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.
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.
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.
Honda announced i-VTEC via a special article in the tech section of their official web-site. As usual, TOVA's japanese article specialist Kaz Mori has translated that article which we reproduce in its entirety in the box below.
From now onwards, there will no doubt be countless attempts to second-guess at what Honda will do to i-VTEC. There is all likelihood that Honda will implement i-VTEC on its performance engines. The most probable benefiaries will be the Integra and the Civic, the two models which have always been at the forefront to carry Honda's high-performance flag.
At this point, it is important to highlight again that the basic DOHC VTEC system is more than capable of delivering extremely high specific power outputs. i-VTEC is not needed. Witness the 125ps/litre power delivery of the F20C used on the S2000. Again what i-VTEC do allow is for Honda to go for the sky in terms of specific power output but yet still maintaining a good level of mid-range power. Already extremely authoritative reviewers like BEST MOTORing have complained about the lack of a broad mid-range power from for eg the F20C engine. In a tight windy circuit like Tsukuba and Ebisu, the S2000 finds it extremely tough going to overtake the Integra Type-R in 5-lap battles despite having 50ps or 25% more power. Watching the 'battle' brings one point painfully clear. There is a dire need of power from the F20C below 6000rpm. Every time the S2000 sneaks up behind the ITR, it fails to engage in a good overtaking move because the power from the F20C is surprisingly insufficient. The reason for this is because DOHC VTEC makes do with merging two distinct power curves. To get the extreme power levels of the F20C, the wild cams' power curve are so narrow that there is effectively a big hole in the composite power curve below 6000rpm. What i-VTEC can do to this situation is to allow fine-tuning of the power curve, to broaden it, by varying valve opening overlap. Thus this will restore a lot of mid-range power to super-high-output DOHC VTEC engines allowing Honda, if they so desire, to go for even higher specific outputs without too much of a sacrifice to mid-range power.
For the moment the 2.0l DOHC i-VTEC engine on the STREAM delivers 154ps and revs to around 6500rpm. This is relatively low-tech as far as Honda's DOHC VTEC engines go. There must be a lot of scary DOHC i-VTEC engines on Honda's design boards at the moment. TOVA readers who are interested to look at what's in the rumour mills are encouraged to visit TOVA's main-site, the Temple of VTEC for one of the best source of future Honda models information on the net.
However, readers who prefers to only read about solid facts can rest assured that once the expected super-high-powered DOHC i-VTEC engine comes out, TOVA will be the first to highlight it, just as we are now the first to explain to you what i-VTEC is all about.WongKN