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In the second half of 2020, Allison Transmission, the global supplier of medium-and heavy-duty transmissions and propulsion systems, unveiled its eGen Flex hybrid system for bus applications. Now the first three buses using the system are racking up miles for IndyGo, the public transit system serving Allison’s hometown of Indianapolis, Ind.

The eGen Flex system was developed from Allison’s established H 40 EP hybrid system that’s been on the market for nearly 20 years, with improvements that go beyond the usual fuel economy benefits brought by a hybrid – in this case, up to 25% – and features like regenerative braking.

The eGen Flex is designed to allow the vehicle to operate in full electric (EV) mode up to 50% of the time, depending on duty cycle. The system incorporates geofencing technology that can be used to automatically switch the vehicle from conventional to electric-only operation. This allows transit authorities to mark dense pedestrian areas such as city centers and bus depots as zero-emission zones and when an eGen Flex-equipped bus enters that zone, it goes into engine-off mode and the bus is driven through the system’s Rechargeable Energy Storage System (RESS).

The RESS system uses lithium-titanate (LTO) battery chemistry that Allison said enables faster operational charging and delivers up to 50 continuous minutes – or approximately 10 miles – of engine-off operation. The LTO battery has a rated life of eight years, Allison said.

The eGen Flex drive unit also incorporates a new disconnect clutch designed to provide seamless uncoupling from the engine drive shaft, allowing the integral electric motors to take over after the driveshaft is disengaged and the engine is shut down. In addition, an all-new Dual Traction inverter is 50 lb. lighter and one-third the size of the H 40 EP version and incorporates new switching modules that Allison said delivers longer service life and improved system efficiency.

The first two eGen Flex-equipped vehicles, Gillig BRT 40-ft. low-floor buses with Cummins B6.7 diesel engines, entered service in late August 2022, with the third commencing operations in mid-December. Over their first eight months, the buses have accumulated more than 110,000 operating miles over an east-west route in the downtown area of Indianapolis International Airport. The buses have operated in engine-off, full EV mode for nearly 25,500 miles and the percentage of EV operation, not counting start-stop events, has ranged from 19-40%.

“After eight months in use, IndyGo is very pleased with the flexibility Allison Transmission’s eGen Flex buses offer, allowing us to reduce our carbon footprint in some of the most densely populated, high-air pollution pockets of our city,” said Inez Evans, IndyGo president and CEO. “This technology represents the next step in our efforts to reduce our dependence on fossil fuels, enhance quality of life for residents and do our part to protect the environment.”

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