Method and system for measuring and balancing cylinder air-fuel ratio

A method, comprising:

• storing at least a threshold amount of ammonia in a selective catalytic reduction (SCR) system;
• and
• balancing torque output of each cylinder of a multi-cylinder engine based on an offset of each cylinder determined while commanding engine operation at a series of lean air-fuel ratios (AFRs), in response to determining that the stored ammonia amount is at least the threshold amount;
• wherein exhaust gas from the engine remains above a first temperature threshold and below a second temperature threshold during the balancing.

A system, comprising:

• a spark ignition engine including a plurality of cylinders;
• a selective catalytic reduction (SCR) device coupled in an exhaust passage of the engine;
• and
• a controller with computer readable instructions stored in non-transitory memory that, when executed during engine operation, cause the controller to:
• operate in a lean imbalance testing mode responsive to determining that at least a threshold amount of ammonia is being stored in the SCR device;
• and
• identify a cylinder air imbalance based on a torque produced by each of the plurality of cylinders relative to a mean torque produced by all of the plurality of cylinders while operating in the lean imbalance testing mode.