Related: Automotive Powertrain
Hitoshi Aoki is the large project leader of the Acura RLX hybrid. The official name of the car is “RLX Sport Hybrid SH-AWD.” And if you sounded out the words for the acronym, that’s “Super Handling All-Wheel Drive.”
Either way, it’s a mouthful.
Yet when asked about the development of this car, whether it was created to be efficient or a performance car, Aoki answers, through an interpreter, with one word.
When asked whether the RLX Sport Hybrid has a drive motor in the rear in order to achieve a rear-drive capability (the non-hybrid RLX is front-drive)—actually all-wheel drive—or to provide torque vectoring (creating a yaw moment during cornering through the adjustment of the motor speed so that more power goes to the outside cornering wheel and the inner wheel can be braked through the application of negative torque), Aoki answers with the same single word: “Both.”
It is a car that is meant to be efficient and powerful. It is engineered so that it provides not only the ability to accelerate quickly, but to be able to go quickly through a curve, not just a straight line.
Which goes to two words that Aoki uses when describing what the engineering team worked toward achieving in the development of the vehicle:
Inomama: at the will of the driver
Fast and controllable.
And yet there still is the aspect of efficiency to the vehicle. The car is rated at 28 mpg city, 32 mpg highway, and 30 mpg combined. Notable when you take into account that the vehicle is rated at 377 hp, making it the most-powerful Acura available right now. (The 2005 NSX was available with a 290-hp V6; presumably the next NSX will trump the RLX Sport Hybrid, but until it shows up in a showroom and not just auto shows . . .)
Like the original RLX—which Aoki and his colleagues refer to as “RLX P-AWS” with the modifying acronym signifying “precision all-wheel steering”—the RLX Sport Hybrid has a 3.5-liter, all aluminum V6 under its hood that produces 310 hp @ 6,500 rpm. But the hybrid has an idle-stop feature and, consequently, revised cam timing so that the restarts are smooth.
The additional power comes from three electric motors. There is a 35-kW electric motor that is integrated with a seven-speed dual clutch transmission (which can be used simply as an automatic or with gear selection via paddles). This motor performs three functions. (1) It supplements the engine as needed in driving the front wheels; (2) it provides regenerative braking for the front wheels; (3) it works as a generator, transforming engine power into electricity for recharging the lithium-ion battery pack.
(Speaking of the battery pack: contained within the “Intelligent Power
Unit” (IPU) it is rated at 1.3-kWh and consists of 72 cells. The IPU also holds a 12-V DC/DC converter, junction board, and electronic control units for the motors and batteries. It is located behind the rear seat of the vehicle. This reduces the cargo capacity in the trunk to 12-ft3 from 15.1- to 15.3-ft (depending on the trim package) in the non-hybrid RLX. However, it offers 38.8 in. of rear seat legroom, which handily bests competitors including the Lexus GS Hybrid, Mercedes E-Class, BMW 5 Series, and Audi A6.)
The other two motors, both 27-kW, are housed in the Twin Motor Unit, a die-cast aluminum housing that is located in the rear, where a differential in an all-wheel-drive vehicle is typically located. (And while this is an all-wheel-drive car, unlike a conventional one, not only is there no rear diff, there is also no driveshaft, both of which mean less weight and fewer friction-related losses.) These motors are mounted back-to-back such that each powers one of the rear wheels. They can also provide negative torque (think: regenerative braking) to each wheel. And a clutch allows motor decoupling from the wheels.
Asked about the big challenges that faced the team in developing this three-motor parallel hybrid setup, Aoki responds that it was making sure that they could coordinate the speed of the rear motors as required. It’s one thing for the rear wheels to be at different speeds during cornering, but another entirely when simply propelling the car forward.
The RLX Sport Hybrid can launch purely as an electric vehicle, using the rear motors. Then as there is more pedal depression, the engine takes over and the front motor acts as a generator to charge the li-ion battery pack. Low-speed cruising is handled by the rear motors alone; the engine is not powering the wheels; high-speed cruising is accomplished via the engine. When there is hard acceleration from a stop, all three motors as well as the engine are brought into play; if it is a rolling hard acceleration the front motor isn’t used to power the wheels. Engine power and rear motor power are used to provide AWD capability under slippery conditions. When the vehicle is decelerating, the front and rear motors supplement the hydraulic braking system; some brake energy is captured and put in the battery pack.
(Speaking of the braking system: The car features an electric servo brake to provide a linear, stable braking feel during regenerative braking. And speaking of feel, there is a reactive force accelerator pedal with an electric mechanism in place of the typical spring mechanism that provides sensory feel through the pedal based on driving conditions (e.g., if the car is climbing a slippery hill, there is greater pedal resistance to encourage the driver to not get on the accelerator too much; if the forward collision warning system determines a crash is imminent, the pedal will push back even before there are visual and audible warnings.)
Asked why they developed a three-motor hybrid system rather than a more-conventional two-motor system for the RLX Sport Hybrid, Hitoshi Aoki uses more than one word in his answer, although one word is certainly the most operative: “If we had one, it would be really, really boring.”