# Numerical Performance Investigation of a Hybrid eCVT Specialized Agricultural Tractor
## 3 Hybrid eCVT Powetrain  
### Architecture and Components  
The proposed hybrid architecture consisted of a downsized thermal unit and two reversible electric machines. The main component of the transmission was a two-stage planetary gear that allowed for achieving a power-split hybrid configuration, and thereby electronic continuously variable transmission (eCVT). In detail, the ICE was mechanically connected through a clutch to the first electric machine (EM\({}_{1}\)). The drive shaft was also linked to the sun gear of the second planetary gear stage and, with a clutch, to the power take off (PTO). The rotor of the second electric machine (EM\({}_{2}\)) was mechanically connected to the sun gear of the first planetary gear stage, whose ring gear was fixed to the chassis. The carrier of this first stage was connected to the crown of the second planetary gear stage. Finally, the carrier of the second stage was connected to the drive shaft of a two speed gearbox which output were the rear and front axles differentials. Figure 4 shows the proposed powertrain layout. This configuration allowed for having no direct connection between ICE and wheels. However, with a proper control, it could allow for the joint cooperation with the electric machines to have full power at different vehicle speed levels. Through the disengagement of the clutch between ICE and EM\({}_{1}\), the powertrain was able \begin{table}
\begin{tabular}{l c c c} \hline \hline  & **Shredder** & **Atomizer** & **Rotart Harrow** \\ \hline Min power (kW) & 12 & 32 & 36 \\ Mean power (kW) & 19 & 36 & 44 \\ Max power (kW) & 26 & 40 & 52 \\ Vehicle speed (km/h) & 5 & 5 & 5 \\ ICE angular speed (rpm) & 2244 & 2244 & 2244 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Relevant characteristics of proposed implement-related activities [22]. Figure 3: Typical agricultural gearbox configuration for maximal travel speed coverage. to operate in full electric mode using the two electric machines together for the best use of the electric energy stored in the battery pack. Lastly, the position of EM\({}_{2}\) in this architecture enabled regenerative braking, although the impact was negligible if not for very special work scenarios. The use of a downsized ICE for this HeCVT tractor was motivated by the following considerations:  
* Specialized tractors are usually equipped with high power diesel engines but, for a large part of their operative life, they work at medium-low load, which translates into suboptimal overall efficiency, and thus excessive fuel consumption. Instead, a downsized ICE working at higher percentage loads that are closer to the nominal design value can achieve better fuel efficiency. * Tractors' emission limits are imposed by the NRMM regulation, which classifies engines according to their application and nominal rated power. The larger the engine, the higher the pollutant production per unit of work. Therefore, an oversized diesel engine has stricter restrictions about the pollutants that it is allowed to produce; thus, it would require large volumes for exhaust gas aftertreatment systems. This is not compatible with compact specialized tractors. The power and torque curves of the downsized engine considered for the HeCVT tractor are shown in Figure 5. In this case, the proposed torque and power curves were also derived from data available online for commercial diesel engine units having nominal characteristics as close as possible to the desired ones. The properties of the two electric machines are reported in Table 3. The numerical model of the EMs defines electrical losses as the sum of the following terms: fixed converter losses, ohmic losses (torque-dependent), and iron losses (speed-dependent) due to eddy currents. In some difficult situations, electric machines can count on a temporary boost function. It consisted of an increase in available torque due to the use of a higher level of current.