From the beginning, the RDX was 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. On one hand, 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 solution comes in the form of a turbocharger- a first-ever application for an Acura production vehicle. The RDX has an innovative variable flow turbocharger that overcomes the shortcomings of conventional turbo designs and delivers an unusually broad powerband with almost no lag in throttle response. Acura’s turbocharger differs from other variable flow turbo designs in that airflow is controlled before it enters the turbocharger, rather than within the turbocharger which is common with 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 inline 4-cylinder, the RDX uses a 2.3-liter DOHC 16-valve inline 4-cylinder engine. Like the TSX, the RDX uses i-VTEC® intake valve control that combines Variable Valve Timing and Lift Electronic Control (VTEC®) with Variable Timing Control™ (VTC™). With the variable flow turbocharger working in unison with i-VTEC®, the RDX delivers strong power and torque, excellent fuel economy and low emissions.
Rated output for the 2.3L engine is 240 horsepower at 6,000 rpm and 260 lb-ft of torque at 4,500 rpm. The RDX has an EPA city/highway/combined fuel economy rating* of 19/24/21 mpg (RDX 2WD) and 17/22/19 mpg (RDX SH-AWD™).
*Based on 2012 EPA mileage estimates. 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.
With its rear-mounted exhaust manifold feeding directly into the turbocharger, the use of a close-coupled catalytic converter and its advanced Programmed Fuel Injection, the 2012 RDX complies with EPA TIER 2 – BIN 5 and CARB LEV II ULEV emissions standards.
The RDX’s Sequential SportShift 5-speed automatic can function as a conventional automatic transmission or (at the driver’s option) it can be shifted manually via steering-wheel-mounted paddle shifters. 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 is available 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 wheels. In addition, via torque vectoring the SH-AWD™ system can overdrive an outside rear wheel to generate a yaw moment that helps reduce understeer, improve steering accuracy and increase cornering power.
ENGINE BLOCK, CRANKSHAFT AND OIL PAN
The RDX utilizes a die-cast aluminum cylinder block that has cast-in iron cylinder liners with 86mm bores. The cylinder block is a 2-piece design that supports the five main bearings with a single cast-aluminum bed-plate assembly to help maximize rigidity and minimize noise and vibration. For even greater rigidity, the oil pan is a stiff aluminum 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 2.3L engine. The cylinder block has been reinforced and additional structural considerations stiffen the engine’s mating surface with the transmission.
The RDX uses a forged-steel crankshaft with a 99mm stroke. To reduce friction and improve durability, the crankshaft journals are micropolished. Special forged-steel connecting rods are used to handle the engine’s high power output. The RDX has special aluminum pistons with thick crowns and super hard Nickel-Phosphorus (Ni-P) plating in the piston ring grooves to provide a long-wearing surface with excellent heat resistance. A high-capacity oil pump supplies special oil jets directed at the underside of the piston crowns to help dissipate heat.
CYLINDER HEAD / VALVETRAIN
The RDX cylinder head is constructed of cast aluminum and features four valves per cylinder that are driven by dual overhead camshafts (DOHC). The intake valve head diameter is 35mm while the exhaust valves measure 30mm. An automatically adjusted silent-type cam drive chain runs in an oil bath for maximum durability and is maintenance free. For better cooling, the RDX has small-diameter 12mm spark plugs with a long 26.5mm reach. The added reach allows the cylinder-head coolant jacket to be larger, with more coolant volume for detonation resistance.
The RDX engine features Acura’s acclaimedi-VTEC® system that teams Variable Timing Control™ (VTC™) with Variable Valve Timing and Lift Electronic Control (VTEC®) to provide optimal camshaft phasing. By allowing the intake valve lift and valve timing to be adjusted to suit the engine’s specific operating parameters, i-VTEC® provides substantial performance, efficiency and emissions improvements.
Variable Valve Timing and Lift Electronic Control (VTEC®)
VTEC® adjusts the lift and opening duration of the intake valves to help the engine produce a combination of strong low-rpm torque and excellent high-rpm power. At low rpm, the timing of the two intake valves is staggered and their lift amount is asymmetric within each cylinder- creating a swirl effect within the combustion chambers. Increased swirl effect generates improved blending of the air/fuel mixture along with better distributes the mixture throughout the combustion area within the cylinder- helping promote improved burn speed and combustion stability. As engine rpm builds, VTEC® transitions to a high-lift, long-duration intake cam profile for improved high-rpm engine output.
