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Old 06 Feb 2008, 05:30 pm
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Default 2009 Acura RL - Powertrain

Chicago - Feb 06 —


The 2009 RL features an all-new powertrain consisting of a 3.7-liter SOHC VTEC® V-6 engine. Replacing the previous 3.5L V-6, the new 3.7L delivers more power, increased torque, more refined valvetrain, and an overall package that is lighter than the V-6 it replaces. The crankshaft of the RL is positioned transversely (side to side) rather than longitudinally (front to rear) which allows the RL engine to be packaged more tightly for better handling agility, without sacrificing interior comfort. Rated at 300 horsepower, the new high-output 3.7L V-6 is as powerful as some V-8 engines, yet the V-6 is more lightweight for better acceleration and more nimble handling. Compared to the old 3.5L V-6, the new 3.7L offers 9-percent more torque between idle and 3,000 rpm-- the rpm range most people operate the engine during normal daily driving. The 3.7L V-6 gets a new engine bay cover assembly that simplifies the under hood appearance as well as focuses more attention on the new engine.

The 3.7L V-6 is teamed with a new Sequential SportShift 5-speed automatic transmission with revised shift parameters, a more robust torque converter, and two new forms of transmission gear activation. The previous RL's multi-gated shifter has been replaced with a new straight-gate style console-mounted shifter. Or, for more spirited driving, a new "smart" F1-style steering wheel paddle shifter system is standard that now works when the transmission is in Drive (formerly the paddles only operated when in Manual mode). Grade Logic Control is paired with revised transmission shift programming to deliver improved performance and a crisper driving feel.

To deliver exceptional handling performance and to maximize all-season traction, the 2009 RL comes standard with revised Super Handling All-Wheel Drive™ (SH-AWD™) that incorporates new control parameters that deliver quicker, more precise all-wheel-drive response. SH-AWD™ is the first all-wheel drive system in the world to distribute 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 torque vectoring direct yaw control feature makes the cornering character of the RL exceptionally neutral when under power. The result is a safer, more predictable, more enjoyable driving experience.


  • 3.7-liter, SOHC, V-6 engine produces 300 horsepower at 6,300 rpm and 271 lb-ft of torque at 5,000 rpm

  • VTEC® (Variable Valve Timing and Lift Electronic Control) for intake and exhaust valves (a first ever application for a SOHC engine)

  • 11.2:1 compression ratio

  • Two-piece, dual-stage magnesium intake manifold

  • Drive-by-Wire™ Throttle System

  • Computer-controlled Programmed Fuel Injection (PGM-FI) with multi-hole fuel injectors

  • Direct ignition system

  • Detonation/knock control system

  • Variable flow exhaust system

  • Maintenance Minder™ system optimizes service intervals

  • 100,000 miles tune-up interval

Emissions/Fuel Economy

  • High-flow, close-coupled catalytic converters plus under floor catalytic converter

  • High capacity 32-bit RISC processor emissions control unit

  • Meets EPA TIER 2 - BIN 5 and CARB ULEV-2 emissions standards

  • EPA estimated 16/22/19 mpg (city/highway/combined)

Noise, Vibration & Harshness (NVH)

  • 60-degree cylinder V-angle for smooth operation

  • Automatically tensioned, maintenance-free serpentine belt accessory drive

Sequential SportShift 5-speed automatic with paddle shifters and Grade Logic Control

  • Quick-response Sequential SportShift allows semi-manual operation

  • New straight-gate style console-mounted shifter

  • Steering-wheel mounted F1®-style paddle shifters (now applicable in Drive and Manual modes)

  • Coordination between Drive-by-Wire™ Throttle System and transmission makes for quicker, smoother shifts

  • Wide gear ratio spacing for good fuel economy

  • Advanced shift-hold control limits upshifts during spirited driving

  • Grade Logic Control System reduces gear "hunting" on steep hills

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

  • Fully automatic, full-time traction and handling system

  • Distributes torque between the front and rear wheels and between the left and right rear wheels to provide torque vectoring to directly control the yaw moment of the vehicle

