1998 Acura 2.3CL -- Powertrain

4/20/1997 9:00:01 PM

The 2.3CL is powered by a completely re-engineered single-overhead-camshaft, 2.3-liter inline four-cylinder engine. Designed to produce additional horsepower and torque with a significant reduction in noise and vibration. Like all other Acura engines, this powerplant is made of aluminum alloy and is equipped with cast-iron cylinder liners. It features four valves per cylinder-two intake, two exhaust-and a single overhead camshaft. The engine is also equipped with Programmed Fuel Injection (PGM-FI) and an onboard diagnostic system (OBD-II). Peak output is 150 horsepower at 5700 rpm and 154 lbs-ft of torque at 4800 rpm.

For exceptional power at all engine rpms combined with excellent fuel economy, the engine utilizes the unique Variable Valve Timing and Lift Electronic Control (VTEC) system first pioneered by Acura on the exotic NSX sports car. By automatically changing the timing, lift, and duration of the intake valves, the VTEC system contributes to abundant low-rpm torque and impressive high-rpm horsepower.

Transmissions for the 2.3CL include a refined 5-speed manual and a new optional electronically controlled 4-speed automatic equipped with the Grade Logic Control System. A direct-acting electronic cruise control system is standard equipment.

Like all other Acura engines, the engine block of the 2.3CL inline four-cylinder is an aluminum pressure casting with cast-iron liners, a design known for its light weight, high rigidity, and excellent long-term durability. The new block is designed with a deep skirt and extensive webbing, adding rigidity and reducing engine noise and vibration. Additional bottom-end rigidity comes from a larger, stronger bearing-cap beam.

The engine block incorporates a second-order balance system that cancels the inertial forces common to large-displacement 4-cylinder engines. The system consists of two parallel shafts on either side of the crankshaft, 81mm (3.19 in.) above its centerline. Driven by a toothed belt, the balance shafts rotate in opposite directions at twice engine speed. Eccentric weights built into the shafts generate inertial forces that counteract the second-order forces created by the motion of the pistons and connecting rods. This system differs from other balancing systems in that it is designed to minimize vibration in the mid-to-high-rpm range, as this is the area where the engine operates much of the time.

To enhance the performance of the power-cylinder CL model displacement was increased to 2.3 liters (2254 cc) by enlarging the bore 1mm to 86mm and lengthening the stroke by 2mm to 97mm, thereby maintaining the undersquared design of the previous engine to provide excellent low RPM torque. The 2.3CL engine features a new lightweight gravity-cast aluminum alloy with a highly rigid, ultra short skirt design and full floating wrist pins to minimize friction and reduce noise.

The engine's drop-forged single-plane steel crankshaft and connecting rods have been redesigned to be stronger and operate with less friction, much like the V-6 components. The crankshaft has 2mm-thicker shoulder webs (20mm to 22mm) and the journal diameter has been increased 5mm (50mm to 55mm).

The I-section, drop-forged steel connecting rods have a completely new design and are considerably lighter than their predecessors (475g vs. 578 g), which helps to minimize vibration. Pin-journal (big end-bearing journal) diameter has been reduced from 48mm to 45mm. Rod thickness is down from 24mm to 20mm and the bolt size is smaller. Like the V-6 rod bolts, those of the 4-cylinder engine are torqued to the plastic region of the bolt material in order to ensure a solid union between the bearing cap and the connecting rod.

The 2.3CL engine's cylinder head is pressure cast in aluminum, for light weight, increased accuracy, and improved breathing due to more precisely formed ports and runners. The head features a 4-valve-per-cylinder valvetrain with pent-roof combustion chambers. Each valve is smaller and lighter than the valves in a 2-valve combustion chamber and therefore has less reciprocating mass. This allows the engine to be revved to a higher rpm, helping to extend the engine's power range. Two intake and two exhaust valves also have greater valve area than comparable two-valve designs, so engine volumetric efficiency improves. Valve actuation is via rocker arms and a hollow single- overhead camshaft, driven by a glass-fiber reinforced toothed belt. The compression ratio is 8.8:1. Unleaded regular fuel is specified.

For a superior blend of power, torque, and fuel economy, the 2.3CL engine utilizes the innovative Variable Valve Timing and Lift Electronic Control (VTEC) system.

At low rpm, the intake valves of the engine follow a set of low-lift, short-duration cam lobes with timing that optimizes cylinder filling at low engine speeds. To ensure complete and rapid combustion, the timing of the intake valves is staggered and their lift is asymmetric, creating a swirl effect within the combustion chamber. This concentrates a richer fuel mixture near the spark plug for efficient combustion.

At engine speeds above 2300-3200 rpm (depending on engine load), the intake rocker arms are actuated by a high-lift, long-duration cam lobe designed to optimize high-rpm output. The result is an extremely efficient engine with a broader, flatter torque curve and smooth power delivery throughout the engine's operating range.

