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Thread: Does fuelling full tank cause an increase in fuel consumption in a carburetted engine

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    Unhappy Does fuelling full tank cause an increase in fuel consumption in a carburetted engine

    hi guys in most of the bikes with carburettor i have found that on a full tank the bike gives lesser mileage. i had a question popping up. does anyone have an answer to it?

    when we put 10 ltrs of fuel as compared to 1 lit the pressure in the jet is greater (using same principle as hygroscopic pressure) as the fuel is not pumped as in FI it only flows out of an overhead tap. This can possibly cause a greater flow rate of fuel out of the jet making the mixture richer. The carburettor works on the principle of sucking fuel by the flowing intake air and greater pressure in the fuel needle means greater fuel sucked out.
    If the fuel tank is a cuboid, pressure due to 10 lt of fuel will be 10 times the pressure due to 1 lt of fuel. that to me sounds like the air-fuel mixture becomes significantly rich in a full tank situation.

    I have a question ... this is the theory i have written... physics says so... is there something to check it? does it really happen...
    in every bike i have seen a drop in mileage when the tanks are fuelled full.

    This might not show in the pickup as richer mixture does not mean more and more power... it just means engine runs cool and more unburnt fuel.

    can anyone device a way to check it guys?
    innovation is the key i guess...


    Thanks
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    Well, even I noticed such a thing 1-2 years ago (but only the mileage part).

    The simplest method is that, when the mixture gets too rich, the smoke is very intense smelling.


    Personally, I full-tank my pulsar 180 often, and still can't make it a "bit richer". It still heats up terribly!

    Which points out that maybe Full-Tanking results in "Other Wastage" of petrol, and the AFR still remains the same at all times ... Maybe it leaks and evaporates somehow ...
    Last edited by Samarth 619; 06-02-2009 at 04:45 PM.

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    I always fill petrol upto the brim in both my bikes&I get consistent mileage!
    In fact, even the manual recommends filling upto the brim, as this reduces losses due to evaporation.
    Filling up petrol higher than the filler cap may lead to leakage.
    In any case, the mileage remains same, regardless of filling less or more fuel.
    Quench my thirst with gasoline!

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    Quote Originally Posted by Braveheart View Post
    hi guys in most of the bikes with carburettor i have found that on a full tank the bike gives lesser mileage. i had a question popping up. does anyone have an answer to it?

    when we put 10 ltrs of fuel as compared to 1 lit the pressure in the jet is greater (using same principle as hygroscopic pressure) as the fuel is not pumped as in FI it only flows out of an overhead tap. This can possibly cause a greater flow rate of fuel out of the jet making the mixture richer. The carburettor works on the principle of sucking fuel by the flowing intake air and greater pressure in the fuel needle means greater fuel sucked out.
    If the fuel tank is a cuboid, pressure due to 10 lt of fuel will be 10 times the pressure due to 1 lt of fuel. that to me sounds like the air-fuel mixture becomes significantly rich in a full tank situation.

    I have a question ... this is the theory i have written... physics says so... is there something to check it? does it really happen...
    in every bike i have seen a drop in mileage when the tanks are fuelled full.

    This might not show in the pickup as richer mixture does not mean more and more power... it just means engine runs cool and more unburnt fuel.

    can anyone device a way to check it guys?
    innovation is the key i guess...


    Thanks
    The error here lies in the basic premise...."when we put 10 ltrs of fuel as compared to 1 lit the pressure in the jet is greater (using same principle as hygroscopic pressure) as the fuel is not pumped as in FI it only flows out of an overhead tap."

    The fuel goes into the 'float chamber' which is so named as there is a 'needle and float' arrangement to allow fuel as it gets used up. And that the 'float chamber' more or less remains full all the while the engine is running. So the 'hydrostatic pressure' on the jet always remains the same irrespective of whether the carb is being fed from a full tank or a full 12000 ltr tanker
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    Quote Originally Posted by Old Fox View Post
    The error here lies in the basic premise...."when we put 10 ltrs of fuel as compared to 1 lit the pressure in the jet is greater (using same principle as hygroscopic pressure) as the fuel is not pumped as in FI it only flows out of an overhead tap."

    The fuel goes into the 'float chamber' which is so named as there is a 'needle and float' arrangement to allow fuel as it gets used up. And that the 'float chamber' more or less remains full all the while the engine is running. So the 'hydrostatic pressure' on the jet always remains the same irrespective of whether the carb is being fed from a full tank or a full 12000 ltr tanker
    Thanks Sandeep Sir for throwing light on this!
    So would this mean that the mileage of a vehicle (assuming all other factors Air fuel mixture, carb setting to be common) will not vary whether we are filling say 2 litres or full tank while fueling up?
    Quench my thirst with gasoline!

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    ^^^ Nopes, as explained by Old Fox, amount of fuel filled will not make any difference wrt the carb. The chamber will obviously always remain around full. "Hygroscopic pressure" btw has no relation here! I don't think there there is any such term to being with.

    However, evaporation from tank, weight of the fuel, etc, can make a difference.

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    aah thanks Old fox for being the mythbuster of xbhp
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    Quote Originally Posted by Sarvajit View Post
    Thanks Sandeep Sir for throwing light on this!
    So would this mean that the mileage of a vehicle (assuming all other factors Air fuel mixture, carb setting to be common) will not vary whether we are filling say 2 litres or full tank while fueling up?
    No, Sarvajit, the quantity of fuel in the tank has no appreciable influence on the fuel consumption of the engine that it feeds.

    The 'float - needle' arrangement in the carb is analogous to the 'ball-valve' thing in overhead water tanks. When the fuel level inside the float chamber (where the jets are located) drops below a pre-set datum, the needle drops and allows fuel into the carb. As the level rises, the float rises and pushes the needle up to close the orifice through which the fuel is flowing in. And hydrostatic pressure acting on so small an area as the needle orifice cannot generate a downforce greater than the 'up-force' of the float due to the fuel below it. So whether there is 2 ltrs or 10 in the tank above it, the 'needle-valve' will only allow the required amount of fuel into the carb.

    And BTW, an increase in fuel flow due to increased hydrostatic pressure will in any case make the carb overflow which is exactly what happens when the needle valve seat gets worn out and fuel keeps dribbling past it even when the needle valve is pushed up by the float.
    Last edited by Old Fox; 06-02-2009 at 07:22 PM.
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    that was some piece of information.
    we all should thank Old Fox for being Old Fox....!
    Its not about the BHP or the CC, its about one common religion called Biking!!!

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