2008 Acura RDX - Powertrain
Acura's new RDX shares much in concept with the TSX. Though one is an SUV and the other is a sports sedan, both vehicles are designed to blend strong performance with agility and precise handling. The unique positioning of this new Entry Premium SUV within the Acura product line guided the development of a special powertrain for the RDX. A conventional 4-cylinder engine would lack the horsepower and torque needed to make the RDX a strong performer. On the other hand, a 6-cylinder would add extra weight.
A perfect solution came in the form of a turbocharger - a first for an Acura production vehicle. The RDX has an innovative new variable flow turbocharger that overcomes the shortcomings of conventional turbo designs and delivers an unusually broad powerband with little or no lag in throttle response. In addition, Acura's turbo design provides this benefit with high reliability. This is because airflow is controlled before it enters the turbocharger, rather than within the turbocharger as in traditional variable-flow designs. Acura's variable flow turbo design thus eliminates moving components that work directly in the stream of hot exhaust gases.
Architecturally similar to the TSX's 2.4-liter in-line four, the RDX applies a 2.3-liter DOHC 16-valve in-line 4-cylinder engine. Like the TSX, the RDX uses i-VTEC® valve control that combines Variable Valve Timing and Lift Electronic Control (VTEC®) with Variable Timing Control™ (VTC™). With the variable flow turbo working in unison with i-VTEC®, the RDX delivers strong power and torque, excellent fuel economy and very low emissions.
Rated output for the engine is 240 horsepower @ 6000 rpm and 260 lb-ft of torque @ 4500 rpm. That gives it the highest torque output of any engine in the Acura lineup. The RDX is EPA rated at 17/22* city/highway mileage, which puts it ahead of some less powerful SUV competitors.
*Based on 2008 EPA mileage estimates, reflecting new EPA fuel economy methods beginning with 2008 models. Use for comparison purposes only. Do not compare to models before 2008. Your actual mileage will vary depending on how you drive and maintain your vehicle.
|2008 Acura RDX|
|Engine||DOHC i-VTEC Turbocharged I-4|
|Horsepower SAE net @ rpm||240 @ 6000|
|Torque SAE net @ rpm (lbs.-ft.)||260 @ 4500|
|EPA estimated mileage City/Highway||17/22*|
|Emissions certification (CARB/EPA)||LEV II ULEV/Tier-2 Bin-5|
|Fuel type||Premium unleaded|
With its rear-mounted exhaust manifold feeding directly into the turbocharger and the use of a close-coupled catalytic converter, the RDX uses its advanced Programmed Fuel Injection (PGM-FI) to meet strict CARB LEV II ULEV and EPA Tier-2 Bin-5 standards.
The RDX's 5-speed automatic with Sequential SportShift can function as a conventional automatic transmission or, at the driver's option, be shifted manually via steering-wheel-mounted paddles. The electronically controlled Drive-by-Wire Throttle System™ and automatic transmission work together to execute shifts, resulting in exceptionally quick and smooth gear changes.
To maximize available traction with handling balance and responsiveness, the RDX comes standard with Super Handling All-Wheel Drive™ (SH-AWD™). This all-wheel-drive system progressively distributes the optimum amount of torque not only between the front and rear axles but also between the left and right rear wheels. The system's direct yaw control helps reduce understeer, improve steering accuracy and total cornering power.
