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Old 07-23-2016, 08:14 AM   #85
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Interesting point ... I've never really given much thought to idle time. I tend to keep idling to a bare minimum. My coach has a MaxxForce engine that works with a regen system (rather than DEF) ... and my perception is that letting the engine idle any more than absolutely necessary will increase regen requirements. I'd guess that in most cases I don't spend more than 15-20 minutes idling in association with a campground stop (combined idling upon arrival and idling in preparation for departure).

I'll have to do some googling and see if I can identify a "fuel burn @ idle" figure for my coach engine. .. and then noodle how to best separate gallons spent idling in order to calculate moving MPG. Currently, the MPG # I use when calculating fuel estimate is essentially an average of "cherry picked" trips where my calculated MPG (calculated the old fashioned way of Total Miles Drive / # of Gallons replaced) of the MPG observed on selected trips that I know didn't involve the use of the Aquahot and/or generator. Obviously this isn't an exact science.

Thanks for the suggestion! If/when I revise my calculator - I'll repost.
SpaceNorman
An accurate MPG figure can ONLY be obtained if one uses ALL of the fuel burned when calculating the MPG.

IMO if the fuel for the Aquahot or generator is coming from the same fuel tank used by the engine, anyone who doesn't include the fuel burned by his Aquahot or generator, (or the fuel used idling), when calculating his MPG is fooling himself.... (and reporting more MPG than they're actually getting)

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Old 07-23-2016, 08:19 AM   #86
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An accurate/honest MPG figure can only be obtained if one uses *ALL of the fuel burned* when calculating the MPG. IMO if the fuel for the generator comes from the on board fuel tank anyone who doesn't include the fuel burned by his generator when calculating his MPG is fooling himself.... (and reporting more MPG than they actually get) Mel '96 Safari
Mel,

"Fooling himself" might be a little strong? But in principle I agree...

That is what the 10.3mpg number is on my spreadsheet... total fuel usage, including generator and Cummins... over the total miles driven.

I still think it's fun, and actually even useful, to have an idea of what the engine by itself will do. Whether I use the generator or not is a subjective choice and depends on where we camp, where we travel, what the weather is like, and who may or may not be with us, etc. How much fuel the cummins uses per mile is not so much of a choice.

Much of the generator usage on this particular tour of the US was due to over-nighting at Walmarts and having to keep the generator running in order to keep my elderly mother comfortable. There was also a lot of desert travel - across the Mohave at 112... and through southern Arizona. We also had issues with the alternator that forced me to disconnect it and rig the wiring so we could charge the coach battery with the generator for the last 800 miles of the trip. Every trip will be different.

Fuel costs what it costs... but knowing the numbers can be helpful when planning and saving for a trip. And now having said THAT... I will admit that on a 3+ week trip, the 60 gallons of diesel that I estimate went through the generator, with a cost of around $150, would never make or break a trip like that. I can basically plan trip expenses at 10mpg and be covered for all fuel requirements.

-cheers
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Old 07-23-2016, 08:22 AM   #87
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Mel s,

Oops... it looks like I was responding to you post at the same time you were editing it.

-cheers
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Old 07-23-2016, 08:30 AM   #88
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I have a propane generator, might explane some of mpg on mine. I'd much rather have a diesel if this one ever stops running i'll change it out .
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Old 07-23-2016, 09:30 AM   #89
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Agreed.

I'm one of those spreadsheet geeks... so below is the data from our last trip.

I used a constant for gallons per hour of generator fuel in order to try and separate the fuel usage of the cummins from the onan... The generator manual says .9gph is the fuel usage at full load. There is nothing in the manual that says what no load usage is, but I have read it's probably around .3gph. It's a 7500kw diesel... it's not the inverter type that regulates rpm based on load... It runs 1800 rpm all the time. I settled on an estimate of .5gph to run the generator.

As you can see... over the course of our 7500 mile trip, the cummins averaged 11.3mpg. FYI... For every .1gph change you make up or down on the generator, you get a .2mpg change up or down (respectively) on the cummins.

