A-VTEC

[Plex]

We�ve been hearing rumors of a next generation VTEC technology for years. In 2005, Honda finally confirmed that they were well along on the development path of their next generation Advanced VTEC (A-VTEC) system. Apart from promises of superior economy and performance, scant details were revealed. Our suspicions were that this would be a system that could continuously vary lift or duration (akin to BMW�s innovative Valvetronic system), rather than using a stepped change as both VTEC and i-VTEC have used. As tidbits of information began to coalesce into a meaningful picture, some of our readers pointed us to a patent filed by Honda for a continuously variable lift system. After poring over the details of this public document, it would seem that the cat is out of the bag. With a high degree of certainty, we believe that this patent does indeed represent the next generation A-VTEC valvetrain technology.

As you can see from the drawings, the implementation of A-VTEC is classic Honda technological innovation. It is simple and compact, with a minimum of moving parts and easy adjustability. The key to the system lies in an innovative circumferential, cam-centric rotationally adjustable drum mechanism. To put it more simply, the intake camshaft (and the intake camshaft only in the proposed implementation) is surrounded by a drum that rotates around the same axis as the camshaft. The position of this drum is determined by a computer controlled gear.

What makes this rotating drum so important is what it carries � a new intermediate rocker shaft that moves with the drum. Explaining the function of this drum is probably best done by first covering how a traditional valvetrain works. Take, for example, a traditional DOHC i-VTEC setup. A camshaft lobe acts upon a follower (underneath the cam). This follower (using a roller to reduce friction) acts upon the valve tip underneath it, often multiplying the lift of the cam through the use of a lever ratio. While various and sundry alterations have been made to such systems over the years, the basic principle is the same. The A-VTEC system changes things in several ways. First, the camshaft is located below the follower. While this is not novel, it is new for Honda DOHC engines (Honda SOHC engines typically locate the camshaft underneath a follower as well). Outside of some small changes in valvetrain mass and inertia, and cylinder head dimensions, this relocation of the follower isn�t all that significant in and of itself. The significance all lies within the intermediate rocker shaft.

According to the technical drawings on the patent (which are not necessarily to scale of course), the intermediate shaft is able move on its drum over a range of about 45 camshaft degrees. Theoretically it could move by a much larger amount if so desired. This has two effects on valve timing. First, it alters cam timing by a similar amount (about 45 degrees in the case of the drawings). Second, it alters valve lift and duration. At one extreme (with the drum rotated fully counterclockwise in the illustration), valve lift and duration are minimized and valve opening is fully advanced. At the other, lift and duration are maximized and valve opening retarded. The variation in lift and duration between minimum and maximum positions is determined by the profile of the intermediate follower.

For anyone acquainted with the basics of camshafts, the benefits of such a system will be obvious. But in case you�ve never dealt with reading a cam card (like most of the population), here�s a brief synopsis. First, advancing a camshaft tends to improve low end power. This is not a hard and fast rule, but a general principle that holds true over a wide range of conditions. Conversely, retarding a camshaft tends to improve high end power. Next, reducing camshaft duration tends to improve low end power, while increasing improves high rpm output. Advanced readers will have already noticed that the A-VTEC system alters both advance and duration simultaneously in a way that optimizes either low or high rpm performance. Finally, during periods of maximum advance and minimum duration (low rpm optimization), lift is also minimized (theoretically close to zero if desired). While lift variation is not the major factor in rpm optimization (more lift generally means more power everywhere), it is highly useful if you want to do something like eliminate the throttle (as BMW has done with Valvetronic).

So, now that we know the basics of A-VTEC, where, how and what can we expect from the application of this technology in real world Honda engines? First of all, the patent sheds a couple of clues for us. The first is in the specific language that outlines a single camshaft with both intake and exhaust lobes � in other words, a SOHC engine. While it would seem unusual for Honda to take what most would deem a backward step, A-VTEC would seem to apply very well to a SOHC design. The timing variation on the intake valves would behave very similarly to the intake cam changes afforded on current DOHC i-VTEC engines. And the changes in lift and duration would offer peak output similar to that of current DOHC i-VTEC engines as well. Furthermore, the elimination of the second (high rpm) lobe on current SOHC VTEC engines would allow the implementation of 2-stage VTEC on the exhaust side instead of the current fixed (and compromised) exhaust profile on SOHC VTEC applications.

The patent is also quite clear that the specifics of the valvetrain geometry are not fixed to just those depicted in the drawings. This certainly leaves room for a DOHC application. In fact, if the cam timing functionality of A-VTEC eliminates the need for the VTC portion of i-VTEC on the intake side, we could see VTC being applied to the exhaust side of Honda�s DOHC engines. Personally, I am not convinced that a performance application of A-VTEC could be maximized without VTC on the intake cam, but I�ll defer to the expertise of the Honda engineers on that one.

Based upon what is known so far, we believe that A-VTEC will likely be applied in an economy application first. Whether this is in a SOHC or DOHC engine is up in the air at this point. But the elimination of throttling losses and the total control of lift and duration certainly makes Honda�s claims of double digit gains in fuel economy over existing Honda engines believable. And, even in focused economy application, I see no reason for Honda to sacrifice any top end power. In fact, I foresee low and midrange torque gains with modest increases in high rpm power as being completely reasonable.

The more curious among you might want to know how A-VTEC compares to the only other continuously variable lift and duration system on the market today � BMW�s Valvetronic. While BMW certainly trumped the rest of the market on getting such a system into production, Honda�s additional years of research seem to have paid off with several advantages. Take a look at the drawings here - http://www.bmwworld.com/technology/valvetronic.htm You will notice several things about BMW�s application. First, Valvetronic increases the height of the valvetrain and cylinder head quite substantially. Second, BMW�s intermediary actuator (I can�t quite call it a follower) is quite large, adding extra moving mass in the valvetrain (perhaps explaining the rpm limitations of the first generation Valvetronic engines). Finally, BMW�s implementation does not alter valve timing by moving the cam centerline. This means that in order to optimize low lift valve timing, BMW�s VANOS VTC system _must_ be applied as well to advance valve opening. By comparison, VTC control seems far less critical on A-VTEC. So, in summary, reduced size, mass and self regulating VTC all would appear to be advantages of A-VTEC over Valvetronic. Of course, the proof is in the pudding. Or, as I like to say, implementation is more important than theory and we still need to see Honda�s implementation. But based on past performance, I have little doubt that A-VTEC will be very effective.

A-VTEC will most likely first appear in the 2008 Accord, which makes the introduction of that model even more highly anticipated than it already was. We were also led to believe that A-VTEC will first appear on a 4 cylinder engine. Will it be SOHC or DOHC? Will it be a variation of the K-series or R-series, or a whole new engine? Perhaps most intriguing to me is whether or not this technology will appear on the J-series V6? If it works as predicted, the life of the J-series could be extended significantly. Class leading top end power could be matched with vastly improved midrange torque and fuel economy � all while remaining smaller and lighter by virtue of using a SOHC design. Pay close attention in the coming months for official news from Honda on this exciting new technology.

Click on link below for drawings.

Temple of VTEC

[3stagevtec]

^ saw this article before.. really hoping to see this technology in the next generation NSX V10..
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