Why are intake valves and exhaust valves different in size?

The size of the intake valve and exhaust valve is determined according to the amount of inflation and exhaust. To understand this problem, we have to talk about the following three situations.

(1) Influence of airflow resistance;

Theoretically speaking, the inflation volume and the exhaust volume are equal, and the opening and closing time of the exhaust valve and the intake valve are also equal. But in fact, this is not the case, because the throttle valve commands the speed and the valve opens and closes. After inhalation, the gas cylinder should be filled with mixed gas equal to atmospheric pressure (about 1 kg/cm2). However, when the gas flows through the intake pipe and the intake valve, it will be resisted. In order to overcome these resistances and make the mixture flow at a certain speed, a certain pressure must be consumed, so the air pressure in the cylinder is always lower than atmospheric pressure. The density of gas varies with pressure, and the density decreases at low pressure, so the actual weight of inhaled mixture decreases. To solve and reduce the influence of airflow resistance, it is necessary to increase the cross-sectional area of airflow channel at the intake valve, that is, to increase the area of the valve. In the case of thin air in plateau area, the intake area should be increased, otherwise the engine will not reach the rated power at all in these areas.

(2) Influence of engine speed:

Under the condition that the size of the original air duct remains unchanged, the suction pressure will change according to the speed and load. When the throttle valve is kept fully open, the speed varies with different loads. Assuming that the inflation weight is 1, when the rotational speed increases from 700 rpm to 2 100 rpm, the pressure decreases from 0.95 to 0.80, indicating that the air density in the cylinder decreases and the air flow rate cannot keep up with the inflation demand. Because when the rotation speed is increased to three times, the movement speed of the piston is also accelerated to three times, and the airflow speed is also accelerated to three times to ensure that the air with the same weight breaks into the cylinder. However, due to the increase of resistance in the airway at high speed, the airflow cannot increase in the same proportion. In other words, the inflation effect is better when it is slowed down, and as a result, the average effective pressure and torsion are also greater (torsion is proportional to the average effective pressure).

(3) Velocity contrast coefficient of intake and exhaust airflow:

The mixed gas entering the cylinder enters when the piston moves from top dead center to bottom dead center and the piston stroke volume forms a vacuum (that is, the air pressure difference). Of course, the piston stroke speed is faster than the air flow speed. It is of course good to have more fresh air mixture entering the cylinder, but the speed of air flow can't keep up with the speed of piston action, which will create conditions for air intake and increase intake valves to meet the needs of air intake.

Exhaust gas is the exhaust gas in the cylinder. Automatic drainage starts from the downward stroke of the piston about 120', 180. When the rear piston starts to ascend, the exhaust gas is behind the front piston, and the exhaust gas is pressed out at the upward speed of the piston. It can be said that the upward speed of the piston and the exhaust speed are equal, which is why the exhaust valve is smaller than the intake valve. Generally, the exhaust diameter is two-thirds of the intake diameter. The diameter of the exhaust valve is designed without affecting the exhaust.