How much fuel can these things really use?

Discussion in 'Motorhome Chat' started by errpaul, Jan 6, 2014.

  1. errpaul

    errpaul Funster

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    So we have just got back from a last minute trip to Disney in the van (just decided before new year...lets go tomorrow! Love having a motorhome).
    Anyway, my question is does anyone know how much extra fuel can really be used running things whilst driving.

    Maybe a bit sad, but we used approx 12l less diesel coming home than going! Same route, cruise control set to the same speed, approx same weight (within 70kg or so).
    The only difference was we had the fridge on 12v going there, along with a habitation heater which is controlled from the cab. The fridge was on halfway home and the habitation heater occasionally on.

    Could they really use this much more fuel or am I missing something??:Eeek:

    I was thinking if that was the case then running fridge on gas whilst driving is a better option?
    Blown air heating whilst driving would probably be better too.

    Any ideas?

    Cheers
     
  2. aba

    aba

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    anything that uses engine power has the potential to use more fuel the alternator on the van just charging the engine battery will put a certain amount of load on the engine if you then run the fridge on 12v it puts around 20amps of extra load on the alternator so the engine has to work slightly harder to compensate add to that air con lights wipers heater blower and you can be up at over 100 amps fairly quickly.
    also a slight hill will increase consumption as will wind.

    running gas appliances when driving may be frowned upon as in the case of an accident you have a much higher chance of bursting into flames.
    however there are special valves that allow you to use gas on the move which cut off the gas supply.

    and before someone says about lpg cars they have safety cut offs too which is why 90% have to be started on petrol before they switch over to gas.
     
    Last edited: Jan 6, 2014
  3. errpaul

    errpaul Funster

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    I never realised it was so much. I know they would use a little extra, but working out the mpg there was a 20% difference!

    On the subject of using gas whilst driving, is this different with the gaslow type refillable system?
     
  4. Mikeco

    Mikeco Funster

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    I think that you are right about being sad, the probable cause is likely to be the direction of the wind.

    Mike
     
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  5. Phill D

    Phill D

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    check my figures but Paris to say Salisbury is around 360 mile and you used 2.6 gallons more.... so thats around 1/2 mile per gallon difference, i think!

    that could be anything, from head wind / tail wind coming home.
     
    Last edited: Jan 6, 2014
  6. hilldweller

    hilldweller Funster Life Member

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    Wind !

    We've had constant strong winds lately, if you had a tail wind one way and a head wind the other -- Bingo.

    It does not take a lot, just look at the difference in fuel from 60 to 70. A steady 5 mph wind would do that to you. +5 one way -5 the other.
     
  7. haganap

    haganap Funster Life Member

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    absolutely what he said ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    2 factors, weight and wind and Aircon, (yes thats 3)

    never heard of running a fridge, lights or anything else making a difference, but I also know Abba knows his stuff so stand to be proved wrong.:thumb:
     
  8. aba

    aba

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  9. callumwa

    callumwa Read Only Funster

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    Agree with the wind being the reason...
    Even a slight head wind will make a noticeable effect on a motorhome..

    I drove an 800 mile round trip just after New Year in the car (much more streamlined than a motorhome)..
    400 miles there, 400 miles back...

    Going 46.8 mpg, coming back 58.9 mpg, only difference strong SW'ly headwind on the first trip.


    :thumb:
     
    Last edited: Jan 6, 2014
  10. Jim

    Jim Ringleader

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    It will definitely be wind Paul, all things being equal, even if only a little is working against you it can really make a difference to you mpg
     
  11. Phill D

    Phill D

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    sorry finger trouble on calculator its more like 3 mpg extra. still could be hills and wind direction
     
  12. Yorick

    Yorick Funster

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    Not sure about the fridge, but I know that air-con does decrease MPG by as much as 5% :Smile:
     
  13. TheBig1

    TheBig1 Funster Life Member

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    or more downhill than uphill on the return along with weather conditions, stop start traffic etc. 1/4 of a mile per gallon of fuel is trivial on the scale of things over all

    worrying about the little things in life ends with missing the good bits in my experience
     
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  14. FULL TIMER

    FULL TIMER Read Only Funster

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    The heater's, assuming you are talking diesel blown air such as eberspacher only use about 300ml per hour when running flat out in boost mode dropping down to around 100ml per hour on lowest setting. I would say that your route probably has more uphill sections on the way there, other things such as wind will also effect the economy as will being stuck in traffic jams
     
  15. John Laidler

    John Laidler Funster

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    Your fridge might be drawing about 50W of power, give or take, which is less than a tenth of a horse power. Of course your MH isn't going to be running at full power so let's assume it was producing an average of 50 bhp during the journey. That's just a guess but probably isn't far off. Now let's say you average 25 mpg at an average speed of 50 mph which means it consumed 2 gallons every hour. We can now say to produce 50 bhp for one hour takes two gallons so to produce a tenth of a horsepower for one hour will take 0.1 * 2/50 = 0.004 gallons per hour. Approximately of course as the figures are guestimates and I've ignored losses in the generation of the electrical power but essentially your fridge is running on next to no fuel.

    A diesel burning habitation heater would consume a lot more but even our oil heated house only gets through something like a gallon every 24 hours even in winter.
     
  16. wasp

    wasp Funster

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    Is it downhill coming back ??:Rofl1::Rofl1::Rofl1::Rofl1::Rofl1: Sorry couldn`t resist
     
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  17. errpaul

    errpaul Funster

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    Thanks for all the replies.....wind seems to be the favoured answer then. We were battered from all directions there and back, and didn't really notice it struggling more at any time due to the wind, but then I wasn't going quickly, and cruise control would've helped.
    Only reason I noticed was as it is a relatively new van for us and the first time we have taken it to France, or any distance that has required more than a tank of fuel one way.

    In the grand scheme of things it's not a lot, but just thinking if I can save a tenner or more per tank by flicking a switch to a different setting on fridge for example why not. :BigGrin:
     
  18. safejourney

    safejourney Read Only Funster

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    Motorhome wasn't parked as flat in France so the fuel gauge showed extra in it.
     
  19. Mastercamper

    Mastercamper Read Only Funster

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    Look at a globe - South is definitely down hill, North is uphill. Must make a difference.

    Obvious answer only travel East/West or West/East.

    I couldn`t resist either!!
     
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  20. Techno

    Techno Funster Life Member

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    The alternator output should make no difference it is being driven by the engine continuously at whatever revs your engine is running at.
    Switching on the aircon will engage the compressor which will then be driven by the engine and increase load and fuel consumption.
    The weather has been circulating around UK /western europe in a anticlockwise direction making the wind S to SW so a headwind down tailwind back
     
    Last edited: Jan 6, 2014
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