Powertrain design and energy management of a novel coaxialseries-parallel plug-in hybrid electric vehicle Abstract：For a series plug-in hybrid electric vehicle, higher working efficiency can be achieved by the drive system with two small motors in parallel than that with one big motor alone. However, the overly complex structure will inevitably lead to a substantial increase in the development cost. To improve the system price-performance ratio, a new kind of series-parallel hybrid system evolved from the series plug-in hybrid system is designed. According to the technical parameters of the selected components,the system model is established, and the vehicle dynamic property and pure electric drive economy are evaluated. Based on the dynamic programming, the energy management strategy for the drive system under the city driving cycle is developed, and the superiority validation of the system is completed. For the studied vehicle driven by the designed series-parallel plug-in hybrid system, compared with the one driven by the described series plug-in hybrid system, the dynamic property is significantly improved because of the multi-power coupling, and the fuel consumption is reduced by 11.4% with 10 city driving cycles. In a word, with the flexible configuration of the designed hybrid system and the optimized control strategy of the energy management, the vehicle performance can be obviously improved. Keywords：plug-in hybrid electric vehicle；series-parallel hybrid system；multi-power coupling；dynamic programming；energy management1 Introduction Electric vehicle (EV) holds significant potential for not only transforming how the world moves, but also for increasing energy security and reducing carbon emissions and other pollutants 1. Pure electric vehicle (PEV) driven by one or more electric motors is undoubtedly the ultimate goal of the EVs development. Todays PEVs are commonly powered by rechargeable lithium-ion batteries, which are also being used to store renewable energy. Nevertheless, because the store energy of the lithium-ion batteries is limited by their chemistry, the batteries energy density is low. Due to theimpracticality of excessive battery assembly, the driving range of the PEVs is relatively short, which led to a worldwide decline in their use 2. To remedy the deficiencies of PEVs, hybrid electric vehicles (HEV) are developed, which combines a conventional gasoline powered engine withsome form of electric propulsion. HEVs have the potential to reduce fuel consumption and emissions comparing to conventional vehicles, thanks to the presence of a reversible energy storage device and one or more electric machines. Due to the existence of the mode of engine generating or driving, the vehicle can meet the requirements of long distance travel without using large-capacity batteries 3,4. However, because the electricity of the vehicle is entirely converted from fuel oil, the fuel economy improvement is limited and the environmental image is still evident. As the transition models from HEVs to PEVs, plug-in HEV (PHEV) has gradually become the preferred solution of EV at the present stage. The battery of PHEV can becharged by an on-board power supply to ensure a certain continued mileage with pure motor driving 5,6. Due to that the running mainly depends on pure electric mode,PHEV plays a more important role in fuel saving, emission reduction and environmental protection than other kinds of HEVs. Owing to the simple configuration and control strategy, the series PHEV that is known as extended-range EV has been successfully manufactured. Since a range extender is installed, the battery can be recharged during the vehicle running, the problem of short driving range faced by PEVs might be mitigated 7. But the range extender is not coupled to the wheels and does not directly power the vehicle, the drive motor and the battery must be designed large enough to meet the power requirements of the vehicle, which leads to a high cost but low performance product. The Parallel PHEV is another kind of PHEVs, in which both the drive motor and the engine work together to power the vehicle and the cost of the rechargeable battery is acceptable. With the prediction of the driving condition, the performance of PHEVs can be significantly improved 8.The composition of parallel hybrids makes them more efficient for the vehicle highway driving at higher, more constant speeds, but it does not apply to driving in the city because their drivetrain structure increases the strain on the engine in stop-and-go driving situations. To make use of the advantages of series and parallel PHEV and avoid their shortcomings, the series-parallel PHEV is developed. Because the driving modes of series and parallel can be freedom chosen, it is particularly suited to the complex driving cycle in a big city, such as operation in low-speed, frequent acceleration and deceleration, often p