Acura RLX Sport Hybrid has impressive economy

  • 12-Dec-2013 10:01 EST
2014_Acura_RLX_Sport_Hybrid_15.jpg

The Acura RLX Sport Hybrid exterior features LED headlamps and fog lamps.

The "power costs fuel" rule has been repealed by the 2014 Acura RLX Sport Hybrid midsize sedan. It's the most powerful vehicle the Honda luxury division has ever built at 377 hp (281 kW), but U.S. EPA fuel economy is an impressive 28 mpg city/32 mpg highway. There's been no decontenting to save weight, as this version introduces major upgrades to its Super-Handling All Wheel Drive (SH-AWD) system, using electric motors to add both a higher degree of precision as well as improved fuel economy.

The RLX hybrid system incorporates three electric motors, one per rear wheel, each rated at 27 kW and 54 lb·ft (73 N·m), and one in front rated at 35 kW and 109 lb·ft (148 N·m). Because the peaks occur at different rpm, integrating all three with the 3.5-L V6 rated at 310 hp (231 kW) produces the combined rating of 377 hp (281 kW) at about 6000 rpm. Peak torque is 377 lb·ft (511 N·m) at 4600 rpm.

The V6 is direct-injected, and has Honda's Variable Cylinder Management (cylinder deactivation), i-VTEC (timing and lift control), along with two-stage oil pressure control and an engine oil warmer. As a full-feature hybrid, it has an electric-drive A/C compressor so there is no loss of cooling during an idle stop. An active engine control mount combines with retarded intake valve timing to smooth restarts, and in our test drive the restarts were absolutely seamless.

The RLX can operate in an electric vehicle (EV) mode using only the two rear electric motors, getting power from the lithium-ion battery pack. The rear motors can launch the vehicle, in a mode called EV launch, and also can operate in low-speed cruise up to about 50 mph (80 km/h), in a mode called EV cruise. The front-drive powertrain propels the car on gentle acceleration and operates the front motor as a generator to charge the battery pack (engine-drive mode) and also at high speed (engine-cruise mode). On hard acceleration, the three motors assist the engine (hybrid mode).

Three motors in regen braking

On deceleration, the engine is disconnected from the transmission, and front and rear motors operate as generators to recharge the battery pack—all three in regenerative braking mode. In slippery driving conditions, the engine and two rear electric motors operate to provide AWD. The vehicle is unlikely to be accelerating, so the front motor is not being used at this time. However, if battery pack capacity is low, the engine also uses the motor-generator to recharge the batteries.

The pack is small (this isn't a plug-in hybrid), rated at 1.3 kW·h and 260 V, and combined with its IPU (Intelligent Power control Unit) is behind the rear seat. It takes relatively little trunk compartment space, leaving 12.0 vs. 15.3 ft³ (340 vs. 433 L) for the non-hybrid. The motors' control system or PDU (Power Drive Unit) is in the underbody area that in a non-hybrid would be occupied by the propeller shaft.

The front powertrain transmission is a "seven-speed"—a planetary gearset for first plus a six-speed wet-type dual clutch transmission (DCT). The motor is at the front of the transmission, the planetary gearset next, followed by the DCT.

Mechanical response time for the hybrid system is 10 ms, according to Yasuo Kitami, Hybrid System Assistant Large Project Leader.

The non-hybrid SH-AWD uses electromechanical clutch packs to distribute torque between the rear wheels. The hybrid version provides more precise distribution with the individual rear electric motors because they operate independently of the front powertrain.

Those rear motors are in a dual motor housing with a double pinion gear system. The front drive system electronics set a target force for the rear motors using steering angle and its rate of change, yaw moment, lateral acceleration, and vehicle speed.

On turns, the outside wheel motor turns that wheel faster. As in the SH-AWD non-hybrid, this provides positive torque to help push the wheel through the turn. When cornering in a tight turn, the inside wheel slows down, which creates a pull (negative torque), so the inside motor operates (regenerative braking) as a generator, and can feed more power to the outside wheel motor. The overall effect is push-pull.

Transitions a challenge 

Although three electric motors provide precision and calibration flexibility for the control electronics, the transitions from one mode to another are a challenge, Kitami told AEI. Going from EV launch to starting the engine and driving the front wheels was one example he cited. Another was the change to AWD if the need arose from a change in road surface. Here again, in the AEI road test the transitions were smooth.

Honda has been developing custom versions of its ACE (Advanced Compatibility Engineering) front body structure to get good ratings for all new vehicles in the IIHS (Insurance Institute for Highway Safety) narrow offset barrier test. The RLX is the latest example. The Acura Division stated it expects Good ratings in this test as well as four other IIHS tests (frontal and side collision, rear/head, and roof strength). The new RLX body is 55% high-strength steel by weight. Of that total, 7% is hot-stamped 1500 MPa yield grade, 7% is 980 MPa, and 6% is 780 MPa.

The RLX has a 112.2-in (2849-mm) wheelbase and 196.1 in (4979 mm) overall length, and the SH-AWD hybrid system adds weight, so even with aluminum hood, door panels, front fenders, and both bumper beams, it's up over 300 lb (136 kg) to 4312 lb (1956 kg) vs. the front-drive RLX.

28/32 vs. 20/31 mpg

The 28/32 mpg fuel economy numbers for the Sport Hybrid are impressive, certainly compared with the lighter front-drive car's 20/31.

Improvements in headup displays, and the industry's growing belief that it can help reduce driver distraction, will be tested in the RLX. Its HUD is Acura's first, and shows road speed, compass heading, turn-by-turn navigation, active cruise control setting, and lane-keeping assist, as well as torque vectoring from the SH-AWD.

The RLX competes against the Infiniti M and Lexus GS, both of which also are available in hybrid versions.

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