Proportion is one of three “P”s that Honda R&D engineers and designers focused on when developing the all-new 2015 Acura TLX, the other two being prestige and performance—understandable targets for a luxury sport sedan.
“The size of the outgoing TL didn’t clearly communicate its underlying performance capabilities or its position as a true midsize sports sedan,” Mike Accavitti, Senior Vice President & General Manager, Acura Division, explained at the TLX’s media launch event in rural Virginia in late July. “At the same time, the TSX had the right proportions to make it a successful midsize sports sedan, it just lacked the full performance chops and the refinement of its bigger competitors.”
The new TLX, therefore, is positioned between the two outgoing sedans in size. Compared to the larger TL, the new sedan is 0.5 in (13 mm) lower in height at 57.0 in (1448 mm), 1.0 in (25 mm) narrower at 73.0 in (1854 mm) wide, and its overall length, at 190.3 in (4834 mm), is nearly 4 in (102 mm) shorter—all of it taken out of the overhangs.
The 109.3-in (2776-mm) wheelbase is unchanged from the outgoing TL, “so the spacing between the front and rear passengers remains the same—the same knee clearance,” said Mat Hargett, Large Project Leader (aka, Chief Engineer) of the 2015 TLX.
“Integral throughout the design and styling process is aerodynamics,” Hargett said. The frontal area of the TLX is 1.5% smaller than the TL, contributing to a CdA reduction of 15%. Acura’s internal coast-down testing, conducted back-to-back with competitors such as the BMW 328i and Mercedes-Benz C-Class, revealed best-in-class aerodynamics for the TLX, he noted.
Development of the new TLX occurred largely in the U.S., Hargett shared with Automotive Engineering. Of the 30-40 team members, about 95% are located in the U.S., he said.
“Other than the new fusion system—the integration of the camera and radar—which was mainly led in Japan, the new SH-AWD (Super Handling All-Wheel Drive) system, which was a co-development between the U.S. and Japan, and the 2.4-L powertrain team for both the engine and transmission, [development] was all done in the U.S.,” he said.
With development of the next-generation NSX supercar also taking place in Ohio, will there be any technology-sharing between the two cars, on systems such as the next-gen SH-AWD? “There probably will be,” Hargett offered. “Some of the improvements in the logic and things like that, I would imagine they’ll use. But I’m not intimately involved with that.”
The NSX is expected to debut in the U.S. next year.
In response to perceived performance deficiencies in the two outgoing cars, the new TLX gets two “Acura-exclusive” direct-injected i-VTEC engines, two new transmissions (go to http://articles.sae.org/13432/ for more on the “world’s first” eight-speed DCT (dual clutch transmission) with torque converter, and two next-generation precision handling technologies—SH-AWD and Precision All-Wheel Steer (P-AWS).
“We brought more new powertrain technology to bear [in the TLX] than any Acura model ever,” said Accavitti.
The 2.4-L four-cylinder engine produces a peak 206 hp (154 kW) and 182 lb·ft (247 N·m).
“This is not a carryover engine from TSX,” Hargett stressed. In addition to the direct injection, engineers increased the compression ratio from 11:1 to 11.6:1, improved power output by 5 hp (4 kW), and increased peak torque by 5% (up to 10% in the mid-range).
The engine has a new dual-stage intake to improve torque. “At lower speeds, you use the long runner; at 3900 rpm, you switch over to the short runner to keep a wider torque band, from about 1000-5000 rpm,” Hargett said. Mated to the 2.4-L is the eight-speed DCT.
Though the TLX is larger and heavier than the outgoing TSX four-cylinder, its EPA fuel-economy ratings of 24 mpg city (+2 mpg), 35 mpg highway (+4 mpg), and 28-mpg combined, are better.
A new 3.5-L V6 engine replaces the 3.5- and 3.7-L engines in the TL. The new powerplant gains 10 hp (8 kW) at 290 hp (216 kW) and 13 lb·ft (18 N·m) at 267 lb·ft (362 N·m) compared to the previous 3.5-L unit. “Although peak torque is up 4-5%, it’s up in some places by 12% for a flatter, wider torque band,” Hargett said.
Variable Cylinder Management automatically deactivates three of the engine’s six cylinders under light engine loads. TLX SH-AWD models also add a new automatic idle-stop feature, with a new 28-V Active Control engine mount that helps to reduce engine vibration during restarts; peak vibration is reduced by 27%, according to Hargett.
Acura’s first application of a nine-speed automatic transmission, developed with ZF, is mated to the V6. “Geared toward performance, the first seven gears are lower than the outgoing 6AT’s sixth gear (2.067:1), and the top two gears are higher for better fuel economy,” Hargett said. The 9AT’s first overall ratio (gear x final drive) of 16.597:1 is 33% lower, and ninth gear’s 1.690:1 is 18% higher.
The front-wheel-drive TLX with V6 and P-AWS has an EPA fuel-economy rating of 21/34/25 mpg city/highway/combined (a 17% increase for highway vs. the 2014 TL with 6AT). The TLX SH-AWD is EPA-rated at 21/31/25 mpg (a 19% FE-ratings gain).
For the next-generation SH-AWD system, Honda engineers redesigned the entire system, including the rear differential, Hargett said. The new rear-drive unit is constantly overdriven by 2.7% (compared to 1.7% on the previous system), providing a greater torque-vectoring effect across a broader spectrum of driving situations.
“We spin the rear wheels 2.7% faster than the fronts; that effectively allows us to create that yaw moment at a 15-m radius [vs. 21 m previously], which means at intersections, where people drive every day,” Hargett explained.
