Lexus RX 500h Hybrid takes a page from Honda’s playbook
The 2023 Lexus RX has just been unveiled! It comes with four different powertrains:
- The base RX350 is powered by a turbocharged 2.4-liter four-cylinder engine, which connects to an eight-speed automatic. Front-wheel drive is standard, all-wheel drive is an option.
- The hybrid RX350h has a 2.5-liter four-cylinder engine paired with an electronically-controlled continuously variable transmission (ECVT) and all-wheel drive. The ECVT is also known as Toyota Hybrid Synergy Drive (HSD), which is the system that Toyota has been using since the first Prius, although it is improved with each new generation. It has two electric motor-generators, a planetary gear set and no clutches.
- A plug-in hybrid model, dubbed RX450h+, will join the lineup later. Lexus hasn’t released specs on it yet, but it will be likely based on the RX350h.
- And finally, the RX500h F Sport Performance combines the turbocharged 2.4-liter engine and electric power, delivering it to all four wheels. This hybrid system is different from the RX350h, and this is the model that I am going to discuss in more detail.
The RX500h uses a new hybrid transmission that forgoes Toyota’s signature HSD system and instead uses a dual-clutch direct-shift six-speed automatic gearbox (DSG) aided with a single electric motor. The electric motor is positioned between the combustion engine and the transmission, which has a clutch on both sides.
The rear wheels are driven with an “e-axle”, basically an electric motor paired with a differential. There is no mechanical linkage between the engine and the rear wheels, so in this regard the system is similar to the RX350h.
One can try to trace the origins of the new hybrid system from the GS450h, introduced by Lexus in 2006. its Lexus Hybrid Drive system (LHD) combined ECVT power-split device with a two-stage motor speed reduction device.
Ten years later, Lexus developed the Multi Stage Hybrid Transmission for the Lexus LC500h coupe, which replaced a two-stage transmission with a four-stage one to increase torque and reduce engine RPMs at high speed.
I suppose that the engineers realized that combining a rubbery character of the ECVT with four noticeable shifts of the mechanical gearbox would provide an unacceptable driving experience, so they programmed the ECVT to have three fixed shift points, obtaining nine shift points altogether (three ECVT “shifts” per each of the first three gears in the “speed reduction device” plus one ECVT setting for the fourth gear in the “speed reduction device”, ten “gears” altogether).
The marketing department turned the old-school shifts into a sporty-like feature that ostensibly allow a more direct control of the car’s acceleration.
The RX500h has six gears in the transmission, and probably by this time the engineers figured that the ECVT did not provide the advantages it was designed for, so they replaced the power-split device with a single electric motor, producing a system remarkably similar to Honda’s i-DCD system (intelligent Dual-Clutch Drive), which also utilizes a pancake-shaped electric motor sandwiched between the internal combustion engine and the dual-clutch transmission.
It is interesting that Honda employed i-DCD on full range of models from the small Fit to the MDX Sport crossover, and it seems to scale well, although the smaller variants of this transmission use dry clutches, and the larger versions use wet clutches.
As for the Hybrid Synergy Drive, it seems that despite having beautiful design from the engineering standpoint, it does not scale up, which is why the RX500h has a a more traditional setup with a multiple-gear transmission and clutches. Once again, Lexus’s marketing department have spun the kicks in the butt as “rhythmic shifts of the 6-speed automatic transmission [that] serve up pleasurable driving”.
Toyota uses a similar approach for its Tundra pick-up, although in a longitudinal setup, combining a relatively traditional 10-speed automatic transmission, a single electric motor and a torque converter in place of a clutch pack.
