Introduction

Multipurpose electric vehicles are becoming increasingly popular around the world owing to their several advantages, such as zero emissions, keen intelligence, and eco-efficiency (Chan and Chau, 2001; Hu et al., 2021). The growing focus on sustainable development in the agricultural sector requires the adaptation of suitable technological approaches that enhance agricultural productivity. Sustainable development requires raising agricultural productivity through appropriate technology and energy sources to meet consumer and agricultural industry demands without compromising the environment. Consequently, adopting electric vehicle (EV) technologies is one of the solutions that have been used in the agricultural sectors due to their better performance, energy conversion efficiency, and environmental friendliness (Pratama et al., 2013; Kim and Heo, 2019; Christiaensen et al., 2021). Besides, the increasing expenses associated with fuel and the implementation of global rules on pollutant emissions have led manufacturers to exhibit a rising tendency toward adopting electric powertrain systems in various sectors and agricultural practices (Lee et al., 2016; Dépature et al., 2017). The current focus of continuing research is on the development of ecologically sustainable electric motors and power transmission systems as alternatives to the prevalent utilization of internal combustion engines in agricultural practices (Kim et al., 2011a; Barreras et al., 2012; Kang et al., 2018).

Despite the advancements in agricultural production and the expansion of small-scale farming, a significant portion of agricultural practices continue to be dependent on manual labor (Andrew et al., 2003; Hunter et al., 2017). Due to the increasing number of aged farmers and the decline in the available workforce population, along with the ongoing trends of rapid urbanization and industrialization, there is an urgent need for a focus on the improvement of farmer-friendly agricultural machinery. The situation can be effectively compensated by small-sized battery-operated vehicles (Ali et al., 2019; Wang et al., 2021).

The primary source of agricultural transportation has been served by tractors. For small-sized agricultural land and an aging agricultural workforce, tractors might be found inconvenient due to their heavy weight and the need for maintenance skills (Ali et al., 2021a). The exploratory research on a versatile framework for small-sized, battery-operated agricultural electric platforms is still lacking in terms of being accommodating to the elderly and women in agricultural field work (Masrur, 2021). For agricultural tasks like spraying, spreading manure, harvesting, and transporting farming supplies and materials, electric-powered transport vehicles have grown in popularity recently (Ali et al., 2021b; Parekh et al., 2022). Challenges faced by the elderly and women farmers in navigating through land irregularities, sloping terrain, slippery wheels, and the tearing of mulching vinyl for transportation purposes with different models of vehicles. The adoption of 4WD electric vehicles could have a significant impact on increasing productivity and efficiency in the agricultural sector by making it possible to overcome the inconveniences and difficulties faced by farm workers while operating the platforms.

Selecting the appropriate power transmission devices is crucial for the development of 4WD agricultural electric vehicles, requiring the strengthening of mechanisms due to working load fluctuations and duration (Park et al., 2010; Sim et al., 2010). The limitations of battery charge-holding capacity have created issues for electric vehicles, making it challenging to deliver regular power for heavy and long-duration activities in agricultural fields (Deng et al., 2020). In order to achieve the successful development of agricultural electric vehicles, it is necessary to consider battery life and select appropriate motors, gear ratios, and charge-holding capacities (Kim and Park, 2012).

On the other hand, the appropriate working load and speed must be capable of being output by the powertrain of the vehicle, depending on the type of work and soil conditions (Jung, 2013). Current power transmission systems have shown promise by replacing the traditional transmission with individual motors and speed reducers on each wheel, enabling high-efficiency uninterrupted transmission during agricultural operations. To design a multipurpose agricultural vehicle, the electric drive unit specifications for wheel drive should be chosen based on the agricultural working loads of the proposed vehicle. The electric motor and reducer must be carefully considered in order to manage the high torque generated during farm work and to establish a suitable reduction gear ratio for effective farming operations (Kim et al., 2019).