  • Outer rear wheel is capable of being overdriven up to 5.7-percent during cornering

  • Understeer is greatly reduced during hard cornering


The new 3.7-liter VTEC® V-6 is the largest and most powerful engine ever fitted to an Acura sedan and it incorporates many of the refinements and improvements that have been developed for other Acura powerplants. The new RL engine has a smooth-firing 60-degree V-angle, compact overall dimensions, and weighs 17.2 pounds less than the smaller displacement 3.5L engine it replaces. Aluminum alloy cylinder block construction (including unique aluminum cylinder sleeves) saves weight and improves cooling, while the first ever SOHC use of VTEC® for both intake and exhaust valves helps push power to an all-time high.

A special high-flow intake system, higher compression ratio, and a new design of close-coupled catalytic converters matched with high flow exhaust make the 3.7L engine the most powerful naturally aspirated 6-cylinder engine in its class.


The RL's lightweight, heat-treated die-cast aluminum-alloy cylinder block has cast-in-place aluminum cylinder liners. These high-silicon sleeves dissipate heat better than iron liners. The silicon aluminum sleeves also allow a closer piston-to-cylinder clearance for better oil control, improved emissions, and less operating noise. A new mechanical etching process during manufacturing exposes silicone particles embedded in the aluminum sleeves, which provide a hard piston-ring sealing surface. The block also incorporates a deep-skirt design for rigid crankshaft support and minimized noise and vibration.


The new 3.7L V-6 uses a forged steel crankshaft for high strength with minimum weight. Designed with special raised crowns, the pistons increase the compression ratio (relative to the RL's previous 3.5L) from 11.0:1 to 11.2:1. This elevated compression ratio is possible due to an oil jet system that sprays cooling oil on the underside of the piston crowns to help keep temperatures in check. New heavy-duty steel connecting rods are forged in one piece and then the crankshaft ends are "crack separated" to create a lighter and stronger rod with a perfectly fitted bearing cap.


Like other current-generation Acura V-6 engines, the RL powerplant uses single overhead camshaft (SOHC) cylinder heads. However, for 2009 the new RL's 3.7L engine employs an all-new, industry-leading VTEC® rollerized rocker arm design for both the intake and exhaust valves. The lightweight heads are made of pressure-cast, low-porosity aluminum. To further save weight and reduce parts count, the cylinder heads incorporate unique integrated exhaust port castings that allow the optimal positioning of a primary close-coupled catalytic converter on each cylinder bank. The result of this design is the reduction of exhaust emissions during cold start-up due to quicker catalytic converter "light off".

Unique new camshafts are 25-percent lighter than the previous 3.5L camshafts they replace. Assembled from hollow tubular steel shafts with splined steel lobes and journals that are pressed in place, the 3.7L's camshaft design is a first for Acura.

The 3.7L's intake port design has an optimized shape that contributes seven horsepower alone (relative to the old 3.5L design) to the RL's 300 horsepower total. Intake valve head diameter in the new engine is 36mm (an increase of 1mm), yet it weighs 13-percent less due to a reduction in material in the valve head. The exhaust valves measure 30mm in diameter, the same as in the previous 3.5L engine.

To ensure positive sealing, a unique 3-layer shim-type head gasket is used. A single Aramid-fiber reinforced belt drives the overhead camshafts.

VTEC® (Variable Timing and Lift Electronic Control)

Acura VTEC® (Variable Timing and Lift Electronic Control) is a major contributor to the new 3.7L engine's gains in horsepower and torque. The engine features the first ever application of VTEC® on a SOHC engine for both the intake and exhaust valves. The system operates the engine's 24 intake and exhaust valves in two distinct modes, so that the operation of the valves continually changes to optimize volumetric efficiency, combustion of the air/fuel mixture, and to increase exhaust flow.

At low engine rpm, the valves have lower lift and are open a shorter period of time. At high engine speeds where breathing is critical, the valves switch to a high-lift, long duration mode to deliver improved volumetric efficiency. The VTEC® changeover point occurs at 4,900 rpm and is undetectable to the driver.

