Increased fuel consumption using headlights

Discussion in 'Electronics' started by TomTanderson, Oct 9, 2008.

  1. TomTanderson

    TomTanderson Hardcore MB Enthusiast

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    I've been meaning to ask about this for a while but the thread about DRLs has finally spurred me to action!

    I have always thought that using electrical items made no difference to fuel consumption as the alternator runs all the time driven by the fan belt (or some other enginey bit ;)), generating electricity whether it is used or not.

    Reading posts here from people who seem to know what they are talking about this doesn't seem to be the case. Can somebody explain, or point me in the right direction of a reasonably technical article, how a modern alternator works and therefore how it uses more fuel to produce more electricity?

    Ta
     
  2. popuptoaster

    popuptoaster Hardcore MB Enthusiast

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    its pretty simple really, you cant get something for nothing, if the alternator is having to provide more power, it needs more power to turn it, the magnetic fields it uses resist turning forces, an alternator doesn't create electricity, it converts rotation into electricity, as your brakes convert rotation into heat, or your power steering pump converts rotation into hydraulic pressure, there are also increased mechanical losses as doing more work will put more stress on bearings and drive belts.


    PS. yeah it does get way more technical than that, but who needs to know the details? :D
     
  3. Swiss Toni

    Swiss Toni Hardcore MB Enthusiast

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    So, having expended energy achieving a given speed, we waste energy by braking and so convert it into heat.
    If instead we convert it into stored electrical energy, isn't that getting something for free? (ie which would otherwise be converted into heat)

    :confused:


    PS
    The earth is flat
     
  4. Diesel Benz

    Diesel Benz Hardcore MB Enthusiast

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    This is what hybrid cars do already. I understood BMW does a bit of brake energy saving even with the standard alternator (battery charged heavier when braking).

    A bit of theory here:
    http://en.wikipedia.org/wiki/Alternator#Automotive_alternators

    As already mentioned, it takes more power from the engine when the alternator produces more electrical power. If it all was there already when the alternator was spinning at a given speed, we would not need nuclear power plants but a single hydroelectric power plant would be sufficient "since its generator is already spinning anyway".

    I did not put a wiki link for the earth topic, we already know it is flat.
     
  5. Ted

    Ted MB Club Veteran

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    Popuptoaster has it.
    I used to demonstrate it by connecting two 1/2 mag tape motors together.
    When they were disconnected they were very easy to spin.
    When connected together, the one you were spinning became MUCH harder to turn, and the emf generated would then spin the other motor.
     
  6. BTB 500

    BTB 500 MB Club Veteran

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    You only have to listen to a generator running. You can clearly hear the engine working harder as soon as anything is connected to the output socket.

    With cars there is also often a momentary dip in RPM if you turn on a heavy electrical load (headlights, heated rear window, etc) with the engine idling. Then the engine management catches up and applies more power to get the correct idle speed.
     
  7. donruffd

    donruffd Active Member

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    yeah thats very true, these electrical items take a very big charge to start off, the engine has to work harder to power all these items im pretty sure sum that hardly does any night driving will hardly ever change their fan belt as opposed to a night driver
     
  8. Fandango

    Fandango Hardcore MB Enthusiast

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    I'm not disagreeing just interested :

    Do the headlamps not run directly from the battery which gets charged anyway, whether the lamps are on or not - unlike a generator?
    I understand it will need more input but isn't this almost negligible?
    Can anyone find any figures for fuel consumption with and without lights?
     
  9. Diesel Benz

    Diesel Benz Hardcore MB Enthusiast

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    Well this is a bit of a chicken and an egg thing but different in the sense that the energy to run the lights would anyway come from the alternator (say eventually, temporarily lights may be power by the battery e.g. if the engine is not running but that energy needs to be filled back from the alternator in which case the alternator would source both the lights and charge the battery).

    The fuel consumption increase is pretty marginal, assume the lights take some 100 W, the alternator efficiency is very high and running the car at steady speed would take some 15 to 25 kW perhaps?
     
