October 14, 2004

Honda's Variable Cylinder Management
by Jim Kerr

Honda's popular Odyssey minivan has undergone a major redesign for 2005. From bumper to bumper, this vehicle is loaded with new technology. One of the more interesting features of this new van is their Variable Cylinder Management. This system is found on the Odyssey Touring model and in the new Honda Accord Hybrid. It allows the i-VTEC V6 engine to operate on three cylinders or on all six.

Click image to enlarge

Honda was not the first to try a cylinder deactivation concept. Cadillac tried to cut out cylinders on a V8 engine back in the early 1980's with limited success (no success if you talk to technicians of the time!). Modern electronics have now made this concept possible. Chrysler was the first to bring it to market on the V8 Hemi engines in the Chrysler 300C and the Dodge Magnum RT. General Motors has their first light duty truck system just appearing in the marketplace. Honda's Variable Cylinder Management (VCM) technology is different than Chrysler and GM's.

While Chrysler and GM are using solenoids to activate special lifters that prevent the valves from opening, Honda is using their i-VTEC technology to stop the valves on three cylinders from opening. The Honda i-VTEC engines uses overhead camshafts with a pivoting cam follower riding on the camshaft. Two rocker arms on either side of the cam follower are interlocked with the cam follower so that every time the follower moves, the rocker arms open the valves.

To deactivate the valves, hydraulic oil pressure, controlled by the computer with a solenoid, moves a synchro pin that interlocks the rockers and cam follower. Now the cam follower is still free to move as the camshaft rotates but the rocker arms are no longer linked to it. This synchro pin will move back and forth, linking or unlinking the rocker arms to control valve operation.

2005 Honda Odyssey 3.5 i-VTEC cutaway. Click image to enlarge

It seems impossible for parts that are moving at several hundred cycles per minute to be linked or unlinked in a fraction of a second by a moving pin, but that is the principle that Honda's VTEC engines are designed on. It has proven itself in racing and millions of kilometres in passenger vehicles over the years.

With the valves deactivated on three cylinders, the engine now provides the economy of a four cylinder engine. There are some frictional loses with all the moving parts. When more power is needed, the computer activates the valves and the engine has 255 horsepower from all six cylinders. A light on the Odyssey's dash indicates whether the engine is operating in economy or full power mode but this light isn't directly related to cylinder deactivation. Generally, when the economy light is on, you are probably operating with three cylinders but not necessarily. The switch over from 3 to 6 cylinder operation or back to 3 cylinders again is so smooth that you cannot feel it inside the van even when trying to.

Six cylinder engines are easy to balance but the power pulses on a three cylinder engine are so far apart that some vibration is inherent. Normally the switching would be noticeable but Honda uses some neat technology to hide any vibrations. First, the engine is "drive by wire". The computer instantly controls the throttle so that engine power does increase or decrease during switching events.

While "drive by wire" is now common with many manufacturers, Active Noise Control is unique to this vehicle. Active Noise Control (ANC) uses sound waves generated from the audio speakers to cancel out any booming noise from the powertrain. The ANC controller uses a microphone to detect any noise and then generates a signal 180 degrees out of phase to cancel out the original noise.

2005 Honda Odyssey 3.5 i-VTEC VCM components. Click image to enlarge

The cancelling sound waves are emitted from the front and rear speakers during 3 cylinder operation, idling and at-start running. During 6 cylinder operation, the system is off. ANC sounds a bit like science fiction but it has been used by the military for stealth technology and now it has found another purpose in creating a smooth quiet ride.

Active control engine mounts are also used to damp the cylinder switching. Controlled by the engine computer, two engine mounts, one in front of the engine and another behind, use solenoids to damp fluid movement in the mounts. The computer monitors engine rpm and actually anticipates vibration during cylinder switching so the mounts can maintain a smooth quiet ride under all conditions.

Honda's VCM provides the power of a sporty V6 with the economy of a small four cylinder, all combined with technology that keeps the interior smooth and quiet. This technology is so good, you don't even know it's there except when filling up at the gas pumps.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

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