Pumping systems

A fuel injection pump for supplying fuel to an internal combustion engine comprising a plunger reciprocable within a bore, a cam for imparting inward movement to the plunger to displace fuel from the bore through an outlet, a throttle through which fuel can be supplied to the bore to effect outward movement of the plunger, governor means for adjusting the setting of said throttle whereby the amount of fuel delivered through the outlet can be varied, first valve means communicating with a point intermediate the throttle and the bore, said first valve means when opened lowering the pressure downstream of the throttle to a drain pressure, second valve means for controlling the pressure applied to the outer end of the plunger so that in use, it is maintained at a value which is higher than said drain pressure, the arrangement being that when said first valve means is opened the plunger will be moved inwardly by the pressure at its outer end, by an amount to ensure that it cannot be actuated by the cam, circuit means for controlling the operation of said first valve means, said circuit means receiving a fuel demand signal from said governor means and an engine speed signal, said circuit means operating to open said first valve means when the fuel demand signal is zero and the engine speed is above a predetermined value, said circuit means being arranged to cause closure of said first valve means when the speed falls to said predetermined value and if the governor means provides a fuel demand signal, and further circuit means operable to effect momentary opening of said throttle when the demand is zero and the engine speed has fallen to a further predetermined value less than said first mentioned predetermined value to prime the passages of the pump including the bore, and to move the plunger outwardly by an amount so that when said governor means provides a fuel demand signal to open the throttle, fuel will immediately be supplied by the pump.