  10. BTB 500

    BTB 500 MB Club Veteran

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    Philips say that to provide 100W for 100 km uses approximately 0.13 litres of fuel.

    I drove to the South of Germany and back in August ... out in daylight (mostly), and back at night. Same speed both times (cruise control virtually all the way) but fuel consumption was definitely a bit worse on the night drive, by about 1.5 mpg IIRC.
     
  11. wemorgan

    wemorgan MB Club Veteran

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    Here's a report on DRL and dipped headlight. Pages 13 onwards discusses the environmental effects including fuel consumption.
     
  12. glojo

    glojo MB Club Veteran

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    Hi Bill,
    Your figures are amazing.

    Was the weight of the vehicle EXACTLY the same for both directions and were you going up and down the exact same gradients in equal amounts?

    I am NOT doubting for one second that using headlights will consume energy which in turn means fuel, but if your suggesting the difference is solely down to using the headlights, then how many do you have? :devil:

    [​IMG]

    Turning off every single electrical gizzmo and closing all the windows will save fuel, but by crikey is it worth it? Driving can be quite tiring and why put extra stress into a journey worrying about saving a cupful of fuel?

    John
     
  13. Dieselman

    Dieselman Banned

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    Bill's about right.
    All lights on is about 140W so 0.25 hp. A car needs about 8-10hp to do about 60mph, so that means an additional load of 3%.

    3% of say 40mpg = 1.2mpg.
     
  14. BTB 500

    BTB 500 MB Club Veteran

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    I think they reckon on 160W average electrical load with headlights on (including interior lighting of dash and switchgear etc.).

    Lower temperature at night may also affect fuel consumption (cooler air is denser, so more aerodynamic drag at a given speed ... but not sure how engine efficiency would change?).
     
  15. Dieselman

    Dieselman Banned

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    It goes up due to greater density.
     
  16. popuptoaster

    popuptoaster Hardcore MB Enthusiast

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    back when i were a yoof i had a morris marina pickup truck with spot lights all over it, if you switched em all on when it was only ticking over it would stall, had about 900watts of lighting on it.
     
  17. stevesey

    stevesey Hardcore MB Enthusiast

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    DM is that true at part throttle?

    Max power goes up becuase you can squeeze more mixture in at full throttle, so with the losses being fairly constant at any particular rpm I can see efficiency increases at full throttle.

    However at part throttle, I'm not so sure what happens, to get the same power you will need to close the throttle plate slightly (so a smaller amount of more dense mixture enters the clyinders). But the same mass of mixture is being moved, so vaccum would be higher, the pumping losses should be the same (or higher due to the increased vaccum, I'm not sure). I suppose the charge will need less compression, but by the time you get to the compression stroke I would have thought the charge temperature is pretty constant (doesn't vary with external temp).

    A quick google was inconclusive - although I did find some references to warm air intakes improving efficiency at part throttle (at the expense of max power).
     
  18. BTB 500

    BTB 500 MB Club Veteran

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    I had a Vauxhall Viva HC, which unbeknownst to me had a thermal fuse (bimetallic strip) on the supply into the fusebox. Hacking down a sunken lane in Cornwall late one night with both extra spotlights and stick-on rear window heater (remember those?) on, the power went and we were instantly in complete darkness :eek: for a very long couple of seconds till it cut back in.

    I also added spots to my 600cc Fiat 126, which only had a dynamo. The current draw with headlights and spots on was more than the maximum output of the dynamo, so they had to be used sparingly! :)
     
  19. Dieselman

    Dieselman Banned

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    Wot's a frottle.?

    Diesel man Diesel.. :D

    The cylinder will fill with the same amount of air charge whether under load or not, so greater density equals more oxygen so more output as it ignites.

    You can feel the additional performance when driving in cold fog at partial load, not only full load.
     
  20. popuptoaster

    popuptoaster Hardcore MB Enthusiast

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    sounds like you've had the same sort of cars as me. :D
     

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