2004 Acura 3.5 RL -- Powertrain
The 3.5 RL is powered by an all-aluminum, 3.5-liter, single-overhead-camshaft, 90-degree V-6 engine that produces 225 horsepower at 5200 rpm and 231 lb.-ft. of torque at 4700 rpm. Four-valves-per-cylinder, a three-stage variable induction system, a sophisticated PGM-FI fuel-injection system, a variable silencer and an advanced air intake help to provide both high torque at lower engine speeds and excellent breathing at higher engine speeds. The 3.5 RL meets LEV (Low Emission Vehicle) standards. It is also designed to run for 105,000 miles before its first scheduled tune-up.
The engine is located longitudinally, moving weight to the rear of the car for a 59/41 weight distribution, an optimal allocation for a front-wheel-drive sedan. The longitudinal mounting also yields reductions in noise, vibration, and harshness (NVH), because it allows the use of softer, optimally positioned engine and transmission mounts.
An electronically controlled, 4-speed automatic transmission, with Grade Logic Control is standard on every 3.5 RL.
The block of the RL engine is made of cast aluminum with cast-in iron cylinder liners, a design emphasizing light weight, rigidity, and outstanding long-term durability. Designed with deep skirts and extensive webbing for added rigidity, the crankcase minimizes noise and vibration. The RL engine, a slightly undersquare design with a bore of 90.0 mm and a stroke of 91.0 mm, yields a displacement of 3474 cc.
The generous main bearing journal supports add rigidity and significantly reduce vibration in the 300-to-800 Hz range. By optimizing stiffness, the resonant frequency of the engine block is an impressive 20 Hz -- meaning that this engine transmits very little vibration through the rest of the vehicle.
PRIMARY COUPLE BALANCER
The rotating balance shaft, mounted to the right side of the engine block, further reduces vibration transmitted to the passenger cabin. The belt-driven balancer counteracts the rocking tendency inherent in 90-degree V-6 designs.
To reduce friction and enhance durability, the forged steel crankshaft is microfinished, an extremely precise finishing process developed originally for the Honda Formula One engine program. The surface of the main bearings has a unique, pyramidal structure, allowing space for enhanced oil retention, reduced friction and improved long-term durability. This sophisticated metallurgy is another benefit gained from the Honda Formula One program.
BLOCK-FORGED CONNECTING RODS
The 3.5 RL has connecting rods forged using a unique, advanced-technology process, called block-forging, that results in parts of unprecedented strength and uniformity. The RL engine is the first automobile engine in the world to utilize this advanced technology.
Because of the imprecise nature of the conventional forging process, the connecting rods in other manufacturer's engines must be sorted according to weight, with rods of approximately equal mass assigned to each particular engine to achieve proper reciprocating balance. Even in rods that happen to have equal overall mass, the distribution of metal density throughout conventional rods can vary enough to affect strength and engine balance.
In marked contrast, the rods of the RL are forged with a "block-forging" system in which the steel blank is struck, at extremely high pressure, from five directions at once. This process has no allowance for excess metal; giving it nowhere to go in the ultra-precise block-forging dies. Accordingly, the blanks that will be forged into RL rods are machined before the forging process, to exacting mass tolerances. In other words, the material that will become an RL connecting rod has been precisely measured and balanced before it ever enters the forging dies.
As a result of this process, RL connecting rods are among the most precisely balanced of any rods found in a production-automobile engine.
FULL-FLOATING PISTON PINS
To reduce engine noise caused by contact between the pistons and piston pins, the RL engine uses a full-floating piston pin design that reduces the clearance between each pin. Typically, a piston pin is press fit in the rod, and a larger clearance between the pin and the aluminum piston allows the piston to rock freely. In the more innovative RL design, the pin moves freely in the rod and the piston rocks freely on the pin; the piston pin is essentially free to "float" between the piston and the rod. This advanced design results in a significant 5-decibel reduction in noise.
The RL engine features a four-valve-per-cylinder valvetrain with pentroof combustion chambers. A flexible, toothed belt drives a single camshaft for each cast-aluminum cylinder head. Hydraulic lash adjusters are used for quieter running and reduced maintenance.
The combustion chamber is designed with a generous "squish" area formed by the piston and cylinder head - promoting rapid and complete combustion by increasing the turbulence of the fuel air mixture around the centrally located platinum tipped spark plug.
