Transport trial

London buses are trialling a new kind of hybrid propulsion system that promises to save fuel and lower emissions.

The GM-Allison Hybrid EP-System combines direct power from an electronically controlled diesel engine with electric power from an energy storage system.

Transport for London (TfL) is trialling the buses as part of a study into the performance of different hybrid systems and their various specifiable components such as controls, batteries and transmissions.

The hybrid propulsion system has already been used in bus fleets throughout 115 cities worldwide.

Transport for London will trial 10 buses in total. Five of the hybrid buses are part of the East London Bus Group where they will carry passengers on the 276 route between Newham General Hospital and Stoke Newington. Metroline will operate a further five on the E8 route between Ealing Broadway and Brentford.

The trials contribute to the mayor’s target of a 60 per cent reduction in emissions across London by 2025.

In addition to its environmental benefits, the new hybrid buses could also be more pleasant for commuters and local residents because hybrid buses are less noisy.

The complete GM-Allison Ep40/50 systems consists of the Ev DriveTM module, which serves as the vehicle transmission, the dual power inverter module, the energy storage system, which is based on advanced nickel metal hydride batteries, two electronic control modules and the electronic driver interface with integrated display.

From stationary, the electrical starter motor draws power from the battery, mounted on the roof of the bus, to set the vehicle smoothly and almost silently into motion. Then as the vehicle picks up speed, the 340hp diesel engine comes into action, offering a direct power supply to the drivetrain as required and eventually feeding the battery with additional power. An automatic controller ensures that the battery is maintained at optimum charge. At cruising speed, the battery cuts out altogether and operates like a conventional drivetrain.

The key contributor to hybrid efficiency is the regenerative braking system, the recapture of energy normally lost during braking. When the driver brakes to slow the vehicle, the kinetic energy that is normally dissipated in the brakes as heat is instead transferred back into usable electric energy by an electric motor which, operating in reverse, acts as a generator and charges the battery. It is estimated that 40 per cent of the energy used to accelerate a bus with the GM Allison two-mode hybrid system comes from the energy saved during regenerative braking.