The intake system was simplified in shape to reduce induction resistance and noise. A larger twin-chambered air box designed to dampen resonant intake tract noise replaces the previous engine's smaller, single-chamber damper. The new box is 10.7 liters in capacity, compared to the older unit's 8.2 liters. The larger box also eliminates the need for a second resonant-frequency damper and an additional side branch. The new intake system reduces intake noise by as much as 5 dB.

The 2.3CL intake manifold has been redesigned to add more power and lower emissions. The individual cast-aluminum runners have revised dimensions to better take advantage of the different air-flow characteristics of the 2.3-liter engine.

Fueling the 2.3CL engine is a Programmed Fuel Injection (PGM-FI) system. PGM-FI uses a 16-bit microprocessor that monitors throttle position, engine temperature, cylinder firing position, intake manifold pressure, atmospheric pressure, exhaust gas oxygen content, and intake air temperature. On the basis of these measurements, the microprocessor meters fuel at the correct fuel-air ratio for the best balance of drivability, power, fuel economy, and exhaust emissions under each operating condition. It then activates each fuel injector at precisely the right moment. To improve fuel atomization, the engine also uses an air-assist fuel-injector system.

The CL Series exhaust system was designed to offer light weight, minimal noise and vibration, and excellent emissions control. The exhaust manifold uses thin-wall iron casting technology for weight savings. Optimized bracketry and rubber mountings reduce low-frequency vibrations and strengthen the system against impact damage.

A 5-speed manual transmission is standard on the 2.3CL. In addition to gear ratios matched to the output of the 2.3-liter four-cylinder engine, the 5-speed manual transmission features a double-cone synchromesh 2nd gear for improved shifting smoothness and feel. The cable operated 5-speed now also utilizes a smaller diameter friction disk to help reduce shift effort. A new friction material is more wear resistant, improves clutch feel and helps prevent judder.

Like the 3.0CL, the 2.3CL now benefits from a new direct-control automatic transmission. The transmission shifts precisely with reduced shift shock, is compact and weighs less than the model it replaces. It also has less gear noise and vibration, is more efficient, incorporates the grade logic control system and is better integrated into the cruise control system.

The automatic is constant-mesh type, with three parallel shafts: input intermediate and output. Shift management is controlled by a new 16-bit microprocessor operating two new linear-shift solenoids. Since the new control is linear, clutch engagement is more progressive than in the older model transmission. The result is smoother shifting under all conditions, whatever the driving style.

The lock-up torque converter helps minimize fluid-coupling slippage by mechanically coupling the engine to the transmission during certain driving modes, such as steady-state cruising. The benefit is better mileage. The lockup feature works in 3rd and 4th gears, and also maintains lockup in 3rd and 4th gears during deceleration. The new design absorbs vibration and judder better during engagement and also an increase in efficiency, and therefore higher mileage.

The Grade Logic Control System is designed to minimize gear hunting on uphill and downhill driving, and to enhance engine braking when traveling downhill. Drawing upon information supplied by the throttle-angle sensor and vehicle speed, the Grade Logic Control System can determine the slope of a hill by comparing this information with a map stored in the engine computer. Based on this information, Grade Logic Control modifies the shift schedule to hold the transmission in a lower gear for better uphill response or downhill engine braking. This system reduces gear hunting, and reduces shifting by as much as 50%, producing a more refined driving experience.

A reflection of the quality built into the new powerplants, the engine's first scheduled tune-up occurs at 100,000 miles. During that time, only routine inspections and fluid changes are required. Auto-tensioning mechanisms compensate for any belt stress fluctuation on the serpentine accessory drive belt and the toothed cam-drive belt. The roller-follower design of the valve gear cuts friction and wear to the point that that the screw-type tappet clearance adjusters needn't be checked until the 100,000-mile mark. Long-wearing platinum-tipped spark plugs are scheduled for replacement at 100,000 miles. The CL engines represent two of the four Acura engines that requires no scheduled tune-up for the first 100,000 miles.

To minimize engine vibration both at idle and at high engine speeds, Acura engineers developed a special electronically controlled hydraulic engine mount for CL models equipped with the 4-speed automatic transmission. The mount features an exterior valve and two chambers filled with fluid. The chambers share the same hydraulic fluid by means of two sets of orifices-one large and one small.

At idle, the large set of orifices is used, allowing fluid to flow smoothly between the chambers. Above idle speeds, a signal is sent to the valve, which then engages the smaller set of orifices. Changing the orifices alters the resonant frequency of the engine mount and damps out excessive vibrations. The different vibration characteristics of the inline 4-cylinder engine equipped with the 5-speed manual transmission allows the use of a more conventional hydraulic engine mount.

For superior performance and smoothness, the CL uses a direct-acting cruise control system. When the cruise control is activated, a lost motion mechanism frees the throttle wire, allowing the throttle to be opened and closed without the friction of accelerator pedal movement. The system also helps reduce "hunting" when holding a preset speed.