The RL uses a unique new multi-arm VTEC® rocker arm system that allows each of the valves to be controlled by its own low-speed cam lobe. The result is better air/fuel mixing in the cylinders that improves both combustion speed and combustion stability. When the engine reaches 4,900 rpm, the powertrain control module (PCM) triggers the opening of an electric spool valve that routes pressurized oil to small pistons within the VTEC® rocker arms. As a result, small pistons slide into position to lock together the rocker arms for a given cylinder, which then follow a single high-lift, long-duration cam lobe (increasing high rpm intake valve lift by 27.9-percent and exhaust valve lift by 10-percent). The intake and exhaust valve timing and duration are unique to the RL. The result of the new VTEC® system is more power and torque, improved emissions, and better fuel economy. Plus, the new VTEC® rocker arm assemblies offer twice the durability of the former design as well as offer roller arm tips to reduce operating friction.


The RL doesn't use a conventional throttle cable, but instead has smart electronics that connect the throttle pedal to the throttle valve inside the throttle-body. The result is less underhood clutter, lower weight as well as quicker and more accurate throttle actuation. Plus, specially programmed "gain" between throttle pedal and engine offers improved drivability and optimized engine response to suit specific driving conditions. The throttle profile of the new RL has been reworked to provide low speed control, followed by progressively building throttle opening response at higher speed.

Acura's high-tech Drive-by-Wire™ Throttle System establishes the current driving conditions by monitoring throttle pedal position, throttle valve position, engine rpm speed, and road speed. This information is used to define the throttle control sensitivity that gives the RL's throttle pedal a predictable and responsive feel that meets driver expectations. With the new 3.7L V-6, a 69mm diameter throttle body delivers a 12-percent flow increase over the previous 3.5L unit and it operates via revised drive-by-wire throttle mapping for increased gain-- thus generating improved mid-throttle power and feel.


The RL's V-6 engine features Programmed Fuel Injection (PGM-FI), which continually adjusts the fuel delivery to yield the best combination of power, low fuel consumption, and low emissions. Multiple sensors continually monitor critical engine operating parameters such as intake air temperature, ambient air pressure, throttle position, intake airflow volume, intake manifold pressure, coolant temperature, exhaust-air ratios as well as the position of the crankshaft and the camshafts.


The new 3.7L V-6 uses a dual-stage intake manifold that is designed to deliver maximum airflow to the cylinders across the full range of engine operating rpm. The 2-piece manifold is constructed of cast-magnesium to be very light.

The dual-stage induction system significantly boosts torque across the engine's full operating range by working in concert with the VTEC® valvetrain. Within the manifold are two separate butterfly valves that are operated electrically by the engine's powertrain control module (PCM) to provide two distinct modes of operation via changing plenum volume and intake airflow routing.

At lower rpm the manifold valves are closed to reduce the overall volume of the plenum. The effective result is an increase in the length of the inlet passages for maximum resonance effect as well as to amplify pressure waves within each half of the intake manifold at lower engine rpm. The amplified pressure waves significantly increase cylinder filling and torque production throughout the lower part of the engine's rpm band.

As the benefits of the resonance effect lessen with rising engine speed, the intake manifold valves open at 3,800 rpm to interconnect the two halves of the plenum, thus increasing its volume. The inertia of the mass of air rushing down each intake passage helps draw in more charge than each cylinder would normally ingest. The inertia effect greatly enhances cylinder filling along with the torque produced by the engine at higher rpm. An electric motor, commanded by the powertrain control module, continually adjusts and controls the intake's valves.


The 3.7L makes use of special, multi-hole fuel injectors that mount in the lower intake manifold and spray directly towards the cylinder head intake ports. The multi-hole injector design provides better fuel atomization for higher horsepower, improved fuel economy, and better cold weather start-up.

Direct Ignition and DETONATION/Knock Control

The RL's powertrain control module (PCM) monitors engine functions to determine the best ignition spark timing. An engine-block-mounted acoustic detonation/knock sensor "listens" to the engine, and based on this input, the PCM can retard the ignition timing to prevent potentially damaging detonation. The 3.7L V-6 has an ignition coil unit for each cylinder that is positioned above each spark plug's access bore.

