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Onan Fuel Burn - JC

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Old 02-15-2014, 12:02:14 PM
tim876 tim876 is offline
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Default Onan Fuel Burn - JC

I have a 1978 JC GAS 12.5 kw generator in my BlueBird Wanderlodge. Finally managed to get the unit running really well (would have been much easier if I found this list first). I have done a full service on the unit including:

electric fuel pump (old mechanical wasn't keeping up)
spark plug wires and new dist. cap
oil + filters (including fuel and air)
condensor and coil and points
carb cleaned and working well including automatic choke
air/fuel set (was running REALLY rich before)

My question is: what should the fuel burn per hour be on this unit and is there anything I can do to make it more efficient?

I am currently seeing about 1.3 gallons per hour (gas). The coach is diesel and the built in generator tank is 33 gallons...which equates to just over 1 full day (approx 25 hours) of runtime and a whole bunch of money in fuel! This is with what I would call a medium load (two electric coach heaters, battery charger).

Is this as good as it gets?
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Old 02-15-2014, 12:35:02 PM
Max Thompson Max Thompson is offline
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Default Re: Onan Fuel Burn - JC

Not knowing what the KW draw of the heaters is, I am going to guess your right in line with “normal”. The JC engine burns around 1 GPH at no load 1.3 @ ¼, 1.6@ ½ , 2.0 @3/4 load. The full load consumption is not relevant in your situation since with a 12.5 gen-end, you can not fully load the JC engine, it is capable of around 16 KW on gasoline fuel & you can not utilize its full potential with the 12.5 gen-end. Judging by the fuel consumption, you are running at about ¼ load for the engine’s ability.

It sounds like you need to calculate the maximum KW you “need”. It may be that a 7.5 JB would be a better choice for your RV. I do not know how big a “Wanderlodge” is, but 50 amps per leg is a lot of power for a motor home. If you could get by with ½ the engine, it looks like a savings in fuel of about $2.37 per hr. which would pay for the swap pretty quickly.
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Old 02-15-2014, 12:38:08 PM
JerryLee JerryLee is offline
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Default Re: Onan Fuel Burn - JC

I have noticed that others on this forum say to expect to use about one gallon of gasoline per hour with no load. If that's accurate, 1.3 gallons per hour with a medium load sounds plausible. I'm sure the experts will weigh in shortly and give you better information.
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Old 02-15-2014, 12:42:11 PM
tim876 tim876 is offline
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Default Re: Onan Fuel Burn - JC

Thanks for the info Max. The BB Wanderlodge is a 35 foot class A coach with three roof airs, three large-ish electric heaters and has other pretty heavy draw systems if things were to all be on at once (including clothes dryer). This is the factory installed unit. I think I may go with some other methods of getting power into the batteries and just rely on the Onan for limited use rather than trying to run all the time (roof airs would be only real need). I really appreciate your precise info...reassures me that I have her running right.
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Old 02-15-2014, 01:13:29 PM
len k len k is offline
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Default Re: Onan Fuel Burn - JC

JC has a big engine (4-cylinder) and big engines have a high baseline no-load fuel consumption. IF you don't need all the power of the JC, can go with a smaller engine gen to lower your baseline no-load fuel usage. Which in your case is the majority of your fuel useage because you are not using much of the JC's ability. All lightly loaded gens have this problem. The key question as Max said, is do you need all of JC's available power or can you get by with something smaller.

Looks like your drawing < 3.5kw , if you changed gen from JC to 6.5kw NHE type your fuel usage for that load would drop in ~ half, as Max said.

15kw JC ---1 GPH @no load ,,, 1.3 @¼ ,,, 1.6@½ ,, 2.0 @3/4 load ,,, ~2.4@full load ,, (JC= 12.5 or 15 kw genhead)
7.5kw JB ---- 0.6 @no-load
7kw NHM rv - 0.4 @no-load ,,,,,,,,,,,,,,,,,,,, 0.7@½ ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 1.3 @full load ,,,(same for 6.5kw NHE---rv )

Could go with diesel for lower fuel consumption, typically ~1/2 of gasoline

Last edited by len k; 02-15-2014 at 11:46:22 PM. Reason: diesel. clean up table
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Old 02-15-2014, 11:26:53 PM
Isaac-1 Isaac-1 is offline
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Default Re: Onan Fuel Burn - JC

Since your main engine is diesel, have you considered swapping in a 12 KW diesel DJC, or for that matter swapping out a DJC engine on your JC generator end? The 12DJC is rated at 1.05 gph full load, so it would also give you better fuel economy as well as a single type of fuel on board.

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Old 02-16-2014, 12:59:33 AM
Tim0477 Tim0477 is offline
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Default Re: Onan Fuel Burn - JC


I highly doubt you want to modify your mobile home, but that would be the ideal situation for those who are efficiency minded to make use of the waste heat from the generator engine. If you replaced the J series generator with something liquid cooled, you could conveniently use the waste heat for hot water heating and coach heating. Onan makes some units with the little Kubota diesel engines that are nice. I suppose it would even be possible to use the waste heat to run the clothes dryer or at least as a pre-heater to reduce the size of the electrical element needed to dry the clothes.

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