A method of operating an internal combustion engine having pilot subchambers communicating with main combustion chambers, the internal combustion engine configured in use to deliver a main fuel injection of a maximum quantity of fuel to the main combustion chambers when the internal combustion engine is operated at maximum load. The method includes delivering a pilot fuel injection of at most 10% of the maximum quantity to the pilot subchambers, igniting the pilot fuel injection within the pilot subchambers, directing the ignited fuel from the pilot subchambers to the main combustion chambers, and delivering a main fuel injection of a main quantity of fuel to at least one of the main combustion chambers receiving the ignited fuel, with the main quantity being at most 10% of the maximum quantity.

A method of operating an internal combustion engine having pilot subchambers communicating with main combustion chambers, the internal combustion engine configured in use to deliver a main fuel injection of a maximum quantity of fuel to the main combustion chambers when the internal combustion engine is operated at maximum load. The method includes delivering a pilot fuel injection of at most 10% of the maximum quantity to the pilot subchambers, igniting the pilot fuel injection within the pilot subchambers, directing the ignited fuel from the pilot subchambers to the main combustion chambers, and delivering a main fuel injection of a main quantity of fuel to at least one of the main combustion chambers receiving the ignited fuel, with the main quantity being at most 10% of the maximum quantity.

A plasma jet plug includes: a tubular insulator having an axial hole; a center electrode disposed inside the axial hole; metal shell disposed on an outer circumference of the insulator; and an orifice electrode electrically connected to the metal shell and disposed on a front side of the insulator. A plasma generating cavity is formed by a surface of the center electrode, an inner surface of the insulator, and an inner surface of the orifice electrode. In the plasma jet plug, a shortest path length D1 of a surface path is greater than or equal to 5 times an aerial gap G, where the surface path extends, inside the cavity, from a surface of the center electrode via an inner surface of the insulator to an inner surface of the orifice electrode, and the aerial gap G is a shortest distance between the center electrode and the orifice electrode.