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Old 11-15-2016, 04:49 PM   #1
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Default 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
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Old 11-15-2016, 05:53 PM   #2
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Originally Posted by booster View Post
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
I don't know how or if it affects the math, but wouldn't these two factors affect the bottom line:

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.
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Old 11-15-2016, 06:04 PM   #3
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I don't know how or if it affects the math, but wouldn't these two factors affect the bottom line:

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.
My assumptions would be that we are just talking generating watts by the Onan, 2800 of them, so it wouldn't be tied to charging batteries. It would be just how much gas it takes to generate 2800 watts. Then you assume it takes the van engine the same amount of gas to generate 2800 watts. Once you know how much per watt it takes, then you can apply it to the watts from the engine generator.

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.
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Old 11-15-2016, 08:23 PM   #4
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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.

Mike
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Old 11-15-2016, 08:53 PM   #5
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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.

Mike
Why is that? Watts are watts AFAIK. I would think the only difference is thermal efficiency of the engine, with the Onan generator and the alternator having similar efficiencies. I think an Onan is 7hp. 160 amps X 12.5 = 2000 watts. 2800 watts/2000 watts = 1.4 more power from the Onan. 1.4 X your 5hp for alternator would be 7hp, which is same as the Onan.

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.
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Old 11-15-2016, 10:49 PM   #6
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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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