The design and development of an Integrated Propulsion System – Phase 1: the wheel architecture

The paper presents the design and development of an integrated powertrain system where the suspension system is also included in the wheel together with the electric machine. Based on a patent of one of the authors, the solution was virtually developed and tested. Different advancements were performed during the development phases for choosing the final architecture. Currently, the integrated propulsion system is manufactured and implemented on a L6E road vehicle to be tested and optimized.


Introduction
The future of the sustainable transportation excellent includes the integrated propulsion systems as one of the pioneers in the road vehicles new era.The road vehicles powertrains current trends clearly express the concerns about the harmful emissions and the electric vehicles as one viable solution.While many electric vehicles powertrains use the individual components operating independently, the integrated propulsion systems approach the interconnectivity between the individual components, achieving the potential of the electric machine, the battery package, the power electronics and the control systems to simultaneously operate in perfect unison.The integrated propulsion systems have several advantages including the energy efficiency and the range increase, together with the dynamic behavior improvement and adaptive power management.The innovative level for the integrated propulsion systems is brought by their modularity together with the multi-choice connectivity between the integrated components.While the multiple design solutions are available, the space utilization demands are strictly defined by each of them.Most of the solutions approaches the integration of the individual components within the vehicle-axle, resulting an electric driven axle (e-axle), and within the wheel, resulting an electric-drivenwheel (e-wheel).These two different possible solutions are very well researched by most of the road vehicles` after-market suppliers, being already tested and validated.The many challenges for these solutions come during the implementation level, where the plug-and-play solution is highly demanded.
The e-axle is an integrated propulsion system that brings in the same unit the electric machine, the power electronics, the transmission and sometimes also the battery package.The e-axle have compact design, being a great space-saving solution that can easily be installed in a road vehicle chassis and can be connected to the suspension system.If the battery package is part of the same unit where the electric machine is installed, the cooling systems are also integrated.
The e-wheel is an integrated propulsion system that brings the electric machine inside the wheel.Being known as in-wheel or hub-motor, this solution is innovative by eliminating the traditional components that belong to an axle or to a drivetrain (shafts, differential etc.), freeing space and winning less vehicle weight in comparison with traditional electric or conventional powertrains.However, this solution is quite complex, because of the wheel assembly and of the difficulties that are brought by the unsprung weight that can affect handling and comfort.
This paper approaches the solution where the electric machine together with the suspension system are hosted inside the wheel.The objectives of this solution include improving efficiency, handling and comfort.One of the authors` patent represents the main starting point of this research.

The state of the art
The approached state of the art consists of presenting the already developments for road vehicles suspension systems and in-wheel motors used for road vehicles.Several innovations were presented during the last years for both the road vehicles suspension systems and for the in-wheel electric machines for road vehicles use.
The suspension system is the main system that offers ride comfort, handling and stability at the same time.It consists of an elastic component and a damping component.Several solutions for the suspension systems are known, including the mechanical, the adaptive, the hydraulic or the pneumatic suspension.Although the excitation forces can be transmitted in a vertical, longitudinal or lateral direction current suspension systems mainly isolate forces in the vertical direction.In many situations, multi-directional insulation can significantly improve safety and comfort.The already made research shows that the inwheel suspension is advantageous, bringing added value to conventional suspension systems.In [18] the in-wheel suspension consists of suspensions arms that are equidistant set around the central wheel hub to provide shock absorption even if the rim is rigid.In [1] some other in-wheel suspension systems solutions were studied, and one prototype was tested.In [5], different in-wheel suspension systems were virtually tested, also considering the energy consumption increase and the difficulties in correct steering, while a new concept was presented and tested.
In addition to the suspension systems components, the tire was in-deep research subject for its elastic behavior as part of the wheel.Some well-known tire manufacturers developed airless tires, with promising partnerships for reaching series manufacturing [13,17].In addition to these solutions, one tire manufacturer together with one rim manufacturer presented the flexible wheel [9,12].The body airless tire was studied by [3,4,7], different design solutions being analyzed during different operation scenarios and for different manufacturing materials.
The in-wheel motors are already well-implemented in different transport solutions, from scooters to utility and construction vehicles.The hub-motor architecture was implemented around 1895 by Lohner-Porsche, its advancements being continuously tested since then.Many after-market road vehicles suppliers are producing in-wheel motors as part of their strategy to contribute to electric vehicles development [11,14,15,16,19].As the competition between suppliers is continuously increasing, similar solutions to the hub-motors are also implemented, one of them consisting of the near wheel motor [10].Based on the existing solutions, researchers studied different design improvements [8] and virtually tested the e-wheel dynamics.

