Emerging Electrification of Military Ground Systems

­While the implementation of electric components in the military will require time and intention, emerging technology in the semiconductor industry, coupled with recent strides in the commercial automotive sector, simplify such efforts.

This article will explore the various options of electrification in military applications, providing examples of projects along the way. Starting with current trends and future predictions, the electrification of military systems is both an attainable and necessary goal of the 21^st century.

Background

The U.S. military began using electrified systems, such as electric servos, before exploring electric propulsion systems, like hydraulic drives. From weight saving advantages to faster and more accurate adjustments (e.g., gun turret), simplified maintenance, and streamlined logistical support, this transition proved both tactically and financially beneficial to the military. But it’s not the only advancement the military has made: more recently, the military increased test and evaluation of electric vehicles (EVs), like electric traction motors to drive tracks and wheels, as part of its effort to change and enhance operational capabilities.

In addition to improved performance, another benefit of electrified military systems is efforts to combat climate change. For example, as climate change continues to impact extreme weather events, military vehicles can be used to power a field hospital or an emergency landing strip for natural disaster aid. Furthermore, this approach would eliminate the necessity for towed or airlifted generators and the fuel required to power them reducing the logistical demands required in disaster situations.

Latest trends and future predictions

Diesel

Diesel – or the military version, JP-8 – has been the fuel of choice for decades when powering military ground vehicles and auxiliary power units, such as generators and A/C systems. Back in the 1980s, the U.S. military and NATO nations released the Single-Fuel Concept (SFC), reducing the number of fuel variations and trimming the need for multiple fuel storage and distribution systems.

Diesel comes with its own set of drawbacks and advantages. For example, diesel contains about 10% more energy per volume than gasoline. Diesel in vapor or liquid form does not ignite as easily as gasoline, greatly reducing accident risk and simplifying handling during transport, fluid transfer, and refueling operations. For reference, the US Army planning estimates for one division are up to half a million gallons of diesel fuel per day (see figure 2). The cost of one gallon of diesel can be as high as $400 per gallon (Pentagon “fully burdened cost”). U.S. military operations in Iraq and Afghanistan consumed around 68 million gallons per month.

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While diesel engines have fewer moving parts than modern gasoline engines, allowing them to be more reliable and durable, they are also heavier than gasoline engines, increasing overall vehicle or equipment weight. The increased weight can create increased logistical challenges when moving individual equipment or entire brigades or divisions.

Trade-offs between temperature and power are also another factor in running diesel engines. While diesel engines can produce more power (especially low-end torque needed for towing) than gasoline engines given the same amount of fuel, they also run hotter. Exhaust gas can easily reach 200°C at the tip of the exhaust and 600°C or more at the engine. Hot engines and exhaust gases make detection through thermal imaging and turning vehicles into targets very easy. While there are efforts on the way to camouflage and reduce heat signature of an internal combustion engine, not generating one would be the preferred option. Electric motors, batteries and power circuits run at peak temperatures around 100°C, and cooling can lower temperatures further.

A final consideration of diesel engines is noise signature. Diesel engines in military ground vehicles are around 100dB at 50 meters, put simply, they are loud (see Figure 3). In extreme cases, noise levels can reach over 120dB at 50 meters. Mufflers on generators can reduce the noise down to about 70dB, which is about as loud as a TV. However, mufflers are not always an option for large military vehicles as their engine’s performance depends on getting exhaust gases and heat out of the engine as fast as possible.

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Hydraulic systems

Hydraulic systems have been the backbone of actuators on heavy equipment, tanks, armored fighting vehicles, artillery and any other system that has rugged and very heavy moving parts. Hydraulic systems have proven reliable over decades but, like any mechanical system, require proper maintenance. The number of components to build a hydraulic system to perform a certain task can be significantly higher than that of a comparable electric system. To name a few components, think about pumps, hoses, fittings, pressure regulators, pipes, hydraulic rams, and hundreds of gallons of fluid. These components are all made from high-strength materials such as steel and reinforced rubber to handle several thousand PSI pressures.

