Simulation-Based Assessment of Energy Consumption of Alternative Powertrains in Agricultural Tractors

Alternative powertrains have been increasingly implemented in different types of on-road vehicles for increasing energy efficiency and reducing emissions [1,2] and electrification is also on the way for off-road machinery [3,4]. The recent technological developments in powertrain electrification [5] and increased fossil fuel prices are also starting to make alternative powertrains and fuels relevant options for agricultural tractors. Unlike passenger vehicles, agricultural tractors have not yet been the most interesting application for powertrain electrification. The uncertainties about future developments regarding fossil fuels, environmental legislation, and emission standards have increased interest in the development of hybrid electric, fully electric, and fuel cell hybrid powertrain solutions [6]. Therefore, it is reasonable to believe that powertrain electrification will also be one of the major technology trends for agricultural tractors in the coming years. Recent scientific research results indicate that there could be a significant potential to increase energy efficiency with alternative powertrains [7]. The main architectures for suitable alternative electrified powertrains have been studied and the benefits of using electric power for numerous agricultural implements have been well recognized [8,9]; however, most of the existing research studies evaluating alternative powertrains for agricultural tractors focus only on single powertrain options and, therefore, a balanced comparison between the different technologies is required. This research presents a comparison—in terms of energy consumption and operational performance—by taking into account the most relevant alternative powertrain options for agricultural tractors. This article is a revised and expanded version of a paper entitled “Simulation of Alternative Powertrains in Agricultural Tractors” [7], which was presented at EVS36 in Sacramento on 12 June 2023.

Some agricultural tractor manufacturers have introduced new concepts for alternative powertrains and have launched prototype tractor models; they are even starting to produce versions of hybrid electric powertrains, but large-scale electrification still has many challenges to overcome. Several companies and research institutions are working on prototype battery electric tractors to reduce greenhouse gas emissions and dependence on fossil fuels in agriculture. John Deere has planned to launch an electric tractor by 2026, the small electric tractor by Fendt (e100 V Vario) has already been launched, and CNH Industrial is developing the New Holland T4 Electric Power and Farmall 75C Electric, which are both lithium-ion battery-powered all-electric utility tractors. Research has been ongoing to improve battery technologies for electric tractors. Increased energy density, longer battery life, and faster charging times are crucial aspects of the success of electric agricultural tractors [10]. The integration of electrified tractors with precision agriculture technologies is also a growing area of interest for manufacturers. Some governments have been offering incentives and subsidies to encourage the adoption of electric vehicles in agriculture. These policies aim to reduce emissions, promote sustainable farming, and support the transition to cleaner energy sources.

Powertrain electrification has spread steadily from passenger cars to utility vehicles and, today, even to heavy on-road vehicles [11,12]. There is also increasing development for off-road machinery, especially since 2021 as energy costs have increased exceptionally and there are many uncertainties surrounding the use of fossil fuels in the future. Higher technology costs can be a major barrier to using alternative powertrains in agricultural tractors, although previous research on heavy vehicles and off-road machinery suggests that the higher development and component costs can be paid off with benefits when assessing the cost on a lifecycle basis [13,14]. The electrification of farm vehicles started with small-sized machines, for example, there are already electrified versions of telehandlers and small loaders available for purchase [15]. Because modern agricultural tractors are used for a wide variety of field operations, road transport, and other supporting work such as front-end loading or mixer wagon operation, there are several different variants of basic agricultural tractors. However, from very small-sized tractors (engine power 16]. This similarity might limit the opportunities to introduce new powertrain designs and favor the minimal modification of the existing layout to avoid too many modifications in the production lines. This is also the case due to the multipurpose aspect of agricultural tractors, providing a universal operator for a vast variety of farm purposes.