Close-Coupled Catalysts and Variable FLOW exhaust system

Like the previous RL engine, the exhaust manifolds of the new 3.7L V-6 are cast directly into the aluminum alloy cylinder heads to reduce weight, decrease parts count, and to create more space. The result of the unique casting design is to position the two primary catalytic converters much closer to the combustion chambers. The 600 cell-per-square-inch, high-efficiency catalytic converters mount directly to the exhaust port of each cylinder head for extremely rapid converter "light-off" after the engine starts. A significant weight savings is realized by eliminating traditional design exhaust manifolds.

Downstream of the close-coupled catalytic converters, a hydroformed 2-into-1 collector pipe carries exhaust gases to a secondary (single, 300 cell-per-square-inch) catalytic converter located under the passenger cabin floorboard. This high-flow secondary catalytic converter has a revised shape and a large 62mm outlet (up from the previous 55mm unit). To balance the engine's need for proper exhaust backpressure at low rpm and higher flow at high rpm, the RL's dual exhaust system incorporates a variable flow rate feature. An exhaust pressure-operated valve in the system has two specific operating modes. The low speed mode has a flow rate of 130 liters per second, but when the 3.7L engine crests 4,000 rpm, the exhaust pressure raises enough to open the special valve which in turn increases exhaust flow to 150 liters per second.


The new 2009 RL 3.7L V-6 engine gains displacement, horsepower and torque without an increase in emissions. It meets the tough EPA TIER 2 - BIN 5 and CARB ULEV-2 emissions standards, and is certified to this level of emissions performance for 120,000 miles.

A number of advanced technologies are factors in the emissions performance. The unique cylinder head-mounted close-coupled catalytic converters light off more quickly after engine start up, and a 32-bit RISC microprocessor within the powertrain control module (PCM) boosts computing power to improve the precision of spark and fuel delivery. Particularly right after startup, better fuel atomization is provided by the high-efficiency multi-hole fuel injectors used to deliver fuel to each cylinder.

The RL's V-6 engine features Programmed Fuel Injection (PGM-FI) which continually adjusts the fuel delivery to yield the best combination of power, low fuel consumption, and low emissions. Multiple sensors constantly monitor critical engine operating parameters such as intake air temperature, ambient air pressure, throttle position, intake airflow volume, intake manifold pressure, coolant temperature, exhaust-to-air ratios as well as the position of the crankshaft and the camshafts.

To further improve emissions compliance, the 3.7L V-6 makes use a new after-cat design exhaust gas recirculation (EGR) system that allows cleaner, cooler EGR gas to be fed back into the intake system. An EGR system, especially one that delivers a cleaner/cooler charge, reduces pumping loss for better fuel economy.


Due to the increased displacement of the RL's new 3.7L V-6, a new starter motor with 18-percent more power is used. To ensure consistent starting, the RL has a 1-touch starter system that maintains starter engagement until the engine starts, even if the driver releases the ignition switch. Simply turn the starter switch (no clumsy keys are required) and the engine automatically starts up.


With its 60-degree V-angle and compact, rigid and lightweight die-cast aluminum-alloy cylinder block assembly, the new 3.7L V-6 powerplant is exceptionally smooth during operation. Other factors that help reduce engine noise and vibration are a rigid forged-steel crankshaft, die-cast accessory mounts, and a stiff cast-aluminum alloy oil pan that reduces cylinder block flex.


The RL's 3.7L V-6 requires no scheduled maintenance for 100,000 miles, other than periodic inspections and normal fluid and filter replacements. The first tune-up includes water pump inspection, valve adjustment, replacement of the camshaft timing belt and the installation of new sparkplugs.

Maintenance Minder SYSTEM

To eliminate unnecessary service stops while ensuring that the vehicle is properly maintained, the RL has a Maintenance Minder™ system that continually monitors the vehicle's operating condition. When maintenance is required, the driver is alerted via a message on the Multi-Information Display (MID).