- Turbocharged and intercooled 2.3-liter inline-four engine, 16-valve DOHC i-VTEC®
- 240 horsepower at 6000 rpm
- 260 lb-ft of torque at 4500 rpm
- i-VTEC® variable valve timing system
- Variable flow turbocharger
- Air-to-air intercooler
- Mobil 1® synthetic oil
- Oil-life monitoring system
- Drive-by-Wire Throttle System™
- Direct ignition system
Transmission and Drive System
- Super Handling All-Wheel Drive™ (SH-AWD™)
- Sequential SportShift 5-speed automatic transmission with F1®-style paddle shifters
- Advanced Shift-hold Control and Grade Logic Control
- Torque converter specially designed to match turbo power characteristics
- Specially tuned lockup clutch damper
- Transmission fluid cooler
Noise, Vibration, and Harshness (NVH)
- Dual chain-driven balance shafts mounted in oil pan
- Maintenance-free silent-chain camshaft drive
- Aluminum cylinder block with cast-in iron cylinder liners for light weight and durability
- Block and 1-piece aluminum crankshaft carrier for improved strength
- EPA certified 17/22* mpg (city/highway)
- Meets strict CARB ULEV-2 and EPA Tier-2 Bin-5 standards
- Rear-facing exhaust ports, aluminum exhaust manifold and close-coupled catalytic converter provide fast catalytic converter light-off
- Jet-pump purge system captures evaporative emissions from the fuel tank, directing emissions into the intake system for proper burn-off
CRANKSHAFT, ENGINE BLOCK AND OIL PAN
The RDX utilizes an all-new turbocharged engine. The die-cast lightweight aluminum block has cast-in iron cylinder liners with 86mm bores. The block is a 2-piece design that fully supports the five main bearings with a single cast-alloy bed-plate assembly to maximize the rigidity and minimize noise and vibration. For even greater rigidity, the oil pan is a stiff aluminum alloy die casting.
To handle the 20-percent increase in horsepower and a 60-percent increase in torque output due to turbocharging, there are numerous strengthening measures throughout the engine. The cylinder block has been reinforced and additional structural reinforcements stiffen the engine's mating surface with the transmission.
The RDX uses a forged-steel crankshaft with a long 99mm stroke. To reduce friction and improve durability, the crankshaft journals are micropolished. Special forged connecting rods are engineered to handle the engine's high power output. The RDX has special alloy pistons with thick crowns and super hard Ni-P plating in the piston ring grooves to provide a long-wearing surface with extra heat resistance. A high-capacity oil pump supplies oil jets directed at the underside of the piston crowns to help dissipate heat.
CYLINDER HEAD / VALVETRAIN
The RDX cylinder head is a low-pressure aluminum alloy casting that features four valves per cylinder driven by dual overhead camshafts. Due to turbocharging, the compression ratio is set at 8.8:1 and Premium Unleaded fuel is required for best resistance to detonation. Valve sizes are 35mm intake and 30mm exhaust, as in the TSX. An automatically adjusted silent-type cam chain is maintenance free and runs in an oil bath for maximum durability. For better cooling, the RDX has small-diameter 12mm spark plugs (the TSX has 14mm plugs) with a long 26.5mm reach (the TSX has 20.5mm reach plugs). The added reach allows the cylinder-head coolant jacket to be larger, with more coolant volume for better cooling and detonation resistance.
The RDX engine features i-VTEC, which adds Variable Timing Control™ (VTC™) to Variable Valve Timing and Lift Electronic Control (VTEC®) to provide optimal camshaft phasing. By allowing the valve lift and valve timing to be adjusted to suit the engine's operating parameters from moment to moment, i-VTEC provides substantial performance, efficiency and emissions improvements.
Variable Valve Timing and Lift Electronic Control
VTEC® adjusts the lift and opening duration of the valves to help the engine produce stronger low-rpm torque and excellent high-rpm power. At low rpm, the timing of the two intake valves is staggered and their lift is asymmetric within each cylinder; this creates a swirl effect within the combustion chambers. With better mixing in the cylinders, burn speed and combustion stability are improved. As engine rpm builds, VTEC® transitions to a high-lift, long-duration cam profile for improved high-rpm engine output.
Variable Timing Control
The i-VTEC® system incorporates VTC™, which continuously adjusts the intake cam timing to suit engine operating conditions. This precise control of cam timing improves power output, fuel economy and emissions performance.
