2011 Acura RL - Powertrain
The 2011 RL features a 3.7-liter SOHC VTEC® V-6 engine that delivers outstanding power and torque to provide brisk acceleration, effortless cruising and excellent fuel efficiency 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 high-output 3.7L V-6 is as powerful as some V-8 engines, yet the V-6 is lighter for better acceleration and more nimble handling. The 3.7L helps ensure that drivers have plenty of torque available during normal driving by generating roughly 80-percent of peak torque available at a mere 1,000 rpm. The 3.7L V-6 features an engine bay cover assembly that simplifies the under hood appearance as well as focuses more attention on the engine.
The 3.7L V-6 is teamed with a new Sequential SportShift 6-speed automatic transmission with revised shift parameters, a more robust torque converter, and two forms of transmission gear activation. For simplicity the RL utilizes a straight-gate style console-mounted shifter coupled with racing-inspired steering wheel paddle shifters for more spirited driving. Grade Logic Control is paired with exclusive transmission shift programming to deliver improved performance and a crisper driving feel.
To deliver exceptional handling performance and to maximize all-season traction, the 2011 RL comes standard with a Super Handling All-Wheel Drive™ (SH-AWD™) system that incorporates control parameters that deliver quick, precise all-wheel-drive response. The RL's 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 more predictable, more enjoyable driving experience.
Powertrain at A Glance
- 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-alloy 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
- High-flow, close-coupled catalytic converters plus under floor catalytic converter
- High capacity 32-bit RISC processor emissions control unit
- EPA TIER 2 - BIN 5 and CARB LEV II ULEV emissions compliance
- EPA estimated 17/24/20 (city/highway/combined) mpg fuel economy
Noise, Vibration & Harshness (NVH)
- 60-degree cylinder V-angle for smooth operation
- Automatically tensioned, maintenance-free serpentine belt accessory drive
Sequential SportShift 6-speed automatic with paddle shifters
- Quick-response Sequential SportShift automatic transmission allows semi-manual operation
- Straight-gate style console-mounted shifter
- Steering wheel mounted racing-inspired paddle shifters
- Coordination between Drive-by-Wire™ throttle system and transmission makes for quicker, smoother shifts
- 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
- SH-AWD™ distributes torque between the front and rear wheels as well as between the left and right rear wheels to provide torque vectoring
- Outer rear wheel is capable of being overdriven up to 5.7-percent during cornering to create a yaw moment of the vehicle to help reduce understeer
The 3.7-liter VTEC® V-6 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 replaced. An aluminum cylinder block (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 the RL's output to an all-time high.
A special high-flow intake system, a high compression ratio, and a unique design of the close-coupled catalytic converters (matched with high flow exhaust) make the 3.7L engine one of the most powerful naturally aspirated 6-cylinder engines in its class.
The RL's lightweight, heat-treated die-cast aluminum 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 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 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. 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.
CYLINDER HEADS / VALVETRAIN
Like other current-generation Acura V-6 engines, the RL powerplant uses single overhead camshaft (SOHC) cylinder heads. However, the 2011 RL's 3.7L engine employs a VTEC® rocker arm design for both the intake and exhaust valves. The lightweight cylinder 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".
A first for Acura, the RL uses exclusive hollow camshafts are 25-percent lighter due to hollow tubular steel shafts with splined steel lobes and journals that are pressed in place.
The 3.7L's intake port design has been carefully optimized to help boost the RL's horsepower. Intake valve head diameter in the 3.7L engine is 36mm and it weighs 13-percent less than a typical valve due to a reduction in material in the valve head. The exhaust valves measure 30mm in diameter.
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 3.7L engine's 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 and 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 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 nearly undetectable to the driver.
The RL uses a unique 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. The small pistons then slide into position to lock the rocker arms for a given cylinder together, which arms 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 VTEC® system is more power, reduced emissions and better fuel economy.
DRIVE-BY-WIRE™ THROTTLE SYSTEM
The RL doesn't use a conventional throttle cable, but instead has smart electronics that connect the throttle pedal to a throttle valve inside the throttle-body. The result is less underhood clutter and 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.
Acura's 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.
PROGRAMMED FUEL INJECTION (PGM-FI)
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.
2-PIECE DUAL-STAGE INTAKE MANIFOLD
The 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 alloy 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. This 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.
