Better hybrids through intelligent battery use

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Improving the quality of the air we breathe in our cities while still allowing vehicular traffic will require huge effort from vehicle manufacturers and OEMs, none of whom has a ‘silver bullet’ solution.

At the core of the debate is the reliance on greater use of hybrid and electric powertrains, but in order to improve the popularity of hybrid technology there are still barriers to purchase that must be overcome.

Cost, performance, operating range and battery life are foremost among these barriers, and again there isn’t one clear course of action – which is why we see so many concurrent development programs being undertaken.

What if we pursued a more radical approach? Assuming no breakthroughs in battery chemistry are just around the corner, the shortcomings of HEVs (hybrid and electric vehicles) could be greatly alleviated if we stopped using batteries incorrectly and instead played to their technical strengths.

The ideal duty cycle for a battery is a low power demand over a sustained period, resulting in a slow rate of discharge, followed by an equally gradual recharge phase without subjecting the cells to too deep a level of discharge. This isn’t much like the typical automotive use pattern, where brisk acceleration demands high power that draws heavy current from the battery and in turn limits range, with the driver often compelled to drain the battery almost entirely in order to get home. So frequently is this cycle repeated, that the lifespan of a battery pack (typically 3,000-5,000 charge/discharge cycles) is insufficient to last the life of the vehicle, necessitating expensive replacement.

Adding an energy recovery system (ERS) to the vehicle can provide all the energy necessary for acceleration during stop-start driving, alleviating the energy recovery demands that shorten battery life and push up electrical system costs. An ERS using a modern high-speed flywheel has the opposite characteristics to a battery – providing high power density whereas a battery provides high energy density.

A flywheel ERS also transfers energy at a high power rate during rapid charge/discharge cycles with no practical limit on the number of cycles. These are the precise characteristics required for successful long-term operation in urban traffic.

The latest self-contained, high-speed flywheel technology is compact, and at around one fifth the cost of an equivalent e-hybrid system, very cost-effective. This enables use of a combined system using a smaller motor and battery pack that can weigh less while providing longer battery life, greater range, easier packaging – and a lower overall cost.

ERS applied to an internal combustion engine (ICE) was originally seen as a direct competitor for electric hybrid systems. While development continues on ICE ERS applications, recognition of the complementary properties of ERS and battery systems has opened up exciting new possibilities and, as such, is clearly on the radar of development and test engineers.

In addition to improving the viability of an EV by adding flywheel energy recovery, ERS can also enhance a hybrid vehicle. A low power electric micro-hybrid for example, configured with 48V architecture, could rely on mechanical ERS to provide acceleration. This would allow the ICE to be downsized and optimized for a restricted range of operation; vehicle efficiency is increased by no longer having to over-specify the ICE or electric motor and also provides power for maximum acceleration.

Improving the sales of low-emission vehicles such as HEVs is easier if the vehicles are fun to drive and require no significant changes in drivers’ habits. By harvesting the energy that would otherwise be wasted during braking, flywheel ERS provides a satisfying experience for the driver through the immediate availability of strong acceleration, without generating harmful emissions, either at the roadside or from the power station.

Tobias Knichel has extensive experience in developing efficiency improving technologies and was one of the first employees at Torotrak acquisition Flybrid. Before joining the Torotrak Group, Knichel worked for the Opel DTM racing team in various engineering roles.

August 25, 2016

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