Patent ID: 8261718

Claim:
A compound pump assembly for use with a fuel source, the compound pump assembly comprising: a housing; a low pressure pump disposed in the housing and including a low pressure pump inlet fluidly coupled to the fuel source and a low pressure pump outlet, the low pressure pump having a continuous low pressure pump inlet pressure at the low pressure pump inlet; a high pressure pump disposed in the housing and including a high pressure pump inlet fluidly coupled to the low pressure pump outlet, a lubrication fluid inlet, a high pressure pump outlet, and a piston bore fluidly coupled to the high pressure pump inlet and the high pressure pump outlet; a piston moveable within the piston bore; a first annulus formed in the piston bore; a second annulus formed in the piston bore and spaced from the first annulus, the second annulus being fluidly coupled to the low pressure pump inlet; and a lubrication fluid source fluidly coupled to the lubrication fluid inlet and the first annulus, the lubrication fluid having a continuous lubrication fluid pressure that is greater than the low pressure pump inlet pressure.