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Methods and systems are provided for synergizing the benefits of engine water injection in a hybrid vehicle system. During engine operation, in response to a change in driver demand, the controller may opt to switch between water injection states while using stored power assist. The selection may be

Methods and systems are provided for synergizing the benefits of engine water injection in a hybrid vehicle system. During engine operation, in response to a change in driver demand, the controller may opt to switch between water injection states while using stored power assist. The selection may be based on the combination of water injection and stored power offset that provides the highest engine efficiency.

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1. A method for a hybrid vehicle including an engine configured with water injection and a modular hybrid transaxle (MHT), comprising: for a desired power level, comparing a first fuel economy without water injection and a first amount of stored power offset from an energy storage system to a second

1. A method for a hybrid vehicle including an engine configured with water injection and a modular hybrid transaxle (MHT), comprising: for a desired power level, comparing a first fuel economy without water injection and a first amount of stored power offset from an energy storage system to a second fuel economy with water injection at a first adjusted engine speed-load and a second amount of stored power offset;responsive to the second fuel economy exceeding the first fuel economy, and a higher than threshold water availability, injecting an amount of water into the engine and changing to the first adjusted engine speed-load; andresponsive to the first fuel economy exceeding the second fuel economy or a lower than threshold water availability, operating the engine without water injection, and changing an engine speed-load to a second adjusted engine speed-load. 2. The method of claim 1, further comprising, responsive to the second fuel economy exceeding the first fuel economy, and lower than threshold water availability, maintaining engine operation without water injection, changing the engine speed-load to the second adjusted engine speed-load, and using stored power from the energy storage system to meet a deficit between engine output and the desired power level. 3. The method of claim 1, wherein the first and second amounts of stored power offset are based on a state of charge of the energy storage system and the desired power level. 4. The method of claim 1, wherein the first adjusted engine speed-load is based on knock limit and friction limit of engine operation with water injection, and the second adjusted engine speed-load is based on knock limit and friction limit of engine operation without water injection. 5. The method of claim 1, wherein changing to the first or the second adjusted engine speed-load includes transitioning from a default engine speed-load via adjustments to a speed ratio of the MHT, wherein the first adjusted engine speed-load includes a lower engine speed and a higher engine load as compared to the default engine speed-load, and the second adjusted engine speed-load includes a higher engine speed and a lower engine load as compared to the default engine speed-load. 6. The method of claim 1, wherein an engine power output with water injection enabled and the first adjusted engine speed-load is the same as an engine power output with water injection disabled and the second adjusted engine speed-load. 7. The method of claim 1, wherein the lower than threshold water availability is due to one of a lower than threshold level of water in a water reservoir of a water injection system coupled to the engine, a lower than threshold quality of water in the water reservoir, and degradation of a component of the water injection system, the component including a water injector and a water pump. 8. A method for a hybrid vehicle, comprising: propelling the vehicle via an engine operating with a water injection state selected based on driver demand and water availability on-board the vehicle; andin response to a change in driver demand, adjusting the water injection state based on each of the change in driver demand, a state of charge of an energy storage system, and the water availability, wherein the adjusting includes selecting between maintaining a current water injection state and transitioning to an alternate water injection state based on fuel economy in each of the current and alternate water injection states with a stored power offset based on the state of charge, wherein the current water injection state includes one of a water injection enabled state and a water injection disabled state, and wherein the alternate water injection state includes the other of the water injection enabled state and the water injection disabled state. 9. The method of claim 8, wherein the stored power offset applied with the current water injection state is different from the stored power offset applied with the alternate water injection state, and wherein the stored power offset includes one of an increase in the state of charge via charging of the energy storage system using engine torque, and a decrease in the state of charge via discharging of stored power to supplement the engine torque. 