Variable Valve Timing: Description and Operation

Fuel and Emissions System Description - VTEC/VTC

VTEC/VTC

- The i-VTEC system has a variable valve timing control (VTC) mechanism on the intake camshaft in addition to the usual VTEC.
This system improves fuel efficiency and reduces exhaust emissions at all levels of engine speed, vehicle speed, and engine load.
- The VTEC system changes the intake valve lift and timing by using more than one cam profile.
- The VTC system changes the phase of the intake camshaft via oil pressure. It changes the intake valve timing continuously.











VTC System

- The VTC system makes continuous intake valve timing changes based on operating conditions.
- Intake valve timing is optimized to allow the engine to produce maximum power.
- Cam angle is advanced to obtain the EGR effect and reduce pumping loss. The intake valve is closed quickly to reduce the entry of the air/fuel mixture into the intake port and improve the charging effect.
- The system reduces the cam advance at idle, stabilizes combustion, and reduces engine speed.
- If a malfunction occurs, the VTC system control is disabled and the valve timing is fixed at the fully retarded position.





VTC System Diagram

The VTC oil control solenoid valve controls oil pressure to the advance chamber/retard chamber of the VTC actuator according to a signal from the ECM/PCM. When the VTC actuator vanes rotate to the advance or retard position, the intake camshaft timing is changed.
CMP sensor A detects the intake camshaft angle and sends the information to the ECM/PCM. The ECM/PCM controls the VTC oil control solenoid valve as the intake camshaft angle changes.






VTEC System

Intake valve side

- The VTEC system changes the cam profile to correspond to engine speed. It maximizes torque at low engine speed and output at high engine speed.
- The low lift cam is used at low engine speeds, and the high lift cam is used at high engine speeds.
- The rocker arm oil control solenoid switches the intake valve side of the VTEC system on and off; the solenoid is controlled by the ECM/PCM.
- The rocker arm oil pressure switch detects VTEC system oil pressure and sends this information to the ECM/PCM.










VTEC System Operation

At low engine speed, the rocker arm oil control solenoid is turned off by the ECM/PCM. Oil pressure from the rocker arm oil control valve does not enter the intake rocker shaft. Each intake rocker arm is separated by a return spring and the intake secondary rocker arm is lifted by the low lift cam lobe.





At high engine speed, the rocker arm oil control solenoid is turned on by the ECM/PCM. Oil pressure from the rocker arm control solenoid enters the primary intake rocker arm via the intake rocker shaft, and it moves the VTEC switching piston in the rocker arm.
This causes the VTEC switching piston to slide into the intake mid rocker arm and the intake secondary rocker arm, locking the rocker arms together. Both intake rocker arms are lifted by the high lift cam lobe.