VTEC
VTEC (standing for Variable valve Timing and lift Electronic Control) is a system developed by the car manufacturer Honda to improve the efficiency (and thus power) of its internal combustion engines.
Introduction to VTEC
In the regular four-stroke automobile engine, the intake and exhaust valves are operated by the lobes on a camshaft. These lobes' shape specify at what points in the cycle each valve opens. The problem is that when the engine is operating at high engine speeds, the optimal points for valve opening and closing are different than at lower engine speeds, due to the behavior of the gases (which have physical limitations on their flow which do not change as engine RPM increases). The ideal engine would have fully variable valve timing, in which the valves would always open at exactly the right point for the engine speed in use.
In practice, this cannot be done with current technology in production cars, although research efforts to design practical computer-controlled valves continue. Therefore, today, the vast majority of automobile engines operate with fixed valve timing that is a compromise. Normal passenger car engines are set up to work well at low engine speeds, since that's where they spend most of their time. Performance-tuned engines have cam profiles that are optimised more towards high RPM operation, where the greatest power can be obtained, but this means that low speed operation is compromised. Anyone who's heard a racing car or highly-tuned hot rod sitting at idle, and heard the engine sounding like it's barely capable of keeping running at that speed, will understand the compromise that has been made.
SOHC VTEC
Honda's VTEC is a simple and fairly elegant method of improving the situation, although it does, of course, introduce extra complexity. Instead of one cam per valve, there are two - one optimised towards low-speed operation, one to high-speed operation. In low-speed operation, the low-speed cam pushes the valve, and the cam follower for the high-speed cam is connected to nothing. As engine RPM increases, increased oil pressure pushes a pin, connecting the high-speed follower to the lower-speed one. After that point, the valve opens and closes according to the high-speed profile, which opens the valve further and for a longer time.
DOHC VTEC
This was the original VTEC scheme. For DOHC engines, variable timing was available for both inlet and exhaust valves, while for SOHC engines, it was only possible to use it for inlet valves.
SOHC VTEC-E
Honda's next version of VTEC, VTEC-E, was used in a slightly different way; instead of optimising performance at high speed, it was used to increase efficiency at low speeds. At low speeds, only one of the two inlet valves is allowed to open, increasing the fuel/air mixture's swirl in the cylinder and thus allowing a very lean mixture to be used. As the engine's speed increases, both valves are needed to supply sufficient mixture, and thus a sliding pin as in the regular VTEC is used to connect both valves together and start the second one moving too.
3-Stage VTEC
Honda's next version, 3-stage VTEC, combines the features of both. At low speeds, only one intake valve is used. At medium speeds, two are used. At high speeds, the engine switches to a high-speed cam profile as in regular VTEC. Thus, both low-speed economy and high-speed efficiency and power are improved.
i-VTEC
The latest version features cam phasing as well. A hydraulic pump rotates (phases) the camshaft to vary valve timing continuously. This can smooth out the famous VTEC surge as well as improve power and efficiency.
VTEC in motorcycles
Apart from the Japanese market-only Honda CB400 Super Four Hyper VTEC, introduced in 1999, the first worldwide implementation of VTEC technology in a motorcycle occurred with the introduction of Honda's VFR800 sportbike in 2002. Similar to the SOHC VTEC-E style, one intake valve remains closed until a threshold of 7000 rpms is reached, then the second valve is opened by an oil-pressure actuated pin. The dwell of the valves remains unchanged, as in the automobile VTEC-E, and little extra power is produced but with a smoothing-out of the torque curve. Critics maintain that VTEC adds little to the VFR experience while increasing the engine's complexity. Drivability is a concern for some who are wary of the fact that the VTEC may activate in the middle of an aggressive corner, upsetting the stability and throttle response of the bike.
References
Honda Motor Co., Ltd. (2004). . Retrieved Sep. 16, 2004.
Driving with VTEC
The original VTEC technology did not do all that much to improve engine power or efficiency at low speeds, though it did mean that Honda did not need to consider high-speed operation at all for its low-speed cam profile. Thus, this has led some to accuse VTEC of being more hype than actual improvement for the average driver. The counter-argument is that VTEC means that the higher-speed power is there if the driver needs it, and that VTEC allows a smaller, more efficient engine to still have high power output available.
VTEC does mean that the engine needs to be running at high engine speeds to develop maximum power, which requires the constant attention of the driver to keep the power in the optimal RPM band for high-speed driving. Some feel this is an interesting driving challenge, while others find it annoying.