Ueka et al. (2013) developed a prototype electric tractor capable of operations under diverse terrains and working conditions. To optimize the driving strategy, it developed specific work modes for different activities and verified the tractor utility by examining its working hours and performance. To maximize driving strategy, distinct work modes for various tasks were examined and validated through an analysis of tractor performance and working hours. To adapt different working load characteristics under typical working conditions, Xu et al. (2014) designed a hybrid series electric machine drive system while ensuring the effectiveness and capacity of transmission systems. Most of the studies required proper allocation of power output to the power drive unit to control driving strategy according to working load data under various load measurement conditions, e.g., battery power, and types of work (Mocera and Somà, 2020). To develop the power transmission system for multipurpose agricultural vehicles, several researchers have explored for ensuring sufficient power by investigating the efficiency of the power transmission parts during agricultural product transportation on various off-road conditions and slopes. However, it is essential to measure the agricultural working loads for various agricultural and driving operations under typical off-road conditions, particularly considering the slope of the ground in highland and mountainous areas for assessing the power drive unit during field operations.

According to the requirements, a commercial speed reduction gear was selected for the design of the power transmission unit of the 4WD electric vehicle, and the gear ratio was investigated through experiments to confirm the working efficiency under typical off-road conditions. Therefore, the objective of the study was to evaluate the gear reduction ratio of a small electric-driven agricultural vehicle platform designed for conducting agricultural activities on farmlands to ensure maximum speed and load-bearing capacity.

Materials and Methods

Overview of a power transmission system of the 4WD multi-purpose electric vehicle platform

A low-capacity and small-sized (gross weight 4,022 N) multipurpose vehicle platform was designed for agricultural transportation, as illustrated in Fig. 1. The main components of the 4WD electrical vehicle were batteries, a controller, individual wheel motors, a reducer, and a battery management system (BMS). The vehicle was equipped with a direct-drive power transmission system, utilizing a gear-to-gear mechanism. An electric source with a voltage range of 220 - 230 V charged the batteries (4 × 100 amp), which provided enough power to run the electric motors integrated with commercial speed reduction gears into the powertrain. To boost the rotation of the wheels, a speed reducer was chosen, which efficiently transferred power to the axles.

The design and performance evaluation of the vehicle is significantly influenced by the power requirements for agricultural operations. The motor-driven system is affected not only by the size and structure of the vehicle but also by the complex working conditions and environmental safety considerations. In order to operate the off-road vehicle amidst agricultural field conditions, the selection of characteristics, including motor choice based on the total force, becomes necessary. Thus, the primary power source chosen for the 4WD electric vehicle was an electric motor. To evaluate the overall power requirements of vehicles under different driving conditions, an investigation was conducted to characterize the performance of the developed vehicle. Initially, an analytical power analysis was undertaken to calculate the power requirement of the 4WD electric vehicle based on equations for torque (load) measurement. The parameters were established according to the working loads and experimental surface characteristics. The assumed required power was employed to select the commercial motor based on the maximum workloads and the farmers preferences.

Fig. 1.

Overall layout of the four-wheel-drive (4WD) multipurpose electric vehicle. GH, gear head.

The selected electric motor, powered by a 24 V of direct current (VDC) battery, transferred the output power through the gear shaft to the vehicle wheels. The electric vehicle (EV) motor drive supplied high torque at low speeds, which was needed for starting and accelerating, and high power at high speeds, which was needed for traveling. The specifications of the electric motor are presented in Table 1.

On the other hand, the high-voltage battery is an essential element for the battery-powered agricultural electric vehicle. Therefore, four individual batteries with a capacity of 100 amps and 12 V each (ITX100-12, ATLASBX, Global Battery Company, Goyang, Korea) were selected and attached to the vehicle. The motor was powered by the batteries. Firstly, the batteries were connected in series to produce 24 V. Then a parallel connection was made between 24 V to assist in the generation of high electrical current or power for long-duration usage. In the battery connection, power is supplied from the battery pack to the distribution system. The distribution system then conveys the power to the respective motors of the 4WD electric vehicle. The motors receive power and are controlled in terms of speed by the speed control unit. The speed control unit modulates the power input to the motors based on the desired speed settings. Overall, the power distribution and speed control of the 4WD electric vehicle were facilitated through these interconnected components and systems. A flow diagram representation of the battery connection and motor power distribution of the 4WD electric vehicle is shown in Fig. 2.

Fig. 2.

A flow diagram representation of the battery connection and motor power distribution of the four-wheel-drive (4WD) electric vehicle.

