grimreaper Posted November 15, 2006 Share Posted November 15, 2006 why is it that when you get to just under 1/2 a tank of petrol the fuel gage goes down really quick? Quote Link to comment Share on other sites More sharing options...
Bobbeh Posted November 15, 2006 Share Posted November 15, 2006 Good question, similarly why does it take so long to go down after you just fill it up Quote Link to comment Share on other sites More sharing options...
ChrisSZ Posted November 15, 2006 Share Posted November 15, 2006 coz when the tank's full the car weighs more so you drive slower. Half empty it and things are lighter so it's play time (more pedal to the metal) :) LOL Quote Link to comment Share on other sites More sharing options...
JustGav Posted November 15, 2006 Share Posted November 15, 2006 coz when the tank's full the car weighs more so you drive slower. Half empty it and things are lighter so it's play time (more pedal to the metal) :) LOL Half-empty... half-full... all a matter of perspective. Quote Link to comment Share on other sites More sharing options...
jagman Posted November 15, 2006 Share Posted November 15, 2006 Its because the fuel guage works on heat created by a small current coil ,when full tank ,the heat is higher and retained hence the needle slow to drop at low levels heat is lower and dissipates into the air easier hence needle drops faster Quote Link to comment Share on other sites More sharing options...
leonv Posted November 15, 2006 Share Posted November 15, 2006 cause there is a magnet by the Empty light and it pulls it down when it passes the halfway mark, almost like the point of no return! How are we supposed to know honestly!! Leslie Quote Link to comment Share on other sites More sharing options...
Guest gzaerojon Posted November 15, 2006 Share Posted November 15, 2006 and another thing while we're on the subject of fuel. why is it a bad thing to run your tank to near empty, when the fuel pump pick up point is at the bottom of the tank?? Quote Link to comment Share on other sites More sharing options...
SupraStar 3000 Posted November 15, 2006 Share Posted November 15, 2006 I thought it was becasue the tank tapered towards the bottom like an hour-glass (egg timer) Quote Link to comment Share on other sites More sharing options...
RobSheffield Posted November 15, 2006 Share Posted November 15, 2006 I think its the act of gravity on the needle arm itself Quote Link to comment Share on other sites More sharing options...
Rayman Posted November 15, 2006 Share Posted November 15, 2006 and another thing while we're on the subject of fuel. why is it a bad thing to run your tank to near empty, when the fuel pump pick up point is at the bottom of the tank?? Because the less fuel in the tank the more the dregs will accumulate near that fuel pick up id say. More fuel means the crap is spread out. Quote Link to comment Share on other sites More sharing options...
Kranz Posted November 15, 2006 Share Posted November 15, 2006 The tank is sort of shaped like an egg on its side. The level sender is a float arm like the float in a toilet cistern. It runs a contact against a variable resistance, giving an output to the guage. When the tank is full the weight of the fuel causes the bottom of the tank to sag a little, increasing the tank's volume. As fuel is used the shape of the tank changes, and the sag reduces as the fuel is used. This reduces the volume of the tank and sort of offsets the drop in fuel level so that the sender can almost stay in the same position... Hence the needle hanging around the full mark for ages. There comes a point where the tank shape stops changing so much and the float starts to drop. At about half a tank the shape is back to normal and as fuel is used the level drops in the narrowing tank (lower half of the egg on its side shape), meaning that the level drops faster the lower it gets. So the last 1/4 disappear really fast! And you shouldn't run a tank really low as sediment in the bottom of the tank can get drawn into the pump inlet screen, blocking it and also blocking the fuel filter. Each time you run really low this blockage occurs more and more until it may cause a restriction. EDIT - Oh, and this saggy tank effect will happen more in summer than winter! Quote Link to comment Share on other sites More sharing options...
Kranz Posted November 15, 2006 Share Posted November 15, 2006 God, have I bored everyone to death? Quote Link to comment Share on other sites More sharing options...
jagman Posted November 15, 2006 Share Posted November 15, 2006 saggy tank theory made me laugh! revisit the bi metal strip /heat theory, all cars have this saggy tank then? some cars have a linear guage but they use a chip ,and it costs more to produce so a coil/ bi metal strip is used and because it uses heat which is retained ,it stops the needle bouncing up/down when the fuel sloshes about which would happen if the guage was a moving coil instrument or moving magnet instrument-you have your answer to the initial why? Quote Link to comment Share on other sites More sharing options...
Kranz Posted November 15, 2006 Share Posted November 15, 2006 Polyethylene tanks do go saggy with the weight of fuel jagman. Honest. I've seen it for myself on the Ford Focus. I spent 6 months designing the run dry system that stops them running out of fuel. Its possibly a stack up of things, resistance tolerances, arm positions, guages etc too. Quote Link to comment Share on other sites More sharing options...
jagman Posted November 17, 2006 Share Posted November 17, 2006 poly tanks may distort a bit , but the fuel level float and its pivot are fixed to the unit which drops in the top of the tank,so even if the tank sagged the fuel level would drop relative to the top of the tank so the guage would underread when full and over read when empty the opposite of what you now get,- back to the bi metal heat thing which has another benefit that the needle will not fluctuate when the cars voltage varies between battery voltage and alternator voltage . as a side note on instruments also of little value,the water temp guages are not linear so the distance between C and N is usually not the same temp difference between N and H , so if you show a reading over the normal setting car running hot and start pannicking it may be only a few degrees C a sign that all is not well but not imminent melt down about to happen,should it continue to rise then panic Quote Link to comment Share on other sites More sharing options...
chilli Posted November 17, 2006 Share Posted November 17, 2006 It's deliberate by design. When you are near empty you need more accuracy to see when you are about to run out When you have nearly a full tank do you care if it is 11 gallons or 12 - not really the scale is deliberately non-linear with more accuracy at the low end on every car I've ever driven/owned I think - it's like a standard design feature on analog fuel guages.. Quote Link to comment Share on other sites More sharing options...
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