There are numerous different methodologies used to vary valve timing. Each manufacturer has its own name for its own VVT system. Toyota uses VVT-i®, Honda uses VTEC®, Mitsubishi uses MIVEC®, and the list goes on. Let’s see how Toyota’s VVT-i system works.


The VVT system shown in the video above is a variation of Toyota’s VVT-i, though Honda has a similar system called VTC. In this system, the ECU receives signals from the camshaft position sensor, crankshaft sensor, oil temperature sensor, mass air flow sensor (MAF), and the engine coolant temperature sensor and uses the information to adjust its output signal to an oil control valve. This valve acts as a hydraulic actuator, rotating a rotor (which is connected to the camshaft) inside a housing, which is connected to the crankshaft via a timing chain. Once the ECU has changed the cam phase angle, the ECU continues to receive inputs from all of the sensors and continually adjusts the oil feed to the rotor.

Like electronic throttle control, this is a closed-loop system, which means that the difference between the current camshaft phase angle and the optimal camshaft angle is the “error signal” that is sent to the ECU. The computer uses the error signal to adjust its output to the actuator to get the camshaft phase angle where it needs to be.


Cam Changing

Other VVT systems change the shape of their camshaft lobes, not just the camshaft phase angle relative to the crankshaft. Changing the cam profile not only affects valve lift (how far the valve opens), but also valve duration (how long the valve stays open). The image above demonstrates the features of a camshaft lobe that affect valve lift and duration.

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At higher engine speeds, many VVT systems change to a more aggressive (i.e. high lift and high duration) cam lobe profiles. Some variable valve lift systems shift the camshaft axially so that a higher profile lobe engages the cam follower, producing more valve lift. Others, like Honda’s VTEC (yo), lock a high profile rocker arm to the low speed rocker arms via a hydraulically actuated pin. A more aggressive cam lobe activates this high profile rocker arm and provides the intake valve with more lift, allowing more air into the cylinder.


For our more visually-inclined readers, Engineering Explained provides a great breakdown of how Honda’s VTEC system works.

How VTEC Works - A Simple Explanation