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07-27-2015, 09:14 AM
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#1
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Senior Member
Join Date: Dec 2012
Posts: 257
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Generac diesel generator, fuel usage question
If running the generac diesel 7500 (50amp) generator with one air conditioner running, let's say it uses 1/2 gallon per hour of fuel. Will my fuel usage go up if I turn on a second air conditioner?
Why I ask, When I run both or only one, I don't hear the RPM's increase or even hear the generator working any harder. So I am wondering if the fuel usage is the same regardless what load.
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"Mrs. CC. Magna" 36' 1993 Country Coach, Magna, 300hp Cummins, 5 speed Alison trans, #5016
127,000 miles and counting...
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07-27-2015, 09:31 AM
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#2
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Senior Member
Monaco Owners Club Freightliner Owners Club iRV2 No Limits Club
Join Date: Jan 2011
Location: Avon Lake, Oh
Posts: 2,958
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It will increase only slightly. The biggest increase is the load created by the first one.
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2000 HOLIDAY RAMBLER ENDEAVOR
40 FT--330HP CAT
2 SLIDES-TOAD 2012 focus
Fulltime-Home is where we park it.
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07-27-2015, 12:33 PM
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#3
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Senior Member
Join Date: Feb 2010
Location: Sunnyvale, CA
Posts: 1,195
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You have a fixed rpm generator, so unless you have a very keen ear, the differences between load levels (unless you catch it just at the moment the load changes) are hard to identify. If you are running both a/c units and the compressors cycle on and off at various times, you should hear the load differences, but once it's going, they are quiet enough that it always sounds the same
Steve
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The Green Machine -- 2000 Mountain High Coachworks Summit (Spartan chassis / Cummins ISC)
...and F-Troop: Fearghus, Fiona, and Frankie (Cairn Terriers)
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07-27-2015, 12:45 PM
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#4
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Senior Member
Holiday Rambler Owners Club
Join Date: Aug 2010
Posts: 9,751
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Same thing as running 1600rpm on a flat highway, and running 1600rpm climbing a grade. Engine spinning same rpm, more fuel and air climbing the hill !
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Ben & Sharon
2008 43' Holiday Rambler Scepter PDQ
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07-27-2015, 02:11 PM
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#5
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Senior Member
Join Date: Feb 2010
Posts: 4,654
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The generator consumes a certain amount of fuel just to operate regardless of load so we can call that overhead.
Past that it converts fuel btu into watts by some conversion factor.
So first ac will use the most ßo to speak then second very little compared to first one as the generator is already running
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Tony & Lori
1989 Country Coach Savannah SE
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07-28-2015, 11:42 PM
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#6
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Senior Member
Join Date: Dec 2012
Posts: 257
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Sounds great, then both airs running is the way to go. Thanks folks.
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"Mrs. CC. Magna" 36' 1993 Country Coach, Magna, 300hp Cummins, 5 speed Alison trans, #5016
127,000 miles and counting...
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07-29-2015, 12:19 AM
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#7
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Senior Member
Join Date: May 2013
Posts: 335
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My generator manual says 1/2 GPH at half load and 1 GPH at full load. Mine is a constant speed 10KW so I guess it's somewhere around 0.1 GPH per 1000 watts but as others have said, it takes some fuel to run even with no load. I would think efficiency in converting diesel to electricity would increase with higher loads.
1994 Holiday Rambler Navigator
Cummins 8.3
Allison MD3060
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1994 Holiday Rambler Navigator
300HP Cummins 8.3
Allison MD 3060
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07-29-2015, 12:45 AM
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#8
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Senior Member
Country Coach Owners Club Solo Rvers Club iRV2 No Limits Club
Join Date: May 2011
Location: Vancouver, WA
Posts: 37,725
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Our 8.0 KW Generac ran at a constant 1800 rpm and never really changed pitch when more load was added. As I remember the manual stated that it used 1.32 GPH at full load and .72 at half.
Our present 12.5 Onan is also a constant speed and again I don't notice any difference under more load but it does burn a little more fuel too.
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2009 45' Magna 630 w/Cummins ISX 650 HP/1950 Lbs Ft, HWH Active Air
Charter Good Sam Lifetime Member, FMCA,
RV'ing since 1957, NRA Benefactor Life, towing '21 Jeep JLU Rubicon Ecodiesel
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12-27-2015, 10:32 PM
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#9
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Senior Member
Join Date: Dec 2012
Posts: 257
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Quote:
Originally Posted by Mr_D
Our 8.0 KW Generac ran at a constant 1800 rpm and never really changed pitch when more load was added. As I remember the manual stated that it used 1.32 GPH at full load and .72 at half.
Our present 12.5 Onan is also a constant speed and again I don't notice any difference under more load but it does burn a little more fuel too.
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You are right, ours is constant as well, I am a little under your GPh but we have a 7500 kW. So that makes sense. I would have expected some change when the load increased but nope.
Does yours shake and shutter when you turn it off, while it is finishing up its last few rotations.
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"Mrs. CC. Magna" 36' 1993 Country Coach, Magna, 300hp Cummins, 5 speed Alison trans, #5016
127,000 miles and counting...
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12-28-2015, 08:47 AM
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#10
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Senior Member
Join Date: Oct 2014
Location: Alberta
Posts: 598
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The speed of the generator is what gives you the 60 cycles per second sine wave. That's why the speed is constant. As you increase the load it will consume more fuel but it remains at the same rpm.
2009 Safari Cheetah 40'
350 HP Cat
2014 Ford Focus
Air Force One
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12-28-2015, 08:51 PM
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#11
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Senior Member
Join Date: Dec 2012
Posts: 257
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Quote:
Originally Posted by G Schulz
The speed of the generator is what gives you the 60 cycles per second sine wave. That's why the speed is constant. As you increase the load it will consume more fuel but it remains at the same rpm.
2009 Safari Cheetah 40'
350 HP Cat
2014 Ford Focus
Air Force One
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There you go. perfectly good explanation! Thanks.
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"Mrs. CC. Magna" 36' 1993 Country Coach, Magna, 300hp Cummins, 5 speed Alison trans, #5016
127,000 miles and counting...
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12-28-2015, 09:24 PM
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#12
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Senior Member
Country Coach Owners Club Solo Rvers Club iRV2 No Limits Club
Join Date: May 2011
Location: Vancouver, WA
Posts: 37,725
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Quote:
Originally Posted by G Schulz
The speed of the generator is what gives you the 60 cycles per second sine wave. That's why the speed is constant. As you increase the load it will consume more fuel but it remains at the same rpm.
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Onan, as do other manufacturers, makes variable speed units too. In fact the 7.5 Onans are generally variable speed units.
Here is one: http://power.cummins.com/content/rv-qd-6000 and read up on the different units for RV's, where you will see they do indeed make many variable speed units for the RV market.
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2009 45' Magna 630 w/Cummins ISX 650 HP/1950 Lbs Ft, HWH Active Air
Charter Good Sam Lifetime Member, FMCA,
RV'ing since 1957, NRA Benefactor Life, towing '21 Jeep JLU Rubicon Ecodiesel
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12-29-2015, 01:47 PM
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#13
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Senior Member
Join Date: Jun 2012
Posts: 106
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Quote:
Originally Posted by bobioknight
Does yours shake and shutter when you turn it off, while it is finishing up its last few rotations.
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I have the 7.5kw but it shakes too on a sudden shutdown.
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2008 Gulf Stream 8327 Fred, 300hp Cummins, Allison MH1000, Freightliner, 24k lbs chassis
2004 Ford Ranger 4x4 5spd, 4k lbs, flat towed
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