VTEC OPERATION SYSTEM

What does the VTEC system in a Honda engine do?

    There is a book entitled How Car Engines Work, telling about the valves that let air into the engine and let exhaust out of the engine. It also tell about the cam shaft that controls the valves. The camshaft uses rotating lobes that push against the valves to open and close them. This is an animation from How Car Engines Work to help understand how the cam shaft opens and closes the valves:

    It turns out that there is big relationship between the way the lobes are ground on the camshaft and the way the engine performs in different RPM ranges. To understand why this is the case, imagine that we were running an engine extremely slowly - at just 10 or 20 RPMs, so it took the piston seconds to complete a cycle. It would be impossible to actually run a normal engine this slowly, but imagine that we could. We would want to grind the cam shaft so that, just as the piston starts moving downward in the intake stroke, the intake valve would open. The intake valve would close right as the piston bottoms out. Then the exhaust valve would open right as the piston bottoms out at the end of the combustion stroke and would close as the piston completes the exhaust stroke. That would work great for the engine as long as it ran at this very slow speed.

    When you increase the RPMs, however, this configuration for the camshaft does not work well. If the engine is running at 4,000 RPM, the valves are opening and closing 2,000 times every minute, or 3 to 4 times every second. When the intake valve opens right at the top of the intake stroke, it turns out that the piston has a lot of trouble getting the air moving into the cylinder in the short time available (a fraction of a second). Therefore, at higher RPMs you want the intake valve to open prior to the intake stroke - actually back in the exhaust stroke - so that by the time the piston starts moving downward in the intake stroke the valve is open and air moves freely into the cylinder during the entire intake stroke. This is something of a simplification, but you get the idea. For maximum engine performance at low engine speeds the valves need to open and close differently than they do at higher engine speeds. If you put in a good low-speed camshaft it hurts the engine's performance at high speeds, and if you put in a good high-speed camshart it hurts the engine's performance at low speeds (and in extreme cases can make it very hard to start the engine!).

    VTEC (which stands for Variable Valve Timing and Lift Electronic Control) is an electronic and mechanical system in some Honda engines that allows the engine to effectively have multiple camshafts. As the engine moves into different RPM ranges, the engine's computer can activate alternate lobes on the camshaft and change the cam's timing. In this way the engine gets the best features of low-speed and high-speed camshafts in the same engine. Several of the links below go into the actual mechanics of the VTEC system if you are interested.

    Several engine manufacturers are experimenting with systems that would allow infinite variability in valve timing. For example, imagine that each valve had a solenoid on it that could open and close the valve under computer control rather than relying on a camshaft. With this type of system you would get maximum engine performance at every RPM. Something to look forward to in the future.  

Variable Timing Control (VTC) Operating Principle

How does VTC effect VTEC system

    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.