UK-based automotive engineering specialist Zytek has announced its involvement in GM’s hybrid pickup programme.

The firm’s involvement began at the project’s inception, when the company, which has extensive experience in engine control technology and electric drivetrains, was commissioned to conduct a control systems architecture study to identify the best way to manage GM’s advanced hardware design. The essence of the engineering problem was to deliver the environmental benefits of a hybrid configuration whilst retaining all of the existing vehicle performance and driveability characteristics, including towing capability, as demanded by US consumers.

After successfully completing the study, GM’s hybrid engineering team elected to continue working with Zytek on the complete control systems development and integration process that would ultimately deliver a production vehicle.

Zytek’s engineering team worked closely with GM to help its engineers implement the necessary changes to the base engine and transmission control system (PCM), including the definition and application of hybrid control algorithms to maximise the effective use of the electric machine and energy storage capability.

The GM hybrid, available for the extended cab versions of the 2004 Chevrolet Silverado and GMC Sierra, is a ‘mild’ hybrid design using a comparatively small electric machine – a 13kW ISAD (Integrated Starter-Alternator-Damper) from Continental and GM’s Vortec 5300 5.3litre V8 engine. The hybrid’s energy storage pack and battery management electronics are stored beneath the vehicle’s rear bench seat.

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The hybrid pickup achieves fuel savings of 12% in normal use with emissions performance equivalent to the high standards set by the original vehicle.

Fuel savings come from three principal areas. The highly developed stop / start capability of the hybrid configuration allows the engine to be shut off entirely when the vehicle is at slow speeds or stationary; the fuel supply is also shut off in a wider operating envelope of vehicle deceleration with closed throttle, whilst EM applied torque during engine braking re-charges the batteries. Further fuel economy is gained by sophisticated control of the conventional automatic transmission including maximising the efficiency gains of a locked up torque converter.

For Zytek’s engineering team, the biggest challenge was ensuring that, with all the extra complexity in the drivetrain hardware and controls, the hybrid truck still drove like its conventionally-powered counterpart. For the user, the only indication that they are driving a hybrid vehicle is the zero RPM indicated on the instrument pack along with the silence as the engine stops when the truck becomes stationary.

“We needed to be very careful that the vehicle didn’t recover too much energy during coast,” said Neil Heslington, programme manager at Zytek. “US drivers won’t accept pronounced engine braking.”

In operation, the hybrid functionality is controlled by the Zytek-designed hybrid control module (HCM). This evaluates the vehicle’s state and issues instructions to the powertrain control module (PCM) which controls the petrol engine and transmission, and Continental’s starter-generator control module (SGCM) which controls the ISAD and power conversion tasks. Battery management is handled by the energy storage control module (ESCM) also from Continental, which communicates the state-of-charge and effective state of health of the battery bank.

“Control system integration is vital in a successful hybrid implementation “said Heslington. “The complex control solution needed to cover all possible usage scenarios required considerable technical effort. Energy management strategies that work effectively in all environmental conditions along with transparent driver interfaces take some developing, for example lead acid batteries don’t always accept the desired levels of power, so systems have to work to create the optimum conditions. Also engine start, for example, occurs when the driver begins to lift their foot from the brake, before they touch the throttle.”

The power and flexibility of the ISAD was used to overcome many of the driveability and NVH issues associated with the hybrid powertrain. Inevitably, operation of a hybrid vehicle involves many additional driveline torque reversals as the electric machine switches from motoring to generating as the vehicle acceleration changes. The ISAD and the ICE were used intelligently during these reversals, to improve the overall smoothness of the powertrain system.

The potential to generate extra electrical power offered by the electric motor used in the hybrid is put to good use elsewhere on the vehicle, In addition to the normal 12V electrical system the 42V energy storage system supports an electro-hydraulic power steering pump and an inverter providing power for four 120V AC power points – two in the cab and two in the truck bed – which allow the user to run power tools or domestic appliances directly from the truck while in the field.

Zytek’s experience with hybrid transmission systems goes back to the mid 1990s. It engineered the revolutionary Panoz Q9 hybrid electric racing car, which campaigned successfully in the US during 1998 and has built some of the world’s most efficient full-electric vehicle transmissions.