VVT (variable valve timing) literally means that the opening and closing time of the engine valves can be adjusted to achieve a variable timing effect through a certain unique technology.
Because the engine's valve train is used to adjust the engine's intake and exhaust effects to ensure the engine's efficiency under certain working conditions. However, the engine's working conditions are constantly changing, so the opening and closing of the valves at a fixed time certainly cannot meet the engine's intake efficiency requirements under all working conditions.
Therefore, the valve advance and delay change time can be achieved through hardware mechanisms, and the precise control of the electronic control system can achieve intelligent variable valve adjustment within a certain range. This technology is what we usually call VVT variable valve timing. If an electronic control system is added, it is electronic variable valve timing. For example, Honda's ivtec, Toyota's vvt-i, etc. They have the following advantages over engines without variable valve timing:
Improve fuel economy
Improve low-speed stability and torque output
Help increase power
Reduce emission pollution
Compared with engines without VVT, fuel economy will be improved by about 10%-20%,
A complete cycle of a four-stroke engine includes: intake, compression, power, and exhaust. Since each stroke requires the piston to move 180 degrees from the top dead center to the bottom dead center, the crankshaft actually rotates 720 degrees in the entire cycle.
The camshaft is the main body of the engine to complete the valve timing. The camshaft is driven by the crankshaft through the timing belt, but the intake valve and exhaust valve only need to be opened once in a complete stroke, so the gear ratio between them is fixed at 2:1. That is, the crankshaft rotates twice, and the camshaft only needs to rotate once.
In theory, shouldn't the valve be opened and closed strictly every 180 degrees? For example, the valve is opened when the piston starts to move downward during the intake stroke, and the valve is closed before the piston reaches the bottom dead point and is ready to move upward; the exhaust valve is opened just before the end of the work during the exhaust stroke, and the piston moves upward to expel the exhaust gas. In theory, isn't this valve timing quite suitable? But reality often does not allow it, because the operation of the engine is extremely complex and changeable, whether it is resistance, friction, intake efficiency, temperature, pressure, exhaust gas circulation and other factors will affect the comprehensive performance of the engine. Compared with the valve timing system, the engine's intake efficiency plays an extremely important role in its operation, and the valve timing system is directly related to the valve timing.
The ideal working condition of the engine is sufficient intake and clean exhaust. Therefore, although the fixed valve opening and closing time is very consistent with the engine's valve timing requirements under certain circumstances, the fixed opening and closing time cannot meet different valve timing requirements for the ever-changing engine working conditions. In general, the engine's intake volume cannot meet the fuel as much as possible and the exhaust gas cannot be completely eliminated. Therefore, before VVT came out, everyone extended the opening and closing time of the intake and exhaust valves by changing the cam structure. This method can ensure that the valve opens in advance before the engine starts to inhale and closes late after the intake ends to obtain more charging efficiency. The valve opens in advance before exhaust and closes late after exhaust to improve exhaust efficiency. At this time, a situation will occur: the intake valve opens when the exhaust valve is not closed, and the angle of the crankshaft rotation during the opening of the intake valve and the exhaust valve is called the "valve overlap angle".
The existence of the valve overlap angle is entirely set to meet the actual operating conditions of the engine, because the intake efficiency of the engine under actual operating conditions cannot reach a coefficient of 1 (self-priming). Therefore, this method can improve the intake and exhaust efficiency as much as possible, so that the combustion efficiency and exhaust efficiency will be improved accordingly. In fact, the valve overlap angle caused by the cam is similar to a simple "fixed valve timing" effect, which can ensure that the engine achieves the "early opening and late closing" effect of the valve under all working conditions. As for how to make it "variable", it is the variable timing technology to be discussed below, because it can be said that the operation of VVT is based on the valve overlap angle.
Since the valve overlap angle is fixed, if I want to achieve the maximum intake efficiency of the engine under different speed conditions, improve the exhaust efficiency, reduce pumping loss, and improve EGR efficiency so that the engine can achieve different effects under different working conditions. For example, at low speed, reduce the valve overlap angle to ensure combustion efficiency, stability, economic pumping loss, etc.; at high speed, increase the valve overlap angle to improve intake efficiency and increase power output.
How to achieve it? Structural principle
How to achieve it? If the camshaft is rotated one angle in advance or delay under specific working conditions, it can ensure that the cam advances or delays the top pressure of the valve push rod, thereby advancing or delaying the opening and closing time of the valve.
The VVT phaser is used here, which is an actuator that adjusts the rotation of the crankshaft. The structure is a closed liquid chamber of a flat cylinder with several independent liquid chambers. Each independent liquid chamber is divided into two small liquid chambers, namely the lag chamber and the advance chamber, by the rotor blades. The rotor is connected to the camshaft, and the phaser is controlled by electronic hydraulics. It receives information from the ECU to pressurize the hydraulic oil in the lag chamber and the advance chamber, causing the side with greater hydraulic pressure to drive the rotor to rotate in advance or later by a certain angle, and finally realize the variable valve opening time through the camshaft.
For example, under high-speed conditions, I need the intake valve to open more in advance. At this time, the ECU will issue an advance opening command to the electronic control system. The electronic control system controls the hydraulic oil to squeeze the advance chamber of the phaser liquid chamber. The hydraulic pressure drives the fan blade to rotate, thereby driving the camshaft to rotate an angle. At this time, the cam contacts the valve push rod in advance and slowly opens the valve opening.
Summary: VVT is actually a compromise solution. Although it can improve low-speed torque and high-speed power and improve good fuel economy, the output of mid-stage conditions is weak. Since the cam stroke is fixed, it is impossible to take into account the opening and closing. The final calibration is to take a compromise solution after N experiments. Now many car companies are also equipped with dual VVT (DVVT), which not only takes care of the intake side but also takes care of the exhaust side to achieve better results as much as possible for both intake and exhaust. Compared with VVT, the fuel economy and performance improvement will be better.
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