For further reference... our rig is 35' long and weighs 19,000 empty (well... with me in it). We do not pull a toad.

You can see how it's impossible to check mileage on a single tank. The trip mileage ranges from 9.4 to 13.5... The reason is because on a large fuel tank (ours is only 90 gallons) you can easily be off perhaps 10 gallons from one fill up to the next depending on how level you are sitting at the station. You have to take a running average... and if you can... make the starting and ending fill ups at the same pump.

-cheers

Well thought out. Lots of information.
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Old 07-24-2016, 09:08 AM   #90
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SpaceNorman
An accurate MPG figure can ONLY be obtained if one uses ALL of the fuel burned when calculating the MPG.

IMO if the fuel for the Aquahot or generator is coming from the same fuel tank used by the engine, anyone who doesn't include the fuel burned by his Aquahot or generator, (or the fuel used idling), when calculating his MPG is fooling himself.... (and reporting more MPG than they're actually getting)

Mel
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I disagree, Mel. The fuel burned by generator or aqua hot has no relation to miles driven, only to hours used. The only common factor is the storage tank.

If you want to measure MPG, you have to isolate those gallons whose efficiency can be measured relative to distance. If you want to measure Total Fuel Consumption, that's a different thing.

I'm not a big spreadsheet geek as Piker claims to be, but I do keep one on which I note the Gen Hobbs time at each fill up, and thus I can separate the fuel burned by the Gen between refuelings. This leaves me with a "Net Fuel to Engine" figure, which is the only way to accurately calculate MPG (if you want to).

Using your total tank method, I could say "Four days ago my fresh tank was full at 100 gallons. Now it's empty. My God! I drank 100 gallons of water in four days!" ;-)


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Old 07-24-2016, 10:40 AM   #91
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I disagree, Mel. The fuel burned by generator or aqua hot has no relation to miles driven, only to hours used. The only common factor is the storage tank.
If you want to measure MPG, you have to isolate those gallons whose efficiency can be measured relative to distance. If you want to measure Total Fuel Consumption, that's a different thing.
I'm not a big spreadsheet geek as Piker claims to be, but I do keep one on which I note the Gen Hobbs time at each fill up, and thus I can separate the fuel burned by the Gen between refuelings. This leaves me with a "Net Fuel to Engine" figure, which is the only way to accurately calculate MPG (if you want to).
Using your total tank method, I could say "Four days ago my fresh tank was full at 100 gallons. Now it's empty. My God! I drank 100 gallons of water in four days!" ;-)
John & Diane, fulltiming since '12 02 DS, FL, Cat330, '04 Element NHSO RVM103
JFXG
Are you saying when the generator and/or Aqua Hot is running while miles are being put on the coach it doesn't affect the overall MPG?
(If that's the case my "engine" gets better mileage than the 8.5 MPG my "coach" gets).

However because I put the diesel fuel for everything that burns diesel into the same diesel tank I include all of the fuel I use from that tank when calculating the MPG of my "coach".
(To me how many MPG my "engine" gets is irrelevant).

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Old 07-24-2016, 12:47 PM   #92
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SpaceNorman
An accurate MPG figure can ONLY be obtained if one uses ALL of the fuel burned when calculating the MPG.

IMO if the fuel for the Aquahot or generator is coming from the same fuel tank used by the engine, anyone who doesn't include the fuel burned by his Aquahot or generator, (or the fuel used idling), when calculating his MPG is fooling himself.... (and reporting more MPG than they're actually getting)

Mel
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I understand your rationale. I suspect the answer to what is the correct approach to calculating MPG is related to what you intend to use your MPG #'s for. In my case - I use my MPG data to project future fuel requirements.

If I don't account for fuel used by the generator and Aquahot - who's fuel consumption is driven by time used independent of the coach motor running, I won't get an accurate picture of what my fuel requirements for a future trip will be. The amount of time spent sitting in a campground and/or boondocking and relying on the generator - has a huge impact on the amount of fuel a given trip will need.

