VTEC
VTEC (described as Variable Valve Timing & Lift Electronic Control, but stands for Valve Timing Electronically Controlled) is a system developed by Honda to improve the volumetric efficiency of a four-stroke internal combustion engine, resulting in higher performance at high RPM, and lower fuel consumption at low RPM.
[citation needed] The VTEC system provides the engine with valve timing optimized for both low- and high-RPM operations.
In basic form, the single cam lobe and follower/rocker arm of a conventional engine is replaced with a locking multi-part rocker arm and two cam profiles: one optimized for low-RPM stability and fuel efficiency, and the other designed to maximize high-RPM power output.
At the switch point, a solenoid is actuated that allows oil pressure from a spool valve to operate a locking pin, which binds the high-RPM rocker arm to the low-RPM ones.
The improvements in low-RPM performance, which is where most street-driven automobiles operate a majority of the time, occur in trade for a power and efficiency loss at higher-RPM ranges.
Timing refers to an angle measurement of when a valve is opened or closed with respect to the piston position (BTDC or ATDC).
Due to the behavior 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.
VTEC was introduced as a DOHC (dual overhead camshaft) system in Japan in the 1989 Honda Integra XSi,[1] which used the 160 bhp (120 kW) B16A engine.
The United States market saw its first VTEC system with the introduction of the 1991 Acura NSX,[4] which used a 3-litre DOHC C30A V6 with 270 bhp (200 kW).
Additionally, the center lobe on the camshaft cannot be utilized by both the intake and the exhaust, limiting the VTEC feature to one side.
The primary exhaust rocker arm contacts a low-profile camshaft lobe during low-RPM engine operation.
The earliest VTEC-E implementation is a variation of SOHC VTEC which is used to increase combustion efficiency at low RPM while maintaining the mid range performance of non-vtec engines.
This induces swirl of the intake charge which improves air/fuel atomization in the cylinder and allows for a leaner fuel mixture to be used.
3-Stage VTEC is a version that employs three different cam profiles to control intake valve timing and lift.
In newer version of 3-Stage i-VTEC combined VTC and PGM-FI to allow ECU to control full range of mode to archive greater fuel economy improvements and performance.
The intake phase varies from fully retarded at idle to somewhat advanced at full throttle and low RPM.
However, the valvetrain has the added benefit of roller rockers and VTC continuously variable intake cam timing.
At low RPM only one valve on the intake opens fully, promoting combustion chamber swirl and improved fuel atomization.
According to Honda, VCM technology works on the principle that a vehicle only requires a fraction of its power output at cruising speeds.
Honda's advanced VTEC technology departs greatly from its previous incarnations by no longer relying on switching between two sets of lobes on a given camshaft.
Advanced VTEC motors still use the now standard oil pressure controlled variable cam gear angle mechanism.
With these two technologies combined Honda has developed an infinitely variable valve timing and lift system ("VVTL").
With the introduction of i-VTEC the systems gained infinitely variable valve timing but still only staged lift i.e. High-Low.
The camshaft is surrounded by a partially open drum which has secondary rocker arms attached to it via a pivoting point.
These secondary rocker arms, which have a varying depth profile (similar to cams), are directly actuated by the camshaft, in a scissor-like manner.
The drum will only rotate to advance or retard the position of the secondary rocker arms, to take advantage of their varying profiles.
Thus, through varying the position of the drum about its axis, each cam profile is changed to an optimal height for maximum engine performance without sacrificing fuel efficiency at lower speeds.
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.
Honda seemed to agree, as their VFR1200, a model announced in October 2009, came to replace the VFR800, which abandons the VTEC concept in favor of a large capacity narrow-vee "unicam", i.e., SOHC, engine.