A powertrain control unit monitors camshaft position, ignition timing, exhaust oxygen content and throttle position, and then it commands a VTC™ actuator to advance or retard the intake cam optimizing engine output and reducing emissions. At idle, the intake cam timing is retarded to deliver a stable idle and to reduce oxides of nitrogen (NOx) emissions. The intake cam is progressively advanced as rpm builds, so the intake valves open sooner and valve overlap increases. This reduces pumping losses, which increases fuel economy and further reduces exhaust emissions due to the creation of an internal exhaust gas recirculation effect. By continuously optimizing the amount of intake cam advance based on the operating conditions, the RDX engine develops high peak power without compromising running manners at a full range of other operating speeds.
DRIVE-BY-WIRE THROTTLE SYSTEM™
Smart electronics connect the RDX throttle pedal to the throttle-body butterfly in the intake tract, optimizing engine response to suit driving conditions. An adjustable "gain" between the throttle pedal and engine offers a significant step forward in drivability.
The Drive-by-Wire Throttle System™ establishes the current driving conditions by monitoring throttle pedal position, throttle-body butterfly position, road speed, and engine speed. This information is used to define the throttle control sensitivity. This gives the RDX throttle pedal a predictable and responsive feel that meets drivers' expectations.
VARIABLE FLOW TURBOCHARGER
A variable flow turbo is the key element that allows the RDX's 4-cylinder engine to perform like a larger-displacement 6-cylinder engine, while still returning small-displacement fuel economy and very low emissions.
One of the challenges of turbocharging is the difficulty in sizing the turbo for optimal performance at all engine speeds. A small turbo spins up quicker, reducing lag (the delay between throttle-pedal movement and a substantial increase in boost pressure), but doesn't have enough flow for the best high engine-rpm performance. Conversely, a large turbo takes longer to spin up, but delivers maximum high rpm performance.
The new Acura variable flow turbo is designed to combine the best qualities of both small and large turbochargers, while avoiding their respective downsides or introducing any reliability concerns. The variable flow turbo increases exhaust gas velocity into the turbo housing at low speeds by restricting the inlet area with a movable valve, enabling the impeller to speed up quicker. At higher engine speed where exhaust gas flow is abundant, the valve opens progressively to maintain optimum exhaust gas flow into both the inner and outer scroll of the turbo housing.
Both the movable flow control valve and the wastegate that limits maximum boost pressure are powered by diaphragm-type actuators that are controlled by electric solenoids, which are in turn controlled by the engine's Electronic Control Unit (ECU). Under full-throttle operation, the flow control valve typically begins to open at 2000 rpm, and is fully open by 2500 rpm. Flow control valve position is determined by exhaust gas flow and not directly by engine rpm. Maximum boost pressure, which is dependent on temperature, altitude and other factors, is 13.5 pounds per square inch (700mm Hg).
Acura's variable flow turbo is designed for maximum durability. Unlike competitor variable-vane turbo designs that put the pivot point of moving parts in very high-temperature areas of the turbo housing, the Acura variable flow turbo has fewer moving parts inside the hot turbo housing, and is designed to shield the control valve pivot from the highest temperatures.
RDX's turbocharger is positioned to the rear of the engine, close to the exhaust ports for maximum transfer of the exhaust velocity to the turbo. Water passages in the aluminum exhaust manifold help keep the temperature within the exhaust system at optimum levels for both the turbocharger and catalytic converter. The turbo housing is made of austenite stainless cast iron for durability when exposed to high-temperature exhaust gases.
When the engine is shut off, coolant continues to circulate due to thermal convection, helping to prevent long-term heat-related damage to the turbo bearings.
COLD-AIR INDUCTION SYSTEM AND INTERCOOLER
An air inlet is positioned in the front fascia to supply the RDX engine with cold, dense air (see the Body & Chassis section). From there, ducting carries cold air to a large air filter assembly, and then on to the turbo located behind the engine block. From the turbo, compressed intake air flows forward over the engine via an air-to-air intercooler which lowers the temperature of the intake charge. Intake air exits the intercooler and passes through a drive-by-wire throttle body, a manifold and then into each cylinder.
