VTEC Engine - intelligent-VTEC Next Generation

The last evolution of Honda's VTEC system was back in 1995 where they introduced the now-famous 3-stage VTEC system. The 3-stage VTEC was then designed for an optimum balance of super fuel economy and high power with driveability. For the next 5 years, Honda still used the regular DOHC VTEC system for their top power models, from the B16B right up to the F20C in the S2000. Now Honda have announced the next evolution of their legendary VTEC system, the i-VTEC.

The i stands for intelligent : i-VTEC is intelligent-VTEC. Honda introduced many new innovations in i-VTEC, but the most significant one is the addition of a variable valve opening overlap mechanism to the VTEC system. Named VTC for Variable Timing Control, the current (initial) implementation is on the intake camshaft and allows the valve opening overlap between the intake and exhaust valves to be continously varied during engine operation. This allows for a further refinement to the power delivery characteristics of VTEC, permitting fine-tuning of the mid-band power delivery of the engine.

Variable Timing Control Operating Principle
Honda's VTC operating principle is basically that of the generic variable valve timing implementation (this generic implementation is also used by by Toyota in their VVT-i and BMW in their VANOS/double-VANOS system). The generic variable valve timing implementation makes use of a mechanism attached between the cam sprocket and the camshaft. This mechanism has a helical gear link to the sprocket and can be moved relative the sprocket via hydraulic means. When moved, the helical gearing effectively rotates the gear in relation to the sprocket and thus the camshaft as well.

The drawing above serves to illustrate the basic operating principle of VTC (and generic variable valve timing). A labels the cam sprocket (or cam gear) which the timing belt drives. Normally the camshaft is bolted directly to the sprocket. However in VTC, an intermediate gear is used to connect the sprocket to the camshaft. This gear, labelled B has helical gears on its outside. As shown in the drawing, this gear links to the main sprocket which has matching helical gears on the inside. The camshaft, labelled C attaches to the intermediate gear.

The supplementary diagram on the right shows what happens when we move the intermediate gear along its holder in the cam sprocket. Because of the interlinking helical gears, the intermediate gear will rotate along its axis if moved. Now, since the camshaft is attached to this gear, the camshaft will rotate on its axis too. What we have acheived now is that we have move the relative alignment between the camshaft and the driving cam-sprocket - we have changed the cam timing !

VTC and other implementations of generic variable valve timing can only change the relative alignment between the camshaft and its driving sprocket. What this effectively does is to change the relative timing between the intake and exhaust cams and thus their valve opening cycles or the intake and exhaust valve opening overlaps. Note that no other valve timing parameters, eg amount of valve lift or absolute valve opening duration can be varied. The only thing that VTC varies is the valve opening overlaps. VTEC is able to vary all valve timing parameters but current implementations does so in two or three distinct stages (or profiles). Adding VTC allows the valve opening overlaps to be continously varied and thus enables the power delivery from the standard VTEC system to be further fine-tuned. The greatest impact will be to the mid-band power delivery of the engine. Most importantly, VTC (and generic valve timing systems) will not replace VTEC but enhance its effectiveness.

The current state of i-VTEC
At the moment, i-VTEC is only implemented in the relatively low specific power output engine of the new JDM Honda STREAM van. Used in the 2.0l DOHC i-VTEC engine, it allows Honda to boost low-end and mid-range power of that engine, a characteristic very desirable for that model. In my opinion, VTC is the most significant innovation that Honda introduced to i-VTEC. Other important innovations includes the changing of the engine orientation (as well as its rotation direction). i-VTEC engines are mounted such that the intake valves faces the front of the vehicle and the exhaust valves the rear, just like the Japanese Grand-Touring Championship racing cars. Other improvements are in the important areas of fuel economy and emissions. [Article Source..]