2003 Honda Accord -- Drivetrain - Part 3
7/29/2002 1:55:36 PM
Major V6 components
Cylinder Heads with Integral Exhaust Manifolds
One of the most innovative aspects of the new Accord V6 is the design of its cylinder heads. Made of pressure-cast, low-porosity aluminum, these lightweight components have tuned exhaust manifolds as integral parts of the casting, a unique feature that improves overall packaging and permits optimal positioning of the primary close coupled catalytic converters.
While the new V6 maintains the basic SOHC design, 4-valves-per-cylinder and VTEC, configurations, refinements significantly boost operating efficiencies. As before, each camshaft is inserted into the head from the front of the engine-an approach that saves weight and complexity by eliminating the need for bolt-on cam caps. And the crankshaft still drives each cam via a fiberglass-reinforced toothed timing belt. However, the diameter of the intake valves has been increased to improve breathing and the ports have been reconfigured to more ideally match the flow characteristics of the new, high-inertia intake manifold.
Finally, the VTEC (Variable valve Timing and lift Electronic Control) system has been changed from a 2-rocker to a 3-rocker design for improved throttle response at low rpm and increased ultimate top-end power.
3-Rocker VTEC System
Honda used expertise gained from its successful racing programs to develop the innovative VTEC system more than a decade ago. Since then, various forms of VTEC have become familiar features on many of the company's engines. VTEC makes it possible to vary the relative timing and lift of the intake valves to optimize overall performance, netting good low-end torque as well as improved high-end horsepower.
Basic operation of the 3-rocker VTEC setup used on the 2003 Accord's V6 is similar to that of the 2-rocker version used on the DOHC 4-Cylinder, where rocker arms fitted with low-friction roller followers actuate the intake valves.
The heart of the VTEC system is a unique camshaft and rocker arm system. For each cylinder's set of two intake valves, there are three rocker arms and three corresponding lobes on the camshaft. The two outboard lobes each have a profile maximizing cylinder swirl to suit low- to mid-rpm operation. The third or center cam lobe has a dramatically different profile designed for longer duration and higher lift. This lobe profile is designed to optimize breathing and horsepower production at high engine speeds.
At low engine rpm, the outboard lobes operate the valves. During high-speed operation the VTEC computer sends a signal to a spool valve, which in turn delivers engine oil pressure to small pistons in the rocker arms. Oil pressure causes the pistons to move, locking all three rocker arms together. Once locked, the rocker arms are forced to follow the center cam lobe, increasing top-end performance. The crossover from low lift to high lift occurs in 0.1 seconds and is virtually undetectable to the driver.
At the core of the Accord's lightweight V6 is an exceptionally strong, rigid aluminum-alloy block that employs cast-iron cylinder liners to enhance durability. Die-cast and heat-treated, the block has a high natural resonant frequency and an optimal 60-degree V-angle, features that contribute to the engine's inherently smooth, quiet operation as well as to its relatively compact size. A short deck height also improves its overall packaging efficiency.
Crankshaft, Connecting Rods & Pistons
Internal components of the Accord V6 deliver an outstanding combination of efficiency and durability. The central element is an extremely rigid forged steel crankshaft with microfinished journals that minimize friction and improve durability. Compression ratio of the lightweight cast-aluminum pistons has been increased to 10.0:1, up from 9.4:1 in the previous six.
The precision-contoured piston crowns have a unique charge-centralizing design that maximizes volumetric efficiency and a large squish area that facilitates more complete combustion and leads to decreased emissions. The pistons are fitted with full-floating wrist pins to help eliminate the "slapping" noise that can occur on cold starts.
The crank and pistons are linked by compact connecting rods that employ a special weight-saving, direct-thread fastener in place of the conventional nut-and-bolt configuration typically used on other engines.
Programmed Fuel Injection (PGM-FI)
The new Accord V6 is fitted with the latest iteration of Honda's sophisticated programmed fuel-injection system. It's controlled by a 32-bit microprocessor that uses a comprehensive array of sensors to monitor throttle position, intake manifold pressure, coolant temperature, intake air temperature, atmospheric pressure and the oxygen content of the exhaust gases, as well as the relative positions of the camshafts and crankshaft. Based on the input it receives, the PGM-FI signals an efficient multi-orifice injector for each cylinder to introduce the proper amount of atomized fuel at the precisely timed instant to ensure ideal combustion. Internal feedback circuitry allows the PGM-FI to custom match its real-time operation to accommodate the specific air-fuel conditions that exist in each individual cylinder.
Electronic Throttle Control
Another factor that contributes to the world-class performance of the Accord's new V6 engine is an all-new electronic throttle control (ETC) system. This system controls the throttle during transmission shifts for improved smoothness. It also allows for throttle control to be incorporated in the traction control system and integrates the cruise control function into the ETC. This computer controlled drive-by-wire (DBW) package is a feature that will be incorporated into other future Honda powerplants. Key system components include an accelerator position sensor, electronically controlled throttle body, DBW driver unit, and the main electronic control unit (ECU).
High Inertia Intake Manifold
Air passing through the ETC enters through a new, high-inertia intake manifold with a runner design that's specifically matched to optimize output characteristics. It features a unique plenum configuration that effectively transitions incoming air from a negative to a positive waveform. This helps create a natural supercharging effect and is designed to complement the new intake port design.