If I happen to boondock to 66 days - and burn 1.5 gallons of diesel per day using the Aquahot (which is at the very low end of the daily fuel consumption range per the folks at Aquahot) and run the generator for just 2 hours per day (burning .54 gph - which is the "50% load" rate that the folks at Onan says it uses) - I'll empty my 100 gallon fuel tank without having driven an inch.

While I understand your logic - if I'm looking to project future fuel need, I can't not factor in fuel used to sources other than the coach engine using a method that doesn't approximate how those sources use fuel (hint - and that isn't based on the # of miles driven).
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Old 07-24-2016, 01:08 PM   #93
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JFXG
Are you saying when the generator and/or Aqua Hot is running while miles are being put on the coach it doesn't affect the overall MPG?.......

Yes, that is precisely what I am saying.

It may affect your MPT (miles per tank), and it may affect your total trip fuel. It may affect your indoor comfort, and it may affect the temperature of the hot water at your galley sink. But it will have no affect at all on the number of miles that your power train can move your coach down the road on one gallon of fuel.


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Old 07-24-2016, 01:23 PM   #94
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To illustrate my point about the impact of fuel consumed by the generator and the Aquahot - here's a screen shot from my "Fuel Calculator" for our upcoming trip to Arizona. We'll be gone for a total of 61 days (leaving in mid September, returning in mid November). Based on our route plans - Googlemaps says we'll be traveling a total of 5,344 miles over the course of the trip.

Although we're going to be staying in campgrounds that all have electric power available - I'm forecasting that over the course of the trip - we'll have 4 travel days where it will be warm enough that running the generator for a couple of hours to run the house AC while driving will be necessary.

I'm also factoring in 61 days of Aquahot use @ 1.5 gallons of fuel per day. This rate of fuel burn is in the middle of the 1-2 gallons per day that Aquahot says is normal when connected to shore power. For the most part - Aquahot usage will be for hot water ... however, we'll be spending a little more than a week travelling thru the mid-upper Midwest on our way home in early November - so we'll likely be using the heat at least a little bit.

As the tool shows - based on these assumptions - I'm expecting we're going to need 97 gallons of fuel to meet the demands of the generator and Aquahot. We'll need another 764 gallons of fuel for the engine to cover the 5,344 miles based on an assumption that we'll see 7 mpg from the engine itself. Those 97 gallons represent more than 11% of our total anticipated fuel requirements.

My point is ... anticipated generator and aquahot fuel requirements over a long trip can have a significant impact on your overall fuel requirements. Enough that trying to factor those requirements in using a method other than "total miles driven" just makes good sense.

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Old 07-24-2016, 01:31 PM   #95
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According to Fuelly - Track and Compare your MPG, I am presently averaging 7.5 mpg in my Dutch Star. Best mileage I have gotten is 10.4 mpg. Worst mileage is 5.79 mpg (did a lot of running the generator to keep the dogs from getting overheated).
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Old 07-24-2016, 07:00 PM   #96
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I average around 7.2 with an ISL400, 47,000 pounds loaded with toad. I do the speed limit usually also, so if it's 75mph that's what I set my cruise to I know I could probably get better if I slowed down some under the limit on those faster roads but oh well.
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Old 07-25-2016, 08:21 AM   #97
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Interesting conversation and great points for both sides. Here is what I have gleaned from the conversation:

1) If you are only going to drive and use the engine it is good to know the engine efficiency.

2) If you are driving and plan to use the engine, generator and aqua hot it is good to know the operating fuel efficiency.

Since motorhome travel is different for everyone both efficiencies are interesting. I calculate both.

I like to know how the engine is performing and how my driving style affects fuel economy. I have found that running the engine around 1750 instead of the recommended by Cummins 1650 gives better fuel economy.

I also like to know the combined fuel efficiency as I can predict the approximate cost of a trip overall. This one is not as important because I fuel when the tank is starting to get low and will pay whatever the best price I can find within the range I have available. As a side note: The price of fuel is the most variable item.
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