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A typical 10 tonne Midi-bus driveline includes an electric retarder device, located at the transmission output, which is used during vehicle braking thereby reducing service brake usage and wear. The retarder is activated via the brake pedal and operates down to speeds of 1 kph. To reduce cost and complexity it may be possible to delete the electric retarder when an infinitely variable transmission (IVT) powertrain is used in place of the standard 5 speed automatic transmission (5AT). The purpose of this study is to determine the amount of service brake usage, in terms of energy dissipation, for both the 5AT (with retarder) and IVT (without retarder) over the Millbrook London Transport Bus (MLTB) drive cycle. Validated MATLAB® dynamic models of both drivelines are used to calculate retarder and service brake usage.

A mild hybrid powertrain with crankshaft mounted integrated motor generator (IMG) and torque controlled infinitely variable transmission (IVT) has shown clear potential for fuel economy (FE) enhancement. It also makes significant driveability and performance improvements possible which are a condition for customer satisfaction and subsequent marketability. The hybrid powertrain supervisory control strategy presented here uses the energy recovered during braking events for power assist, hence improving FE and driveability compromises. This is achieved by operating the engine at its best brake specific fuel consumption (BSFC) point during steady state conditions without deteriorating the transient response as a result of the comparatively fast IMG torque response. This paper demonstrates the launch manoeuvre and general driveability improvements achieved in simulation with validated models.

The IVT control system can be viewed as having two distinct roles, namely that of steady state and transient torque management. Steady state management functions consist of setting engine power and transmission reaction torque to achieve optimal fuel economy, emissions and driver demanded wheel torque. The transient torque management function defines additional engine and transmission reaction torques, based on known inertia and plant responses, to manage the transition between these steady state operating points according to the driver's wishes, subject to plant constraints. This paper gives a basic overview of the IVT steady state control functions, leading onto a detailed description of the transient torque management function. The software functions are illustrated in block diagram format, using simulation verification data plots and vehicle data logs for validation purposes.