World’s first affordable diesel hybrid powertrain

Connecting these devices with the final drive is a new, highly innovative gearbox developed by Xtrac to Zytek’s specification. Power inputs are received from the engine and starter/generator on one shaft and from the traction motor on a second shaft. These are blended under electronic control and distributed to the front wheels via the existing driveshafts.

When the vehicle pulls away, torque is delivered only from the electric traction motor. As speed increases, electric and diesel drive blend seamlessly until torque from the electric drive is reduced to zero. A very high input drive ratio allows the diesel engine to operate within the most efficient part of its power curve, with a low engine speed allowing efficient combustion and keeping frictional losses to a minimum. The broad power range of the dual mode system, combined with the phasing of electric and diesel drive at the ends of the vehicle speed range, allows the gearbox to operate with just two ratios - one for diesel drive, one for electric drive - without compromising NVH or economy.

“Achieving the target length, with the required inputs and outputs, was a real challenge,” says Robin Price, Business Development Manager with Xtrac. “We met Zytek’s specification through some very sophisticated CAE analysis and use of our own X36 gearsteel to allow smaller, lighter components.” The firm’s experience designing compact, high-power gearboxes such as that in the 2006 Le Mans winning Audi ensures that the solution will also be highly durable.

The new gearbox is just 138mm long, allowing the highly integrated hybrid transmission, including the gearbox and both electric motors, to package in the same space as the six speed Getrag automated manual transmission it replaces.

Powerful vehicle control

Key to obtaining the optimum efficiency from a hybrid powertrain is the interaction between the engine and hybrid control systems, and it was for this reason that Zytek elected to develop a brand new diesel control system. Taking the place of the existing unit on the car, the Zytek DEMS (Diesel Engine Management System) interfaces with every sensor and actuator on the standard engine and calculates the optimum fuel rail pressure, EGR (Exhaust Gas Recirculation) rates, turbo boost pressure, swirl valve position, and ultimately fuel injection quantity, timing and number of injections. The control system runs up to 5 injections per combustion event, depending on temperature, engine speed & load, and NVH requirements.

The Zytek DEMS also communicates with other systems via the standard vehicle’s LIN and CAN databusses, with a further CAN bus added to interface with the additional componentry. Building on the company’s history of developing petrol engine control systems, DEMS was developed and calibrated by Zytek in less than 14 months. The calibration enables the vehicle to exceed Euro IV emissions regulations without any other modifications to the engine beyond the fitment of a particulate trap.

Integrated into the DEMS control unit is a separate control board to manage the hybrid systems on the vehicle. In this configuration, the unit is referred to as the EHCM (Engine & Hybrid Control Module). To calculate the optimum blend of diesel and electric torque, the EHCM compares the throttle position, representing the driver’s demand for power, with factors such as battery state of charge, vehicle speed and safety inputs (required to validate the torque request, part of the safety system fitted to the vehicle). When decelerating, the EHCM calculates the optimum blend of friction and regenerative braking based on the position of the brake pedal. When only low rates of retardation are required, only regenerative braking is used. For higher rates of retardation, regenerative braking is blended with conventional friction braking using the vehicle’s standard braking system.

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