Procedure of gear reduction ratio determination

The multipurpose electric transport vehicle was driven using an electric motor, and the output power came to the wheel axle through a speed reducer. To provide sufficient power to the vehicle wheel, a selection of gear reduction ratios is required to conduct the agricultural work at various payload conditions. The gear reduction ratio should be selected to be greater than or equal to the minimum value that can be included in the range within the performance curve of the electric motor under working load conditions. According to the desired traveling speed of the vehicle, a minimum gear ratio of 50 was applied in this study to be within the performance curve (motor-TN curve) of the chosen electric motor. A maximum ratio of reduction gear was selected considering the operating load conditions, and a low ratio of reduction gear was selected, recognizing the maximum traveling speed condition of the vehicle. The maximum travel speed of the existing electric vehicles was considered to be 4 km/h. The average value of the torque data was calculated at various payload conditions, and the measurement was applied to design the drive unit of the vehicle. The maximum ratio of reduction gear was calculated to satisfy the maximum traveling speed of 4 km/h using the maximum rotational speed of the motor selected in this study of 2,800 rpm (at the loaded condition), where the wheel radius of the vehicle was 0.19 m. The minimum gear reduction ratio was calculated using the following equation.

where, GR is the gear reduction ratio, Wheelrpm is the maximum motor rotational speed in rpm, R is the wheel radius of the vehicle in m, and VVehicle is the vehicle traveling speed in m/s.

The chosen gear reduction ratio was applied in the developed electric vehicle prototype, and the vehicle was rigorously tested and validated to ensure that the projected results corresponded to the actual performance. The evaluation included load measurements with the real field conditions under various load conditions and driving scenarios. The selected motor and gearhead attachments with the vehicle are shown in Fig. 3.

Fig. 3.

Electric direct current (DC) motor and gearhead selection: electric DC motor (A), gearhead (B), and motor and gearhead attachment to the four-wheel-drive (4WD) vehicle wheel axle (C).

Workload data collection on various farm roads

A data acquisition system was prepared to collect the torque data during the experimental trial under real field conditions. A load-measuring torque sensor (TRS605 FUTEK, Advanced Sensor Technology Inc., USA) was fixed to the rear wheel axle to measure the wheel axle torque of the electric vehicle. The torque sensor and the electric motor driveline were fixed with an alignment as they rotated at the same speeds. A motor rotation speed was controlled by an inverter (SV-iG5A; LS Electric Co., Ltd., Korea). To record sensor data, the NI 6212 and LabVIEW software (version 2010, National Instruments Corp., Austin, USA) were used. The placement of the torque sensor and the torque data measurement procedure are shown in Fig. 4. A high-precision inclinometer (SST420; Shanghai Vigor Technology Development Co., Ltd., China) was placed on the vehicle body to measure the inclination of the three experimental roads (asphalt, concrete, and grassland).

Fig. 4.

System for measuring wheel load using a load measurement device. Power transmission unit (A), Torque measurement process (B). DAQ, data acquisition; PC, personal computer.

The 4WD agricultural electric vehicle transportation activities were carried out at the Chungnam National University agricultural field facility in Republic of Korea in order to assess the performance of the power transmission systems. The vehicle was loaded with solid concrete bricks, resulting in trail payloads of 981, 2,942, and 4,903 N used for the experiment. The performance of the vehicle was assessed using the payloads of the vehicles at maximum of 4 km/h driving speeds on three different farm roads (on asphalt, concrete, and grassland) as shown in Fig. 5. To collect the gear torque data (load) of the vehicle, a data acquisition system was prepared. It was used to calculate the torque on the rear wheels by recording the rotational speeds of the wheel axle. Loads were evaluated to measure the torque on approximately 0 - 4°, 4 - 8°, and 8 - 12° sloped roads available in the experimental fields. For each experiment, the procedure was repeated three times and the averaged data was used for the power requirement evaluation. The reduction gear performance of the vehicle was evaluated according to the maximum payload capacity (maximum load 4,903 N) and slopes (maximum inclination up to 8 - 12°).

Fig. 5.

Field experiment for a multi-purpose electric vehicle during off-road conditions on concrete (A), asphalt (B), and grassland (C).