Direct Ignition and Knock Control
Ideally balancing performance, economy and low emissions demands precise control of the spark timing as well as an effective ignition system to ensure optimum burning of the air-fuel mixture under all operating conditions. To help accomplish this goal, the a new sensor determines the onset of engine "knocking" and modifies spark timing before this potentially troublesome detonation can do any damage. The system allows the engine to run with a greater amount of spark advance and a higher compression ratio than the previous V6, increasing efficiency. A compact, high-energy ignition coil positioned directly atop each respective plug bore in the cylinder head activates each spark plug in the new engine.
High-flow Exhaust System with Close-Coupled
The high efficiency exhaust system incorporates several key elements that work in concert with the engine's uniquely designed cylinder heads to help boost performance, reduce tailpipe emissions and trim weight. Major system components include two close-coupled primary catalytic converters, a secondary underfloor catalytic converter, a centrally positioned, high-flow resonator and dual rear silencers. Integrating the exhaust manifold into the head casting allowed the primary catalytic converters to be mounted directly to the exhaust orifice. This location ensures an extremely rapid light-off for the high-efficiency 900-cell per square inch converters, which directly contributes to the engine's exceptionally low emissions.
A new high flow hydroformed 2-into-1-collector pipe that transfers exhaust gasses to the secondary 350-cell converter also reduces exhaust backpressure. A new design for the silencer further reduces backpressure. The net result is a 30-percent drop in backpressure compared to the previous Accord V6. These improvements account for 15 of the extra 40 horsepower the new V6 develops. Eliminating a flange on the rear portion of the exhaust pipe and adopting a new, more compact design for the rear silencers also trimmed weight from the system. Because these smaller silencers are easier to package, the rear of the new Accord has a more refined appearance.
Accord V6 with Manual Transmission Sport Sound
The V6 engine in the Accord Coupe fitted with the 6-speed manual transmission features exclusive revisions to the air intake system and a modified single-chamber resonator. This provides the "Ultimate Sport Accord" with a more aggressive intake sound during acceleration, yet retains the sophisticated character of other Accord models under normal cruising conditions.
5-Speed Manual Transmission for 4-Cylinder Engines
The manual transmission paired with the Accord 2.4-liter 4-cylinder engine is a new lightweight, compact 5-speed, housed in a rigid die-cast aluminum case. Multi-cone synchronizers used on first through fourth gears contribute to a smoother, more fluid shift feel, while helping reduce throw distances by 50mm. A repositioned shift lever further facilitates quick, direct gear changes.
The clutch assembly is an equally compact design that features low-torsion springs in the pressure plate to keep pedal effort low and eliminate judder while ensuring a smooth, progressive engagement.
5-Speed Automatic Transmission for 4-Cylinder
The all-new 5-speed automatic transmission replaces the previous 4-speed automatic. It is lightweight and compact and designed to provide best-in-class performance and fuel economy. It also reduces shift shock and improves shift smoothness, thanks to a new linear solenoid with direct control.
The new transmission also features an updated grade logic control system. By using sensors that monitor throttle position, vehicle speed and acceleration/deceleration and then comparing these inputs with a map stored in the transmission's computer, the system is able to determine when the vehicle is on an incline and adjust the shift schedule for improved climbing power or downhill engine braking.
5-Speed Automatic Transmission for V6 Engines
The 5-speed automatic transmission used in V6 Accord models is different from the one in 4-cylinder models, but similar to the one used in new Acura 3.2 TL. This wide ratio transmission's lower gears provide quick acceleration while the tall top gear ratios result in low cruising rpm levels for reduced noise and lower fuel consumption. One difference from the TL transmission is the addition of the ETC system to further enhance shift smoothness by momentarily closing the throttle (reducing torque) at shift points.
Because the transmission shares the idler and third-gear clutches, the transmission provides five ratios in a unit about the size of a conventional 4-speed transmission. The transmission also incorporates a first gear one-way clutch for smoother shifts, plus a heat exchanger that controls and moderates transmission operating temperatures for both durability and improved shifting smoothness.
Linear solenoids provide precise, real-time control of the clutch on/off pressure. This superior clutch-engagement accuracy allows the grade logic control to operate smoothly under all conditions. For added refinement, a bearing supports the idler shaft.
To manage overall powertrain operation, the Powertrain Control Module (PCM) provides precise management of the transmission-engine interaction. For instance, by limiting engine output torque and/or transmission clutch pressure, hard driveline shocks are limited. The system also prevents the engine from exceeding 5000 rpm when the transmission is in neutral or park. It also has an upgraded grade logic control system similar to the one in the transmission for 4-cylinder engines.
6-Speed Manual Transmission For V6 Coupe
To maximize its appeal to enthusiasts, the new V6 Accord Coupe is available with a close-ratio 6-speed manual transmission closely related to the unit in the Acura 3.2CL Type S, creating the "Ultimate Sport Accord". A low (higher numerically) first gear improves acceleration off the line while a tall overdrive sixth gear provides for good fuel economy under freeway cruising conditions.
Prime weight-minimizing elements in this new 6-speed include a high-pressure-cast aluminum alloy casing and a compact gearset made of high-strength steel. Brass multi-cone synchronizers on first through fourth gears, and a single-cone synchro on the reverse gear, help improve shift feel and smoothness while a relatively large gear-mesh contact area decreases internal noise.
The ideally positioned shift lever features short, precise throws and incorporates a positive lockout mechanism that prevents a driver from accidentally shifting into reverse when the car is moving forward. Matching transmission to engine is a new self-adjusting hydraulic clutch that's fitted with a vibration-minimizing dual-mass flywheel.