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Diesel hybrid has a ‘software’ gearbox
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01/02/2007
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Software and control techniques replace clutches and gears in a new type of high-efficiency diesel hybrid drivetrain, writes Roger Bishop. Developed by a UK automotive engineering consultancy, it is said to be highly cost-effective compared with current systems and can be quickly implemented within the constraints of a compact European passenger car.
A demonstration vehicle (built within the UK government’s Ultra Low Carbon Car Challenge), delivers 85g of CO2/km compared with 121g/km for the already efficient standard vehicle, a 1.5litre, three-cylinder smart forfour Cdi.
Current passenger car production hybrids are either parallel or dual mode systems. Pure series hybrids offer high efficiency at low speeds but this greatly reduces as speed increases so their application tends to be limited to vehicles like inner city buses. Parallel hybrids have the internal combustion engine permanently coupled to the wheels so are more efficient at high speeds but are more difficult to optimise for low-speed duty cycles. Dual mode transmissions can operate in series or parallel modes but are more complex, larger and more expensive.
The solution being applied by UK automotive engineering consultancy Zytek is a dual-mode hybrid that eliminates these compromises by using sophisticated control strategies to replace clutches and complex epicyclic gearsets. This reduces component count, eliminates wearing friction surfaces, reduces packaging volume and substantially reduces manufacturing costs.
Engineers decided the new hybrid should be built around a small, high efficiency diesel engine using common rail injection and sophisticated after-treatment. They also work well in hybrid transmissions because the electric drive can be used to disguise turbo lag and give exceptional driveability. DaimlerChrysler was extremely supportive and provided a vehicle and technical data for the project.
Programme manager Neil Cheeseman decided to develop a bespoke hybrid transmission that would directly replace the existing gearbox without any changes to the vehicle body or powertrain mounting points. The dual mode architecture has two motors: astarter/generator permanently engaged with the engine and a traction motor. Both are specifically designed for their application and engaged with the drivetrain when required via a dog synchromesh system similar to that found in a manual transmission.
A sophisticated control system replaces the synchro cone by automatically matching the speed of the input shaft and the lay shaft to allow seamless shifts with zero torque interruption.
The starter/generator is a 43kW DC brushless permanent magnet device operating at speeds up to 3,600rpm. It is integrated into the gearbox bellhousing and sized to allow engine cranking at temperatures down to –40°C. It also provides torque assist when required to boost acceleration. Supplementing drive torque in this way allows a single very high gear to be engaged at low revs (1,600rpm) to keep the engine operating in its most efficient range. Cooling is provided by means of a water jacket integrated within the diesel engine’s cooling system.
The traction motor is a 50kW AC brushless permanent magnet device mounted on the end of the gearbox and capable of operating at up to 11,500rpm. Cooling is by a dedicated system shared with the inverter. Testing of the traction motor has shown a peak efficiency of more than 97%. Although it shares the same fundamental architecture as the starter/generator, Zytek has chosen a different internal construction to optimise it for low speed, high torque.
Connecting these devices with the final drive is a highly innovative gearbox developed by Xtrac that sits between the two electrical machines. Power inputs are received from the engine and starter/generator on one shaft and from the traction motor on a second shaft. These power inputs are distributed to the front wheels via the gearbox and the existing driveshafts. The gearbox shows a high mechanical efficiency normally expected of a manual transmission.
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.
The new gearbox is just 138mm long, allowing the hybrid transmission, including the gearbox and both electric motors, to fit the same space as the six-speed Getrag transmission it replaces.
Controller examines all the sensors
Key to obtaining optimum efficiency from a hybrid powertrain, says Zytek, is the interaction between the engine and hybrid control systems. This was behind the company’s decision to develop a completely new diesel control system. The DEMS (diesel engine management system) interfaces with every sensor and actuator on the standard engine and calculates optimum fuel rail pressure, 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 five injections per combustion event, depending on temperature, engine speed/load, and NVH requirements. It also communicates with other systems via the vehicle’s LIN and CAN buses, with a further CAN bus added to interface with the additional componentry.
Integrated into the DEMS control unit is a separate control board to manage the hybrid systems. 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 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.
Battery cells are cooled with cabin air blown through the battery compartment, exiting through new vents in the rear of the vehicle. As the vehicle is a plug-in hybrid, an onboard charger has been fitted that can recharge the hybrid battery in under seven hours from 0-100% state of charge.
The inverter supplies not only the 300V three-phase power for the motors, but also the 1,800W of 12V power for the vehicle’s electrical systems.
Battery charging is via three routes: regenerative braking; the engine-driven starter/alternator; and by plugging the vehicle in to a conventional domestic 13A electrical socket using a connection hidden under the rear number plate.
Additional technologies. An electrically driven water pump reduces parasitic losses by closely controlling the flow of coolant. And the brake vacuum pump has been supplemented with an electric pump so that braking assistance can be provided even when the engine is off. There is also a 6kW electric water heater to provide cabin warmth when the vehicle is operating as an electric vehicle, and a 5kW electric air conditioning compressor to provide cabin cooling requirements. The electric water heating element is also used to dissipate energy when the batteries are full and the vehicle is slowing using regenerative braking. This is said to help the engine and cabin heating system warm up more quickly and provide a more consistent brake feel.
Driving modes. Drivers can select one of three control calibrations: ‘Economy’ is intended for city driving – engine use is minimised and maximum power is limited to increase the range on electric power; ‘Drive’ is for mixed usage – full power is available and the engine will automatically stop/start when appropriate; ‘Sport’ disables the stop/start function and allows maximum power. The high regen mode is available in all of these calibrations, allowing the driver to store more regenerative energy.
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Author
Roger Bishop
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