MULTI-HOLE FUEL INJECTORS
The 3.7L engine 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 efficiency 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
The exhaust manifolds of the 3.7L V-6 are cast directly into the aluminum cylinder heads to reduce weight, decrease parts count and create more under-hood 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, 350 cell-per-square-inch) catalytic converter located under the passenger cabin floorboard. This high-flow secondary catalytic converter has a large inlet and outlet to enhance exhaust flow. 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 system flow to 150 liters per second.
The 2011 RL 3.7L V-6 engine meets the tough EPA TIER 2 - BIN 5 and CARB LEV II ULEV 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. 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 an 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 efficiency.
ONE-TOUCH STARTER SYSTEM
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 key is required) and the engine automatically starts up.
NOISE, VIBRATION AND HARSHNESS (NVH) CONTROL
With its 60-degree V-angle and compact, rigid and lightweight die-cast aluminum cylinder block assembly, the 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 oil pan that reduces cylinder block flex.
100,000 MILE TUNE-UP INTERVALS
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 spark plugs.
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 expenditures 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.
SEQUENTIAL SPORTSHIFT 6-SPEED AUTOMATIC TRANSMISSION
To maximize driver control, acceleration and fuel economy, the 2011 RL is equipped with an all-new 6-speed automatic transmission featuring Sequential SportShift with racing-inspired steering wheel mounted paddle shifters and two automatic shift modes.
Though comparable in size and weight to the RL's previous 5-speed automatic transmission, careful engineering of the layout and power flow of the new 6-speed minimizes size, parts count and overall weight.
The Sequential SportShift 6-speed automatic improves launch-feel, acceleration performance and fuel efficiency. Compared to the 5-speed transmission previously used with RL, the new 6-speed has lower gear ratios (higher numerically) in First through Fifth gears and in Reverse. The lower gear ratios improve acceleration and pulling power. A tall final gear allows for relaxed cruising rpm and enhanced highway fuel economy.
For improved reliability, an external transmission cooler is standard.
|Transmission Gear Ratio Comparison|
|Transmission Gear||5-speed AT||6-speed AT||Percentage change|
|Sixth||-||0.556:1||13% higher (vs. 5th gear)|
The 6-speed automatic transmission also includes engineering enhancements aimed at improved performance and fuel economy. Expanded control of the multi-disc lock-up assembly within the torque converter improves the efficiency of power delivery and works with the new gear ratios to provide a 1 mpg improvement in city fuel economy and 2 mpg improvement in highway fuel economy.
The 6-speed transmission can be controlled by a straight-gate console-mounted shifter or a steering wheel paddle shifter system that works whenever the transmission is in either of its two automatic modes. In addition to Grade Logic Control and Shift Hold control, the RL also has Cornering G Shift Control. All of the transmission logic systems work together to automatically alter shift timing based on driving conditions.
The Sequential SportShift 6-speed transmission can be operated in two different fully automatic modes with the console-mounted straight-gate shifter. The D (or "Drive") mode is ideal for most driving situations, and combines fuel efficiency with smooth operation and responsive power when needed. The S (or "Sport") mode is for more performance-oriented driving, and features more aggressive shift mapping to keep engine rpm higher for greater acceleration and response.
In Sport mode the transmission typically operates in the four lowest gears, and won't shift to Fifth gear unless the vehicle reaches very high speed. In Sport mode, the economy-oriented Sixth gear is locked out.
When in the D mode (that is optimized for normal driving), the transmission incorporates an advanced Grade Logic Control System, Shift Hold Control and Cornering G Shift Control— all of which reduce unwanted shifting and gear hunting. The result is smart transmission operation that optimizes fuel efficiency and keeps the transmission in the appropriate gear for the specific driving conditions, thus generating excellent performance and smooth operation.
While traveling up or down hills, Grade Logic Control alters the transmission's shift schedule to reduce shift frequency and improve speed control. A shift map in the transmission computer continually measures throttle position, vehicle speed and acceleration/deceleration, and then determines when the vehicle is on a hill. The shift schedule is then adjusted to hold the transmission in lower gears to boost climbing power or to provide engine braking when traveling downhill.
Shift Hold Control keeps the transmission in its current (lower) gear ratio when aggressive driving is detected, as in the case of decelerating at a corner entry. Shift Hold Control leaves the chassis undisturbed by excess shifting and ensures that power will be immediately available (without a downshift) at the corner exit.
