Method for operating a spark ignited engine

A method for operating a spark ignited engine, including forming a combustible mixture by mixing generally homogeneously a first fuel and air and introducing this mixture into the at least one cylinder in an intake stroke, and compressing the combustible mixture with a piston in a compression stroke thereby introducing a part of the combustible mixture into a pre-chamber. During the intake and/or the compression stroke, a second fuel is introduced into the pre-chamber at an introduction-time before start of combustion, and the second fuel is of the same or different chemical composition and/or concentration with respect to the first fuel, and a spark ignites in the prechamber. An amount of second fuel and/or the chemical composition of second fuel introduced to the pre-chamber and/or spark timing of the pre-chamber and/or an in-cylinder charge temperature is chosen such that a desired duration of combustion can be achieved.