Innovative GM-Allison Hybrid EP-System powers 10 new Optare
Tempo buses for Transport for London, contributing to a cleaner and
quieter London
Bus passengers in East London are the first in the UK to
experience a unique form of hybrid power with four of 10 new Optare
Tempos equipped with the GM-Allison Hybrid EP-System recently becoming
operational. The patented ‘two mode’ parallel hybrid technology combines
direct power from an electronically controlled diesel engine with
electric power from an energy storage system and is already delivering
significant fuel saving benefits, lower emissions, and raising vehicle
performance for bus fleets over 115 cities worldwide.
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.
How the GM-Allison two-mode hybrid system works
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.
