GM wanted vehicles with the Two-Mode Hybrid powertrain in production within four years. For a conventional powertrain project, this timing would be typical, because the engineering team could start with an existing design. Early Two-Mode Hybrid technology had been developed by GM and Allison for use in transit buses now in operation in over 100 cities, but because the technology needed to be “leaner and greener” for passenger car and truck applications, the team had to start from scratch. “Creating well over a million lines of code on a compressed schedule, while ensuring that the control system meets GM’s high quality standards and complying with all legislative regulations and safety standards, was a tremendous challenge,” says Hubbard.

GM engineers also had to optimize the design for several competing objectives, including fuel economy, responsiveness, and driver comfort, while keeping the design flexible enough to use across a range of vehicles. To do that, GM engineers had to ensure that several complex systems worked together to optimize performance. “Integrating the engine, transmission, electric motors, and battery with the power management strategy is the key to hybrid system design,” explains Kent Helfrich, director of Powertrain Software Engineering at GM. “These systems are too complex to build separately and integrate in a final step.”

In addition, GM engineers needed to verify their designs before committing them to hardware.

GM used MathWorks tools and Model-Based Design to develop system models, verify designs using simulation, and generate production code for the Two-Mode Hybrid powertrain control system. This approach enabled GM engineers to explore multiple strategies, accelerate design iterations, minimize prototypes and rework, and verify the control system before hardware was available.

Using MATLAB^®, Simulink^®, and Stateflow^®, GM engineers designed the control system architecture and modeled all the control and diagnostic functions, including hybrid operating strategy, engine start-stop, active damping, shift execution, and propulsion safety.

At the same time, they created detailed plant models for all relevant physical systems, such as the engine, electric motors, battery, hybrid transmission, and vehicle dynamics. Running closed-loop simulations with the plant model, they verified the control system using standard city and highway driving fuel economy tests.

After generating production code for the engine, transmission, and hybrid energy management ECUs using Embedded Coder^®, they further verified the control systems using Simulink Coder™ and hardware-in-the-loop (HIL) simulators with the plant models and test cases from the non-real-time verification phase. Comparing the results of these tests enabled the engineers to identify ECU integration issues.

When the hybrid powertrain hardware became available, the team was able to move quickly from HIL to prototype vehicles for confirmation testing.

The Two-Mode Hybrid powertrain has won several awards, including Technology of the Year from Automobile magazine and Green Car of the Year from Green Car Journal.