INTERVIEW: SIX QUESTIONS FOR LEXUS LC HONCHO KOJI SATOBlog Apr 5th, 2016
The Toyota Motor Corporation advanced the hybrid art at the New York auto show by making two important introductions. The Toyota Prius Prime advances the state of the ultimate-economy hybrid art, and the Lexus LC 500h’s similarly unique new 10-speed two-motor system, making its North American debut, advances the art of the performance hybrid. The goal was to deliver performance almost identical to that of the mainstream 5.0-liter V-8 version while delivering small V-6 fuel economy. We had a chance to touch base with the LC’s chief engineer, Koji Sato, to ask a few questions about this cool new system and to check in on the progress of final vehicle tuning, which is being done with input from MT’s own hot shoe Randy Pobst.
Motor Trend: So tell us a bit more about this hybrid drive. How do you get 10 ratio steps out of a four-speed transmission and a two-motor planetary gear set?
KS: The system hardware for the multistage hybrid system involves motor-generator 1 and motor-generator 2 [connected by an E-CVT planetary gear set], and then after these two motors is [a four-speed] automatic transmission. In the old systems, all the power from the engine was going to the two motor-generators and then to the drive shaft. But in this system, the engine torque can go through two paths. One is directly to the transmission, and then also the electrical force can be added to it. That means that this E-CVT can be realized in many steps here that enhance output power by using the four gears. The main purpose of the system is creating a fun-to-drive feel by hybrid.
With previous [Toyota/Lexus HSD] hybrid systems, the engine revs go high, and then acceleration is linear but not so emotional. But our system can expand the acceleration area for the engine. In this system the automatic transmission helps us to enhance the engine acceleration area. From the low speeds we can create maximum engine output at low rpm by using the motor-generator output. So that’s why we can enhance traction force from low speeds. Plotting traction force versus vehicle speed, with this system the traction force is almost double. It’s more than that of the V-8 hybrid system in the LS even though we are using a 3.5-liter V-6 engine. This is because of the additional powertrain output. The traction force greatly exceeds the tire grip limit, so this hybrid system can make a burnout.
This is why we are bringing a hybrid to this segment of car. The hybrid is more unique and engaging. There are shift paddles, and an M (manual) mode. The hybrid will be slightly slower, but acceleration out of corners will be more responsive. Randy has driven the Hybrid and loved it. It is not track optimized, but the transmission is good for that duty. It will slow down somewhat as the battery is depleted, but this hybrid feels much more mechanically connected. The LC provides two different dynamic recipes—V-8 and hybrid, with the hybrid getting 30 percent better fuel economy than the LS 600h did.
(Final tuning is being completed, so Sato-san could not comment on economy relative to the V-8.)
So that is our image of this hybrid, why we are focusing on the hybrid model. If you think about the motor and engine—the dual source—the motor torque can be implemented much quicker.
MT: Will it accelerate more quickly than the standard car?
KS: The gas model’s 5.0-liter gas V-8 is a little quicker. The big difference is accelerator response when you’re in a corner. Better pickup and more responsive.
MT: On a track, would the battery deplete and slow it down?
(Pointing to several graphs and charts, Sato-san explained that because most of the driving force is coming from the engine output, there would only be certain lower-speed corners where the electric energy would be of great benefit, and hence the car would be slower on a second or third lap when the battery pack was depleted.)
MT: Randy Pobst has sampled the V-8 car three times, making suggestions based on his drive of each prototype stage. Can you relate a few stories about key areas where his input lead to important design changes?
KS: Always, Randy’s drives are fun for me. We respect him and his feedback. Our third trial was in December 2015 at Willow Springs. That car implemented all the conditions that were designed in, with upper body improvements like side doors made 22 kg [48.5 pounds] lighter by using aluminum outside and carbon-fiber inside panels and fenders that save 3 kg [6.6 pounds] using aluminum. All of these items were included. And the brakes were a level better. The transmission was a key point of the checking trial, so we gave him the latest version of the 10-speed automatic. Our intention was to improve the rhythm in driving with close ratio steps. This gives the driver a more emotional rhythm. Here again he approved. Randy basically validated most of our intuitions. But he gave us two homework items.
One was the rear-steer system. Sometimes the rear-steer systems give an unnatural feel for a high-skilled driver like Randy. So we checked the data to find the root cause of this condition. (Sato and his engineers rode along with Randy to feel for themselves what he was saying and then looked for evidence of what he was talking about in the data for the specific corners that he had issues with at the track.) It’s been fixed and checked already, and we’ll give it to him for his next ride (which is scheduled for May 2016).
Brake feel was another request coming from Randy. In some corners at Laguna Seca, he felt he needed more confidence in high-g areas. And with this one also we found out what the root cause was and then modified it. We are already finished, and that one also will be given to him in his next ride.
MT: Will the car still bottom out when driven hard at MRLS, as Randy mentioned in our story?
KS: That one also, we found out what happened there—the suspension damping setting was not enough for the big high-g areas. It’s kind of a tuning-level issue.
MT: And I presume it will be possible to completely switch off the stability control on the production car?
KS: Yes. Now all our vehicles can be operated without VSC.