Variable Timing Control (VTC™)
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, throttle position and exhaust oxygen content- 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 reduced oxides of nitrogen (NOx) emissions. As engine rpm builds, the intake cam is progressively advanced 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 (EGR) 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 over the full range of operating speeds.
DRIVE-BY-WIRE™ THROTTLE SYSTEM
Smart electronics connect the RDX throttle pedal to the throttle-body valve in the intake tract, optimizing engine response to suit specific driving conditions. The Drive-by-Wire™ throttle system establishes the current driving conditions by monitoring throttle pedal position, throttle-body valve position, engine rpm and road speed. This information is used to define the throttle control sensitivity and to generate a responsive feel that meets drivers’ expectations. If performance driving is detected, adjustable “gain” between the throttle pedal and engine offers an improvement in drivability.
VARIABLE FLOW TURBOCHARGER
A variable flow turbocharger is the key that allows the RDX’s 4-cylinder engine to perform like a larger-displacement 6-cylinder engine- all while delivering small-displacement fuel economy and low emissions.
One challenge 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 an increase in boost pressure), but normally doesn’t have enough flow for strong high rpm performance. Conversely, a large turbocharger takes longer to create boost pressure (known as turbo lag), but delivers excellent high rpm performance.
Acura’s variable flow turbocharger was designed to combine the best qualities of both small and large turbos, while avoiding their respective downsides or introducing any reliability concerns. A variable flow turbocharger increases exhaust gas velocity into the turbo housing at low speeds by restricting the inlet area via a movable valve- thus enabling the turbo impeller to speed up quicker. At higher engine speed where exhaust gas flow is abundant, the valve opens progressively (based on engine rpm) to maintain optimum exhaust gas flow into both the inner and outer scroll of the turbocharger housing.
Both the movable flow control valve and the wastegate (the mechanical device that limits maximum boost pressure) are powered by diaphragm-type actuators that are controlled by electric solenoids, which are controlled by the engine’s Electronic Control Unit (ECU). Under full-throttle operation, the flow control valve begins to open at 2,000 rpm, and is fully open by 2,500 rpm. Flow control valve position is determined by exhaust gas flow, not by engine rpm. Maximum boost pressure (which is dependent on temperature, altitude and other factors) is 13.5 pounds per square inch (700mm Hg).
Unlike variable-vane turbocharger 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.
The RDX’s turbocharger is positioned at the rear of the engine, close to the exhaust ports for maximum transfer of the hot, high velocity exhaust gasses. 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. When the engine is shut off, coolant continues to circulate within the turbo housing, helping prevent long-term heat-related damage to the turbocharger bearings.
COLD-AIR INDUCTION SYSTEM AND INTERCOOLER
An air inlet is positioned in the RDX’s front fascia to supply the 2.3L engine with cool, dense air. From there, inlet ducting carries the air to a large air filter assembly, and then on to the turbocharger located behind the engine block. From the turbo, compressed intake air is routed through an air-to-air intercooler that lowers the temperature of the intake charge. As the compressed air exits the intercooler, it passes through the Drive-by-Wire™ throttle body, into the intake manifold and is then routed to each cylinder.
To provide the intercooler with a constant flow of cooling air, the RDX has a large front-mounted air inlet located above the grille that creates a “ram-air” effect for increased air velocity across the intercooler.
Spent combustion gases flow though a liquid-cooled aluminum exhaust manifold (mounted on the rear side of the cylinder head), through the variable flow turbocharger, and then into a close-coupled catalytic converter. From there, exhaust is routed to an under-floor catalytic converter, through a large pre-muffler and then to a high-flow muffler. Dual exhaust outlets fitted with bright finish exhaust tips reside 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 low 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 LEV II ULEV (Ultra Low Emissions Vehicle) emissions standards.
Greatly contributing to low emissions are the aluminum exhaust manifold and turbocharger that are positioned to the rear of the transversely mounted engine. This design allows the catalytic converter to be positioned very close to the turbocharger generating extremely quick catalytic converter “light-off” after start-up. Thei-VTEC® design valvetrain also plays an important role in emissions reduction by fostering internal exhaust gas recirculation (EGR), which significantly reduces the amount of nitrogen oxides (NOx) emitted. 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 delivers low evaporative emissions.
Depending on operating conditions, evaporative emissions from the fuel tank are fed into the RDX’s intake tract to prevent the fuel vapors from escaping to the atmosphere. Whereas a normally aspirated engine always has partial vacuum in its intake tract that pulls in evaporative emissions, a turbocharged engine has high pressure in its intake tract whenever the turbocharger is providing forced induction. Thus, the RDX directs some turbo pressure through a small nozzle to create negative pressure (a vacuum) that pulls evaporative emissions into the engine.
