Electric Axle and Wheel Module Driveline Concepts for Self-propelled Agricultural Machinery and Equipment Carriers

Technical Summary: Electric Axle and Wheel Module Driveline Concepts for Self-propelled Agricultural Machinery and Equipment Carriers

Problem Statement
The electrification of self-propelled agricultural machinery (e.g., combine harvesters, forage harvesters, root crop harvesters) and equipment carriers offers the potential for new design freedoms and reduced energy losses compared to traditional hydrostatic drive systems. However, standard tractor architectures, which rely on a central engine and a single integrated driveline, face significant economic and technical barriers when transitioning to distributed electric drives. Specifically, the cumulative cost of multiple electric motors, power electronics, and reduction gears for a standard four-wheel-drive tractor can result in a total invested motor power of up to 400% of the nominal vehicle power, making central motor configurations more attractive for that specific class.

Conversely, non-standard vehicles such as self-propelled harvesters and equipment carriers often exhibit a distinct separation between process drive power (chopping, threshing) and ground drive power. In these machines, the process drive consumes the majority of power (60–80%), while ground drive requires significantly less. This separation creates an opportunity to electrify the ground drive independently. The challenge lies in defining optimal modular driveline architectures that balance engineering requirements—including load spectra, efficiency, steerability, controllability, braking, structural support, asymmetric wheel loading, and manufacturing costs—without the constraints of a central mechanical transmission.

Methodology
The paper defines and compares two specific design groups of direct electric drivelines without power-split: Axle Modules and Wheel Modules. Both are defined as self-contained mechatronic units integrating the electric motor, transmission, power electronics, control intelligence, steering, and braking systems. They require only a DC power bus and a communication interface from the vehicle chassis.

The authors analyze these concepts against specific engineering requirements derived from the operational profiles of self-propelled agricultural machinery:

• Load Spectra: Analysis of hydraulic pressure data from combine harvesters and forage harvesters reveals that while typical field pressures range between 50–200 bar, short-duration peak events (e.g., accelerating uphill) can reach 450 bar. This necessitates electric motors capable of high short-time peak power.
• Efficiency: The study references efficiency targets for tractor CVTs and specific targets for combine harvester ground drives (Bernhard target), noting that electric drives have the potential to exceed hydrostatic efficiency, particularly at partial loads where hydrostatic systems suffer disproportionately.
• Speed and Manufacturing: The analysis considers the narrower speed ranges of harvesters (3–8 km/h field, up to 40 km/h road) compared to standard tractors, and the economic implications of small-series production, where modular components from specialized suppliers offer economies of scale.
• Comparative Analysis: The paper evaluates both concepts across dimensions of space/ground clearance, suspension, manufacturing costs, steerability, controllability (degrees of freedom), braking systems, speed reduction strategies, free-wheeling capabilities, thermal management, and redundancy.

Key Contributions

1. Definition of Modular Concepts: The paper formally defines "Axle Modules" and "Wheel Modules" as fully integrated mechatronic units with distributed control intelligence and steering, distinguishing them from previous hydrostatic or centralized electric attempts where control remained centralized.
2. Architectural Comparison: A detailed technical comparison is provided, highlighting that:
   • Wheel Modules offer maximum design freedom for frame and suspension, maximum redundancy (limp-home capability with three functional units), and full eight-degree-of-freedom controllability (torque vectoring, individual wheel steering). However, they incur higher manufacturing costs, require motor over-dimensioning to handle asymmetric wheel loads (e.g., side-hill operations), and necessitate complex structural elements to carry process equipment loads.
   • Axle Modules provide lower manufacturing costs, structural rigidity suitable for heavy process equipment, automatic load sharing via a differential (avoiding motor over-dimensioning), and simpler thermal management. Their limitations include reduced design freedom between wheels and lower controllability (two degrees of freedom per axle).
3. Integration of Steering and Braking: The paper emphasizes that both concepts integrate steering actuation and braking systems directly into the module, allowing for a single DC power input. It discusses specific implementations, such as using range brakes for parking functions in wheel modules.
4. Speed Reduction Strategies: The authors analyze the necessity of multi-range transmissions (typically two ranges: field and road) to optimize efficiency across the operating speed spectrum. They reference specific planetary gear arrangements that allow for high reduction ratios and external power-shifted brakes.

Results and Findings

• Efficiency: Electric drives demonstrate a significant efficiency advantage over hydrostatic drives, with experimental data showing a 17–27 percentage point improvement under field conditions, primarily due to better partial-load performance.
• Cost: Current manufacturing costs for electric axle prototypes are significantly higher than hydrostatic equivalents (e.g., 4.5 times higher in one cited case). The paper suggests that cost parity relies on the widespread adoption of modular components across the industry to achieve economies of scale.
• Load Handling: The differential in axle modules automatically handles asymmetric vertical loads, whereas wheel modules require individual motors to be dimensioned for worst-case single-wheel loads, leading to over-dimensioning and increased weight/cost.
• Controllability: Wheel modules enable advanced driving modes such as pull-in-turn, torque vectoring for soil compaction management, and precise Ackermann steering, which are not possible with axle modules.
• Thermal Management: Axle modules benefit from concentrated heat sources allowing for simpler cooling circuits, while wheel modules distribute heat sources, potentially increasing cooling complexity.

Significance and Claims
The paper claims that direct electric drivelines without power-split open a fundamentally new design space for self-propelled agricultural machinery and equipment carriers. It posits that while both concepts are viable, the choice depends on the specific application:

• Axle Modules are presented as the stronger starting point for heavy self-propelled harvesters and root crop machinery where structural rigidity, cost efficiency, and handling of asymmetric loads are primary drivers.
• Wheel Modules are favored for applications where design freedom, redundancy, and advanced controllability (such as torque vectoring for soil protection) are the primary design drivers.

The authors conclude that the transition from centralized mechanical/hydrostatic drivelines to distributed electric modules is already underway in the sector (citing existing hydrostatic implementations). The main barrier to full electrification is currently the higher cost of electric components compared to the energy savings and CO2 reductions they offer. However, the paper suggests that as electric component costs decrease and energy costs rise, the economic viability of these modular concepts will improve. The paper also notes that hybrid configurations (mixing axle and wheel modules in a single vehicle) represent an additional dimension of design freedom for future architectures.