“Under hood generator”

[bmwbob]

We are shopping for a good used Class B van camper.
A number of them are being offered with what they call an “under the hood generator”, which I take to mean a second, larger alternator to provide AC power in the place of a traditional generator.
Can anyone who has used this system give me some info on how well this works to power appliances while you are driving?
Also, is this second alternator equipped with an electric clutch like the AC compressor so that it is not putting a load on the engine when it is not needed?
Thanks!
Bob

[ktmrfs]

BFL13 wrote:
Must be an inverter. The alternator makes DC

well, eventually DC but an alternator generates ac, usually 3 phase and it is possible to get pretty high AC voltages out of an alternator. The AC voltage is then rectified to produce DC

without some extra circuits, it will be 3 phase and not necessarily 60 hz.

The honda inverter generators run an alternator convert to DC, do a DC-AC conversion in the inverter.

[bmwbob]

Gotcha.
Not for me, really. I can’t see hauling 8AGM batteries in addition to everything else.
I have no intention of doing long term boondocking.
Thanks for the info.
Bob

[obgraham]

bmwbob wrote:
So, how many watts was the UHG good for? I’ve not seen one over 600 watts, and that’s well below the 2800:watts of the usual class B rig generator.
Did it have an electric clutch like the one on the AC compressor so that if only drew power from the engine when needed?
Bob

You're trying to equate the UHG to a standard genny. Doesn't work that way. It charges a battery bank, and that's what you live off. It is true that if you need long hours of a/c, the current demands are beyond the capabilities.
The UHG turns when the engine is on. No clutch. I doubt that during the life of the rig you would see significant cost due to increased fuel use.
It's all about getting the right rig for its intended use.

[bmwbob]

So, how many watts was the UHG good for? I’ve not seen one over 600 watts, and that’s well below the 2800:watts of the usual class B rig generator.
Did it have an electric clutch like the one on the AC compressor so that if only drew power from the engine when needed?
Bob

[obgraham]

The UHG was and still is, a viable choice. But it depends just what you are using the RV for.
I had a Roadtrek, with UHG. It charged my bank of 8 AGM batteries. Those powered a 5000 watt inverter, which supplied all the 110 voltage I ever needed -- space heaters, microwave, coffeemakers, tv's. I never once plugged the rig into shore power.
Now since we were out and about almost every day, the UHG kept a full charge on those batteries as I drove around during the day. In addition there was a small solar, which would maintain the batteries while parked, so long as usage was not great.
Where this system was not viable was if you parked up for a number of days, say boondocks with no shore power. Then yes, you had to start the rig and run the engine for perhaps 90 minutes to recharge.

RT's problems were 2: Crooked management (that did them in), and their inability to stick to a configuration, changing it so often that their reliability went to heck because nobody knew how that rig was configured.

The RT Class B's with the UHG worked very well for their purpose.

[dougrainer]

https://www.youtube.com/watch?v=-2gDE2tP0nU

Amazing what I learn from these forums. Our dealership is the largest Class B retailer in Texas and NONE of our Class B van Brands have this System. Doug

[theoldwizard1]

bmwbob wrote:

How many times in this example does energy change its form, and how much efficiency is lost in the process? ??

1st and 2nd Law of Thermodynamics !

Energy is never CREATED, it is just converted from one form to another.
When energy is converted, some is always lost.

[theoldwizard1]

bmwbob wrote:

Also, is this second alternator equipped with an electric clutch like the AC compressor so that it is not putting a load on the engine when it is not needed?
Bob

If this operate like most alternators, if no power is sent to the rotor (the job of a voltage regulator is to control this voltage) then the load on the engine is nearly zero.

[bmwbob]

Sounds kind of like when I want hot water. So, the utility company buys and burns some sort of fuel, making fire and heat. That heats water in a boiler. But I can’t get the hot water from that boiler.
So, this water is heated to the point of making steam. The expanding steam is used to cause the blades of a turbine to spin. The turbine is connected to a generator which produces electricity. The electricity is trucked to my home over miles and miles of copper wire. At my home, the too-high voltage is stepped down by a transformer to what my appliances can use. Now, the electricity is passed through darned near a dead short circuit in my tankless water heater. Cold water is passed over the glowing heating element and becomes what I wanted in the first place: hot water.
How many times in this example does energy change its form, and how much efficiency is lost in the process? ??
Highjacking my own thread (temporarily, I hope!),
Bob

[jkwilson]

BFL13 wrote:
Must be an inverter. The alternator makes DC

Alternator=alternating current=AC.

A rectifier is used on vehicles to convert the AC output to DC and is integrated into the alternator housing in most applications.

But on a variable speed engine, you’d want to generate DC with a generator and convert to AC with an inverter.

So you do the opposite of what sounds right to get the kind of power you want 🙂

[Community Alumni]

For a while I entertained the idea of getting a Class B moho and joined 3 different makes FB groups to get myself familiarized with Class Bs.

If memory serves me right, Roadtrek may have been the pioneer in using the so called “under the hood generator”. I followed the groups for several months and came to the conclusion UHGs systems were fraught with problems and the reason some RV manufacturers use them is strictly to save money.

I'm old-school and prefer old-school generators to an alternator with a fancy name in the engine compartment. Onan recently came out with an inverter type generator for Class Bs and it is very quiet compared to the traditional generators.

[CA_Traveler]

On a van forum UHGs were mated with a Lithium battery so they could charge quicker. Very convenient - just start the engine. But costly, a much larger engine than needed is running when parked, run at a high idle RPM which sounds noisy and maybe harder on the environment. I would like to see a unbiased set of pros and cons.

[valhalla360]

Poking around online, the only "under hood generators" I found were large after market alternators paired with a large inverter to supply AC.

These can work but they come with some downsides:
- You are putting hours on your main engine which is likely far more expensive than a small dedicated generator. Ask about required RPM to generate the rated amperage. Most alternators need to run faster than idle to do more than low output.
- Large alternators get used on cruising boats frequently and due to the heavy loads, if they aren't perfectly aligned and tensioned, they tend to eat up belts and are hard on the bearings supporting the pulleys.

Now if it was mated to something like the hybrid F-150, that would be a different ball game.

[Reisender]

Following. Sounds interesting.