What is Variable Valve Timing?

Figure 1: Solenoid valves of different models.

VVT refers to variable valve timing. It adjusts the amount of intake (or exhaust), as well as the timing and angle of solenoid valves opening and closing according to the operating conditions of the engine. It is intended to optimize the amount of air entering the cylinder and improves combustion efficiency.
VVT is one of the new technologies that have been gradually applied to modern cars in recent years. It can increase the intake charge and charge coefficient of the generator, and further raise the torque and power of the engine.

The Constitution of VVT

A typical VVT includes a valve phaser, an intake and exhaust camshaft position sensor, a crankshaft position sensor, an intake and exhaust VVT solenoid valve, an intake and exhaust camshaft position actuator, oil channels, a VVT control valve filter.

Take Toyota’s variable valve timing- intelligence (VVT-i) as an example. The core working components include a movable piston, a helical spline, a VVT pulley, an oil control valve (OCV), a camshaft angle sensor, a hydraulic pressure, an engine control unit (ECU) and an engine oil pump. The components and structures are shown as follows:

Figure 2: Constitution of Toyota VVT-i.

The Functions of VVT

VVT continuously adjusts the valve opening, closing time and intake air volume of the engine. And thus it plays an essential role in:
1. improving fuel economy and idle stability
2. reducing NOx (nitrogen oxide) and HC (hydrocarbon) emissions
3. increasing the torque at low speed
4. increasing the power at high speed

Types of VVT

The most common VVTs for consumers are VVT-i, Variocam, VTEC, CVVTCS, etc. The names are quite different, but the specific working principles and design ideas are very similar. The following briefly introduces these types of VVT.

1. VVT-i: short for Toyota's “Variable Valve Timing- Intelligence". It is improved from the original VVT system. The VVT-i is a new generation with significantly improved fuel economy.

2. Variocam: the variable valve timing technology used in the engine of the Porsche 911 sports car. Through the valve we can find that it has two positions, and each intake valve has two maximum strokes. Controlling the change in valve travel is two sets of cam controls. One set is the high-speed cam, which is the red part of the cam; the other set is the low-speed cam, which is the cam between the high-speed cams.

Figure 3: Porsche's VarioCam System.

3. VTEC: short for variable valve timing and lift electronic Control system. Its principle is similar to that of Variocam, but the control method is different. There are still high-speed cams and low-speed cams on the camshafts, but because the valves of the Honda engine are driven by rocker arms, they cannot be as compact as the Porsche.

4. CVVTCS: After the merger of Renault and Nissan, many technologies are shared within the group. Among them is the continuous variable valve timing control system (CVVTCS) that Nissan has devotedly researched. The principle is similar to that of Honda VTEC. It also uses hydraulic action to change the angle between the camshaft synchronous toothed pulley and the end of the camshaft, thereby changing the valve timing angle.

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