Dual fuel system and operating strategy metering fuel via varied fuel pump output

A method of operating an engine system comprising:

• feeding a first fuel from a fuel pump into a fuel injector;
• feeding a second fuel into the fuel injector;
• injecting a first fuel charge pressurized in the fuel injector and containing the first fuel and the second fuel into a cylinder in an engine;
• injecting a second fuel charge pressurized in the fuel injector and containing the first fuel and the second fuel into the cylinder in the engine;
• and
• varying an output of the fuel pump to vary an amount of the first fuel in the second fuel charge relative to an amount of the first fuel in the first fuel charge.

A fuel injector comprising:

• an injector housing including a nozzle having formed therein a plurality of nozzle outlets, a first fuel passage extending from a first fuel inlet, and a second fuel passage extending from a second fuel inlet;
• a plunger movable in a plunger cavity fluidly connected to the second fuel passage;
• a spill valve positioned fluidly between the plunger cavity and the second fuel inlet;
• and
• the fuel injector further having formed therein a combined-fuel outlet passage, a nozzle check movable to open and close the combined-fuel outlet passage to the plurality of nozzle outlets, and a check valve separate from the nozzle check and positioned at least partially in the first fuel passage.