Doug:
"and yes they do but the reality is they are not designed to run at high currents and or be spun at speeds exceeding 2000 rpm."
i wouldn't be so fast to condemn all truck alternators, there are good and bad in them just as there is in anything.
check out the following link from prestolite, these alternators are designed to run at peak 6000 rpm and deliver serious chargeing power
even at engine idle. (you have to consider the 3 to 1 step up common on heavy trucks)
http://www.prestolite.com/pgs_products/specs.php?item_detail_id=24319&item_series_id=94&refresh=1148312166are they efficient, i don't know yet, but i would expect them to be higher in overall efficiency than say an ST head generating ac to feed a charger that charges a battery.
if you really want to see some interesting offerings in heavy alternators, check out electodyne
the electrodyne units address all your concerns, thin iron, max copper, low flux leakage, high efficiency etc.
http://www.electrodyne.com/models.htmlclearly i don't think we can write off all automotive alternators in broad sweep, sure most are built at a price point and are designed to put out low currents continuously, but there are those that are designed to deliver high amperage all day long.
even modern car alternators are delivering more continuous amperage than they did a few years ago, with electric fuel pumps, high energy ignition, fuel injection and huge stereo systems, 40 or 50 amps continuous has become the norm.
on heavy trucks the loading can be much higher on a continuous basis, so many manufactures have stepped up and are building high output low rpm units, some probably aren't as efficient as others, only testing will prove one out over the other i suppose.
bob g