PHEVs, which combine a gas or diesel engine with an electric motor and a large rechargeable battery, offer advantages over conventional hybrids because they can be charged using mains electricity, which reduces their need for fuel, said researchers at the University of California, Riverside in the US.
However, the race to improve the efficiency of current PHEVs is limited by shortfalls in their energy management systems (EMS), which control the power split between engine and battery when they switch from all-electric mode to hybrid mode.
While not all plug-in hybrids work the same way, most models start in all-electric mode, running on electricity until their battery packs are depleted, then switch to hybrid mode. Known as binary mode control, this EMS strategy is easy to apply, but is not the most efficient way to combine the two power sources.
In lab tests, blended discharge strategies, in which power from the battery is used throughout the trip, have proven more efficient at minimising fuel consumption and emissions.
However, their development is complex and until now they have required an unrealistic amount of information upfront. "In reality, drivers may switch routes, traffic can be unpredictable, and road conditions may change, meaning that the EMS must source that information in real-time," said Xuewei Qi, a postdoctoral researcher at UCR's Centre for
Environmental Research and Technology (CE-CERT). The highly efficient EMS developed and simulated by Qi and his team combines vehicle connectivity information, such as cellular networks and crowd-sourcing platforms, and evolutionary algorithms - a mathematical way to describe natural phenomena such as evolution, insect swarming and bird flocking.
"By mathematically modelling the energy saving processes that occur in nature, scientists have created algorithms that can be used to solve optimisation problems in engineering," Qi said.
"We combined this approach with connected vehicle technology to achieve energy savings of more than 30 per cent. We achieved this by considering the charging opportunities during the trip - something that is not possible with existing EMS," said Qi.
Together with the application of evolutionary algorithms, vehicles will not only learn and optimise their own energy efficiency, but will also share their knowledge with other vehicles in the same traffic network through connected vehicle technology. "Even more importantly, the PHEV energy management system will no longer be a static device - it will actively evolve and improve for its entire life cycle. Our goal is to revolutionise the PHEV EMS to achieve even greater fuel savings and emission reductions," Qi said. The research was published in the journal IEEE Transactions on Intelligent Transportation Systems.
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