How VTEC Works

VTEC is one of Honda's greatest inventions. Though an undisputed expert in turbo-charging as evidenced by years of Formula-1 domination while Honda was active in the sport, Honda's engineers feels that turbo charging has disadvantages, primarily bad fuel economy, which made it not totally suitable for street use. At the same time, the advantages of working with smaller engines meant that smaller capacity engines with as high power output as possible (ie very high specific-output engines) are desirable for street engines.

Thus Honda invented VTEC which allows it to extract turbo level specific output from its engines without having to suffer from the disadvantages of turbocharging.

VTEC (Variable Valve Timing and Lift Electronic Control) is a valvetrain system developed by Honda to improve the volumetric efficiency of a four-stroke internal combustion engine. The VTEC system uses two camshaft profiles and electronically selects between the profiles. It was invented by Honda R&D engineer Ikuo Kajitani, and was the first system of its kind. Different types of variable valve timing and lift control systems have also been produced by other manufacturers (MIVEC from Mitsubishi, AVCS from Subaru, VVT-i/VVTL-i from Toyota, VarioCam Plus fromPorsche, NeoVVL from Nissan, etc.).

History

VTEC, the original Honda variable valve control system, originated from REV (Revolution-modulated valve control) introduced on the CBR400 in 1983 known as HYPER VTEC. In the regular four-stroke automobile engine, the intake and exhaust valves are actuated by lobes on a camshaft. The shape of the lobes determines the timing, lift and duration of each valve. Timing refers to an angle measurement of when a valve is opened or closed with respect to the piston position (BTDC or ATDC). Lift refers to how much the valve is opened. Duration refers to how long the valve is kept open. Due to the behaviour of the working fluid (air and fuel mixture) before and after combustion, which have physical limitations on their flow, as well as their interaction with the ignition spark, the optimal valve timing, lift and duration settings under low RPM engine operations are very different from those under high RPM. Optimal low RPM valve timing, lift and duration settings would result in insufficient filling of the cylinder with fuel and air at high RPM, thus greatly limiting engine power output. Conversely, optimal high RPM valve timing, lift and duration settings would result in very rough low

RPM operation and difficult idling. The ideal engine would have fully variable valve timing, lift and duration, in which the valves would always open at exactly the right point, lift high enough and stay open just the right amount of time for the engine speed in use.

VTEC Operation

A 4-stroke engine goes through induction, compression, combustion and exhaust strokes to generate power. Before the advent of VTEC, the valves controlling the intake and exhaust strokes were operated according to fixed rules.

If the intake valves were made to open a relatively small amount to privilege drivability at low engine speeds as used in normal driving conditions, the engine would not be allowed to intake enough air at higher engine speeds, sacrificing outright performance. On the other hand, if the intake valves were made to open wide to privilege breathing at higher engine speeds, performance at low engine speeds would be compromised. This is a dilemma that has plagued engines for over a century.

An elegant, simple mechanism Switching between high and low valve lift using two cam profiles and two rocker arms per cylinder. The switch is made using hydraulic pressure to push/release the sliding pin, locking/unlocking the middle rocker arm and the other rocker arm.

At low engine speeds, the pin is retracted, disengaging the middle rocker arm. The valves are operated by the two outside, low-profile cams for a low valve lift.

At higher engine speeds, increased hydraulic pressure pushes the pin, engaging the middle rocker arm. The valves are operated by the middle, high profile cam for high valve lift.