Embracing Acura’s complex powertrain technology on a
spiritual level.
Most residents
of Japan identify themselves as Buddhist. After a private briefing on the
Acura RLX Sport Hybrid SH-AWD’s new powertrain by chief engineer Hitoshi Aoki
and hybrid system engineer Yasuo Kitami, we feel thoroughly enlightened. Makes
sense.
The Accord
hybrid’s new “no transmission” setup is complicated, and the Acura RLX Sport
hybrid’s system is complex enough to induce catatonia
Honda’s simple, effective, but slow-selling Integrated Motor
Assist (IMA) system is all but dead. The Accord hybrid’s new “no transmission”
setup is complicated, and the Acura RLX Sport hybrid’s system – consisting of
three electric motor/generators, a 3.5-liter V-6, and a seven-speed dual-clutch
automatic – is complex enough to induce catatonia. Let us now attempt to
enlighten you.
The big luxury sedan has four powered wheels, with the rears
driven by a pair of electric motors. Known as the Twin Motor Unit, it is very
similar to the TMU that will power the front axle of the new NSX.
The two rear
motors are coupled through a planetary gearset. During acceleration and
braking they act in concert. When the hybrid is accelerating from rest, the TMU
is the sole power provider if sufficient charge exists in the 1.3-kWh battery;
with gentle acceleration, it serves that role up to 50mph.
The big luxury
sedan has four powered wheels, with the rears driven by a pair of electric
motors
Each TMU motor has its own sun gear and
planetary carrier while both share one ring gear. Most of the time, the ring gear is held fixed, allowing each
36-hp motor to independently drive its assigned rear wheel. But while cornering, one motor adds
torque to the outside wheel as the other becomes a generator, slowing the
inside wheel, a phenomenon called torque vectoring. Since the TMU isn’t
coupled to the engine, it can torque-vector independently of throttle position.
The system releases the ring gear above 78mph to limit motor
speed and avoid overloading the power electronics. The TMU can still perform
its torque-vectoring trick above 78mph as long as the motors’ torques are equal
but opposite. And although the RLX hybrid probably won’t see many aggressive
80-mph corners, this capability will prove worthwhile in the NSX at all speeds.
And that’s only half of the powertrain.
Known as the Twin
Motor Unit, it is very similar to the TMU that will power the front axle of the
new NSX
Under the RLX’s hood lies a 310-hp V-6, the seven-speed
dual-clutch tranny, and the third electric motor in one transversely mounted
package. The motor is situated at the end of the transaxle opposite the engine,
a departure from the old IMA design that sandwiched the motor between the
engine and transmission. While the motor’s primary responsibilities are
starting the engine and generating electricity, it also contributes up to 109
pound-feet of torque under full throttle.
The dual-clutch trans receives motor torque through its
odd-gear input shaft while the engine sup-plies torque through both even- and
odd-gear input shafts. It’s an arrangement similar to a regular dual-clutch,
which prepares for a gearchange by engaging the next ratio on the input shaft
attached to the disengaged clutch. In Honda’s transmission, though, those gears
aren’t just spinning along waiting for the pending ratio change, they’re
transmitting electric-motor torque.
