Powertrain

POWERTRAIN OVERVIEW

The RL powertrain is a complete departure from that of its predecessor, the 3.5 RL. Though both vehicles feature 3.5-liter V-6 engines, the new RL benefits from VTEC, a 5-speed automatic with Sequential SportShift, paddle shifters and Grade Logic Control (in place of a 4 AT in the 3.5 RL) and Super Handling All-Wheel Drive in place of front drive in the 3.5 RL. Plus, there are hundreds of other differences, both minor and major. One of the most significant is that like other Acura vehicles, the crankshaft of the RL is now positioned transversely (side to side) instead of longitudinally (front to rear), as was the case in the 3.5 RL. This change allows the RL to be packaged more tightly for better handling agility, without sacrificing interior comfort.

The 60-degree aluminum-alloy 3.5-liter V-6 of the RL features architecture that's related to that of the powerplant in the 2004 MDX. Few parts are shared between the two engines, however; comprehensive changes give the RL powerplant an entirely different character than that of the MDX. This new engine is very lightweight and compact: Compared to the previous 90-degree V-6 in the 3.5 RL, the overall width (measuring across the "V") is 36 mm less, and the length (measuring along the crankshaft) is 61 mm shorter.

The advanced new VTEC™ engine develops 300 horsepower and 260 lbs.-ft. of torque, significantly more than the previous generation 3.5 RL (225 horsepower and 231 lbs.-ft.). Horsepower has increased by 33 percent and torque has risen by 12 percent. Despite this substantial performance upgrade, fuel economy (18/26estimated City/Highway) is better than that of the previous generation 3.5 RL (18/24 estimated City/Highway). And in terms of emissions performance, the RL meets CARB LEV-2 ULEV requirements, a much tougher standard than the LEV standard met by the 2004 3.5 RL.

While it is as powerful as some competitive V-8 engines, the high output V-6 in the RL is lightweight for better acceleration and more nimble handling. To put the power to the ground with a high level of efficiency, smoothness and driver control, the RL comes standard with a 5-speed automatic with Sequential SportShift with paddle shifters to allow finger-tip manual operation.

To maximize available traction and to provide exceptional handling balance and responsiveness, the RL comes standard with Super Handling All-Wheel Drive™ (SH-AWD™), the first and only all-wheel-drive system that distributes the optimum amount of torque not only between the front and rear wheels but also between the left and right rear wheels. The system's direct yaw control makes the cornering character of the RL exceptionally neutral under power.

Powertrain Comparison

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POWERTRAIN AT A GLANCE

Engine

• 3.5-liter, SOHC, VTEC™ V-6 engine produces 300 horsepower at 6200 rpm and 260 lbs.-ft. of torque at 5000 rpm
• VTEC™ (Variable Valve Timing and Lift Electronic Control)
• 11.0:1 Compression ratio
• Two-piece, dual-stage intake manifold
• Direct ignition system and detonation/knock control
• Variable flow exhaust system
• Drive-by-wire throttle system
• Computer-controlled Programmed Fuel Injection (PGM-FI)
• Maintenance Minder System optimizes service intervals
• 105,000-mile tune-up interval

Emissions/Fuel Economy

• High flow close-coupled catalytic converters plus under floor catalytic converter
• High capacity 32-bit RISC processor engine control unit
• Meets CARB LEV-2 ULEV standard, which cuts NOx by 77% compared to LEV-I LEV of previous 3.5 RL
• Estimated mileage of 18/26 mpg City/Highway

Noise, Vibration & Harshness (NVH)

• 60-degree V-angle for inherently smooth operation
• Automatically tensioned, maintenance free serpentine accessory drive
• 5-speed automatic with Sequential SportShift, paddle shifters and Grade Logic Control
• Quick-response Sequential SportShift allows semi-manual operation
• Steering-wheel mounted manual-mode shift control paddles
• Coordination between drive-by-wire throttle system and transmission makes for quicker, smoother shifts
• Wide gear ratio spacing for strong acceleration and good fuel economy
• Advanced shift-hold control limits upshifts during spirited driving
• Advanced Grade Logic Control System reduces gear "hunting" when driving on steep hills
  