The case for electrification

"Electric vehicles are quiet. They have a low heat signature and incredible instant torque, and because they tend to be low maintenance with fewer moving parts, they have the potential to reduce logistics requirements," said Deputy Defense Secretary Kathleen Hicks in a statement in November 2021 at Wayne State University in Michigan.

Compared to diesel powered drive systems, electric drive systems operate almost silently. The only audible noise they generate is the occasional humming from the inverters in the power supply. Power semiconductors in the inverters, which turn on and off thousands of times per second, naturally emit this audible hum. However, they also emit an electronic signature, which an intelligence, surveillance, target acquisition, reconnaissance (ISTAR) system can detect.

Electrification of military vehicles can take place in several stages. For systems on vehicles or ground systems that can convert to electric actuation, one starting point could be simpler auxiliary systems before eventually implementing fully electrified propulsion systems. Breaking down the vehicles by type, there are more than 242,000 wheeled tactical and 170,000 non-tactical vehicles. There are thousands of diesel generators and environmental control units in service around the globe, demonstrating a wide market for electrification.

The global auto industry has shifted to electrified subsystems since the mid-1990s. Motivated by stricter emission laws, engineers and scientists have researched how many watts a system on a car uses and have consequently developed solutions to move them from the mechanical to the electric domain. Examples are electric cooling fans, power steering, and breaking. Implementation took place in incremental steps. One example is electric steering, which first used an electric hydraulic pump, eliminating the belt driven pump, while actuation of the wheels was still done via hydraulics. Now electric power steering means just that: an electric motor moves the drag link and steers the wheels.

Hybrids, Electric Vehicles (EV), and mild-hybrids are all well established in the commercial space. They all use electric power and may still have an internal combustion engine. Hybrids are broken down into several sub-groups, outlined below:

1.         Mild-hybrids are the simplest implementation and have been around more than 20 years. They simply turn off the internal combustion engine when the vehicle idles, which is also called the “Start-Stop” feature. This feature requires the vehicle to know the state of charge of the 12V battery as to not stop the engine with a weak battery, preventing a restart.

2.         EVs are fully electric and no longer have an internal combustion engine. The high-voltage batteries (600V to 800V) require external charging. The dependency on external charging now limits the range of the vehicle (beyond its initial battery range) to any charging network that is available. This is very likely the one factor that will limit military EVs to specific applications rather than broad implementation across the force. The second factor that limits EVs as tactical options may be the lack of redundant of power systems. “What if the batteries or the electrical motor fails?” may be a question on your mind right now. However, given that today’s ground vehicles or generators also lack redundancy, this should be a minimum to no concern.

EVs make sense where their main strengths – silence, no heat signature – make the biggest difference: reconnaissance and security. Ultra-light tactical vehicles in crewed and uncrewed versions powered by electric motors and high-voltage batteries could be the optimal equipment to approach an objective with a much-reduced chance of detection.

3.         Hybrids combine an internal combustion engine with an electric drive. Both supplement each other: the electric drive’s high-torque supports acceleration and acts as a generator to recharge the high-voltage batteries while the internal combustion engine does most work at cruising speed.

4.         Plug-in hybrids add external charging in case the engine or regenerative breaking cannot keep the high-voltage battery pack topped off. Hybrids have smaller battery packs than EVs. They also have extended range capabilities: there still is an internal combustion engine and a fuel tank that can be topped off at a gas station or tactically at a Rearm, Refuel, and Resupply Point (R3P).

The hybrid vehicle traction system is categorized into series and parallel architecture. In a series architecture, electric motors power the wheels either directly or through a transmission. The internal combustion engine operates a generator to charge the batteries, without any mechanical connection (such as a drive shaft) between the engine and the wheels. In parallel architecture, both the internal combustion engine and electric motors have the capability to propel the vehicle. Although this provides redundancy, parallel architectures are inherently more complex due to the inclusion of both mechanical components (such as differentials, driveshafts, and transfer cases) and electric drive systems connected to the wheels.

