Anyone know of a website where I can find out how mechanical fuel pumps work or point me to a decent book?
{Header made less vague - DD}
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have you tried using a search engine ?
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en.wikipedia.org/wiki/Fuel_pump
home.howstuffworks.com/how-to-repair-small-engines...m
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An eccentric on the camshaft moves a diaphragm up and down - this movement combined with one-way valves produces pulses of fuel.
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But the clever bit is that there is a deliberate degree of lost motion between the lever and the diaphragm. This means that if the engine does not call for fuel, the diaphragm remains fully displaced and is not moved again after the first few strokes.
The diaphragm return spring defines the fuel pressure in conjunction with the active area of the diaphragm. As fuel is used by the engine, the diaphragm starts to relax under the influence of its spring and is once again moved by the lever - but only far enough to make up for the volume of fuel used.
This cunning scheme means that the supply of fuel is provided at an almost constant pressure (most people only see these pumps running into an "open circuit") and the linkage is only active to the extent required to make good the volume of fuel used per stroke.
Hope this helps, I had a bit of trouble explaining this. Take one apart - all will be self evident.
659.
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>>Take one apart - all will be self evident.
Mmm not easy to explain without a piccie.
The way I view it is that the only positive drive from the engine/camshaft or electro-magnetic actuator if you think of an old electric SU pump is to retract the diaphragm, making the volume of fuel trapped in the pump itself as large as possible.
There is no direct drive pushing the diaphragm forward to squeeze the fuel out of the pump, the only part that does any actual pumping is the return spring, and hence delivery pressure is largely independent of engine speed.
The were some lift pumps (AC?) on some older diesels which you could feel the diaphragm not returning as pressure built when you worked the lever to prime the system, and closed a bleed valve while still priming.
Number_Cruncher
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My Triumph has one with a lever underneath, for priming the petrol if the car has been unused for a while, to reduce futile engine cranking.
The first few strokes produce a satisfying gurgle of fuel, then the lever goes completely loose with no resistance at all.
There are electric ones working on the same principle. They do a rapid series of clicks, diminishing in speed, and finally stopping. Impatient drivers try and start before the clicking stops, discerning ones wait for the petrol to be delivered first.
I still instinctively wait for the clicking to stop even with a modern injection engine, although I know it is just the ECU self-testing, not the fuel pressure building up.
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