ENGINE ARCHITECTURE
An advanced 3.2-liter, 24-valve, all-aluminum, 60-degree V-6 engine
powers both the CL and the CL Type-S. Remarkably compact and
lightweight, the CL powertrain incorporates a wide variety of
advanced technologies, including the VTEC system, high flow air
filter and throttle body along with a unique, dual-stage induction
system exclusively on the CL Type-S.
ENGINE BLOCK
Die-cast and heat-treated, the compact aluminum block is extremely
rigid, with a high natural frequency and minimal resonant vibration.
The V-6's iron cylinder liners have their bore pitch set at 98 mm
spacing to reduce overall engine size. The free-revving CL engine is
oversquare with a bore of 89 mm and a stroke of 86 mm to give the
engine a total displacement of 3210 cc.
The CL employs a cylinder bank V-angle of 60-degrees, allowing for a compact design with improvements in engine smoothness and reduction in overall bulk and weight. The block has an unusually short deck height, resulting in an overall reduction in the height and width of the assembled engine.
The Type-S engine incorporates separate, spin-cast cylinder wall liners that are cast into the engine block during the initial manufacturing stage. These spin-cast liners allow for a reduction in weight over traditional cylinder liners and provide added heat dissipation to avoid engine knocking. Additionally, by reducing the number of machined surfaces, spin-cast cylinder liners simplify the manufacturing process.
CRANKSHAFT/CONNECTING RODS/PISTONS
A rigid, forged crankshaft and narrow 19 mm connecting rods allow a
reduction in overall engine length as well as width. The connecting
rods in the V-6 do not use conventional nut-and-bolt type fasteners,
instead utilizing lighter bolts that thread directly into the rod and
operate as "plastic-region" fasteners. These bolts allow a downsizing
of the rod bolts while maintaining the proper clamping force and
strength margins.
The V-6 incorporates compact pistons made from lightweight, cast aluminum alloy. The shape of the piston crown helps concentrate the fuel/air charge in the center of the combustion chamber to provide maximum volumetric efficiency, producing a 9.8:1 compression ratio in the CL. The Type-S achieves a 10.5:1 ratio thanks to a larger dome on the piston which provides a smaller combustion chamber volume. The pistons utilize a 3-ring design with the Type-S engine incorporating a unique angled top ring for an improved seal within the cylinder to accommodate the higher combustion pressure.
CYLINDER HEAD
The pressure-cast aluminum alloy cylinder heads reflect Acura's
latest thinking in engine design. Utilizing 4 valves per cylinder and
a compact combustion chamber with a generous squish area, the
cylinder head is a key component of the high output CL engines. The
Type-S engine also features special, race-designed intake valves to
further improve flow into the combustion chamber.
The single camshaft in each cylinder head is installed from the timing-belt end of the engine, eliminating the need for bolt-on camshaft caps &endash; a savings of weight and complexity. Driven by the crankshaft via a fiberglass-reinforced toothed belt, the cams activate the valves by way of friction reducing roller followers.
VARIABLE VALVE TIMING AND LIFT ELECTRONIC CONTROL (VTEC™)
The Variable Valve Timing and Lift Electronic Control (VTEC™)
valve train is a major contributing factor to the CL's remarkable
combination of high power delivery and fuel efficiency. With the lift
and opening duration of the intake valves altered automatically based
on engine rpm, the engine develops both strong low-rpm torque and
maximum high-rpm power.
At low-rpm, the VTEC intake valves follow a set of low-lift, short-duration cam lobes with timing that optimizes cylinder filling. Additionally, the timing of the intake valves is staggered and their lift is asymmetrical, creating a swirl effect within the combustion chambers. This increases burn speed and improves combustion stability and allowable EGR rate. As the engine accelerates through 4400 rpm (4800 rpm on Type-S), the intake rocker arms transition to high-lift, long-duration cam lobes designed to optimize high-rpm output.
To help boost high-rpm power, the camshafts found in the Type-S engine open the intake valves for a longer duration, provide additional lift for the exhaust valves and increase valve overlap.
DUAL-STAGE INDUCTION SYSTEM (CL TYPE-S)
The CL Type-S features a high-performance, dual stage induction
system, which contributes substantially to the output of the 260
horsepower engine. This system uses a large, two-chamber intake
plenum separated by a butterfly valve, which is opened by an electric
servomotor above 3800 rpm.