Military hybrid EVs

Examples of currently tested military hybrid EVs include: ProPulse® diesel-electric hybrid system by Oshkosh, General Motors eISV (Infantry Squad Vehicle) (see Figure 4), or an electrified version of the Bradley fighting vehicle featuring BAE’s HED (Hybrid Electric Drive) and QinetiQ’s Modular E-X-Drive® technology.

Oshkosh has implemented the ProPulse ® technology in HEMTT A3 and MTVR and claims significant fuel efficiency advantages: around 35% fuel efficiency improvement at 20 mph cruising speed and 25% at 60 mph, respectively (ProPulse ® Hybrid Diesel-Electric Systems, Oshkosh Corporation). General Motors eISV is powered by a 400V 3-phase permanent magnet motor in combination with a 400V GM BEV2 lithium ion battery with a 66kWh capacity (GM Defense, LLC).

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Figure 4:  Oshkosh, General Motors electric Infantry Squad Vehicle (eISV)

The U.S. Army’s Rapid Capabilities and Critical Technologies Office (RCCTO) awarded BAE a $32 million protype agreement in 2020 (BAE Systems, July 16 2020), demonstrating the value of electric and hybrid vehicles in the military’s near future.

Semiconductor technology as an enabler for military electrification

Semiconductor companies like Infineon have enabled the commercial automotive sector to engage in the design, development, and sale of millions of electric vehicles (EVs) and hybrids for more than a decade. Extensive customer feedback, billions of miles driven, and millions of engineering hours have contributed to making commercial electrified vehicles highly reliable for both urban and highway use. Furthermore, EVs and hybrids have become increasingly affordable as advancements in technology and manufacturing techniques have matured. Car companies have perfected electric vehicle designs, while semiconductor companies have supported and co-developed the necessary technology, anticipating needs a decade into the future.

Power semiconductor devices, including silicon (Si), silicon carbide (SiC), and gallium nitride (GaN) technologies, are now highly reliable and in mature production. Silicon-based products like IGBTs typically operate up to 700V. Si is cost effective for systems in the EV’s power domain that are not always on. For instance, an EV's traction system may primarily power the rear axle motors and to a lesser extent, the front axle motors during hard acceleration. In this case, the inverters for the front axle may be built with Si components to optimize cost efficiency.

Wide BandGap (WBG) materials like SiC and GaN support the high-power and long-range needs of electric vehicles. Both SiC and GaN can switch at high frequencies when powered with high voltages. Switching high voltages (e.g. 400V - 600V) in EV inverters allows for a more efficient use of available battery power. Moreover, SiC has higher thermal conductivity than GaN, allowing the inverter or on-board charger (OBC) to run at higher temperatures. This reduces the need for cooling, in turn reducing the system’s overall power consumption, complexity, and weight. Especially in systems greater than 50kWh, SiC has the power and cost advantage over Si (IGBT) and GaN.

Semiconductors are available as discrete components, such as individual transistors, or as modules that include transistors and diodes prepackaged by the semiconductor supplier. SiC discrete devices are suitable for use in inverters or on-board chargers with power ratings of approximately 1,200V and 50A, whereas SiC modules can accommodate applications requiring several hundred amperes. Certain semiconductor companies provide SiC modules up to 3,300V. SiC is also the most robust technology of the three based on its avalanche current and short circuit capability.

GaN combines voltages in the 600 - 1000V range with high switching frequencies in the megahertz range. This based on GaN having the lowest RDS(ON) x Qg (>90% lower than Si). Switching at high frequencies and inherent low parasitic capacitance, GaN designs greatly reduce electromagnetic interference (EMI). The reduction of EMI or electro-magnetic signature is of particular interest for tactical vehicle applications as it reduces detectability. However, while GaN allows for higher switching frequency, GaN is less resilient to high operating temperatures than SiC.

Conclusion

As the military continues to explore and integrate electrification in its ground systems, it paves the way for a future where operational effectiveness, logistical efficiency, and environmental responsibility converge. This progressive transition not only enhances mission readiness but also aligns with broader global efforts toward sustainability and innovation.

Infineon