Despite the unit being much more compact and 25% lighter, “we didn’t sacrifice performance,” Hargett stressed. “It still delivers 1200 N·m maximum during torque vectoring left or right, same as the outgoing TL SH-AWD rear differential.”
Engineers reduced the size of the unit by eliminating the electromagnetic clutches and the planetary gears that drive them and integrated the lubrication pump. A hydraulic actuator with single-motor design is employed.
“The single electric motor controls a dual pump, which independently controls the hydraulic pressure to both the right and left clutches,” he explained.
The height of the system is reduced by 8%: “So the floor is now flat for the AWD customer—a huge complaint that we fixed,” he said. Drag is also reduced by 2%.
Engineers also made logic improvements to SH-AWD. “So in cornering and low to mid speeds, instead of just looking for a buildup of lateral g’s in the car, as soon as the customer moves the steering wheel we start torque vectoring in the rear,” Hargett explained. “And now for the first time in our conventional SH-AWD systems, for a highway lane change—with high speeds and smaller inputs—we actually send torque to the rear inside wheel, creating a counter-yaw moment that keeps the car from rotating.”
To improve slippery road performance, SH-AWD has been integrated with Vehicle Stability Assist (VSA). The systems now work cooperatively to optimize torque distribution.
Up to 90% of available torque can be transferred to the front wheels during normal cruising. In hard cornering and under acceleration, up to 70% can be directed to the rear wheels. Up to 100% of the torque sent to the rear axle can be applied to either the left or right rear wheel.
The next-generation P-AWS (1st gen system appeared on RLX) also features all-new hardware and logic. Key components of P-AWS include independent electrically assisted left/right toe-control actuators, special control arms and suspension uprights, and an ECU that is linked via a dedicated CAN to the electric power steering, VSA, braking, and engine control systems.
The system continuously monitors and calculates the correct amount of rear-toe angle input necessary for the conditions, analyzing steering input, engine and road speed, transmission, throttle position, brake pressure, yaw rate, lateral acceleration, and other factors. The maximum amount of rear-wheel toe angle adjustment is 1.8° positive or negative for a total range of 3.6°.
High-speed maneuvering capabilities of both P-AWS and SH-AWD models are further enhanced by Agile Handling Assist (AHA), which uses the TLX’s VSA and six-piston brake modulator to create a yaw moment, helping the driver more easily trace the desired line with smaller steering-wheel inputs. AHA stabilizes the car earlier than conventional VSA.
The SH-AWD version of the TLX is 234 lb (106 kg) lighter compared to the outgoing TL SH-AWD. The two-wheel-drive TLX V6 is 150 lb (68 kg) lighter than its TL predecessor.
In addition to the significant weight savings from the new SH-AWD system, the nine-speed AT is 66 lb (30 kg) lighter than the outgoing 6AT.
“Going from a double wishbone front suspension to a front MacPherson strut is a significant weight down,” Hargett added. “There’s a lot of [weight-reduction examples] all over; you have to attack the vehicle holistically. The brakes—even though they offer 30% better judder performance and reduced stopping distance—are 11 lb (5 kg) lighter vs. the outgoing system.”
Stiffer body for premium feel
The foundation to improve both dynamic performance and passenger comfort is a rigid, lightweight body. To make the vehicle stiffer without adding a lot of weight, engineers applied 60% advanced high-strength, lightweight materials on the TLX.
High-strength steels (HSSs) of 340 MPa (49 ksi) and above comprise 52% of the new TLX’s body-in-white (BIW), with 35% of that being HSS 590. The TLX features a one-piece, UHSS hot-stamped door ring—1500-MPa (218-ksi) Usibor steel from ArcelorMittal—that was first applied on the 2014 Acura MDX, for enhanced occupant protection and body rigidity. Magna’s Cosma International provides the hot stamping.
Aluminum is used for the hood, front and rear bumper beams, and front subframe (hybrid Al/steel), making up 6% of the TLX’s body weight. And a three-piece magnesium steering-hanger beam, located behind the instrument panel, replaces the 26-piece aluminum and steel unit in the previous TL model, saving 2.7 lb (1.2 kg). The magnesium beam comprises 2% of the body weight.
(Go to http://youtu.be/XOSTJzDD-FU to watch Acura engineer Dan Powderly give more details on the TLX’s body-in-white engineering.)
NVH reduction was another focus for the engineering team. “What we’ve been trying to do at Acura is seal the body,” said Hargett. TLX engineers applied a lot of the same NVH technologies from the 2014 MDX (read more at http://articles.sae.org/12225/), but for the first time the company has applied a Dow acoustic spray foam at 10 different locations around the cabin—two spots in the C-pillar, one large placement in the B-pillar, and two in the A-pillar.
The result of this effort is 50% less body leakage over the outgoing TL and best-in-class road and wind noise, according to Hargett.
Despite the extensive lightweight material usage, the TLX’s BIW is heavier than that of the outgoing TL. “The reason: not only crash [performance] but we also wanted to significantly increase torsional stiffness and the mounting rigidities, the suspension fitting points,” Hargett said.
The structure’s torsional rigidity is up 21% vs. the previous TL, and mounting-point rigidity is 25% greater.
“There’s one big thing related to torsional rigidity vs. the outgoing TL—the TL had a fixed rear seat, which is an essential part of the body structure,” Hargett explained to Automotive Engineering. “We wanted to offer our customers 60/40 split fold-down seats. Now all of a sudden, you lose that as a piece of structure in the overall body torsional rigidity.”
To compensate, braces were added where the seats fold down.
“[The BIW] got heavier for a multitude of different reasons,” he said. “So that’s why when developing a car you need to take a holistic view—where you can take weight out and where you can’t…We didn’t want to trade-off performance at all, so we attacked elsewhere.”