Over the last few years, research studies have been carried out to estimate the benefits and feasibility of hybrid electric powertrains in agricultural tractors. For many reasons, compact and medium-sized tractors (engine power between 50 and 100 kW) have often been the baseline for hybridization studies. Troncon et al. (2019) studied the feasibility of hybridization for specialized orchard and vineyard tractors using a mild parallel-hybrid system [17,18]. The challenge was to fit the electric system in a rather limited space and deliver an adequate performance. Their simulated research results indicated that fuel consumption would be 15–35% lower depending on the duty cycle operation. In another study, an ICE-based platform was converted to a parallel hybrid powertrain with a downsized engine and electric motor [19]. The downsizing was about 29% (from 77 kW to 55 kW of engine power), the electric motor maximum power was 60 kW, and the battery size was 25 kWh. The fuel economy savings were evaluated using simulations of high and low power duty cycles, which clearly showed that hybridization had only a marginal benefit on high power cycles (on average about a 5% reduction) and a significant benefit on low power cycles, having a reduction of over 30% on average [19]. Mendes et al. (2019) investigated the hybridization of a tractor backhoe loader by focusing on using electrical power produced by a generator for the hydraulic system with supercapacitors as the energy storage [20]. Simulations on real-world recorded duty cycles indicated over a 50% reduction in fuel consumption. Mocera and Martini (2022) proposed a hybrid eCVT power-split hybridization for a specialized agricultural tractor [21]. Their performance simulations showed that the hybrid tractor would have a comparable performance in typical use of the tractor and fuel savings of 10–20%.

Alternative fuels, such as biodiesel, biogas, e-fuels, or hydrogen for internal combustion engines, have the potential to lower greenhouse gas emissions compared to traditional fossil fuels. This can contribute to mitigating environmental impacts associated with agricultural activities. Some alternative fuels are derived from renewable sources, offering the advantage of sustainability. For instance, biofuels can be produced from crop residue or organic waste, providing a renewable and potentially carbon-neutral energy source. Certain alternative fuels, like biogas, can be produced locally, promoting regional economic development. The adoption of alternative fuels is hindered by the lack of widespread infrastructure for production, distribution, and refueling [22]. Establishing a robust infrastructure is crucial for the successful integration of alternative fuels into agricultural practices. Some alternative fuels have a lower energy density than traditional fossil fuels, which can impact the overall range and efficiency of agricultural tractors [23]. This challenge requires advancements in fuel storage and utilization technologies.

Considering off-road vehicles and machinery in general, agricultural tractors differ from other machinery because they are often used for various purposes and many different types of field operations. Therefore, it is important to develop methods that provide the tools for evaluating the benefits of powertrain electrification of agricultural tractors [24]. Vehicle modeling and simulation methods are a practical and rather fast way of analyzing and comparing different powertrain solutions. Different from many other vehicles, agricultural tractors are used on different types of field surfaces and in different conditions, which creates specific challenges for modeling [25]. Reliably and accurately simulating tire–soil interactions need high-fidelity models, e.g., FEM—(Finite Element Method) or DEM—(Discrete Element Method) based models, that need laborious development and require significant amounts of computational capacity [26,27]. In addition, acquiring reliable validation data for high-fidelity tire–soil interaction models from field operations can be rather challenging [28]. For effectively comparing and evaluating the performance of alternative powertrains, less computationally intensive models are typically used, such as numerical simulation.

This research presents a numerical modeling and simulation approach for evaluating alternative powertrains in agricultural tractors using Autonomie vehicle simulation software [29]. Off-road vehicles and machinery are typically simulated in a different way to on-road vehicles because they usually perform repetitive tasks and do not have a traditional speed profile to follow. Instead, agricultural tractors are simulated based on distance, by giving a target speed based on the distance traveled. Also, as these types of machines often do heavy work, the resistance force from implements must be integrated into the model by, for example, simulating agricultural field work such as plowing or harrowing. Naturally, in typical field work, like field cultivation, the power requirement can consist of a passive draft force or an active power that uses the power take-off (PTO) or hydraulic power in an implement. For evaluating alternative powertrains in agricultural tractors, numerical modeling and simulation provide an effective approach to generating different simulation cases, comparing component sizing, and then evaluating the benefits in several use cases.

