Method and system for transitioning between lean and stoichiometric operation of a lean-burn engine

A method for transitioning an internal combustion engine between a first operating condition and a second operating condition, wherein the first and second operating conditions are characterized by combustion, in each of a plurality of engine cylinders, of a supplied air-fuel mixture having a first and second air-fuel ratio, respectively, and wherein one of the first and second air-fuel ratios is significantly lean of a stoichiometric air-fuel ratio and the other of the first and second air-fuel ratios is a stoichiometric air-fuel ratio, the method comprising:

• identifying at least two discrete sets of cylinders supplied with the air-fuel mixture at the first air-fuel ratio;
• sequentially stepping the air-fuel ratio of the air-fuel mixture supplied to each set of cylinders from the first air-fuel ratio to the second air-fuel ratio;
• and
• wherein the first air-fuel ratio is the lean air-fuel ratio and the second air-fuel ratio is the stoichiometric air-fuel ratio, the method further including:
• determining when the air-fuel ratio of the air-fuel mixture supplied to all but one set of cylinders has been stepped to the second air-fuel ratio;
• and
• stepping the air-fuel ratio of the air-fuel mixture supplied to the one set of cylinders to a third air-fuel ratio rich of a stoichiometric air-fuel ratio.

A system for controlling operation of a lean burn engine having a plurality of cylinders, each cylinder receiving a metered quantity of fuel from a respective fuel injector, and each cylinder receiving an ignition spark from a respective spark plug, the system comprising:

• a controller including a microprocessor arranged to operate the fuel injector for each cylinder to thereby individually control the air-fuel ratio of an air-fuel mixture supplied to each cylinder, wherein the controller is further arranged to transitioning the engine between a first operating condition and a second operating condition, the first operating condition being characterized by a first air-fuel ratio and second operating conditions being characterized by a second air-fuel ratio, one of the first and second air-fuel ratios being significantly lean of a stoichiometric air-fuel ratio and the other of the first and second air-fuel ratios being a stoichiometric air-fuel ratio;
• and wherein the controller is arranged to sequentially step the air-fuel ratio of the air-fuel mixture supplied to each of at least two cylinders from the first air-fuel ratio to the second air-fuel ratio;
• and
• wherein the controller is further arranged to determine when the air-fuel mixture supplied to each cylinder has been maintained at the second air-fuel ratio for a second predetermined time, and to change the air-fuel ratio of the air-fuel mixture supplied to at least one cylinder to a third air-fuel ratio rich of the stoichiometric air-fuel ratio.