The Maintenance Minder™ system monitors operating conditions such as oil and coolant temperature along with engine speed to determine the proper service intervals. Depending on operating conditions, oil change intervals can be extended to a maximum of 10,000 miles, potentially sparing the owner considerable money and inconvenience over the life of the vehicle. The owner-resettable system monitors all normal service parts and systems, including oil and filter, tire rotation, air-cleaner, automatic transmission fluid, spark plugs, timing belt, coolant, brake pads and more. To prevent driver distraction, maintenance alerts are presented on the MID when the ignition is first turned on, not while driving.


To maximize acceleration performance, fuel economy and driver control, the RL has a new 5-speed automatic transmission equipped with Sequential SportShift, F1®-style steering wheel mounted paddle shifters and Grade Logic Control.

The new transmission includes a more heavy-duty case assembly, larger counter shafts, revised gearing, larger gear clutches with improved friction material, along with a new torque converter with a revised clutch lock-up assembly. The transmission is operated via a new straight-gate style console-mounted shifter or by way of a new "smart" steering wheel paddle shifter system that now works when the transmission is in Drive (formerly the paddles only operated when in Manual mode). Grade Logic Control is paired with revised transmission shift programming that delivers improved performance and a crisper driving feel.

Designed for low maintenance and a high level of durability, the RL transmission requires no scheduled service until 120,000 miles when operated under normal conditions. To provide strong off-the-line acceleration coupled with a relaxed, fuel-efficient cruising rpm, the automatic transmission has the widest gear ratio spread of any 5-speed automatic transmission in the RL's class.

Besides the benefit of the 3.7L's added power, when in Manual shift mode the new RL delivers improved performance as a result of new transmission shift map programming. During gear upshifts, the 5-speed automatic transmission holds Third and Fourth gears longer into the engine's rpm range for improved performance. On downshifts, new shift logic has been added that generates 30-percent quicker response, a race-style "double blip" of the throttle to help match gear speeds, the transmission fluid line pressure releases the previous gear quicker, and then the next gear's clutch pack line pressure is applied more aggressively for quicker gear engagement.

Automatic Mode

The Sequential SportShift transmission can be operated in a conventional fully-automatic mode via a new console-mounted straight-gate shifter. When in automatic mode, the transmission incorporates an advanced Grade Logic Control System and Shift Hold Control, both of which work to reduce gear "hunting" and unnecessary shifting.

Shift Hold Control keeps the transmission in its current (lower) gear ratio when the throttle is quickly released and the brakes are applied (as might be the case when decelerating to enter a corner). Shift Hold Control leaves the chassis undisturbed by excess shifting, ensuring that power is immediately available without a downshift.

Grade Logic Control alters the 5-speed automatic's shift schedule when traveling uphill or downhill, reducing shift frequency, and improving speed control. Throttle position, vehicle speed and acceleration/deceleration are continuously measured, then compared with a shift map stored in the transmission computer. The Grade Logic Control System then determines when the car is on a hill; if this is the case, the shift schedule is adjusted to automatically hold the transmission in a lower gear for better climbing power or increased downhill engine braking.

In addition, the transmission has new "smart" logic programming that, when in Drive, allows the use of the steering wheel mounted paddle shifters (previously, the paddle shifters only worked in Manual mode). With the paddle shifters, up and downshifts are made in the same fashion as when the transmission is in Manual mode. However, when operated in Drive, if further paddle shift inputs are not made within a short time span, the transmission returns to its normal fully-automatic Drive mode.

Manual Mode

The Sequential SportShift transmission can be shifted into a sport mode by moving the center console-mounted selector lever downward to a special shift detent labeled "S". When in Sport Mode, the RL's transmission operates exclusively via F1Ò-style paddle shifters mounted on the steering wheel. A digital display in the tachometer face indicates which gear the transmission is in. To heighten control and driver involvement, special shift logic in manual mode delivers quicker, firmer shifts than when in fully automatic mode.

