Transport for London (TfL) is trialling the buses as part of an extensive long-running study into the performance of different hybrid systems and their various specifiable components such as controls, batteries and transmissions. Five of the GM-Allison-equipped buses are with 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 roll-out of these 10 new GM-Allison-equipped buses will contribute to the Mayor's target of a 60 per cent reduction in emissions across London by 2025. Londoners will notice that hybrid buses are also significantly quieter than diesel buses, making bus travel more pleasant for passengers and reducing noise for local residents along bus routes. TfL currently operates more than 2,500 buses equipped with the smooth fully automatic transmissions across London.
The complete GM-Allison Ep40/50 systems consists of the Ev DriveTM module (which serves as the vehicle transmission) the Dual Power Inverter Module (DPIM), the Energy Storage System (ESS) based on advanced Nickel Metal Hydride (NiMH) batteries –, two electronic control modules and the electronic driver interface with integrated display. This unique hybrid architecture combines the efficiency of a direct mechanical connection between engine and drive train of the parallel hybrid system with the advantages in engine management typical of serial hybrid design.
The real invention is the EV DriveTM module which is designed with a concentric arrangement of gearing (planetary gear sets) and two electric machines. This module combines (torque blend) electrical machine power with engine power. Thanks to the unique electrical and mechanical integration, the EV DriveTM does not have fixed gear ratios as does a typical transmission, but its gear, speed and torque ratios are infinitely and continuously variable.
This module forms a Hybrid Electrically Variable Transmission (HEVT). An Electrically Variable Transmission has the potential to combine the continuous control and urban drive cycle efficiency of the series hybrid with the high power capability and high efficiency of the parallel hybrid.
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 340 hp 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. This is why greatest benefits of the technology are experienced in lower speed, start-stop operations such as city buses.
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 percent of the energy used to accelerate a bus with the GM Allison two-mode hybrid system comes from the energy saved during regenerative braking.
Allison estimates that since its system was introduced in 2003, it has contributed to the saving of over 25 million litres of fuel and the elimination of more than 65,000 metric tonnes of CO2. It has been sold into more than 2,200 buses and coaches worldwide and covered over 200 million kms in revenue service.