Technical Altenator question

[Cheesetoast]

ok, i believe my td has a 90amp altenator, would a higher amp altenator affect my fuel economy at all?  what about changing pully sizes? just a thought.  :?:

[Master ACiD]

not really.

an alternator only puts out as many amps as required to maintain a set voltage. what this means is that when you first start the car, the alternator may be putting out 90 amps, but soon after this the amperage tapers off untill it only puts out enough amps to maintain 14.4 or whatever youre alternator is set at.

fir example, my car takes about 5 amps to run with no lights or fans on.  turn on the a/c and theres another 20 amps. headlights, another 20ish. radiator fan turns on, 15 more amps.

radio takes 3 amps. brake lights take 5 iirc.


so if you have alot of crap on youre car, you might need a bigger alternator. but it wont gain you anything unless you actually need it.

[zyewdall]

Now, you could go to one of the high efficiency permanent magnet alternator instead of the standard automobile alternator.  That might give you an extra 0.5mpg or so, if you always run with your headlights on like I do.  They cost a good $600 or so though, so it might not be worth it  :roll:

Micro-hydropower installations use them, and can get 40% more power out of the same site as the same turbine with an automobile alternator, which shows you how much power an automobile alternator is wasting.

[fspGTD]

Alternators sap fuel economy in 3 ways:

* energy draw to develop electrical current.  This "shuts off" when not needed.   So as already discussed, will not change when changing an alternator (unless you can switch to a more efficient unit.)

* static and rotating mass.  Mass takes energy to accelerate.  You khow how a heavy car gets worse fuel economy and takes more gas to get up to speed than a light car...  Well, the static and rotating mass of the alternator gives a similar effect, especially the rotating mass.  The more the alternator is overdriven respective to the crankshaft, the more the alternator's rotating mass will resist any given change in crankshaft speed.  At engine redline, some OEM alternator setups turn nearly 20,000 RPM!  The alternator can be underdriven by fitting a larger pulley (or smaller one at the crankshaft), but the downside is less current development capacity at engine idle speed.

* drag by the cooling fan (and also to a very small extent, bearings.)  Alternators generally have a cooling fan that spins with the rotating assembly to keep the alternator from overheating.  The cooling fan will sap power that is a function of its rotating speed, regardless of how much electrical current it is generating.  Older alternators with the stamped-sheetmetal cooling fans mounted to the pulleys probably have less efficient cooling fans that take extra drag than newer alternators with internal cooling fans.  But higher current capacity alternators will also need larger cooling fans, which impose additional drag.