Conventional high-performance engines typically use a substantial amount of valve overlap to achieve better breathing at higher rpm, with the intake and exhaust valves opening simultaneously at top dead center between the exhaust and intake strokes. At low rpm, however, this allows blowback of the fuel-air mixture, with exhaust gas creating a pressure wave that reverses the flow of the fresh incoming charge. To prevent the resulting unstable combustion-and relatively rough running - at low rpm, the RL camshaft timing has considerably reduced valve overlap, resulting in smoother low-speed running and improved low-rpm torque. The advanced four-valve-per-cylinder design, and the Variable Induction System intake tract, combine to maintain excellent high-rpm power output.
To further reduce noise, the resonant frequency of the timing belt has been tuned by altering both its width and its material. The position of the belt tensioner was also optimized as part of the intensive effort to reduce overall engine noise.
VARIABLE INDUCTION SYSTEM
To tune the intake system volume and length for optimum torque at any rpm, the RL uses a three-stage intake manifold. To enhance cylinder charging by making use of resonance effects in the intake tract, the engine uses a longer intake tract at low rpm, transitions to a mid-length tract in the mid-range, and then is opened completely for high-rpm running - allowing the inertia effect of the moving column of air in the system to enhance cylinder filling at higher engine speeds. The system has been designed and tuned for the special low-rpm/high-torque tuning emphasis of the RL engine.
The RL features an innovative variable silencer. A special valve in the first exhaust chamber within the muffler is able to route the exhaust as needed for quiet operation and, when applicable, reduced back pressure for improved power output. Under normal driving conditions, the valve is closed - channeling exhaust flow through the entire muffler tract for maximum noise reduction. During hard acceleration, however, at 3500 rpm the valve opens - allowing exhaust to flow directly to the muffler's dual exit chambers for improved flow and enhanced engine output.
In addition to its variable silencer, the RL's V-6 engine benefits from a unique air intake-joint shape and a large intake-opening size for enhanced airflow to the intake manifold. The increase in intake airflow allows for a proportionate increase in fuel delivery, with a corresponding increase in ignition timing for enhanced engine power and response.
PROGRAMMED FUEL INJECTION (PGM-FI)
Programmed Fuel Injection (PGM-FI) compensates for varying driving and atmospheric conditions, providing enhanced drivability and engine efficiency in a wide variety of
conditions. Data on coolant temperature, air temperature and density, manifold vacuum, engine rpm, throttle position, crankshaft position, and exhaust oxygen content is interpreted by the Electronic Control Module (ECM), which determines fuel injector duration and timing as well as ignition timing and transmission operation. To reduce the pulsing noise emanating from the fuel injectors, a special sound-dampening unit is mounted at the junction of the fuel delivery tube and each individual injector.
4-SPEED GRADE LOGIC CONTROL AUTOMATIC
The result of an extensive development program, the advanced, computer-controlled automatic transmission of the RL underscores Acura's unwavering commitment to automotive refinement, efficiency, and responsiveness.
The 4-speed transmission uses the innovative Grade Logic Control system to enhance uphill and downhill powertrain smoothness. When driving on steep grades, the Electronic Control Module automatically holds the transmission in the proper gear, even if a reduction in throttle position would otherwise dictate an upshift. This all but eliminates unnecessary shifts and "gear hunting." Also, on downgrades, the transmission holds third gear instead of automatically shifting to fourth, letting engine braking help prevent unwanted velocity.
A refined, lock-up torque converter enhances performance and acceleration while optimizing fuel efficiency and engine braking. Developed to reduce the "creeping" effect at idle, the converter makes the vehicle extraordinarily easy to hold with the foot brake at a stop.
HYDRAULIC SUBFRAME MOUNT
To reduce engine noise under acceleration, the subframe mounts between the engine/transmission/suspension assembly are filled with hydraulic fluid of a specifically tuned viscosity. The resulting softer damping rate helps to isolate the passenger compartment from engine and road noise.
ELECTRONIC/HYDRAULIC ENGINE MOUNTS
To reduce vibration at idle, the RL uses hydraulic engine mounts with two different damping rates: a soft rate that allows the engine/transmission unit to move more freely at idle and a stiffer rate that positions the powertrain more firmly during driving. Between 820 rpm and 850 rpm, the damping characteristics of the mounts change automatically; a small solenoid valve closes, restricting the flow of fluid from chamber to chamber thus increasing the damping rate.
In addition, the main engine mounts have been located very close to the center of mass of the engine/transmission unit, more effectively isolating the primary inertial couple effect at idle. This results in significantly reduced vibration felt at the front seat in the critical 500 to 1200 rpm idle and off-idle range.