To help protect the engine and drivetrain from damage, an array of preventative features are active when the transmission is in Manual mode. In Second, Third and Fourth gears, the shift logic changes and the transmission ECU cuts off fuel flow to the engine if there is a possibility of over revving.

In the rare situation where the fuel cutoff alone is unable to prevent engine over-revving (as could happen on a steep downhill), the transmission will automatically upshift gears to prevent engine damage. When downshifting, the transmission won't execute a driver-commanded downshift that would send the engine beyond its rpm redline as caused by the lower gear. For improved stop and go performance, the new Sequential SportShift transmission will automatically downshift to First gear as the vehicle comes to a stop as to prevent lugging away from a stop if the transmission held itself in Second gear.

Straight-gate shifter assembly

The RL employs a new straight-gate shifter assembly that replaces the previous multi-gate, multi-position shifter. Located in the center console, the new shifter delivers a more intuitive, more easy-to-use design that also takes up less space on the console surface.

The new shifter allows the driver to choose D (allowing the use of First through Fifth gears) or D3 (only allowing the use of First through Third gears). On a downhill, engine braking can be provided by moving the shifter from D to the D3 position that will systematically generate downshifting from Fifth, Fourth, or Third gears depending on the vehicle speed.

Cooperation between Transmission and DRIVE-BY-WIRE

Both shift speed and smoothness are improved by cooperation between the Drive-by-Wire™ Throttle System and the new electronically-controlled, B-type automatic transmission. The engine can be throttled by the engine management system during upshifts and downshifts, thus the function of the engine and transmission can be closely choreographed for faster, smoother shifting. As a result, the peak g-forces (or "shift shock") are reduced significantly during upshifts and downshifts. With the new 3.7L V-6, the Drive-by-Wire™ Throttle System has received special throttle mapping (with increased throttle gain) to generate improved mid-throttle power and feel.

Super Handling All-Wheel Drive™ System

Super Handling All-Wheel Drive™ (SH-AWD™) is an innovative all-wheel drive system that continually distributes the optimum amount of torque not only between the front and rear wheels, but also between the left and right rear wheels. The result of the specific right/left wheel torque distribution (or torque vectoring) is more neutral, accurate steering when cornering under power that front-drive, rear-drive or conventional all-wheel-drive can't equal.

The previous RL's SH-AWD™ system didn't initiate rear side-to-side torque distribution until Second gear. However, on the 2009 RL new control parameters allow rearward torque delivery in First gear. Plus, front-to-rear and side-to-side power transfer happens much quicker with improved precision. The benefit of the revised SH-AWD™ system is improved accuracy, quicker response, and more power able to be applied to a specific rear wheel.

Torque splits are as follows:

  • During straight-line cruising and moderate cornering below half throttle, up to 70-percent of engine torque is delivered to the front wheels.

  • In full-throttle straight line acceleration, up to 40-percent of the power is sent to the rear axle.

  • In hard cornering, up to 70-percent of available torque goes to the rear wheels for enhanced chassis balance. Up to 100-percent of this torque can be applied to the outside rear wheel that can also be overdriven up to 5.7-percent by a built-in acceleration device.

SH-AWD™ incorporates a unique rear differential that continually varies the amount of torque delivered to the left and right rear wheels. SH-AWD™ generates a yaw moment during cornering by specifically routing torque from the engine. When cornering, a planetary gearset overdrives (or accelerates) the rear wheels while individual right and left clutch packs selectively direct torque to either or both rear wheel(s), driving them faster than the average of the front wheels to dramatically enhance the cornering, steering feel, overall handling and stability of the RL. The result is class leading cornering precision as well as enhanced traction in inclement weather.

Direct Yaw Control System Theory

SH-AWD™ counters understeer with a unique Direct Yaw Control system. Rotating an outside rear wheel faster than the average speed of the two front wheels allows the system to generate vehicle "yaw" while turning. By relieving the front tires of some of the work of turning the car, the system reduces understeer and the vehicle stays more balanced and controllable. In addition, with the cornering load more evenly distributed between the front and rear tires, the total cornering grip is increased. In conventional vehicles, cornering is created almost entirely by the steering angle of the front tires. With the RL, cornering is created by steering angle of front tires combined with the extra drive torque supplied by an outside rear tire.