Super Handling All-Wheel Drive™ (SH-AWD™)

• Fully automatic, full-time traction and handling enhancing system
• Distributes torque between the front and rear wheels and between the left and right rear wheels to directly control the yaw moment of the vehicle
• Understeer is reduced when cornering
• Maximum cornering grip is achieved by evening the load on each tire

ENGINE ARCHITECTURE

The 3.5-liter VTEC™ V-6 in the RL is the most powerful production engine in Acura's history and incorporates many of the refinements and improvements that have been developed in other Acura powerplants. The RL engine has a smooth-firing 60-degree V-angle and compact overall dimensions. Aluminum alloy construction saves weight and improves cooling, while free-breathing VTEC™ cylinder heads operate four valves per cylinder.

A high inertia intake system, increased compression ratio, new close-coupled catalytic converters and high flow exhaust help make the RL engine the most powerful normally aspirated 6-cylinder engine in its class. Compared to the previous 3.5 RL, the new RL has gained a total of 75 horsepower. Of that total, the new high inertia intake manifold accounts for 15 horsepower. Another 40 horsepower was gained in internal engine efficiencies, and an additional 20 horsepower was netted by the variable flow-rate exhaust system.

ENGINE BLOCK

The lightweight, heat-treated die-cast aluminum-alloy block has cast-in-place iron cylinder liners. These thin-wall, centrifugal-cast iron liners help reduce the block's overall length and weight. With their rough outer surfaces, these liners bond securely to the surrounding aluminum during the manufacturing process, enhancing liner-to-block rigidity and heat transfer. The block also incorporates a deep-skirt design for rigid crankshaft support and minimized noise and vibration.

CRANKSHAFT/ PISTONS/CONNECTING RODS

The forged crankshaft of the RL is similar to that of the MDX, but with revised counterweights to accommodate the weight of higher compression pistons. With their taller, reinforced crowns, these new pistons raise the compression ratio (relative to the MDX) from 10.0:1 to 11.0:1. The previous generation 3.5 RL had a compression ratio of 9.6:1. The elevated compression ratio is a key element in the horsepower gain compared to the previous generation engine. Part of the reason this elevated compression ratio is possible is an oil jet system that sprays cooling oil on the underside of the piston crowns to keep temperatures in check. New steel connecting rods are forged in one piece and then the crankshaft ends are broken , creating a lighter and stronger rod with a perfectly fitted bearing cap.

CYLINDER HEADS / VALVES

Like the MDX and the TL, the RL uses cast alloy single overhead camshaft cylinder heads that incorporate tuned exhaust manifolds as an integral part of the casting. Made of pressure-cast, low-porosity aluminum, these lightweight components improve overall packaging, enhance exhaust flow and allow the optimal positioning of a primary close-coupled catalytic converter on each cylinder bank. To ensure positive sealing, the RL has a three-layer type head gasket like that of the MDX, TL and NSX. A single Aramid-fiber reinforced belt drives the overhead camshafts. The RL cylinder heads have 36mm diameter intake valves and 30mm diameter exhaust valves. As a point of reference, the MDX has 35mm intake and 30mm exhaust valves.

VTEC™ (VARIABLE TIMING AND LIFT ELECTRONIC CONTROL)

Acura VTEC™ (Variable Timing and Lift Electronic Control) is a new addition to the RL, and is a major contributor to the engine's large gains in horsepower and torque. The system operates the 12 intake valves in two distinct modes, so that the operation of the intake valves changes to optimize both volumetric efficiency and combustion of the fuel-air mixture. At low engine speeds, the intake valves have low lift and are open a comparatively short period of time during cylinder filling. At high engine speeds where breathing is critical, the valves switch to high-lift, long duration mode to deliver the best volumetric efficiency. The VTEC™ changeover point is undetectable to the driver and occurs at 4950 rpm.

