recommended diesel generator make???

[Audiomaker]

Here's the chart I have between a 7kw and 8kw (both 3cyl Kubotas)



For the 1kw difference in output power, it's averaging about .08 gallons per hour difference.
Run 12hrs/day, that's a gallon per day, or in my case 1/3 of my fuel tank capacity in a month.

Many APU's burn about .10 gallons/hr while powering your small loads and running their own A/C units (or heat from the heat exchanger).
That's close to the *difference* between the 7kw and 8kw if I'm reading it right.

So I suppose it's dependent on what one wishes to do.  The big one for RV's seems to be running 2 or more air conditioners (usually 3) at once.
I can only run 1 of my 3, so for me it's get on the grid in the summer or sweat, but when not running those A/C units, I might be saving over $100/mo running lights, computers, and charging batteries (which will power the microwaves and tools and such).

Add 250lbs of APU and I get one more A/C only when I need it, perhaps use it most of the time over my 7kw, and have a backup.

Does that logic follow?
I am only throwing out there what makes sense to me because I'm new at this (RV'ing), and I'd love to double check my thoughts.

[TomC]

This is not accurate, and you're not following my line of thought. If you only want to creat 2kw worth of power, either an APU or a 8kw genset will create that. The only difference in the fuel burn is the amount of fuel necessary to power the larger engine-which on a Diesel is very small. Even running the APU instead of the genset, I doubt you'd see much more then a quart or 2 daily in fuel burn. You'd have to put on thousands of hours to make up the difference in the about $10k price for the APU.

What I could see is a home made single cylinder air cooled Diesel belt driving a 200 amp 12v alternator to keep your batteries up. 200amps @ 14.1volts equals 2820 watts (4hp engine to power)-or enough to power an inverter without battery drain.  Good Luck, TomC

[Oonrahnjay]

...     What I could see is a home made single cylinder air cooled Diesel belt driving a 200 amp 12v alternator to keep your batteries up. 200amps @ 14.1volts equals 2820 watts (4hp engine to power)-or enough to power an inverter without battery drain.  Good Luck, TomC 

    I understand what you're saying here, Tom, but what would you expect for mechanical losses generating and electrical losses inverting?  As a ball park, I'd guess about 10% for each.  So if your little engine could make enough power to actually drive the alternator enough to make 200 amps, that would make actual, usable electrical power at a roof-top air conditioner at about 2200 watts (or roughly 18 Amps @ 120V).  Is it practical to start and run a rooftop A/C at 18Amps in "real life"?
    I'm not saying that your principle is wrong, but I'm thinking that a theoretical 200A at 12V is going to be pretty close to the bone in real-life -- meaning more engine, more alternator, more inverter would be needed (if my guess about the practical amount of current needed is correct).  And that would up the fuel consumption and reduce the "marginal" fuel cost saving. 
   What do you think?   BH   NC   USA

[wg4t50]

TomC,
I do not think you would enjoy a single cylinder 1800 rpm diesel air cooled, they are very noisey as in big noise, Onan had a great single cylinder diesel,r 3 kw, burnt .32 gal per hour at full load, great, they ran for ever, the issue was the noise & vibration, not having a water jacket sure produced noise.
The noise was the demise of them.
Dave M

[Audiomaker]

This is not accurate, and you're not following my line of thought. If you only want to creat 2kw worth of power, either an APU or a 8kw genset will create that. The only difference in the fuel burn is the amount of fuel necessary to power the larger engine-which on a Diesel is very small. Even running the APU instead of the genset, I doubt you'd see much more then a quart or 2 daily in fuel burn. You'd have to put on thousands of hours to make up the difference in the about $10k price for the APU.

What I could see is a home made single cylinder air cooled Diesel belt driving a 200 amp 12v alternator to keep your batteries up. 200amps @ 14.1volts equals 2820 watts (4hp engine to power)-or enough to power an inverter without battery drain.  Good Luck, TomC

I guess I'm missing your line of thinking Tom, but we'll get it straight

I'm not sure what you are saying is inaccurate?  The chart I posted is right out of my Power Tech manual.
I would agree with you to an extent that fuel burned = kw generated.  For the most part it is just mechanical losses from moving the larger engine parts.  Still, there comes a point of whatever is required to maintain the 1800rpm at idle (or at loads requiring only idle).
The generators in the Queen Mary would require gallons per hour to power a single 100w light bulb, vs less than a gallon per day for a smaller generator doing the same thing. There is no questioning that larger machines cost more to run even under the same load... so where is the crossover point?

Comparing a 5, 7, 10, or 15kw generator, it might only come out to a gallon a day difference, but that also doesn't account for increased emissions, often noisier, and higher maintenance costs.
Take for instance my 7kw.  With generators, you are supposed to change the oil every 100 hours (every 5 days continuous).
My generator requires exactly 1 gallon of oil....weekly @ $30.  If it required 2 gallons per week, I just ate $120/mo extra without considering the fuel difference, cost of larger (or more) replacement parts...overhaul cost...and an additional couple hundred pounds of weight without adding any redundancy to the system.

I've really looked at my generator usage.  I've lived off grid a few times in my life for 3 months or more, and usually the only thing I can find that requires 10kw+ is A/C and heat.  Almost everything else...either in a home, or RV, can be powered without about 5kw.  That includes hair dryers and such if on a rated inverter system, so only A/C, and heat (water too) are requiring more since they run full time.
Of course, this all comes from my own frame of reference.  Maybe most buses have electric clothes dryers and such?  I'm ignorant here.

What I do know is that an inverter doesn't require me to put oil in it every week.

These days truckers live on the road for weeks, and most of the guys doing it are using APU's.  Whether it's hype or not, most APU manufactures quote about an 80% difference in fuel consumption vs idling the prime mover.  Some of the numbers you see on idling semi's is in in the range of 1 to 1.5 gallons (per hour) with A/C running and such, and these are waaay more advanced engines that what you will find in any generator (computerized efi...etc)... they've got laptops hooked to the things figuring out how to save a gallon, but still they use 8 times what the little APU does to allow the trucker to camp at the Travel America.

I also agree with you that a $10k APU isn't for everyone, but neither is a $10k Onan Quiet Diesel.   Both units are pretty expensive new.
My wallet buys used, so I see APU's in the $1k to $2k range... which is less, or around the same as a decent used diesel generator.
Hypothetically (and I mean to test this), a smaller generator like my own, combined with an APU and an inverter system might run the 2-3 air conditioners that a 10kw generator would, but be cheaper to run during the other 3 seasons since you could pick and choose as required.

Heck, If I could design my own rig, and it's compartments, I'd probably line up 3 identical APU's side by side... each with it's own A/C compressor (going to 3 different areas), each with it's own heat exchanger (one to heat air, one to heat the block, and one to heat water), and each with it's own generator head or 48v alternator.
I think that would be a very versatile system as you could have your AGS start and stop them as required automatically, and talk about security in redundancy...yay!

I suppose we all have our own thoughts on this, and I don't mean to hijack this thread any further, but the OP's request included a 32" compartment, and I guess that's how we got on the APU topic in the first place.  I can't think of much else diesel that could be made to fit in there.

[TomC]

I realize using a 1 cylinder air cooled would sound like hitting a hammer on a steel plate. The better choice is a Kubota (or any other manufacture) smallest 2 cylinder water cooled.

Powering a 2800 inverter at 14.1vdc takes about 200amps. You don't have to worry about starting an A/C since a typical 2800watt inverter will have over 5000watts of surge power-that can be drawn from the batteries. Remember, you just want an alternator that keeps the batteries up, and a 200amp will do that. Good Luck, TomC