Direct Yaw Control is a significant advance over conventional drive systems. To deal with high power output, front- or rear-drive systems generally use some type of limited-slip differential to maintain traction while under power. However, a linking effect of the inside and outside drive wheels in these systems resists turning. This linking effect is a factor that works against the front tires as they attempt to turn the vehicle. Conventional AWD systems have a similar linking effect between the inboard and outboard tires as well as between the front and rear axles, thus causing a similar resistance to turning. This is part of the reason why traditional AWD systems typically lack the more nimble feel of the best two-wheel drive systems. By using drive torque to help rotate the car, the RL is more responsive, neutral and predictable, while simultaneously offering all of the usual foul-weather benefits of all-wheel drive.

Electronic Controls and Parameters

The logic and control of SH-AWD™ is integrated with the RL engine's Electronic Control Unit (ECU) and Vehicle Stability Assist™ (VSA®) ECU. The Engine ECU provides engine rpm, throttle position, intake manifold pressure, and transmission gear ratio data. The VSA® ECU provides data on lateral g, yaw rate, wheel rotation speed and steering angle. The SH-AWD™ ECU monitors the status of the acceleration device and the distribution of right and left Direct Electromagnetic Clutch torque. Traction is calculated based on the information from the engine ECU. During a hard acceleration situation, lateral g and steering angle are used to calculate the torque split between the right and left rear wheels. At the same time, this data is used to control the total amount (ranging between 0.6- and 5.7-percent) of rear wheel acceleration.

SH-AWD™ System Layout

SH-AWD™ is a seamlessly operating, full-time all-wheel drive system that requires no driver interaction for operation. A torque transfer unit is bolted directly to the front-mounted transaxle. Attached to the front differential ring gear is a helical gear that provides input torque to the transfer unit. A short horizontal shaft and a hypoid gear set within the case turn the propeller shaft ninety degrees and move it to the vehicle center line. Lightweight carbon fiber dual driveshafts carry power to the rear differential unit.

Torque vectoring takes place in the rear differential which contains three planetary gear and clutch sets. Torque from the driveshaft passes through the differential's first clutch/planetary gearset, which as a unit is called the Acceleration Device.

Output torque from the Acceleration Device is channeled to a hypoid gear that turns the output 90-degrees to drive the rear axle shafts. A matched pair of Direct Electromagnetic Clutch systems (one on each side) send power to each rear wheel. These clutch systems can be controlled as a pair to alter the front/rear torque split, or depending on the situation, the rear wheels can receive between 30- to 70-percent of the engine's total output. The right and left Direct Electromagnetic Clutch systems can be controlled independently, to allow up to 100-percent of the total rear axle torque to go to only one rear wheel.

Acceleration Device

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

The Acceleration Device 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 traditional straight-line acceleration mode). During cornering, the carrier is coupled with the case, and the output shaft speed increases up to 5.7-percent. A speed sensor at the hypoid gear (located downstream of the Acceleration device) provides a constant 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 are two identical Direct Electromagnetic Clutch systems that control the amount of drive torque that reaches each rear wheel. With the 2009 RL, the speed and accuracy of this electromagnetic clutch system is far more precise than previous versions thus providing an improved limited-slip differential function. In addition, the acceleration device incorporates a low- and high-range that delivers different levels of rear wheel overdrive depending on vehicle velocity and lateral acceleration. A computer-controlled electric coil controls the pressure applied to a clutch, which slows the sun gear within a planetary gearset to modulate the torque being sent to the rear wheel. The amount of torque transmitted to each rear wheel continually varies between zero and 100-percent, depending on driving 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 improve vehicle stability. A search coil sensor allows the ECU to estimate the clutch plate coefficient of friction (which changes with heat build-up) 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, the coils provide a feedback loop that the ECU uses to adjust voltage to the electromagnetic clutches thus compensating for clutch wear.

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