10. The method of claim 8, wherein the selecting includes: transitioning to the alternate water injection state when the fuel economy associated with the alternate water injection state with the stored power offset is higher than the fuel economy associated with the current water injection state with the stored power offset by more than a threshold amount, and a higher than threshold level of water being available in a water reservoir; andmaintaining the current water injection state when the fuel economy associated with the alternate water injection state with the stored power offset is lower than the fuel economy associated with the current water injection state with the stored power offset, or higher by less than the threshold amount, or a lower than threshold level of water being available in the water reservoir. 11. The method of claim 8, further comprising, while maintaining the current water injection state, compensating for a deficit in the driver demand via torque from a motor coupled to the energy storage system. 12. The method of claim 8, further comprising, in response to maintaining the current water injection state, operating the engine with a first adjusted speed-load profile while maintaining a power level of the vehicle using motor torque, and in response to transitioning to the alternate water injection state, operating the engine with a second adjusted speed-load profile while maintaining the power level of the vehicle using motor torque. 13. The method of claim 12, wherein the first adjusted engine speed-load profile is based on a knock limit of engine operation with the current water injection state and the second adjusted engine speed-load profile is based on a knock limit of engine operation with the alternate water injection state. 14. The method of claim 12, wherein the current water injection state includes the water injection enabled state and the first adjusted engine speed-load includes a lower than default engine speed and a higher than default engine load, and wherein the alternate water injection state includes the water injection disabled state and the second adjusted engine speed-load includes a higher than default engine speed and a lower than default engine load. 15. The method of claim 12, wherein the vehicle includes a modular hybrid transaxle (MHT) and wherein operating with the first adjusted speed-load profile includes selecting a first speed ratio of the MHT that matches the first adjusted speed-load profile, and wherein operating with the second adjusted speed-load profile includes selecting a second, different speed ratio of the MHT that matches the second adjusted speed-load profile. 16. The method of claim 12, wherein a power level of the engine is maintained at each of engine operation in the current water injection state with the first adjusted speed-load profile and engine operation with the alternate water injection state with the second adjusted engine speed-load, and wherein the power level is a powertrain output of the engine determined as a product of engine load and engine speed. 17. The method of claim 8, wherein operating the engine in the water injection enabled state includes injecting an amount of water into the engine via one or more of direct injection into an engine cylinder via a direct water injector, port injection into an intake port upstream of an intake valve via a port water injector, and central injection into an intake manifold, upstream or downstream of an intake throttle, via a central water injector. 18. A vehicle system, comprising: an engine;a motor powered via an energy storage system;a fuel injector for delivering fuel from a fuel tank to the engine;a water injector for delivering water from a water reservoir to the engine;a modular hybrid transaxle (MHT) coupling the engine and the motor to vehicle wheels, the MHT having a plurality of speed ratios; anda controller with computer readable instructions stored on non-transitory memory for: estimating a first efficiency associated with operating the engine with the water injector deactivated and a first power offset from the energy storage system;estimating a second efficiency associated with operating the engine with the water injector activated and a second, different power offset;if the second efficiency is higher than the first efficiency, and a water level in the water reservoir is higher than a threshold, activating the water injector, adjusting an output of the motor to provide the second power offset, and selecting one of the plurality of speed ratios of the MHT to operate the engine with a first modified engine speed-load profile; andif the second efficiency is lower than the first efficiency, or the water level in the water reservoir is lower than the threshold, deactivating the water injector, adjusting the output of the motor to provide the first power offset, and selecting another one of the plurality of speed ratios of the MHT to provide a second modified engine speed-load profile, different from the first modified engine speed-load profile. 19. The system of claim 18, wherein the first modified engine speed-load profile is based on an engine knock limit and engine friction when operating with water injection, wherein the second modified engine speed-load profile is based on an engine knock limit and engine friction when operating without water injection, wherein the first modified engine speed-load profile has a lower engine speed and a higher engine load than the second modified engine speed-load profile, and wherein an output power of the engine with the water injector activated and the first modified engine speed-load profile is the same as an output power of the engine with the water injector deactivated and the second modified engine speed-load profile.