The proposed and applied technologies
The proposed e-wheel architecture should integrate the suspension system and the electric machine.Firstly, based on the presented state of the art and on the patent [2] obtained by one of the authors, five different e-wheel architectures (figure 1) were investigated where the main challenge is the suspension system integration.
The first solution (figure 1.a) consists of a wheel with three Ω type springs (that can be changed to U shape springs), with variable length and an angle of 120 degrees between them.Each spring is mechanically fixed to the wheel rim and to the center of the wheel hub.Inside each spring, a hydraulic shock absorber can be fixed.This solution demands a big diameter wheel (above 18 inches), is relatively easy to build, but with increased difficulty in manufacturing the Ω type springs.
The second solution (figure 1.b) consists of a wheel with curved elastic spokes and additional rubber springs, being fixed to the rim and to the wheel hub.This solution demands a big wheel diameter (above 1303 (2024) 012009 IOP Publishing doi:10.1088/1757-899X/1303/1/0120093 18 inches) and it has few components in comparison with the other five studied solutions.This solution is part of a patent registered in China by the same author and patent owner, under the number CN209159289.
The third solution (figure 1.c) consists of a wheel where the suspension is based on rubber elements and lamellar elastic springs inserted into them, being rigidly fixed to the rim and to the wheel hub.The elastic and absorption behaviors are achieved separately.This solution has no limitations for the wheel diameter, but the manufacturing approach includes the development of a personalized vulcanization mold for each targeted wheel diameter.
The fourth solution (figure 1.d) consists of a wheel with radial elastic elements fixed at the same time to the wheel rim and to the wheel hub.The radial elastic elements must be manufactured using the same material and needs to develop a personalized vulcanization mold for the outer band.
The fifth solution (figure 1.e) consists of a wheel with articulated elements.This solution has the highest modularity from all five studied solutions.Its modularity includes the possibility to easily change the elastic elements, to adapt them following the requests in terms of weight, elastic characteristics, size and volume, by keeping the same mechanical design and structure.The fifth solution was adopted to be investigated in deep.Therefore, starting from its basic design, three different design proposals derived from it were researched, but only one was continued for production and implementation.
The first proposal (figure 2.a) for the chosen solution consists of three different group of levers.Each group have two levers connected to two other levers with connecting rods.This mechanism is blocked when the elements are rigid.To ensure an elastic behavior, the elements have deformation within the elastic limit.Unfortunately, this kind of mechanism offers a reduction of few millimeters for the unsprung mass, being considered not a viable solution for the electric vehicle proposed for testing.
The second proposal (figure 2.b) for the chosen solution consists of six levers connected two by two by a shaft on which a helical spring is mounted.One end of the helical spring is connected to the outer lever, while the other end of the helical spring is connected to the inner lever.While crossing an obstacle, the angle between the levers varies (figure 2 The mathematical model for this design proposal was created and run using a virtual spring design and calculation software [21].The helical spring wire diameter was considered equal with 6 mm, while the external diameter for the helical spring was considered equal with 30 mm.The helical spring was considered to have ten coils.Using these values, the mathematical model calculates the helical spring life cycle and the torque depending on its deformation (figure 3).Unfortunately, the chosen spring with the presented characteristics breaks.Increasing the helical spring wire diameter represents a solution but is not acceptable being very hard to be further implemented and not appropriate for the unsprung mass.The third proposal (figure 4) for the chosen solution keeps the same design approach as the second proposal, but with an upgrade.The previous proposal with only one helical spring on each median axis has a low level of redundancy.If the spring breaks, the function of the suspension is completely compromised.The upgrade consists of using six helical springs instead of three, being aligned symmetrically regarding the wheel median vertical axis.The existence of two helical springs for each group has high level of redundancy allowing the system to work for a limited time even if one spring is broken.
The mathematical model for this design proposal was also created and run using the same virtual spring design and calculation software [21].Preliminary sizing steps were performed with several iterations.Based on the obtained results, together with the requirements for the outer diameter value of about 40 mm and the maximum length of ten coils, it was determined that the diameter of the wire should be over 5.5 mm.Several suppliers were contacted to purchase the most suitable helical spring already manufactured.For each of the most suitable received helical springs where the characteristics are meeting all the constraints, the same virtual spring design and calculation software was used.The chosen helical spring has the following characteristics: the wire diameter value of 5.8 mm, outside diameter value of 41.30 mm, the length of nine coils, the maximum length value of 102 mm.In addition to the suspension system that is integrated into the wheel, the electric machine should also be integrated.Currently, integrating the electric machine into the already chosen design represents a big challenge.The electric machine size represents a major constraint.However, there are different The first available solution (figure 6.a) consists of using two different electric machines that are coupled directly next to the drive wheels.The coupling can be done either by using a spline or grooves on the output shafts of the electric machines.When choosing the electric machines for this solution, the main constraint is to have the output shaft with spindle or with grooves, while the other requirements are strictly related to size, power and easy integration.
The second available solution (figure 6.b) consists of using the already existing electric machine that are installed and are in use on the electric vehicle proposed for testing.As a result of this solution, no important changes are required, only flanges are needed for each of the drive wheels to be connected to the existing electric machines.Based on the comparisons made for the two investigated solutions, adding the size requirement of the hub to achieve its modularity, the wheel architecture is chosen, being presented in figure 7. The flange is part of the wheel hub and must be customized in a modular way for being easily implemented to the existing electric vehicle proposed for testing and to other configurations.The e-wheel will be assembled with the electric drive motor by four different nuts specially configured to obtain the minimum size of the wheel hub.