To provide the intercooler with a constant flow of cooling air, the RDX has a large front air inlet above the grille, with ducting directly to the intercooler, creating a ram effect for increased air velocity across the intercooler.
Spent gases flow though a liquid-cooled aluminum exhaust manifold on the rear side of the cylinder head, through the variable flow turbo, and then into a close-coupled catalytic converter. From there, exhaust is passed to an under-floor catalyst, through an 8-liter under-floor pre-chamber, and then back to a 15-liter silencer. Dual exhaust outlets with bright exhaust tips terminate at the rear fascia. The RDX's exhaust note within the cabin has been specially tuned to deliver a sporty exhaust sound under acceleration, while remaining refined at cruising speeds.
The RDX's turbocharged engine achieves a high output of 104.3 horsepower per liter. At the same time, it meets strict CARB ULEV-2 standards.
Basic engine architecture contributes to the RDX's excellent emissions performance. The aluminum exhaust manifold and turbocharger are positioned to the rear of the transversely mounted engine, allowing the catalytic converter to be positioned very close to the turbo. With the close-coupled multi-cell catalytic converter, light-off is quick after startup. The i-VTEC cylinder head also plays a role in emissions reduction by fostering internal exhaust gas recirculation, which significantly cuts NOx. Special high-flow fuel injectors with a wide dynamic range contribute to the RDX's low emissions and high horsepower, and a variable flow fuel system also contributes to low evaporative emissions. The accuracy of the control of the injector has been increased so that less fuel can be injected during certain situations (such as when at idle).
Depending on operating conditions, evaporative emissions from the fuel tank are fed into the RDX's intake tract, to prevent fuel vapors from escaping to the atmosphere. Whereas a normally aspirated engine always has a partial vacuum in its intake tract that pulls evaporative emissions in, a turbocharged engine has high pressure in its intake tract whenever the turbocharger is providing forced induction. The RDX directs turbo pressure through a small nozzle to create negative pressure and pull evaporative emissions into the engine through a valve before the turbocharger.
To provide maximum lubrication even at high operating temperatures inside the variable flow turbo, the RDX only uses specified synthetic engine oils, such as Mobil 1® to satisfy anti-deposit and anti-deterioration performance requirements. The Maintenance Minder™ system integrated into the instrument cluster calculates synthetic engine oil life based on the vehicle's driving history, and alerts the driver when an oil change and other maintenance needs are required.
NOISE, VIBRATION AND HARSHNESS CONTROL
The RDX's high output in-line 4-cylinder engine features extensive Noise, Vibration and Harshness (NVH) reduction technologies. The aluminum alloy die cast cylinder block with a bed-plate type main bearing cap helps resist vibration. A pair of chain-driven balance shafts further smoothes the inherent vibration of the large displacement in-line 4-cylinder layout. The RDX oil pan is cast aluminum for better strength and sound attenuation than stamped metal designs. A self-adjusting silent-type cam chain and a serpentine accessory drive belt also help minimize NVH.
SEQUENTIAL SPORTSHIFT 5-SPEED AUTOMATIC TRANSMISSION WITH PADDLE SHIFTERS AND GRADE LOGIC CONTROL
To maximize acceleration, fuel economy and driver control, the RDX has a standard Sequential SportShift 5-speed automatic transmission with F1®-style paddle shifters and Grade Logic Control. Scaled to handle the RDX's high horsepower and torque output, the transmission has a standard fluid cooler that is mechanically similar to the RL. Special torque converter settings are designed to complement the RDX's turbo boost curve and power delivery by allowing more initial slip and rpm gain so that torque can build more quickly.
