If I was like most of you with solar panels and large battery banks.
I would find a large (4kw or bigger) dc alternator at whatever voltage your bank is. Run that off a serpentine system similar to how I run my ac generator. I know the military used oddball direct current generators on some things, so I wouldn't think it would be difficult to locate something that would work.
I would then keep the 40 amp 12v alternator and essentially run all engine functions and controllers off of 12v. Have that paired with the bus 12v system.
Keep your 12v vehicle battery and your house battery system seperate. You could then run a large enough inverter to power your air conditioning/house needs. I think there are charge controller that have an autostart function to start the generator when your house batteries are low. Use solar charging for as much as possible, but have the generator as backup power.
If you really want to get creative, you could use an arduino setup to control engine speed relative to power demand.
I think i'm leaning towards this idea. If I go with an "apu" type system, it will have it's own small engine alternator (40 amp) and small starting battery. I think those little Kubuta engine usually require about 500cca starting power. I would keep this completely isolated from all other systems.
Then I would just have two input sources for my house bank, my "main" belt driven alternator that the small kubota drives, in the ~140 amp range, + solar charger/controller. 140 amps is a lot less than 4kw (it's about 1680w) but still should be enough for my needs. My DC loads are going to be pretty light on this build. My biggest consumer will be a dc fridge/freezer which I expect will use around ~80Ah/day during summertime, and 2nd my laptop computer which uses around 80W of power, but will be on 6-8 hours a day. I have a whole spreadsheet with formulas in Google Drive if anyone want's to take a look.
I really like all the Magnum Energy stuff, including their pure sine inverters. They have several other add on options, such as a control panel, a dc shunt that can measure current usage and battery state of charge by measuring Ah in/out once calibrated to the bank size. They also offer an auto-start gen controller.
I already plan on building my own Ardunio thermostat to control this whole system from a heating/cooling perspective. This way I can control engine startup/shutdown etc. For example. if I have the unit set to a/c and it's mid morning and warming up, and the thermostat calls for cooling, it will auto-start the pony engine and start cooling down to whatever set temp I have. Say things cool off and there is no call for cooling for 15 minutes (for example) then the engine will shut down automatically to save fuel.
As far as engine speed, I'd only plan to have two speeds, low idle at ~1400-1500 rpm and high idle at 1800-2000. If I have my set point at 74F and I turn on the a/c and it's 80 in the bus, it'll start up and go into "high power" mode with high blower and high engine speed to cool things down, once it reaches the set point it'll drop to "low" mode, slow the engine, and turn the fan to low and cycle the a/c clutch in/out to maintain set temp. If I creep up in temp, say 1-2 degrees over set temp, I can switch back to "high" cooling mode, etc.
I will also be able to use the Magnum gen auto start controller to start the engine. All I need to do is isolate each with a shockley diode so that they won't back feed each other, and feed both to a single relay, sort of like a simple OR logic circuit. If a/c, OR charging go to +12, relay closes, then starts engine.
As far as engine control, any diesel engine gen controller can handle glow plugs, and engine startup, and also protection from low oil/over temp, etc. You just close a circuit, and the engine starts. Open circuit (relay) and engine stops.
@cadillackid As I have it figured now, with 2000 rpm max engine speed, 10" serpentine pully on my engine, 6" compressor pulley, that puts me at 3320 rpm on my compressor at "max" speed. I could always raise/lower that speed but I think 2000 would be an ideal max for compromise of power output/noise and fuel consumption.
This also puts my alternator at 6,600 rpm provided it has a 3" pulley, which is pretty common. Most alternator peak output at 6000 rotor rpm so that should give me my 140 amps pretty easily.
I have several "good" take off 6.0 Powerstroke alternators lying around that are 140 amp so that's why i'm going down this road. They are also 8 rib micro-v which I think would be perfect.