The implementation
The e-wheel prototype was implemented on an existing L6E category electric vehicle (figure 9) for being on-road tested.Further results are pending to be obtained for optimizing the e-wheel.
Figure 9.The e-wheel prototype used by a L6E category electric vehicle.

Conclusions
The paper presents the stages of research and development for one of the phases of obtaining an integrated propulsion system.This first phase was dedicated to the wheel architecture.Several solutions for the suspension system integrated in the wheel and for the electric machines integrated in the wheel were studied.In-wheel suspension systems represent an innovative solution, being under development by many car manufacturers suppliers.They offer several benefits including more free space for the chassis, less unsprung weight, better handling.In-wheel electric machines are widely used for different electric vehicle categories.They offer several benefits including flexible vehicle design, reduced energy losses, simplified drivetrain.Starting from one author patent, different wheel architectures were virtual IOP Publishing doi:10.1088/1757-899X/1303/1/0120098 tested.Based on the obtained results, one solution was adopted, and its prototype was produced.
Currently, the prototype is tested on a L6E category electric vehicle.
The studies of integrated propulsion systems represent a current and a future concern for most technological companies involved in powertrain electrification.The achieved solution presented in this paper has great potential for being replicated and produced in series.

Figure 2 .
Figure 2. Two of three of the proposed designs for the e-wheel architecture chosen solution [2].

Figure 3 .
Figure 3.The helical spring caculation for the second design proposal.

Figure 4 .
The final accepted design.

Figure 6 .
Figure 6.Investigated solutions for coupling the electric machine to the wheel.

4 .Figure 8 .
The prototype Each e-wheel component was built with respect to the determined sizes and then assembled.The rims (figure8.a)were obtained from existing wheels, type 4Tx16, from which the original hubs were extracted.The new hub (figure8.b)consists of 18 independent plates assembled using nuts, with respect to the sizing of the electric machine.The springs mountings (figure8.c)were realized by cutting from metal sheets, then they were welded to the rims.The outer and inner levers (figure8.d)were manufactured using computer numerical control machines.The springs (figure8.e)were bought from a supplier.The prototype was produced (figure8.f)and the tire was mounted (figure 8.g).The e-wheel prototype with each of its components.
possible design solutions for coupling the electric machine even if its size is larger than the available hub size.