When the valve is closed during low-to-mid rpm operation, a resonance charge condition exists as pressure pulsations between cylinders assist the adjoining cylinder of each bank. In doing so, the volume of air filling the cylinders increases with the resonance energy generated by using the Helmholz principle. This Helmholz principle phenomenon (the same effect that increases bass response in a tuned-port sub-woofer) results in improved cylinder charging.
When this valve opens at 3800 rpm, an inertial charge condition is generated and increased air filling continues when the manifold (plenum) volume is increased. This phenomenon uses the high flow mass to pressurize the manifold port air, producing a natural "supercharger" effect.
This increased airflow and cylinder filling is a primary contributor to the CL Type-S engine's additional power. Additional improvements were made to reduce air intake restriction and increase total air flow volume through a larger 64 mm bore throttle body, a low restriction air intake and a high flow air filter.
PROGRAMMED FUEL INJECTION (PGM-FI)
Controlled by a 16-bit, 32 MHz Central Processor Unit (CPU), the CL's
Programmed Fuel Injection (PGM-FI) system monitors and controls
numerous fuel system functions. Signals indicating throttle position,
engine coolant temperature, intake manifold pressure, atmospheric
pressure, exhaust gas oxygen content and intake air temperatures are
all processed by the CPU. It controls fuel delivery by injectors
mounted in the cast aluminum, tuned-length intake manifold. The
manifold features a large plenum chamber to help maximize airflow and
increase power and torque. While tracking the operation of the engine
with position sensors on the crankshaft and both camshafts, the
PGM-FI CPU is also in constant communication with a similar CPU that
controls the 5-speed Sequential SportShift automatic transmission.
EXHAUST SYSTEM
The CL's high-flow, dual-outlet exhaust system is designed to offer
lightweight, low restriction performance while ensuring excellent
emissions and noise control. To help the Type-S achieve additional
horsepower, its system allows greater exhaust flow through a larger
diameter exhaust pipe, an increased flow catalytic converter and
ultra-high flow capacity dual silencers. These enhancements to the
Type-S increase exhaust flow by over 30 percent compared to the CL.
The Type-S exhaust has also been tuned to provide an aggressive,
performance-oriented note while minimizing noise at cruising speed.
DIRECT IGNITION SYSTEM/KNOCK CONTROL
Proper ignition spark timing is critical to engine performance and
emission control. This timing constantly changes depending on many
factors. If there is too little spark advance for the conditions,
then efficiency suffers; if there is too much spark advance, then
knocking (or pinging) can result in overheating and engine damage.
The CL features a knock-control system to ensure a properly timed spark. Based on a centrally positioned sensor in the block that "hears" the first traces of knocking, the ignition timing is advanced to the point of peak efficiency, but not beyond, even if fuel quality is less than the specified premium unleaded. This fine spark control allows the CL to operate safely and more efficiently with greater spark advance than its predecessor. The ignition system also features compact direct ignition coil units positioned directly in the spark plug access bores.
CONTROL OF NOISE, VIBRATION AND HARSHNESS
Though the CL's powertrain was engineered for uncompromised
horsepower and torque, special effort was devoted to reducing noise,
vibration and harshness (NVH). Key engine attributes were analyzed to
develop engineering targets for smoothness and noise control. Both
the CL and the CL Type-S engines reflect this effort with features
like the compact, rigid aluminum block and its unusually high
resonant frequency, a rigid forged crankshaft, die-cast accessory
mounts and a stiff cast aluminum oil pan.
105,000-MILE TUNE-UP INTERVALS
The CL boasts engine tune-up intervals of 105,000 miles, between
which only routine inspections and fluid changes are required. The
roller-follower design of the VTEC valvetrain cuts friction and wear
to the point that the screw-type tappet clearance adjusters do not
require inspection until 105,000 miles. At 105,000 miles, the
platinum-tipped spark plugs are also due for replacement.
DIRECT-ACTING CRUISE CONTROL
Even though the CL was designed as a true driving enthusiast's car,
an electronic cruise control is provided for convenience on long
trips. When engaged, the cruise control system's Electronic Control
Unit (ECU) directly commands downshifts to the automatic transmission
ECU as required, allowing the vehicle to more closely maintain the
set road speed even in hilly driving conditions. The convenient
steering wheel-mounted cruise control buttons allow the driver to
adjust speed in one mph increments or disengage the cruise control
without touching the brake pedal by pressing the "cancel" button.