A method of adjusting operation of an internal combustion engine includes injecting fuel into cylinders of the internal combustion engine (first fuel operation); obtaining a first fuel exhaust temperature profile during the first fuel operation; injecting two fuels into the cylinders in a duel fuel operation; obtaining a duel fuel exhaust temperature profile; and adjusting the injection quantity and/or an injection timing of one fuel in a cylinder(s), based on a difference between the first fuel exhaust temperature profile and the duel fuel exhaust temperature profile. Other methods of operating with single fuel and using sensors other than exhaust temperature sensors are disclosed.

Methods and systems are provided for actively heating pistons in combustion chambers to decrease a torque imbalance in an engine. In one example, a method for operation of an engine includes determining a variation between compression ratios in a first combustion chamber and a second combustion chamber and operating a piston heating system to apply a targeted amount of heat to a first piston assembly based on the variation between the compression ratios, the first piston assembly including a first piston positioned within the first combustion chamber.

A vehicle includes an internal combustion engine having a plurality of cylinders, a sensor configured to generate a sensor signal correlated with an air-fuel ratio of first and second cylinders of the plurality of cylinders, and a controller in communication with the sensor and programmed to monitor the air-fuel ratio of the first cylinder in response to an indication that air-fuel ratio measurements of the first cylinder satisfy predetermined criteria for current engine operating conditions while air-fuel ratio measurements of the second cylinder do not satisfy the predetermined criteria for the current engine operating conditions.

Methods and systems are provided for diagnosing a degraded fuel injector delivering undesired additional fuel in a variable displacement engine. In one example, a method includes, responsive to an indication of a cylinder air-fuel imbalance, deactivating a subset of cylinders of a multi-cylinder engine, performing a power balance test to determine an output of each cylinder after a duration of deactivation, and indicating that a deactivated cylinder has a degraded fuel injector responsive to the output being lower than a threshold output.

Methods and systems are provided for measuring exhaust gas recirculation (EGR) distribution among individual engine cylinders. In one example, a method may include fluidly coupling a plurality of intake runners of an engine to a vacuum pump, diverting a portion of intake charge gas from the intake runner to a gas composition sensor with the vacuum pump, measuring an oxygen concentration of the diverted intake charge portion with the gas composition sensor, and estimating an EGR concentration of the intake charge based on the measured oxygen concentration.

Methods and systems are provided for diagnosing a degraded fuel injector delivering undesired additional fuel in a variable displacement engine. In one example, a method includes, responsive to an indication of a cylinder air-fuel imbalance, deactivating a subset of cylinders of a multi-cylinder engine, performing a power balance test to determine an output of each cylinder after a duration of deactivation, and indicating that a deactivated cylinder has a degraded fuel injector responsive to the output being lower than a threshold output.

Methods and systems are provided for learning a cylinder-to-cylinder air variation. During conditions when a PFDI engine is operated in a port-injection only mode, prior to port fuel injection, a direct-injection fuel rail pressure may be lowered via direct-injection. Then, prior to a spark event in a port-injected cylinder, the direct-injector may be transiently opened to use the rail pressure sensor for estimating a cylinder compression pressure, and inferring cylinder air charge therefrom.

A method determines a sensor error of a sensor in an exhaust gas system of a motor vehicle. One step of the method involves determining at least one actual sensor signal of the sensor. Another step of the method involves determining at least one target sensor signal of the sensor by means of a model. A further step of the method involves determining the sensor error of the sensor according to a deviation between the actual sensor signal of the sensor and the target sensor signal of the sensor.

Methods and systems are provided for diagnosing a degraded fuel injector delivering undesired additional fuel in a variable displacement engine. In one example, a method includes, responsive to an indication of a cylinder air-fuel imbalance, deactivating a subset of cylinders of a multi-cylinder engine, performing a power balance test to determine an output of each cylinder after a duration of deactivation, and indicating that a deactivated cylinder has a degraded fuel injector responsive to the output being lower than a threshold output.

Methods and systems are provided for actively heating pistons in combustion chambers to decrease a torque imbalance in an engine. In one example, a method for operation of an engine includes determining a variation between compression ratios in a first combustion chamber and a second combustion chamber and operating a piston heating system to apply a targeted amount of heat to a first piston assembly based on the variation between the compression ratios, the first piston assembly including a first piston positioned within the first combustion chamber.