The research results clearly indicate the significant potential to reduce the energy consumption of agricultural tractors by powertrain electrification. Over the years, different electrified powertrain topologies have been proposed for vehicles, and as with other types of vehicles, such as city buses [14], the benefit of electrified powertrains for agricultural tractors is typically dependent on the duty cycle or work task carried out with the vehicle. In many scientific and practical research studies [17,18,19,33,34], parallel hybrid electric powertrain topology has been recognized as being suitable for agricultural tractors. One of the main reasons for this could be that it would not need any major modifications to conventional tractor designs but, instead, could be implemented by adding a motor/generator in the place of the flywheel along with a small battery pack or even supercapacitors [20]. The results indicate that the parallel hybrid electric powertrain would provide meaningful energy savings for medium-sized tractors and, when operating with lighter loads, large-sized tractors. Similar conclusions were drawn in recent research by Beligoj et al. (2022), who evaluated the feasibility and life-cycle cost of a parallel hybrid powertrain for different sizes of agricultural tractors [35]. They concluded that very little energy consumption reduction or cost saving would be attained with large-sized tractor (engine power of 210 kW) hybridization, but small-sized orchard tractors and medium–large-sized tractors with medium workloads would provide considerable savings in life-cycle costs. The lower fuel consumption would offer reductions in operational costs and decrease the carbon footprint of these tractors.

The series hybrid electric powertrain has shown to be less interesting for vehicle applications that do not have very repeatable duty cycles or for which there are several use cases. This is the case for passenger vehicles and for agricultural tractors because these are used for a wide variety of purposes by different types of professional and individual users. The simulation results showed the variable potential of a series hybrid electric powertrain, including notable benefits for the medium-sized tractor but less encouraging results in the case of the large-sized tractor. Nevertheless, more detailed simulations should be performed to evaluate the potential of the series hybrid powertrain for different types of agricultural tasks. In comparison to parallel hybrid powertrains, the series hybrid powertrain could provide the possibility of using the electric power take-off (ePTO) and electrified implements, which would be much harder to accomplish with the parallel hybrid due to the limited amount of on-board electric power [35].

Hydrogen as a vehicle fuel is gaining more and more interest as a method for reducing the use of fossil fuels and harmful emissions. Fuel cell systems have been used as the main power sources in vehicles for a long time, but the technological cost and lack of fueling infrastructure are still barriers that have not been fully resolved. Even though fuel cells can be considered as a mature technology, it is not technologically easy to design an agricultural tractor with a fuel cell system because of the spatial requirements for the stack, hydrogen storage, and auxiliary systems. Recent research by Ahluwalia et al. (2022) concluded that the fuel cell system could be cost competitive for agricultural tractors if the targeted improvements to the cost, performance, and durability of the technology could be achieved [36]. Much more research is needed to find the best solutions for alternative fuels for use in agricultural vehicles. For example, methane or methanol might be preferred over hydrogen because of its low volumetric energy density and adapted infrastructure requirements [37]. As a potential fuel for internal combustion engines, burning hydrogen in an engine also has some challenges in terms of NOx emissions and engine knocking [38], and the storage challenge would remain the same as for the fuel cell systems.

The batteries in electric vehicles have seen tremendous technological development and market success, essentially in all on-road vehicle categories; even for 40 ton heavy-duty trucks and battery-powered tractors have been designed and manufactured. Hence, battery and power electronics technology is certainly mature enough even for heavy-duty machinery. The simulation results show that energy consumption could be reduced by up to 70%, which comes from a much higher powertrain efficiency. However, this higher powertrain efficiency does not mitigate the fact that many agricultural field operations require high power or high workload operation. This ultimately leads to high energy requirements and, therefore, the focus must be on the total amount of required on-board energy. The simulations in this research were performed with the consideration that all the tractor models have the same performance and, therefore, the total weight was limited. It could be said that a higher battery capacity than was used in this research could be installed into battery-powered electric tractors [39]. In this case, the tractor weight would increase, which would have some influence on performance and energy consumption, but this influence would need to be evaluated with more detailed simulations. Another challenge that remains to be resolved is battery charging; it is not clear whether every farm could have access to high-power fast charging. Thus, preliminary studies on the fully electrification of agricultural tractors have concluded that it would be more profitable to have a battery exchange system rather than high-power charging systems [40].

Overall, electrification is being applied to agricultural tractors and there are more possibilities than challenges. More research is needed to evaluate the different use cases, namely duty cycle operations, and, especially, life-cycle energy consumption, emissions, and cost [41]. Available electric power would allow the electrification of many auxiliary devices that could lead to additional savings by reducing the idling losses that are quite important for agricultural tractors [42]; Molari et al. (2019) stated that agricultural tractors may remain idle from 10% to 43% of their entire operating time [43]. This would provide additional savings with electrification because unnecessary idling could be easily avoided by shutting down the engine.