Rich Allott Posted June 16, 2011 Share Posted June 16, 2011 The valve which controls the flow of fuel is in the pump nozzle. The pump nozzle is connected to the pump motor by a flexible hose (which expands slightly if the contents are under pressure.) Assume that (and I'm making up the figures; I've no idea what the delivery pressure is) the pressure in the hose (the pressure at the nozzle) is 17psi before you open the nozzle. You put the nozzle in the tank, press the trigger, the valve in the nozzle begins to open. The pressure in the hose drops as fuel flows into the tank. The pump motor starts to turn to increase the pressure in the hose and pump more fuel. When the pump motor starts to turn, the dials showing the quantity of fuel dispensed and the cost start to turn. As the pressure in the hose falls when the nozzle is opened, a few ccs of 'free' fuel will be dispensed before the pump motor starts to turn. (Petrol costs about 1 penny for 7cc.) When you release the trigger on the nozzle, the flow of fuel from the nozzle stops - but the pressure in the hose is lower than it was when you first put the nozzle in the tank, so the pump motor keeps turning (and the counters keep turning) until sufficient fuel (apparently about about 14cc) has been pumped into the hose to bring the pressure back up to 17psi. The fuel pump may charge you for a penny or two's worth of fuel after you close the nozzle, but it didn't charge you for the first penny or two's worth when you opened it. I read this and thought of James May from Top Gear....... Link to comment Share on other sites More sharing options...
crookesey Posted June 16, 2011 Share Posted June 16, 2011 So what, who cares? Anyone who does, needs to put in his/her order for a life. Link to comment Share on other sites More sharing options...
bookbinder Posted June 16, 2011 Author Share Posted June 16, 2011 So if this happens with pressure in the pump sending it over by 1 or 2 pence why can i get the exact amount at morrisons down meadowhead that never runs over . Link to comment Share on other sites More sharing options...
Total Chaos Posted June 16, 2011 Share Posted June 16, 2011 So if this happens with pressure in the pump sending it over by 1 or 2 pence why can i get the exact amount at morrisons down meadowhead that never runs over . Because they are probably giving you 1 or 2p less petrol,making it look like you got bang on say £30.I am guessing tho. Link to comment Share on other sites More sharing options...
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