System and method for controlling an engine air-fuel ratio

A method for an engine, comprising:

• responsive to a change in a desired air-fuel ratio, adjusting an air-fuel ratio of the engine, where the engine is a dual fuel engine combusting diesel fuel and natural gas, to a determined level by:
• during a first condition, adjusting a speed of the engine;
• during a second condition, adjusting fuel flow of one or more of the diesel fuel and the natural gas to engine cylinders of the engine;
• and
• during a third condition, adjusting an engine bypass valve adapted to divert intake air away from the engine cylinders to a location external to the engine.

A system for an engine, comprising:

• a first intake air duct coupled to a first engine cylinder bank;
• a turbocharger including a compressor at least partially driven by a turbine, where the first intake air duct is coupled to an outlet of the compressor;
• a bypass passage coupled to the first intake air duct, upstream of the first engine cylinder bank, the bypass passage including an engine bypass valve configured to divert airflow from the first intake air duct to a location external to the engine;
• and
• a controller with computer readable instructions stored thereon for:
• adjusting a position of the engine bypass valve in order to deliver a determined air-fuel ratio to engine cylinders in response to one or more of a sensed change in load, an indication of knock, or an indication of misfire.