5-Position Shift Gate and Sequential SportShift Paddle Shifter Operation
The RDX has a 5-position shift gate that simplifies the operation of the transmission. In the "D" position, the transmission operates automatically, selecting among the five forward gear ratios based on the driving conditions. By pulling the lever back into the "S" position, the transmission is placed in "Sport" mode, providing sportier automatic shifts among the first three gears during normal speeds. When initially engaging sport mode, the transmission will automatically shift between first, second and third gears until a paddle shifter is activated, at which time the transmission goes into full manual sport mode.
The Sequential SportShift Paddle Shifters can operate in both the "D" and "S" position. Engaging a paddle when in "D" temporarily places the transmission in manual mode (but without gear holding at redline) and after relatively steady-state cruise conditions are detected, the transmission returns to full automatic operation. Engaging a paddle when in "S" additionally enables gear holding (which allows only driver commanded shifts), only automatically shifting to first gear at a stop, if not driver-selected. The steering wheel paddles are activated when a fingertip pull of the right paddle commands an upshift and a pull of the left paddle commands a downshift. A digital display in the right meter pod indicates which gear the transmission is in when the steering wheel paddles are active.
The RDX transmission can be operated in conventional fully automatic mode ("D" and "S") via the console-mounted 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" on hills and through turns.
Grade Logic Control alters the transmission'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 map stored in the transmission computer. The Grade Logic Control System then determines when the RDX is on a hill; in this case, the shift schedule is adjusted to automatically hold the transmission in a lower gear for better climbing power or increased downhill engine braking.
Shift Hold Control keeps the transmission in its current (lower) ratio when the throttle is quickly released and the brakes are applied (as might be the case when decelerating for a corner). This leaves the chassis undisturbed by excess shifting, while ensuring that abundant power is immediately available without a downshift. This is beneficial in a turbocharged engine where higher rpm helps to assure higher boost pressures and power output.
Manual Mode Protective Features
To help protect the engine and drivetrain from damage, an array of preventative features are active when the transmission is in manual mode (i.e. paddles active). If the driver doesn't command an upshift in time, the transmission ECU cuts off fuel flow to the engine if there is a possibility of over-revving in each gear. While downshifting, the transmission will not execute a driver-commanded downshift that would send the engine beyond redline in the lower gear. The Sequential SportShift transmission will automatically downshift to first gear as the vehicle comes to a stop to prevent lugging away from a stop in a high gear.
Cooperation between 5-Speed Automatic Transmission and Drive-by-Wire Throttle System™
With the RDX's Drive-by-Wire Throttle System™ and electronically controlled automatic transmission, the function of the engine and transmission can be closely choreographed for faster, smoother shifting. As a result, shift shock is reduced significantly during upshifts and downshifts.
SUPER HANDLING ALL-WHEEL DRIVE™
The RDX has standard Super Handling All-Wheel Drive™ (SH-AWD™). The RDX system differs mechanically from the RL luxury sedan, but is similar in principle. This all-wheel-drive system progressively distributes the optimum amount of torque not only between the front and rear axles but also between the left and right rear wheels. The result is neutral, accurate steering when cornering under power that front-drive, rear-drive or conventional all-wheel-drive can't equal.
SH-AWD™ counters understeer by rotating the outside rear wheel faster than the speed of the front axle to yaw the vehicle through the turn while cornering. By relieving the front tires of some of the work of turning the vehicle, the system reduces understeer, and handling balance and controllability are improved. 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. But in the RDX, additional cornering force is created by the steering angle of front tires combined with the extra drive torque supplied by the outside rear tire.
To deal with high power output, conventional front- or rear-drive systems generally use some type of limited-slip device to maintain traction under power. However, the linking effect of the inside and outside drive wheels in these systems resists turning, or creates understeer. This is a factor that works against the front tires as they attempt to turn the car. Conventional AWD systems have a similar linking effect between the inboard and outboard tires and front and rear axles, causing a similar resistance to turning. By using drive torque to actually help turn the vehicle, the RDX can be more responsive, neutral and predictable, while simultaneously offering all of the usual benefits of all-wheel drive.
