Methods and systems for improving hybrid vehicle efficiency

A method for operating a driveline of a vehicle, the driveline comprising an engine and an electric machine, the method comprising:

• adjusting a torque output of the engine in response to a driver demand torque from a driver of the vehicle;
• determining a torque difference between the driver demand torque and the torque output of the engine;
• determining an operating range of the electric machine, in which efficiency of a torque output of the electric machine is less than a threshold, the operating range defined by a present speed and torque output of the electric machine and having a first, positive torque limit defining a positive extent of the operating range and a second, negative torque limit defining a negative extent of the operating range;
• in response to the torque difference being less than the second torque limit, adjusting the torque output of the electric machine to provide the torque difference between the driver demand torque and the torque output of the engine;
• in response to the torque difference being greater than the second torque limit and less than zero, maintaining the torque output of the electric machine at the second torque limit;
• in response to the torque difference increasing from being greater than the second torque limit and less than zero to being greater than zero and less than the first torque limit, transitioning the torque output of the electric machine from the second torque limit to the first torque limit and maintaining the torque output of the electric machine at the first torque limit;
• and
• in response to the torque difference increasing above the first torque limit, adjusting the torque output of the electric machine to provide the torque difference between the driver demand torque and the torque output of the engine.

A driveline system for an engine-driven vehicle, comprising:

• an engine;
• an electric machine;
• a driveline disconnect clutch selectively coupling the engine and the electric machine;
• and
• a controller including executable instructions stored in non-transitory memory for:
• adjusting the engine to output a torque in response to a driver demand torque from a driver of the vehicle;
• determining a torque difference between the driver demand torque and the torque output of the engine;
• determining an operating range of the electric machine, in which efficiency of a torque output of the electric machine is less than a threshold, the operating range defined by a present speed and torque output of the electric machine and having a first, positive torque limit defining a positive extent of the operating range and a second, negative torque limit defining a negative extent of the operating range;
• in response to the torque difference being less than the second torque limit, adjusting the torque output of the electric machine to provide the torque difference between the driver demand torque and the torque output of the engine;
• in response to the torque difference being greater than the second torque limit and less than zero, maintaining the torque output of the electric machine at the second torque limit;
• in response to the torque difference increasing from being greater than the second torque limit and less than zero to being greater than zero and less than the first torque limit, transitioning the torque output of the electric machine from the second torque limit to the first torque limit and maintaining the torque output of the electric machine at the first torque limit;
• and
• in response to the torque difference increasing above the first torque limit, adjusting the torque output of the electric machine to provide the torque difference between the driver demand torque and the torque output of the engine.