NOVI, MI – In an effort to improve the efficiency of the all-new ’08 Accord’s more powerful 3.5L V-6 without a hybrid-electric powertrain or 6-speed gearbox, Honda Motor Co. Ltd. endows its iconic family car with a new 3-stage cylinder deactivation system.
Referred to as Variable Cylinder Management (VCM) and featured on sedan and coupe models with the 5-speed automatic transmission, VCM enables Honda’s most potent engine ever offered in the U.S. (268 hp) to improve fuel economy over the less-powerful 3.0L SOHC V-6 it replaces, while also achieving Partial Zero Emissions Vehicles (PZEV) status.
Honda debuted its first cylinder deactivation system – a 2-stage setup that converted a V-6 to an inline 3-cyl. during light cruising – in the ’05 Accord Hybrid. It since has been fitted to the Odyssey minivan and Pilot cross/utility vehicle.
The new VCM system differs in that at highway speeds, moderate cruising and ascending mild hills, the ’08 Accord’s 3.5L SOHC V-6 switches to a staggered V-4 layout, whereby the left rear and right front cylinders are idled for better efficiency in a greater number of driving situations, the auto maker says.
For maximum gains at low engine loads, VCM reverts to 3-cyl. mode and shuts down the transverse-mounted engine’s entire rear bank of cylinders. Six-cylinder operation is used only at startup and for accelerating.
During initial test drives of the Accord lineup here, the VCM system seamlessly goes about its work, with only a small “ECO” light in the tachometer occasionally illuminating to tell the driver that part or half of the engine is taking a breather.
Although the U.S. Environmental Protection Agency changed its fuel economy calculations for ’08, Honda says the new VCM-equipped V-6 Accord achieves a city/highway rating of 19/29 mpg (11.4/8.1 L/100 km) vs. 18/26 mpg (13.1/9.1 L/100 km) for the ’07 model using the new methodology.
Key to the new VCM system is Honda’s venerable VTEC (Variable Valve-Timing and Lift Electronic Control) technology, dubbed i-VTEC in the Accord.
Of note is the i-VTEC designation for the model’s 4-cyl. engines, as well. However, the DOHC layout of those powerplants allows for the full suite of intelligent VTEC capabilities to be implemented, including variable intake valve, phasing, lift, timing and duration.
For the V-6, only the standard VTEC phasing feature is used, with VCM serving as the “intelligent” aspect of the technology.
When activated by the engine-management computer, VCM employs integrated spool valves/rocker-shaft holders, which selectively direct oil pressure to the rocker arms. The result is the connecting or disconnecting of the rocker arms from the valvetrain and the subsequent closing/deactivation of the intake and exhaust valves, which helps reduce pumping losses and improves efficiency.
The spark plugs in the deactivated cylinders, the auto maker says, continue to fire in order to minimize plug-temperature loss and prevent fouling.
For added smoothness, VCM monitors the by-wire electronic throttle position, vehicle speed, engine speed and gear selection, among other factors, to determine the proper cylinder configuration.
In addition, oil pressure (for the VTEC system) is monitored for sufficient flow, while ignition timing, throttle position and torque converter lockup are optimized to ease the transition from 3-, 4- or 6-cyls.
To manage the added vibration from up to half the engine not firing, Honda adds an Active Control Engine Mount (ACM) feature to the Accord V-6. Consisting of actuators positioned at the front and rear of the engine, ACM receives signals from the main electronic control unit and adjusts to prevent unwanted vibrations being transferred throughout the chassis.
Also featured is the latest version of Honda’s Active Noise Control (ANC) technology, which debuted on the previous Accord Hybrid and is standard on VCM-equipped V-6 models and all 4-cyl. Accords.
Aimed at limiting the booming low-frequency engine noises that invade the interior when VCM is active, ANC uses a pair of overhead microphones and an electronic processor to detect and cancel out unwanted resonances through amplified “reverse-phase” audio signals emitted by the door speakers.
The end result is completely unnoticeable, adding to the eerie smoothness of the VCM’s operation.
Despite the gains provided by VCM, the auto maker refrains from integrating it in V-6 coupe models fitted with the optional 6-speed manual transmission. This primarily is the result of wanting the sportiest variant of the new Accord to have the best engine response, something that is hampered by the added flexibility of VCM.
Rather, the 6MT coupe uses the conventional non-intelligent VTEC system, which allows for stronger low- and mid-range power despite the same overall horsepower and torque ratings as the automatic transmission models.
Other drawbacks to the sport coupe’s performance orientation include the loss of PZEV emissions status and a city/highway fuel economy reduction to an estimated 17/25 mpg (13.8/9.4 L/100 km).