The invention provides a method for operating an internal combustion piston engine, comprising - introducing (S1, IOS) air into a cylinder (3) of the engine, - compressing the air in a first compression stroke (CS1) of the cylinder (3), - injecting (S3, I1) a first fuel into the cylinder (3), - allowing (S4) a first combustion of the injected first fuel with a portion of the oxygen in the compressed air as oxidant, to produce power to a crankshaft (4) of the engine in a first power stroke (PS1) succeeding the first compression stroke (CS1), and to produce residues including oxygen, and - compressing (S5) the residues in a second compression stroke (CS2) succeeding the first power stroke (PS1), - characterized by injecting (S5, I2) into the cylinder (3) a second fuel of a type which is different from that of the first fuel, and allowing a second combustion of the second fuel with the oxygen of the residues as oxidant to produce power to the crankshaft (4) of the engine in a second power stroke (PS2) succeeding the second compression stroke (CS2), and - by controlling the amount of second fuel injected in the step of injecting the second fuel so as for the second combustion to be a stoichiometric combustion.