High pressure pump and method of reducing fluid mixing within same

A fuel system for use with high pressure fuel rail, the fuel system comprising:

• a fuel source;
• a lubrication fluid source having lubrication fluid at a continuous lubrication fluid pressure;
• a low pressure pump 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 that is less than the lubrication fluid pressure;
• a high pressure pump including a high pressure pump inlet fluidly coupled to the low pressure pump outlet, a high pressure pump outlet fluidly coupled to the high pressure fuel rail, a lubrication fluid inlet fluidly coupled to the lubrication fluid source, 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, the piston having a first end exposed to the fuel and a second end exposed to the lubrication fluid;
• a first annulus within the piston bore fluidly coupled to the lubrication fluid inlet;
• and
• a second annulus within the piston bore fluidly coupled to the low pressure pump inlet;
• and
• at least one fuel injector fluidly coupled to the high pressure pump outlet.