Results and Discussion

Working loads under different driving surface conditions

The maximum workload distribution characteristics on the driving surfaces of individual wheels of the developed 4WD electric vehicle were shown in Fig. 6. The toque data was recorded during the experiment to investigate the workload capacity of the vehicle under asphalt, concrete, and grassland road conditions. The analysis of selected reduction gear performances was conducted by considering maximum load scenarios under 8 - 12° random sloping conditions. Torque data fluctuated due to the slope variations during driving in off-road conditions (Kim et al., 2011b; Kim and Park, 2012). On asphalt roads, the load value increased frequently, but on concrete roads, the value fluctuated because of the vibration effects.

The torque value may be changed due to uneven terrain, obstacles, and variable surface friction coefficients on grassland roads. The maximum working load of 25.12, 26.64, and 39.50 Nm was recorded under 4,903 N payloads for asphalt, concrete, and grassland circumstances, respectively at 4 km/h driving speed on maximum slope conditions. These measurements showed the relationship between torque and speed during the experimental activities. The trial data indicated the varied torque characteristics of the 4WD electric vehicle under various road conditions, giving insight into its workload capacity and performance on various terrains corresponding to actual field conditions.

Fig. 6.

Torque characteristics of the four-wheel-drive (4WD) electric vehicle under asphalt (A), concrete (B), and grassland (C) conditions.

The findings obtained from the various payloads of the 4WD electric vehicles under off-road conditions provided useful information into the performance of the developed vehicle on different surfaces, including asphalt, concrete, and grassland roads. The load requirements at maximum sloping conditions for each road type are indicated by the experimental torque data presented in Table 2.

On asphalt roads, an average torque of 4.23 ± 4.37, 6.56 ± 3.59, and 8.77 ± 5.28 Nm was required for the agriculture vehicle to handle payloads of 981, 2,942, and 4,903 N, respectively. Similarly, on concrete roads, slightly higher torque values were observed, with average values of 5.65 ± 2.98, 7.46 ± 4.57, and 11.32 ± 3.84 Nm, respectively. On grassland roads, the torque requirements were found to be 5.43 ± 3.61, 6.65 ± 6.11, and 10.13 ± 5.76 Nm, respectively. Fig. 7 illustrated the torque results for loads of 5,003 N, 6,964 N, and 8,925 N, considering an existing vehicle weight of 4,022 N. The graphical changes in torque values exhibited a partially linear trend across the three different surfaces. The results suggested a similar pattern in the torque requirements for varying payloads and road types. Differences in surface roughness, traction, and resistance can be attributed to the variation in torque values across different road conditions. Smoother surfaces and better traction are generally offered by concrete roads compared to asphalt and grassland roads, which results in lower torque demands. On the other hand, grassland roads pose greater challenges due to their uneven terrain and lower traction, necessitating higher torque values to overcome the increased resistance.

Fig. 7.

The results of torque changing for loads of 5,003, 6,964, and 8,925 N, including the vehicle weight on asphalt, concrete, and grassland roads.

The selection of an appropriate gear reduction ratio is considered crucial in optimizing the performance of the 4WD electric vehicle (Hong et al., 2017). A pivotal role is played by the gear reduction ratio in converting the high-speed, low-torque output of the electric motor into a lower-speed, higher-torque output at the wheels. It can be observed from the torque data obtained in the experiments that significantly higher torque demands are experienced by the vehicle on grassland roads, suggesting the necessity for a suitable gear reduction ratio to enhance the vehicles off-road capabilities. In off-road conditions, the performance of the vehicles could be improved by a higher gear reduction ratio, as the torque output would be amplified, thereby providing better traction and increased power at the wheels (Kichler et al., 2011).

Evaluation of gear reduction ratio considering workload and vehicle speed

The original TN curve (torque and rotational speed curve) of the commercial motor used in the vehicle is shown in Fig. 8A, and the area of interest (AOI) segmented from the original TN curve provided by the manufacturing company to clarify the field experimental torque data inside the original TN curve is shown in Fig. 8B. The actual motor TN curve was tested in the company laboratory and showed the load-bearing capacity of the reduction gear. The motor power was assessed based on the driving operations under various road conditions. To determine the gear reduction ratio of the vehicle, the actual working loads and driving speeds were considered.

Fig. 8.