Electronic Controls and Parameters
The logic and control of SH-AWD™ is integrated with the RDX engine/transmission Electronic Control Unit (ECU), and Vehicle Stability Assist™ (VSA®) ECU. The engine/transmission ECU provides engine rpm, airflow meter and transmission gear ratio data, while the VSA® ECU provides data on wheel speed. The SH-AWD™ ECU also monitors lateral g-forces, yaw rate and steering angle, and the status of rear right and left direct electromagnetic clutch torque. Drive torque is calculated based on the information from the engine/transmission ECU, then the acceleration situation, wheel spin, lateral G and steering angle are used to set the front to rear torque distribution, and torque split between the right and left rear wheels. SH-AWD™ operating parameters include:
- Up to 90-percent of available driveline torque can be transferred to the front wheels during high-speed driving
- During straight line full-throttle acceleration, up to 45-percent of available torque can be transferred to the rear wheels
- In hard cornering during acceleration, up to 70-percent of available torque can be directed to the rear wheels for enhanced vehicle dynamics
- Up to 100-percent of the torque sent to the rear axle can be applied to either rear wheel as the conditions dictate
SH-AWD™ System Layout
SH-AWD™ is a full-time all-wheel drive system that requires no driver interaction for operation. The RDX system differs mechanically from the RL system, but is similar in principle.
A torque transfer unit is bolted directly to the front-mounted transaxle. Attached to the front differential's 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 send power to the rear propeller shaft, which in turn carries it to the rear drive unit.
The rear drive unit of the RL sedan has a clutch and planetary gear set, called the "acceleration device," that can increase the speed of the rear axle up to 5.7-percent. In contrast, the RDX's rear driveshaft is constantly overdriven by 1.7-percent and the resulting overdrive effect is regulated by the left and right rear clutch packs. The difference in the chosen rear axle speed increase is due to the differing handling dynamics of the two vehicles. The RDX achieves the desired SH-AWD™ cornering characteristics with less outside rear wheel speed increase. A matched pair of direct electromagnetic clutch systems, one on each side, control the power delivered to each rear wheel.
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. These clutch systems can be controlled as a pair to alter the front/rear torque split, or be controlled independently, to allow up to 100-percent of the total rear axle torque to go to one side of the vehicle.
An electric coil controls the pressure in each clutch device, which slows the sun gear in a planetary gear set to modulate the torque sent to the wheel. The amount of available rear axle torque transmitted to each rear wheel can vary continuously, between zero and 100-percent, depending on the conditions. Since the clutches are electromagnetically operated, the amount of drive torque delivered to each rear wheel can be controlled quickly and precisely, reducing wheel slip in low traction conditions.
The clutch packs and friction material are specially designed to withstand the small amount of continuous slip between front and rear axles (created by the 1.7-percent speed differential), while ensuring the expected level of Acura durability. An oil-temperature sensor allows the ECU to estimate the clutch plate coefficient of friction (which changes with heat) in each clutch pack and then adjusts voltage sent to the electromagnetic coil that controls each clutch to compensate. To ensure that the amount of torque transmitted remains optimized as miles accumulate a coil provides a feedback loop that the ECU uses to adjust voltage to the electromagnetic clutches to compensate for any clutch wear.
MAINTENANCE MINDER SYSTEM
To eliminate unnecessary service visits while ensuring that the vehicle is properly maintained, the RDX has a Maintenance Minder™ system that automatically monitors the vehicle's operating conditions. When maintenance is required, the driver is alerted via a series of messages on the Multi-Information Display (MID) located in the instrument cluster. The alerts occur well in advance of required maintenance for scheduling convenience.
The resettable system monitors operating conditions such as oil and coolant temperature and engine rotations to determine the proper service intervals. It indicates when maintenance is due on many normal service parts and systems, including oil and filter change, tire rotation, air cleaner, automatic transmission fluid, parts replacement and more. To prevent driver distraction, maintenance alerts are presented when the ignition is first turned on, not while driving unless an urgent need arises.