The diffusion of Full Hybrid and Plug-In hybrid cars is imbued with the engineering charm of the transmissions necessary to couple the different engines that contribute to traction. The market offers a wide variety of solutions, with a differentiation in the architectures and control logics of the powertrain. Therefore, there is no lack of points of appreciation for the work carried out by the technicians of the individual houses, to fine-tune transmissions that appear as real works of high engineering.
One of the best-known architectures in the automotive landscape is the gearbox E-CVT from Toyota. The system sees the two electric motogenerators and the combustion engine engaged on three coaxial shafts, coupled through a planetary gearing. The input information for the control logic are the speed at the wheels and the driver’s torque request: thanks to the planetary transmission, the control unit modulates the rotation speed of the electric motors, so that the consequent engine speed necessary to satisfy the request to the pedal corresponds to the condition of maximum possible combustion efficiency. The nature of the E-CVT is therefore such that, at a given wheel speed, different speeds of the electric motors may correspondwhich are adapted according to the torque demand to allow the thermal engine to work at the maximum efficiency speed and load.
While extremely compact and effective, Toyota’s E-CVT transmission is not the only one on the market. The E-Tech technology for Renault’s electrified models offers, for example, an alternative embodied in the gearbox Multimode. The initial studies on this architecture date back to 2014, when Renault implemented and made it commercial the technology developed by the Formula 1 team, so as to respond to Toyota with a radically different solution from the E-CVT. The Multimode, present on all models of the Renault hybrid range, is a gear cascade system. Contrary to the planetary gearing of the Japanese house therefore, the rotation speed of the different powertrain motors is linked to that of the wheelsalbeit appropriately scaled through the various transmission ratios.
The Renault gearbox has three shafts, on two of which are keyed respectively the electric drive motor and the combustion unit. Overall there are two gears for the electric propulsion and four for the thermal one, which including the respective idlers provide a total of fifteen different combinations for the coupling between the two motors. The selection of the coupling mode based on the driving conditions refers to a map which, according to the speed and torque demand, identifies the most energy efficient configuration. The only combination to be excluded is the first electric gear coupled to the first gear of the thermal engine, as the starting from standstill always takes place entirely in electric. The first electric gear allows you to reach up to 75 km / h, while with the second you can reach even 160 km / h.
A third electric motor generator is added to the two engines already mentioned, the combustion one and the electric traction unit. This is theHSG, which stands for High Start Generator, directly engaged on the third shaft of the Multimode gearbox. Four main functions are associated with the HSG: firstly, it can be used to recharge the battery using the heat engine as a generator, like a traditional series hybrid. Not surprisingly, the E-Tech hybrid powertrain has two dedicated inverters, one for the HSG and the other for the electric traction motor. The second function performed by the HSG is the ignition of the combustion engine, which thanks to the 400 V voltage and 50 Nm of torque is almost instantaneous. Furthermore, considering how the vehicle is always started electrically and therefore the thermal engine takes over when the car is already in motion, the HSG accelerates the combustion engine up to the gearbox engagement speed. Precisely the ability to accelerate the thermal is exploited when shifting: the HSG acts as a synchronizer, regulating the speed of the combustion engine with that of the gearbox output shaft. The transmission is thus presented with frontal clutches and is devoid of synchronizers, a feature that helps to limit overall dimensions and weights, contained in the order of 50 kg. Finally, in certain contexts the HSG supplies torque to the internal combustion engine, indirectly contributing to traction.
Overall, the Toyota E-CVT and Renault E-Tech’s Multimode embody two polar opposite interpretations of hybrid transmissions, but equally valid. In this way the houses have ensured their own design identity, distinguishing themselves from the competition and each proposing a concept of its own invention.
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