The original motor TN curve (A), and area of interest segmented from the original TN curve (B). TN, torque and rotational speed; AOI, area of interest.

Torque data for all driving conditions and payloads were plotted in between the motor performance curve (torque versus rotational speed) shown in Fig. 9. Furthermore, the applicable gear reduction ratio was considered based on the relationship among the motor TN curve, working load, and speed (Kim and Park, 2012; Lyu et al., 2016). The minimum gear ratio was calculated as 50, corresponding to the equation, using the maximum rotational speed of the motor at 2,800 rpm and the wheel radius. Under the experimental field conditions, the maximum rotational speed and the torque of the wheel were 1.79 rpm and 37.50 Nm, respectively. When the torque data were plotted inside the TN curve, they fell appropriately within the electric motor performance curve (TN curve). It was noted that off-road activities were possible within the rated capacity of the electric motor (2,800 rpm). Whenever the ratio of the reducer increased, the rotational speed increased, and the torque value shown was very near the TN curve line, and within the gear reduction ratio of 64, it satisfied the desired working load capacity. If the gear reduction ratio increased more from this level, the working load data would pass the motor TN curve, which might be dangerous for the overall power transmission system of the vehicle (Fig. 9). Therefore, it was confirmed that the gear reduction ratio should range from 50 to 64 to satisfy the off-road agricultural working load conditions and maximum driving speed criteria. In this case, if the lowest gear ratio was selected below 50, the vehicle could not be driven at its 4 km/h maximum driving speed. Therefore, in this study, a gear reduction ratio was set at 50 to satisfy the both maximum working load and driving speed of the electric vehicle.

Fig. 9.

Selection of the gear ratio (maximum 64) considering maximum working load and speed.

The development of a drive system for a 4WD small-sized multipurpose agricultural electric vehicle focused on combining different gear reduction ratios for commercial motors based on the actual working loads and driving conditions. The range of the gear reduction ratio was selected in accordance with a minimum gear reduction ratio to fulfil the desired working load (torque) condition, where a maximum gear reduction ratio meets the intended maximum driving speed of the vehicle. A gear reduction ratio was selected within the range of 50 to 64 to confirm the maximum output and the driving speed (i.e., the working capacity) within the area of the motor TN curve. To achieve the maximum torque and desired driving speeds, the motor specifications must be adjusted, and a multistage reducer with two or more stages should be used (Hadboul and Ali, 2022). Moreover, due to the difficulty of maximum load data used during agricultural work, a certain margin rate should be applied to the load data in consideration of future safety factors.

Conclusion

The research presents a comprehensive performance analysis of an electric vehicle under various agricultural working conditions. The selection of a suitable gear ratio was focused to develop a 1.6 kW 4WD electric vehicle for off-road applications. The results indicated maximum torque requirements of 25.12, 26.64, and 37.5 Nm for payloads of 5,003, 6,964, and 8,925 N, respectively, on asphalt, concrete, and grassland roads, respectively. The findings of this study contribute significantly to the ongoing efforts in developing sustainable and efficient electric vehicles for agricultural applications, paving the way for smarter and greener farming practices. To establish the range of gear reduction ratios, the study selected a minimum gear reduction ratio (1 : 50) that meets the working load (torque) condition and a maximum gear reduction ratio (1 : 64) that aligns with the intended maximum driving speed of the vehicle. Ultimately, the reducer gear ratio was chosen 1 : 50 which maximizes the output and driving speed. The study confirmed that these parameters allowed the vehicle to operate efficiently within the motor TN curve. In some cases, differences between the gear ratio required to satisfy the maximum torque and expected driving speeds may necessitate a change in motor specifications or the use of a multi-stage reducer with two or more stages. Additionally, due to the difficulty in obtaining precise data on the maximum load that occurs during agricultural work, the study recommends applying a certain margin rate to the load data, considering future safety factors. Finally, the study presents a method for selecting a gear reduction ratio using vehicle workload data obtained during a field trial in off-road circumstances. The results will be beneficial for manufacturers aiming to produce vehicles capable of handling multiple tasks in different agricultural fields. A comprehensive analysis of workloads and a thorough consideration of engineering principles will lead to providing well-balanced insights into the selection of suitable gear ratio that enhances the overall performance of electric vehicles and the driving experience.