To provide maximum engine lubrication (even at high operating temperatures within the variable flow turbocharger), the RDX uses Mobil 1® synthetic engine oil to satisfy anti-deposit and anti-deterioration performance requirements. The Maintenance Minder™ system integrated into the instrument cluster calculates 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 inline 4-cylinder engine features extensive Noise, Vibration and Harshness (NVH) reduction technologies. The die-cast aluminum cylinder block with a bed-plate type main bearing cap setup helps resist vibration. A pair of chain-driven internal balance shafts further smoothes the inherent vibration of the large displacement inline 4-cylinder layout. The RDX oil pan is constructed of cast aluminum for better strength and sound attenuation as compared to a stamped steel design. A self-adjusting silent-type cam chain and a serpentine-style accessory drive belt team to help minimize NVH.
SEQUENTIAL SPORTSHIFT 5-SPEED AUTOMATIC TRANSMISSION
To maximize acceleration, fuel economy and driver control, the RDX has a Sequential SportShift 5-speed automatic transmission with racing-inspired paddle shifters and Grade Logic Control System. Scaled to handle the RDX’s horsepower and torque output, the transmission has a special fluid cooler. The torque converter specifications are designed to complement the RDX’s turbocharger boost curve and power delivery by allowing more initial slip (resulting in quicker rpm gain) so that torque can build more quickly.
The RDX has a console-mounted 5-position shift gate that simplifies the operation of the transmission. In the Drive (“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 more performance-minded shifts in the first three gears. 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 (without gear holding at redline), and after steady-state cruise conditions are detected, the transmission returns to full automatic operation. When in “S” mode, engaging a paddle shifter enables gear holding, which allows only driver commanded shifts. The steering wheel mounted paddle shifters are activated via a quick fingertip pull. Using the right paddle shifter commands a transmission upshift while a pull of the left paddle commands a downshift. A digital display within the meter pod indicates which gear the transmission is in when the steering wheel paddle shifters are active.
With the RDX’s Drive-by-Wire™ throttle system and electronically controlled Sequential SportShift automatic transmission, the interaction between the engine and transmission can be closely choreographed for faster, smoother shifting. As a result, shift shock is reduced significantly during upshifts and downshifts.
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) gear 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 at corner exit without a transmission gear downshift.
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. paddle shifters are active in “S” mode). If the driver doesn’t command a gear upshift in time, the transmission ECU cuts fuel flow to the engine to prevent over-revving.
In addition, the transmission will not execute a driver-commanded downshift that would send the engine beyond maximum redline as caused by a change to a lower gear. The Sequential SportShift transmission will automatically downshift to First gear as the vehicle comes to a stop, which prevents the RDX from lugging away from a stop in a higher gear.
SUPER HANDLING ALL-WHEEL DRIVE™
For improved all-weather traction, the RDX is available with Super Handling All-Wheel Drive™ (SH-AWD™). The SH-AWD™ 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. SH-AWD™ can also overdrive a single outside rear wheel to create a yaw moment to further enhance vehicle control. By relieving the front tires of some of the work of turning the vehicle, yaw reduces understeer, and handling balance and controllability are improved.
With the cornering load more evenly distributed between the front and rear tires, the total cornering ability is increased. In conventional vehicles, cornering is created almost entirely by the steering angle of the front tires. With the RDX, cornering force is created by the steering angle of front tires combined with yaw as generated by the extra drive torque supplied by the outside rear tire. The result is more neutral, accurate steering when cornering under power that front-drive, rear-drive or conventional all-wheel-drive can’t equal.
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 and can create 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, 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-force and steering angle are used to set the front to rear torque distribution as well as the 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 normal 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
- Up to 100-percent of the torque sent to the rear axle can be applied to either rear wheel
SH-AWD™ System Layout
SH-AWD™ is a full-time all-wheel-drive system that requires no driver interaction for operation. A torque transfer unit is bolted directly to the RDX’s 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 power to the rear drive unit.
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- thus providing a limited-slip differential function. The electromagnetic clutch systems can be controlled as a pair to alter the front/rear torque split, or can 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 current 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 situations.
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 overdrive speed sent to the rear 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 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 which compensates 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 in advance of required maintenance for scheduling convenience.
The resettable system monitors operating conditions such as oil and coolant temperature and total engine rotations to determine the proper service intervals. Maintenance Minder™ 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 service need arises.