Page 31 - Commercial Vehicle Engineer - September 2021
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For these reasons and others, in the North American market – where cities are imposing ZEZs rather than LEZs, particularly around schools and hospitals – there is a strong case for electric hybrids. There is no need to wait for batteries to recharge; fuel consumption is reduced at speeds above 10mph by 25% compared to conventional diesel engines; and hybrids are significantly less costly to own and run than full-electric vehicles.
It can be the right option for fleets presently unable to invest in charging infrastructure.
This is further strengthened by market confidence in proven technologies: Allison has offered an advanced and dependable two-mode split parallel architecture hybrid solution, the H 40/50 EP system, to bus and coach operators since 2003.
Long range view
Building on the proven H 40 EP and H
50 EP systems, Allison recently launched eGen Flex for North America, a new electric hybrid system for buses and coaches which provides full-electric propulsion and zero- emission capability. eGen Flex deploys electric propulsion selectively rather than permanently and eliminates the need for charging stops by topping-up the energy stored in its batteries through regenerative braking. Its all-electric range of 10 miles is more than enough to get through typical city ZEZs, even with frequent stop-starts.
By using geo-fencing technology, eGen Flex automatically turns off the vehicle’s diesel engine and switches to electric propulsion when entering a ZEZ, then automatically turns the diesel engine back on when leaving the zone. It is also possible to automatically switch to engine-off mode when approaching, standing at, and leaving passenger stops, and when the vehicle enters a depot. To enable this automatic function, geo-fence activation point coordinates are uploaded and updated remotely via telemetry connectivity. No additional action is required from the bus’s fleet operator or driver to enable the EV zones.
Latest battery technology
A key technical highlight of the system
is that it incorporates the latest lithium- titanate-oxide (LTO) battery technology, which has a number of advantages in this application over the nickel metal hydride battery used in the Allison
H 40/50 EP system that eGen Flex supersedes, as well as the nickel manganese cobalt (NMC) batteries used by competitor systems. LTO battery chemistry provides superior cycle-life, enabling more EV events of longer range in any given time period. This technology has also made possible an industry- leading battery design life (dependent
on duty cycle and accessory load requirements) of eight years.
LTO also increases energy density, which enables faster charging even when it is cold, because Li-plating of metallic lithium forming around the anode doesn’t occur. Crucially, LTO also extends the pure electric (engine off ) range capability. Whereas hybrid buses with NMC batteries have demonstrated an all- electric maximum range of about three miles, eGen Flex can run in all-electric mode for up to 10 miles. This future- proofs buses in anticipation of ZEZs getting longer.
Alex Schey
All of these advantages mean that eGen Flex is a manageable step towards a low- carbon or no-carbon future without bus and coach operators having to attempt to get there in one big and expensive leap.
Finally, while eGen Flex has been developed primarily for the bus and coach sector within North America, the same technology has the potential for specialised heavy-duty applications around the
world, such as defence, off-highway and construction. These vehicles often need to operate in LEZs or ZEZs while stationary and operational but may still have a long way to drive on the highway.
Alex Schey is chief commercial officer, electrification at Allison Transmission.

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