The RL uses a 3-rocker VTEC™ system similar to that of the MDX and TL. This configuration allows each of a given cylinder's intake valves to be controlled by its own low-speed cam lobe, allowing for staggered valve opening and lift. (By comparison, with 2-rocker VTEC, a single low-speed cam lobe controls both intake valves for each cylinder).

Better mixing in the cylinders improves both combustion speed and combustion stability. When the engine reaches 4950 rpm, the powertrain control module (PCM) triggers the opening of an electric spool valve that routes pressurized oil to small pistons in the intake valve rocker arms. These pistons slide into position to lock together the three intake rockers in a given cylinder, which then follow a single high-lift, long-duration cam lobe. The intake and exhaust valve timing and duration is unique to the RL.

TWO-PIECE DUAL-STAGE INTAKE MANIFOLD

The 2005 RL uses a new dual-stage intake manifold that is designed to deliver maximum airflow, and accounts for 15 percent of the horsepower (out of 75 horsepower total) gained over the previous-generation 3.5 RL. The 2-piece cast-aluminum manifold is also very light. Compared to the one-piece, dual-stage unit used on the MDX, the RL manifold saves 2.6 lb.

Working in concert with the VTEC™ valve train, the induction system significantly boosts torque across the engine's full operating range. Internal passages and two butterfly valves commanded by the powertrain control module provide two distinct modes of operation.

These valves are closed at lower rpm. In this mode, the three cylinders on each bank draw air from only the nearer half of the manifold's internal chamber, or plenum. The volume of the plenum and the length of inlet passages are tuned to maximize the resonance effect, wherein pressure waves are amplified within each half of the intake manifold at certain rpm ranges. The amplified pressure waves significantly increase cylinder filling and the torque produced by the engine throughout the lower part of its rpm band. Funnel-shaped intake ports-similar to those used on racing engines-are built in at the uppermost end of each intake runner to improve airflow.

As the benefits of the resonance effect lessen with rising engine speed, the butterfly valves open at 4000 rpm to interconnect the two halves of the plenum, increasing its volume. An electric motor, commanded by the powertrain control module, controls the connecting butterfly valves. Now each cylinder draws intake air from the full plenum chamber. The inertia of the mass of air rushing down each intake passage helps draw in more charge than each cylinder would normally ingest. This phenomenon is the same effect produced by a low-pressure supercharger. The inertia effect greatly enhances cylinder filling and the torque produced by the engine at higher rpm.

Acceleration Device

Positioned at the front of the RL rear drive unit, the Acceleration device typically passes torque rearward to the rear axle at very close to a one-to-one ratio. In cornering, however, the Acceleration device's output shaft spins faster than its input shaft.

The Acceleration assembly uses a compact planetary gearset to achieve its speed increase. Hydraulic actuators operate clutch packs that control the planetary gearset. When the input shaft is locked with the planetary gear carrier, there is no ratio change (this is the straight-line mode). During cornering, the carrier is coupled with the case, and the output shaft speed increases up to five percent. A speed sensor at the hypoid gear, downstream of the Acceleration device provides a feedback loop to the SH-AWD Electronic Control Unit to ensure that the system is working properly.

Direct Electromagnetic Clutch Systems

Located on either side of the hypoid gear that drives the rear axle, two identical Direct Electromagnetic Clutch systems control the amount of drive torque that reaches each rear wheel, and provide limited-slip differential function. An electric coil controls the pressure applied to a clutch, which slows the sun gear in a planetary gearset to modulate the torque that is sent to the wheel. The amount of torque transmitted to each rear wheel can vary continuously, between zero and 100 percent, depending on the conditions.

Under deceleration (throttle closed) while cornering, torque to the outside rear wheel is varied to change from an inward to an outward yaw moment, helping vehicle stability. A search coil sensor allows the ECU to estimate the clutch plate coefficient of friction (which changes with heat,) and then adjusts voltage sent to the electromagnetic coil that controls the clutch to compensate. To ensure that the amount of torque transmitted remains optimized as miles and wear accumulate, a coil provides a feedback loop that the ECU uses to adjust voltage to the electromagnetic clutches to compensate for potential clutch wear.

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