How does the engine work? Working principle of variable valve

OHV, OHC, SOHC, DOHC meaning

The installation position of the camshaft of the engine has three forms: bottom, middle and top. The camshaft of the lower-type gas distribution mechanism is located in the crankcase, the camshaft of the middle-type gas distribution mechanism is located in the upper part of the machine body, and the camshaft of the upper-type gas distribution mechanism is located on the cylinder head.

OHV (Overhead valve) has a camshaft at the bottom, the camshaft is arranged at the bottom of the cylinder, and the valve is arranged at the top of the cylinder. The layout of the engine's camshaft. The position of the bottom camshaft is at the waist of the cylinder (the middle position), and the valve is opened by the push rod. A metal connecting rod needs to be connected between the cam of the bottom camshaft and the valve rocker arm, and the cam pushes up the connecting rod to push the rocker arm to open and close the valve. It is mostly used in engines with large displacement, low speed and high torque output.

OHC (overhead cam) overhead camshaft, that is, the camshaft is arranged on the top of the cylinder. According to the number of camshafts, it can be divided into two types: single overhead camshaft (SOHC) and double overhead camshaft (DOHC). Engines with overhead camshafts have faster valve opening and closing motions, which results in higher speeds and smoother running. They are more suitable for engine performance at high speeds. Overhead camshafts are widely used.

If there is only one camshaft at the top of the cylinder responsible for the opening and closing of the intake and exhaust valves, 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, it is called a double overhead camshaft (DOHC).

The gas distribution mechanism mainly includes the timing gear system, camshaft, and valve transmission components (valve, push rod, rocker arm, etc.). The main function is to open and close the intake and exhaust valves of each cylinder in time according to the working condition of the engine. , So that the fresh mixed gas fills the cylinder in time, and the exhaust gas can be discharged out of the cylinder in time. Divided into valve overhead type and valve side type.

When the engine is in a high-speed range, a working stroke of a four-stroke engine takes only a few thousandths of a second. Such a short period of time often causes insufficient intake and exhaust exhaust of the engine, which affects the efficiency of the engine. Therefore, it is necessary to make up for the shortcomings of insufficient intake and exhaust exhaust through early opening and late closing of the valve. In this case, there will inevitably be a moment when the intake valve and the exhaust valve are opened at the same time, and the valve timing is called "valve overlap

Valve timing

The valve timing can be simply understood as the moment when the valve opens and closes.

In theory, during the intake stroke, when the piston moves from top dead center to bottom dead center, the intake valve opens and the exhaust valve closes; during the exhaust stroke, when the piston moves from bottom dead center to top dead center, the intake valve Close and the exhaust valve opens.

In actual engine operation, in order to increase the intake air volume in the cylinder, the intake valve needs to be opened early and delayed closed; similarly, in order to make the exhaust gas in the cylinder cleaner, the exhaust valve also needs to be opened early and delayed closed In order to ensure the effective operation of the engine.

Variable valve timing, variable valve lift

Variable valve timing and lift technology is to allow the engine to freely adjust the "breathing" under various loads and speeds, thereby improving power performance and improving combustion efficiency.

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

The essence of affecting the 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 opening and closing time of the valve, but cannot change the intake air amount per unit time. Lift can meet this demand. Variable valve lift technology can match the appropriate valve lift at different engine speeds, resulting in sufficient torque at low speeds and strong horsepower at high speeds. The system uses a smaller valve lift at low speeds, which is beneficial to increase the turbulence in the cylinder to increase the combustion speed and increase the low-speed torque of the engine, while using a larger valve lift at high speeds can significantly increase the intake air volume, and Increase power output at high speeds.