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VTEC And The New Interceptor
Versatility--it's the key to any engine's operation. Early engines had carburetors without throttles, so they always ran wide-open. As internal-combustion-engine design progressed, adjustable components let engines produce power over a wider useful range of operating conditions. First, of course, came the adjustable throttle. Next was an adjustable spark advance, varying the engine's timing.
More recently, fuel injection and digital ignition have allowed engineers to more closely coordinate and vary fuel delivery and ignition timing. This allows new powerplants not only to produce more power over a wider range of operating conditions (varying altitudes, temperatures, loads, and at engine speeds from idle to midrange to redline), but also allows them to do so while polluting less.
Still, valve actuation is one aspect of engine control that has remained one of the final frontiers. But Honda's VTEC™ changes all that.
A camshaft, or more correctly the lobes on a camshaft, control several aspects of valve actuation: When a valve opens respective to the crankshaft rotation and piston location in the cylinder (valve timing); how far that valve opens (valve lift); how long that valve remains open (valve duration); how fast the valve attains its maximum lift; and the time both intake and exhaust valves are open at once (valve overlap). Engine designers necessarily design camshafts as a compromise, and typically they give an engine optimum performance in a relatively small rpm range. Realistically, this translates to an engine that produces superior top-end power but suffers from a compromised midrange, or an engine that has a broad, generous midrange but produces less top-end power than one might expect for its displacement.
Honda has pioneered several systems to vary valve timing and lift, and the motorcycle division has led the way. In 1985, Honda produced the CBR®400F-2 REV bike, an air-cooled inline-four sportbike with a two-valve/four-valve system very similar in principle but differing in detail from the new Interceptor's. (The CBR400F-2 REV was never sold in the United States.) In addition, Honda has applied various VTEC systems on the Acura NSX sports coupe, Integra GS-R, Prelude, del Sol, and Accord Civic automobiles in various markets worldwide.
HOW IT WORKS
The Interceptor's VTEC is quite elegant in concept. At engine speeds below 7000 rpm, the Interceptor's camshafts open only one intake and one exhaust valve per cylinder. Above 7000 rpm, the Interceptor's cams open both intake valves and both exhaust valves for each cylinder.
The Interceptor's cams are always acting on one intake and one exhaust valve per cylinder. These direct actuation valves (diagonally opposed in the cylinder as viewed from the top) use precise, proven, shim-under-bucket valve actuation with 16,000-mile maintenance intervals. The two remaining valves, which open after 7000 rpm, utilize shimless buckets.
Above 7000 rpm, an oil spool valve, which is actuated by an electric solenoid, sends oil pressure to the hydraulically actuated lifter sleeve inside the inverted buckets of the direct actuation lifters. This positions a spring-loaded engagement pin above each valve stem of the two remaining valves in the cylinder. Consequently, these two remaining valves open and close along with the direct actuation valves, and the engine functions in a conventional four-valve manner.
Throughout all Interceptor VTEC engine speeds, camshaft lift, timing, duration and overlap remain constant; only the number of valves in use varies.
WHY IT WORKS
One of the keys to engine performance is keeping the fuel/air charge moving at a high velocity on its way to filling the cylinder--in the neighborhood of 350 to 450 feet per second. The longer we leave the valves open and the faster the charge moves, the more fuel charge we can stuff into a cylinder--and all that means more power. Mixtures moving in that 350 to 450 fps range rush into a cylinder with considerable force, and continue to fill the cylinder when the intake valve is left open past Bottom Dead Center (BDC) and the piston is on its way up. High charge speeds are easy to maintain at high rpm because as the piston moves down quickly it pulls the mixture into the cylinder quickly, too. But what about lower engine speeds? Cut the charge speed in half, and it has only one-quarter of its previous kinetic energy. That means it doesn't fill the cylinder as efficiently, especially after BDC of the piston travel. This is a particular problem at idle, when the charge speed is at its lowest.
Enter our friend Daniel Bernoulli, the 18th-century Swiss scientist whose famous law tells us of the relationship between pressure and velocity. In essence, our mixture will have to speed up if it passes through a restricted area. (Carburetor venturis are a classic example.) By opening only one intake valve below 7000 rpm, the fuel/air charge velocity remains high because the charge must speed up to rush through the smaller (single-valve) opening. With engine speeds in excess of 7000 rpm, the speed of the piston dropping in its cylinder and consequent high vacuum keeps the mixture speeds right where they need to be, even with both valves open.
These same principles apply to scavenging the exhaust gases from the cylinder as well. Again, high gas speeds promote good evacuation of the exhaust gases from the cylinder and out through the exhaust pipes.
Low-rpm economy and midrange power, plus high-rpm peak power and efficiency--that's the VTEC advantage.
WHAT IT'S LIKE TO RIDE
The Interceptor's designers have gone to great lengths to smooth the two-valve-to-four-valve transition. Ignition timing and fuel-injection mapping do a great deal to make this possible. The Interceptor also features a solenoid-actuated dual-air-intake-duct design that keeps one duct closed during low-speed operation. Like the two-valve/four-valve system in the engine, this one duct/two-duct system keeps intake velocity speed high as well.
Make no mistake--you'll definitely know when your Interceptor shifts from two-valve to four-valve mode. You'll feel the power, yet it will be a smooth increase. You'll hear the difference in both intake and exhaust note as a blood-stirring howl sets up. It'll put a smile in your helmet.
VTEC is one reason the new 50-state Interceptor® makes so much power yet still runs so clean. How clean? Clean enough to meet the California Air Resources Board 2008 emissions standards years early. A three-way exhaust catalyzer, air-injection system, new-generation fuel injection system and sophisticated injector and ignition mapping all work in concert to help the VFR® run clean.
With the 2002 Interceptor, riders get so much more than ever before, all in one package. But then, that's the Honda way.