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ORNL researcher explores impact of motor/generator and battery pack sizing on medium-duty PHEV; optimization framework

GHG emissions in PHEV post-transmission configuration as an example of the optimization study output. Source: A.A. Malikopoulos. Click to enlarge.

Using a new optimization framework, Dr. Andreas Malikopoulos of the Energy & Transportation Science Division at Oak Ridge National Laboratory (ORNL) has explored the impact on fuel economy and GHG emissions of varying the size of the motor/generator and battery pack in pre- and post-transmission hybrid configurations of a medium-duty PHEV. The paper is currently in press in the Journal of Energy Resources Technology.

Broadly, he found that for the PHEV pre-transmission configuration, there is a trade-off between fuel economy and GHG emissions when the motor/generator and battery size increases. In post-transmission PHEV configurations, however, a combination of a big motor/generator size with a big battery size appears to be beneficial both in terms of fuel economy and GHG emissions as it enhances energy recovery during brake regeneration as a result of the physical location of the motor/generator.

For the work, he proposed an optimization framework to facilitate better understanding of the potential benefits resulting from the proper selection of motor/generator and battery size on fuel economy and greenhouse gas (GHG) emissions in plug-in hybrid electric vehicles. The goal of the framework is the appropriate sizing of these these components and thus reducing the PHEV cost to encourage market penetration of PHEVs.

To formulate the optimization problem analytically and reduce computation time, Malikopoulos used a set of polynomial metamodels constructed to reflect the responses produced by changes in the design variables (i.e., motor/generator and battery size). For the evaluation of various vehicle performance indices required for the optimization study, he used Autonomie—a Matlab/Simulink simulation package for powertrain and vehicle model development developed by Argonne National Laboratory.

Using a medium-duty plug-in hybrid electric vehicle (PHEV) as the base, Malikopoulos applied the framework to two different parallel powertrain configurations, pre-transmission and post-transmission, to derive the Pareto frontier—i.e., the curve of Pareto-efficient points—with respect to motor/generator and battery size.

He employed a blended supervisor control strategy that uses a mix of the electric motor and engine to power the vehicle in charge-depleting (CD) mode. For all the PHEV simulations, the state of charge (SOC) of the vehicle was allowed to fluctuate with a delta SOC of 60% (80% initial SOC depleting to 20% SOC). The powertrain had to operate as an all-electric vehicle below vehicle speeds of 25 mph (40 km/h) during CD operation; and the engine had to turn on at vehicle speeds greater than 45 mph (72 km/h) for drivability reasons.

To study the impact of the motor/generator and battery size and the associated trade-offs between fuel economy and GHG emissions, Malikopoulos constructed a general quantitative Pareto-based assessment by varying the weighting factors from 0 to 1. The assessment aims to identify the optimal motor/generator and battery size considering the associated trade-offs of fuel economy and GHG emissions.

Among his findings were:

  • Increasing the battery size has a significant impact on fuel economy, partly attributable to the additional amount of electricity from grid that can be stored and used to power the vehicle. The motor/generator size, on the other hand, impacts fuel economy only in conjunction with a larger battery size.

  • The combination of a large motor/generator and large battery enhances energy recovery during brake regeneration. This is more apparent in the PHEV post-transmission configuration, where fuel economy is noticeably improved.

  • A moderate number of modules seems to be the optimal battery size for both configurations.

  • For a large motor/generator, the impact of a large battery is quite different for the pre- and post-transmission configurations. For the PHEV pre-transmission configuration, GHG emissions are minimal for a combination of a 120 kW motor/generator with a six-module battery. For the PHEV post-transmission configuration, on the other hand, a 120 kW motor/generator in combination with a battery with 10 modules seems to be the optimal GHG solution.

  • The optimization and modeling approach adopted here facilitates better understanding of the potential benefits from proper selection of motor/generator and battery size. This understanding can help us identify the right sizing of these components and thus reducing the PHEV cost. Addressing optimal sizing of PHEV components could aim to an extensive market penetration of PHEVs. Future research should consider the interactions between power management control strategies and these design variables. Simultaneous consideration of both design and power management may reveal more opportunities for substantial improvements in fuel economy and GHG emissions.

    —“Impact of Component Sizing in Plug-In Hybrid Electric Vehicles for Energy Resource and Greenhouse Emissions Reduction”, in press


    • Malikopoulos, A.A. “Impact of Component Sizing in Plug-In Hybrid Electric Vehicles for Energy Resource and Greenhouse Emissions Reduction,” ASME J. Energy Resour. Technol. (in press)



Study in right direction. Still woud be interesting to know comparison with EV having no mechanical link engine-wheels like Fiskar.


PHEVs with 4 to 6 plug-in battery modules would solve the problem on an individual basis.

Owners could start with the minimum number of battery modules and add more latter to suit their individual needs and pocket book.

Secondly, most owners could benefit from delayed purchase of extra battery modules with more recent higher performance technologies and lower cost and less weight per kWh.


A new study published in NATURE reveals that Shale Gas leaks are 4X higher (9+% vs 2.4%) than previously claimed by operators.

The same study concluded that power stations using Shale Gas as feed stock are at par with Coal fired power stations for global warming emissions per kWh produced.

That may have a negative effect on PHEVs and BEVs in USA as more and more of their power stations use Shale Gas.


As emissions are more and more reported by 'Private Operators' and they have a well known practice of under-reporting by 2X to 4X, are we being mislead.

Is our per capita emission about 100 tonnes/year instead of the reported 25 tonnes/year?

Is it save to continue using emission reports from 'Operators'?


It would be more helpful and useful if the study had also taken into account the cost of the larger or smaller genset and the larger or smaller battery capability. Anyone would have guessed that a 110kW genset would require less fuel, and it is fairly obvious that adding a genset allows for smaller batteries.


Yes citizen...a 20 KW genset would normally use less fuel than a 200 KW monster but may not always be enough to accelerate (uphill) with four 300+ lbs passengers and a 4000+ lbs vehicle?

In other words, about 46% (and going up) need larger heavier vehicles. The other 54% (and going down) could do with much lighter vehicles with less power.

We may need two very different type of vehicle?


I once pulled a 3-ton load up a 5% grade at 65 MPH on less than 70 kW.  A sustainer over 50 kW is very oversized unless it is needed to support heavy acceleration.


Yes E-P...most vehicles are over powered and more so year after year? Over 300 hp will be the norm soon?

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