Engine generator effect on mileage
This question has come up a couple of times lately.
Did some rough guess on it based on the fuel an Onan uses for similar power.
Figure about 180 amps average, which appears to be what we are seeing due to thermal output reductions. About 2250 watts depending on the voltage maintained. That is about 80% output of an Onan.
IIRC, a 2800 Onan uses .5 gal/hour at full output, so .8 X .5 = .4 gal/hour
Van at 60 mph, getting 15 mpg would use 4 gallons. Add .4 gallons for the generator and it would use 4.3 gallons. 60 miles divided by 4.4 gallons = 13.6 mpg.
Substantially more than I would have guessed. It will get better quickly as the amps to the batteries decrease, but it could easily be at 180 amps for 1-2 hours depending on battery bank size and SOC.
If you ran the coach air off the inverter to get more cooling, you would probably lose about 1/2 that much so about .7mpg just for the AC.
Math check is encouraged :rolleyes:
1. The Typical converter I see installed on class Bs is 45 amps so the Onan is working, more or less, four times as long to deliver the equivalent amp hours to the batteries.
2. The alternator is providing battery charge voltage directly. There is no voltage conversion loss. The efficiency of the Onan/converter system is decreased by the converter losses.
By similar logic, there would be no conversion to DC for the Onan as we are only looking at how much power it is producing and watts are watts.
Are the numbers perfect? Hardly, as there lots of things not known, like the efficiencies of both systems. Basically a rough guideline to get an idea of what to expect.
Another thing to remember is that this is at 60 mph only. At 30 mph, you would lose about double this amount.
I think comparing to an onan is apples to oranges.
you can kinda guess a 160 amp alternator will drag about 4 or 5 horsepower- this is why drag race cars may not have alternators and run purely on battery power.
The Onan will generate 2800 watts which is 3.75hp, so is actually running over 50% efficiency in the generator section. This is similar to what I always hear for alternator efficiency.
.4 gallons of fuel would generate 19+hp/hours per the converter program. Overall efficiency of the Onan generator would be 3.75/19 = about 20%. That would put the engine at about 38% efficient, which is about where most gas engines run, and would probably be pretty close to what the Chevy gas engine runs. It all kind of adds up right.
Not specifically related to this topic but I figure I'd post this data:
With the Scangauge I've seen:
0.11 GPH at 634 RPM idling with the 5,200 BTU A/C running off inverter.
0.45 GPH at 651 RPM idling with the 5,200 BTU A/C running off inverter and dash air on "Max Air".
The 5,200 BTU A/C is less than a 600W load.
Fuel consumption for the gas engine Onan 2800W is listed in the manual as being:
No load 0.16 gph
Half load 0.28 gph
Full load 0.46 gph
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