltdown sooner than later.

bob g

[canadian REDNECK]

the general rule of thumb is 2 HP per KW, but most auto alternators are only 50% efficient. that leaves us at 4 HP add 1 hp makes 5hp. a GX160-GX190 should work, but i would recommend a 9HP and run it at half rev's (quieter)use around a 6 inch drive pulley on the motor and a 2 in driven on the alternator.

[dnix71]

Car alternators are air cooled. High power at low rpm equals failure from overheating. You need to either mod the fan to provide more cooling at low rpm or make the pulley ratios such that the alt spins fast enough at idle to keep cool. Moving the rectifiers outside the alt and mounting them on a heat sink would help cool the alt as well.

[spottrouble]

For what its worth, I just spent the past 2 evenings parting out a couple of alternator based battery chargers for a large commercial UPS system, they were designed by engineers, but I'm not sure they were good engineers . Power in was 230v 3 phase, this was through a Variable Frequency Drive to power a 15hp 1725rpm electric motor which drove a 220amp 12v truck alternator. The key pads on the VFD's had been taped over, so I'm guessing they were set to run at 1 speed, the pulley on the motor was 6" diameter, alternator used a 2" pulley. Voltage regulator appears to be a commercially produced unit, but data tag was removed. There was another box of electronics with a Condor current regulator, and lots of little "shop made" boards with some circuitry and resistors rats-nested together

So 7.5hp for a gasoline powered unit driving a 100amp alternator sounds about right