Hybrid, Military Off-Road Vehicles Explore Anywhere on the Planet

A systems approach is required to efficiently electrify vehicles and equipment. Because of this, a system integrator that needed to electrify military autonomous amphibious vehicles for a military vehicle original equipment manufacturer (OEM) wanted just that. It wanted a partner that could provide everything they needed to produce a quiet, environmentally friendly, off-road military vehicle. This vehicle needed to be amphibious and autonomous.

The Requirement

An OEM wanted to move away from total reliance on hydrocarbon fuel in its all terrain vehicle (ATV). It wanted to have quiet, autonomous system as well. The system integrator working with the OEM decided to go with a hybrid, military off-road vehicle approach for flexibility. The task was to have minimum compromise on performance due to electrification.

The system integrator needed power generation, traction, and cooling. With all these requirements, weight and space were extremely limited. The team needed motors paired with an inverter that are easy to integrate, delivering efficiently the performance required.

The Systems Approach Solution for the Hybrid, Military Off-Road Vehicle

The manufacturer of the system components worked with the integrator to ensure that the traction motor gave the vehicle the torque and power needed to take on the rough terrain it would encounter while in service. The system included axial flux motors, power electronics, and high performance, seal less pumps.

The high-power, high-torque, efficient axial flux motor had a compact design. The pancake-like form factor of the motor maximized the available space in the vehicle. An axial flux motor’s torque is proportional to the cube of the diameter, and it has no significant influence on the length. Because of this, and axial flux motor has more torque when compared to a traditional radial flux motor of similar size with lighter weight and a smaller footprint. The lighter weight and size was important since the vehicle required five axial flux motors.

The 350-V, three-turn axial flux motor chosen had these specifications:

• Maximum speed –  8,000 rpm 
• Continuous torque – 133 Newton meters (Nm)
• Peak torque for 20 seconds (s) –  342 Nm
• Nominal output power –  64 kilowatts (kW)
• Peak output power for 20 s – 144 kW

In addition to four wheel motors for traction,  one axial flux motor was used to generate power. Operation as a generator had several benefits, including:

• Quiet operation
• Decreased operating costs because idle is not required
• Zero maintenance because of fewer moving parts

Although the five motors are the same, the orientation of their coolant port and phase cable was configured according to the packaging requirements of the integrator. The motors are water cooled, which was a great benefit for the simplification of the hybrid platform design because additional components that would be required for oil cooling were not required.

The inverter needed to be high-voltage with ​functional safety​. The application also required a single drive manage the five motors in the vehicle. The drive selected had the following parameters:

• Nominal voltage – 350 volts direct current (VDC)
• 100% derating voltage – 450 VDC
• Min battery voltage – 128 V
• Peak current for 1 minute (min) – 400 A_rms
• Continuous current for 60 min – 200 A_rms

Finally, for system cooling, ​one, high performing, 24-V sealless centrifugal pump was part of the system. The pump had a minimum operating temperature of -30°C and a maximum operating temperature of more than 75°C.

The pump used has the following benefits:

• The product life is one of the best in the market, with the expected mean time between failure of the pump at 40,000 hours. This supports a robust hybrid platform.
• The pump is sealless, which means that there is no requirement for maintenance.

The Results: A Hybrid, Military Off-Road Vehicle That Met the Requirements

The system-level solution simplified the integration and decreased the time to market for the OEM. The quiet, long-range operation required was secured with this systems approach to the powertrain.The vehicle is in final testing, which is going well. If the success continues, the OEM plans to produce more than 50 vehicles per year.