Alternator vs no alternator vs tiny alternator
 

I calculated the other day that it might have cost me 1.6 mpg to run (2) 65 Watt H6054 halogen headlamps. I assumed 53 mph, Bsfc 0.40 #/hp-hr, and 50% alternator efficiency. Since then I have been thinking about yanking the Bosch OEM alternator out of my '81 Diesel Rabbit, and replacing it with a tiny little Nippondenso unit I got out of a crashed Geo Metro 3 cyl years ago. I could probably put a big pulley on the Nippondenso as well. I have converted most of the commonly used lights to LED, so I am sure the OEM unit is overkill.

By the way, has anyone heard of a manual fuel cutoff retrofit for Bosch Diesel injection pumps?

You can cut the fuel on a diesel rabbit by turning the key off, the mechanical fuel pump has a solenoid valve, a single wire going to the valve, that is the only thing that keeps the engine running is the fuel getting to the pump.
As for the alternator, a smaller alternator isn't going to make much of a change other then the stock alternator an electrical load is an electrical load, spinning the two alternators without a load should be about the same drag, so spinning them with the same amount of load should be about the same.

I was actually thinking of fitting a bigger alternator, reasoning is this:
A lot of the loss in an alternator is the fan and the iron loss, both are speed dependant. if I use a 24V 150A alternator at less than 1/2 speed it should work for 12V if the regulator is changed for a 12V one, and losses should be much less. it would run well below it's rated current so it should stay cool with the lower fan speed too.
I have been thinking of a way to turn it off without disconnecting it, if the warning light wire is raized in voltage to around 17V the regulator will think that is the battery voltage and turn off completely, we can do this with a 6V battery, connect the -ve side to the main battery +ve and the +ve of the 6V one to the warning light. warning light will come on, and alternator shoulf turn off. there are some cars with different connections, probably won't work for those.

Instead of swapping in a smaller alternator, replace your battery with a larger one (deep cycle) and put a kill switch on the alternator. That way the oversized alternator can charge the battery only when needed, like when engine braking. Check out these two threads:
Automatic alternator cut out/regen braking
Deceleration Alternator

If you want to address the fan and spinning losses that George Tyler mentioned, then try an A/C clutch pulley - it's been talked about, but I don't know if it's been done.

The diesel rabbit already has about as large of a battery as you will see in a compact car, I don't think there is space for a larger battery, but it's worth checking anyway.
Your engine doesn't have alot of electrical loads once it's started, of course starting the engine, with the glow plugs and the massive starter for the size of engine you have, can ask alot of a battery, so you do want to make sure it's charged up right away, but once it's been running for a few minutes I suspect that the load on the alternator is quite small, so making sure that the belt is not to tight, that the alternator spins freely and maybe installing a fan with lower drag is going to be your easiest option, like others have suggested installing a field winding cut off switch might help a little too but it also might shorten your battery life so you have to look at what the payback is.

Ryland you are right about cutting the fuel with the key, but that sometimes confuses the glowplug controls and the seatbelt interlock, so I have a switch to do that. I don't use it anymore though because it is not 100% reliable. I tried a normally closed momentary switch for a dome light, but that switch was all over the place and would have anywhere from .3 to 1.2 ohms resistance, that worked about 75% of the time. I replaced it with a Volvo rocker switch that is much more consistent at around .2 ohms, but it is still not perfect. At 2 amps the solenoid coil can't stand much additional resistance in series and still produce enough force to lift the core. A manual control would go around that completely and draw no current.

If I got rid of the solenoid coil the average current drain on the car would drop so much I could probably put the 15 Watts of PV panels I have on it and just toss the alternator. Since I put in Bosch 80010 4 second glow plugs the starting performance is simply spectacular. Even on a cold day (for Arizona) the starter only has to crank for a second or two.

Thinking about this made me curious enough to calculate:

Glow plugs are on a 50 amp fuse so 0.00066 kWh per start.

Starter assuming its 2 hp, 0.0008 kWh per start.

Fuel Solenoid 0.024 kWh per hour use.

Headlamps 65W each, 0.13 kWh per hour use.

15 W PV panels 6 hour average insolation per day 0.09 kWh

So, I conclude I could easily add the PV panels and given my normal commuting duty cycle easily toss the alternator. If I can slack it off and wire the belt out of the way, I could easily reconnect any time I need it. And, if I got a manual control on the fuel flow, I might never need it again.

...how about grafting a "clutch" assembly from an airconditioning compressor onto the front of an alternator = electrically controlled alternator!

The problem with that is your running only off battery which with a load will be around 12v, maybe less depending on load . The alternator only puts out amperage according to load so the regulator adjusts this, it does though maintain higher voltage value of around 14v or so. This does make alternator put out some but at same time any load , the amperage will be more with lower voltage .

You could implement what Honda/Acura does by using a dual output alternator were it output is reduced on low loads and only goes into high mode with decent load on , like AC, headlights etc .

...the a/c clutch is only activated upon voltage application, so when the alternator isn't engaged, it (the clutch) isn't a load.

...the assumption being that the alternator "regulator" is still being used when the alternator is engaged.

smaller alternator
 

i was thinking of relocating and downsizing the alternator in my diesel rabbit
as well as putting it on its own belt and making an adjustable, cam actuated
quick release to remove the belt for longer daytime drives.
i recently scored wearable 4" "headlights" with individual reflectors for each led @ a dollar store!! plan to use them for tail, break lights. how do i bring the voltage down to 4.5 and 3 volts? also will i need power resistors?

i began designing a cable actuated solenoid for the VE pump after seeing a cut away view demo pump @ a fuel injection shop and discussing it with a tech there but then quickly began to realize the power draw is minimal.
so unless you are going all out psycho dont worry about it. if you are still worried it is a simple plunger with a very light spring load. dunno what threads it uses but I would double viton o-ring it if i were to do it.
dont crack your pump removing it the electric one cause its in there good!

how much more efficient would a modern and or smaller alternator be?