When your next tractor is a Tesla
The automotive industry is rapidly shifting toward Electric Vehicles (EVs), but there’s another category of vehicles ripe for electrification: ‘Non-Road Mobile Machinery’ (NRMM), as the regulators put it. This category includes tractors and other agricultural equipment; municipal and property maintenance equipment; materials and goods handling and transportation; and construction forestry and mining equipment.
Electrifying these vehicles will be challenging, because they have more varied duty cycles than road EVs, more complex mechanical requirements, and often need much more power than even the fastest electric sports car. Nonetheless, the promise of lower energy costs, increased productivity and greater reliability, and the pressure from emissions regulation, is driving rapid innovation in many forms of NRMM.
The pollution and climate imperative
The EU began regulating the emissions of NRMM in 1997, addressing the pollution caused by powered garden tools, agricultural and farming machinery, trains, inland waterway vessels, and construction machinery. The EU’s legislation has evolved since then, with its most recent form, Stage V, calling for equipment being sold now to produce 97% fewer particulates than similar equipment produced back in 1997, and 94% less nitrous oxide and hydrocarbon emissions.
Achieving these standards will mean that non-road engines will have to be fitted with mitigation technologies including diesel oxidation catalysts, diesel particulate filters, exhaust gas recirculation systems, and common-rail fuel injection. Engines of more than 56kW will also need selective catalytic reduction modules to cut nitrous oxide emissions.
These systems will be complex to implement and may limit the utility of the vehicles to which they are fitted. Some observers also argue that, because the regulations do not explicitly address carbon dioxide emissions, there could be unhealthy trade-offs between reducing air pollutants and minimising greenhouse gas emissions during the vehicle design process. The pace at which the regulations will be applied also means that some highly polluting NRMM engines may remain in service into the 2030s.
Opportunities for electrifying agriculture
One response to these regulations is the electrification of agricultural equipment. A 2017 article for Rural Engineering, carried out by a team from the Universidade Federal de Lavras (UFLA) in Brazil, reviewed current research on what it will take.
It argued that because tractors sometimes need to deliver very high levels of torque, especially when using accessories such as ploughs, they are often over-specified for day-to-day duties. This reduces their day-to-day efficiency, as well as making them more difficult to electrify.
However, it should be possible to build smaller tractors that can operate for an eight-hour workday from a single battery charge. The transition from Internal Combustion Engine (ICE) to Electric Motor (EM) drive will also give designers an opportunity to change tractor drivetrains. A first step would be to implement mild hybrid systems, in which an EM provides low-speed traction and contributes additional torque when the ICE needs it.
There is also an argument for installing super capacitors alongside rechargeable batteries in these vehicles to provide short periods of very high current delivery when the EMs need to deliver very high levels of torque. The drawbacks of this approach include the relatively low energy density of super capacitors, their size, and cost. On the other hand, their robust ability to store and discharge energy should reduce stress on the batteries and so extend their operating lifetimes.
The electrification of agricultural machinery faces the same battery energy density and cost issues as the car industry. There’s another challenge, too; that of delivering large amounts of charging power in a rural setting. The obvious (but costly) response is to use photovoltaic panels and storage batteries to build up reserves of energy during the operating day that can recharge vehicle batteries overnight.
This would demand sophisticated controllers to make the most of the sun’s energy, as well as efficient power electronics to charge the vehicle batteries.
A hybrid experiment
A team from the University of Padua, Italy, and the agricultural equipment maker Carraro Agritalia, undertook a feasibility study in 2019 on building electric versions of the small tractors used in orchards and vineyards. One reason for the study was that the additional emissions control systems demanded by Stage V NRMM regulations might take up so much space that even small tractors could not work in narrow orchard rows.
The team decided on a mild hybrid architecture so the tractor could handle both light and heavy-duty working. They reworked an 82kW ICE tractor design, downsizing the engine to 56kW and adding a 20KW electric motor. The two power plants are linked in a parallel hybrid powertrain, with an additional clutch so the ICE can be decoupled for pure electric working. Their study showed that in hybrid operation mode, the electric motor could make up for the loss of torque caused by downsizing the ICE. It also found that the electric motor could be used on its own for light duties, including pulling a heavy trailer at a constant low speed during harvesting.
The team proposed an electric motor design to suit their hybrid and noted that the motor controller circuitry would have to manage both the high currents needed to produce the motor’s peak torque, and the dynamics of servicing demands for instantaneous torque.
Carraro is now developing this work. Its webpage details a tractor with a mild hybrid powertrain that uses a 55kW ICE, a 20kW electric motor, and a 25kWh LiFePO4 battery pack. Shifting to the smaller ICE avoids the need to install the extra catalytic converter required by Stage V NRMM regulations for larger engines, saving space and reducing costs.
The Carraro mild hybrid also employs the company’s eTCH90 Long Drop torque-converter based, full-powershift transmission, which it has designed for use in a wide variety of hybrid and full-electric vehicles. Carraro says its hybrid system will cut fuel use and emissions, reduce maintenance costs due to less use of the ICE, and cut noise and vibrations.
Beyond electric agriculture
The tractor example serves as a good model for what is happening in other areas of the NRMM market. Innovation is being driven by both opportunity and regulation: for example, some construction equipment makers are moving to electric power because cities are declaring themselves emission-free zones, in which traditional equipment cannot operate.
A lot of manufacturers are beginning by substituting EMs for ICEs or developing hybrid versions of their most compact machinery. Many innovators are challenged by the rapidly changing duty cycles of NRMM. A digger might travel at low power to an operating site, use a lot of power to lift and move material, and then could recover energy as its bucket returns to rest. The digger may also have to provide mechanical power to drive auxiliary equipment and pumps to drive hydraulics, each with their own duty cycles. The complexity of the aggregate duty cycle makes it more difficult to correctly dimension power units to provide the power and torque necessary to do the job, and to control the resultant system for optimal efficiency.
The energy storage challenge of EVs is leading to extensive innovation in equipment architectures. For example, some manufacturers are building equipment with hot-swappable battery packs. Others are considering integrating energy-storage solutions such as super capacitors and hydraulic accumulators into their designs. It is also worth remembering that there is a whole class of tethered NRMM vehicle, such as the articulated loaders used in underground mining to avoid polluting the air, which are permanently connected to grid power or work with a combination of grid and battery power for greater flexibility.
Tethering equipment might demand a change in working practices, but it provides a new degree of design freedom for those who are prepared to use electrification as an opportunity for a radical rethink of their offerings.
A paper in the journal Energies, published in 2021, found extensive innovation in other areas of the NRMM sector, including street sweepers, refuse trucks, heavy-duty forklifts, excavators, road rollers, loaders, materials handlers, and towing tractors, among others. Although the challenge of full electrification is large in many of these cases, this rapid burst of innovation, coupled with the opportunity to use the evolutionary pressure to rethink product offerings, should see electrification spread rapidly beyond the highway.
If you want to find out more about solutions for electrifying agricultural and construction vehicles from our leading suppliers, visit our non-automotive transportation page. Alternatively, if you have a question or you would like to discuss your design in more detail, you can reach out to our team of field applications engineers in your local language.
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