Variable Timing Electronically Controlled (VTEC)

PURPOSE/OPERATION
The engine is equipped with multiple cam lobes per cylinder, providing one valve timing and lift profile at low speed and a different profile at high speed. Switch-over from one profile to the other is controlled electronically, and is selected by monitoring current engine speed and load.

In general, it would be ideal if the high rpm performance of a racing engine and the low rpm performance of a standard passenger car engine could be combined in a single engine. This would result in a maximum performance engine with a wide power band. Two of the major differences between racing engines and standard engines are the timing of the intake/exhaust valves and the degree of valve lift. Racing engines have longer intake/exhaust timing and a higher valve lift than standard engines. The Honda Variable Valve Timing and Valve Lift Electronic Control System takes this into account. When valve actuation is set for low rpm timing and lift, low rpm torque is better than in a standard engine. When valve actuation is then switched for high rpm timing and lift, output improves to the level given by a racing engine. Until now, few variable valve timing systems have been commercialized. In those that have, only the time that both valves are open (intake/exhaust overlap) could be changed. Honda's system is the first in the world in which both the valve timing and the degree of valve lift can be changed as needed, making it the most advanced valve train mechanism available.

The control system for this mechanism constantly monitors the changes in engine status such as load, rpm and vehicle speed. This information is transmitted to the engine control module. The VTEC solenoid valve, in response to a signal from the engine control module, closes the oil passage to the rocker arm at low speed. This cuts oil pressure to the hydraulic pistons in the rocker arms so the arms operate independently. At high speed, the engine control module opens the VTEC solenoid valve. The increased oil pressure causes the hydraulic pistons to lock the primary, secondary, and mid rocker arms together. The VTEC pressure switch serves as a sensor to determine if the switch-over has taken place in response to the engine control module signal.