What is a variable valve?

In fact, the actual operating speed of the engine is not fixed, but like a person running, sometimes rapid and sometimes slow, so it is particularly important to adjust your breathing rhythm. Let's take a look at how the engine "breathes".

Camshaft

Simply put, the camshaft is a metal rod with multiple disc-shaped cams. What role does this metal rod play in the operation of the engine? It is mainly responsible for the opening and closing of the intake and exhaust valves. The camshaft rotates continuously under the drive of the crankshaft, and the cam continuously presses down the valve (rocker arm or push rod), thereby realizing the function of controlling the opening and closing of the intake and exhaust valves.

What do OHV, OHC, SOHC, and DOHC mean?

You often see the letters SOHC and DOHC on the engine casing. What do these letters mean? OHV refers to overhead valve bottom camshaft, which means that the camshaft is arranged at the bottom of the cylinder and the valve is arranged at the top of the cylinder. OHC refers to overhead camshaft, which means that the camshaft is arranged at the top of the cylinder.

If there is only one camshaft on the top of the cylinder that is responsible for the opening and closing of the intake and exhaust valves at the same time, it is called a single overhead camshaft (SOHC). If there are two camshafts on the top of the cylinder that are responsible for the opening and closing of the intake and exhaust valves respectively, it is called a double overhead camshaft (DOHC).

A metal connecting rod is required to connect the cam of the bottom camshaft and the valve rocker arm. The cam pushes up the connecting rod and pushes the rocker arm to realize the opening and closing of the valve. However, too high a speed can easily cause the push rod to break, so this design is mostly used in engines with large displacement, low speed, and high torque output. The overhead camshaft can omit the push rod, which simplifies the transmission mechanism from the camshaft to the valve, and is more suitable for the power performance of the engine at high speed. The overhead camshaft is widely used.

The role of the valve timing mechanism

The valve timing mechanism mainly includes the timing gear system, camshaft, valve transmission components (valves, push rods, rocker arms, etc.). The main function is to open and close the intake and exhaust valves of each cylinder in time according to the working conditions of the engine, so that the fresh mixed gas can fill the cylinder in time and the exhaust gas can be discharged from the cylinder in time.

What is valve timing? Why is timing needed?

The so-called valve timing can be simply understood as the moment when the valve opens and closes. Theoretically, in the intake stroke, when the piston moves from the top dead center to the bottom dead center, the intake valve opens and the exhaust valve closes; in the exhaust stroke, when the piston moves from the bottom dead center to the top dead center, the intake valve closes and the exhaust valve opens.

In actual engine operation, in order to increase the intake volume in the cylinder, the intake valve needs to be opened in advance and closed later; similarly, in order to make the exhaust gas in the cylinder more clean, the exhaust valve also needs to be opened in advance and closed later, so as to ensure the effective operation of the engine.

What are variable valve timing and variable valve lift?

When the engine is at high speed, the time for each cylinder to inhale and exhaust in one working cycle is very short. In order to achieve high charging efficiency, the intake and exhaust time of the cylinder must be extended, that is, the valve overlap angle must be increased; when the engine is at low speed, too large a valve overlap angle will easily cause exhaust gas backflow, and the intake volume will decrease, resulting in unstable engine idle speed and low low-speed torque.

Fixed valve timing is difficult to meet the needs of both high and low engine speeds, so variable valve timing came into being. Variable valve timing can be adjusted according to the engine speed and working conditions, so that the engine can obtain ideal intake and exhaust efficiency at high and low speeds.

The essence of affecting engine power is actually related to the amount of oxygen entering the cylinder per unit time, and the variable valve timing system can only change the time of valve opening and closing, but cannot change the intake per unit time. Variable valve lift can meet this demand. If the engine valve is regarded as a "door" of a house, the valve timing can be understood as the time when the "door" opens, and the valve lift is equivalent to the size of the "door" opening.