ICE-17 — Variable Valve Timing (VVT)

ASSUMPTION — This lesson covers the general principle; no manufacturer’s proprietary implementation is discussed.

In this lesson you will learn:

• What VVT changes
• How the cam phaser works
• The effects of VVT

1. Why Is It Needed?

Fixed valve timing is optimal only over a narrow speed range. An engine runs over a wide speed/load range, so a single timing cannot be best everywhere.

FACT — VVT shifts the camshaft phase (valve opening/closing timing) with speed and load to optimize gas exchange across a wide operating range.

2. The Cam Phaser

FACT — A cam phaser changes the camshaft’s angular position relative to the crankshaft, usually driven hydraulically (oil pressure) or electrically. The ECU computes and controls the target phase.

3. Effects

• Volumetric efficiency — improved by optimizing intake-valve closing.
• Internal EGR (residual gas) — controlled via valve overlap, affecting NOx.
• Idle/low speed — overlap reduced for stability.

INTERPRETATION — VVT improves low-speed torque and high-speed power simultaneously; it does not change lift, which is a separate technology (ICE-18).

4. Summary

• Fixed timing is suboptimal over a wide range.
• VVT shifts cam phase with speed/load.
• It affects volumetric efficiency, internal EGR and idle stability.

5. Sources

• Bosch Automotive Handbook
• Heywood, Internal Combustion Engine Fundamentals

Next Lesson

• ICE-18 — Variable Valve Lift (VVL).