Cornering G Shift Control is a transmission feature that first debuted on the Sequential SportShift 6-speed automatic transmission. Cornering G Shift Control monitors the speed of each rear wheel independently to determine when the RL is turning.
When the system detects a sufficient speed differential between the rear wheels, it will suppress an unwanted upshift. This prevents the transmission from upshifting during a corner, which could upset the chassis balance and would then require downshifting again at the corner exit when the throttle is applied.
Temporary Manual Operation in "Drive"
Whether in Drive or Sport mode, special transmission logic programming allows the use of the steering-wheel-mounted paddle shifters. When the driver operates the steering wheel-mounted paddle shifters while in Drive, the transmission responds to the driver's shift command and then returns to its normal fully automatic Drive mode if further paddle shift inputs are not made within a short time. This special logic makes it easy for the driver to command a quick downshift without leaving the comfort of Drive mode. When in Sport mode, use of the paddle shifters puts the transmission into full manual mode that remains until another mode of operation is selected with the console-mounted shifter.
By moving the center console-mounted gear selector lever rearward to the detent labeled "S," the transmission is shifted into Sport mode. This mode offers automatic operation with more aggressive shift mapping. A pull on the racing-inspired paddle shifters (mounted on the steering wheel) places the transmission in fully manual mode. A digital display in the tachometer face indicates which gear the transmission is in.
A new double-kick-down feature lets the driver command a sport-minded double downshift— such as from Fifth to Third gear. By pulling on the left downshift paddle twice in rapid succession, the transmission will drop directly to the chosen lower gear ratio. The Drive-by-Wire™ throttle system also creates a "blip" of the throttle to help match gear speeds while downshifting.
To prevent harm to the powertrain when the transmission is paddle shifted by the driver (including during double-kick-down shifts), the system will inhibit potentially damaging shifts. As an added safety measure, the Powertrain Control Module (PCM) can also cut off engine fuel flow to prevent over-revving.
If fuel cut-off is insufficient to prevent engine over-revving, as may be possible when the vehicle is on a steep downhill, the transmission will automatically upshift to prevent damage. On downshifts, the transmission will not execute a driver command that will over-rev the engine.
For improved stop-and-go performance and to help prevent "lugging" the engine, the Sequential SportShift transmission will automatically downshift to First gear even though the transmission has been left in a higher gear, (except in Second gear) as the vehicle comes to a stop. In Manual Mode, when coming to a stop in Second gear, the vehicle will start in Second gear as well.
MULTI-CLUTCH LOCK-UP TORQUE CONVERTER
The new Sequential SportShift 6-speed automatic transmission teams with a special torque converter that not only has a higher maximum torque rating, but also has a unique converter lock-up assembly. The lock-up assembly uses multiple lock-up disks that generate nearly double the facing area of a typical torque converter. The lock-up assembly not only reduces heat build-up during operation, but also features improved overall lubrication that generates better cooling. The torque converter allows for lock-up activation during a much wider range of driving conditions that helps improve fuel economy.
STRAIGHT-GATE SHIFTER ASSEMBLY
The straight-gate shifter is an intuitive and easy-to-use design that takes up little space in the center console. The shifter allows the driver to choose D (using First through Sixth gears) or S, which activates the performance-oriented Sport mode, which has more aggressive shift programming and locks out the use of Sixth gear when operating automatically. When in Sport mode, gear selection via racing-inspired paddle shifters (mounted on the steering wheel) automatically puts the transmission into manual mode, and the driver can select among all six forward gear ratios.
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 electronically-controlled, Sequential SportShift 6-speed 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. 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 and is capable of overdriving either rear wheel. The result of the specific right/left wheel torque distribution (or torque vectoring) is a more neutral, accurate steering when cornering under power that traditional front-drive, rear-drive or conventional all-wheel-drive can't equal. To realize maximum benefit from SH-AWD™, the system is capable of initiating front-to-rear, and rear side-to-side torque distribution in all five forward gears.
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, which also can also be overdriven up to 5.7-percent by a built-in acceleration device.
The RL's 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 (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 speed 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 maximum traction utilization in inclement weather.
Direct Yaw Control
SH-AWD™ helps counter 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. With yaw, by relieving the front tires of some of the work of turning the car, understeer is reduced and the RL stays more balanced and controllable. 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-force, 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-force 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 shaft 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.
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, during 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™ system's Electronic Control Unit (ECU) 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. Starting 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 for varying conditions. 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.