US Patent 9644585 Auxiliary pressure relief valve in single piston fuel pump

A fuel pump has a low pressure fuel inlet (40) and a single piston pumping chamber (10) for supplying high pressure fuel through a pump outlet to an inlet side of a common rail (16). The fuel pump comprises a primary pressure relief valve (12) having an inlet side in hydraulic communication with the inlet side of the common rail (16) and an outlet side in fluid communication with the pumping chamber (10); and an auxiliary pressure relief valve (19) having an inlet side (35) in hydraulic communication with the pumping chamber (10) and an outlet side in direct fluid communication with a fuel passage (37) at the low pressure of the pump inlet (40).