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Why Should the Intake Valve Be Larger Than the Exhaust Valve?

The valve is pressed open by the cam and closed by the valve spring. The intake valve opens when the mixture needs to be drawn into the cylinder, and the exhaust valve opens when the burned exhaust gas needs to be discharged.

Since intake air is “sucked” in and exhaust is “pushed” out, it is more difficult to intake air than exhaust, and the more intake air, the better the combustion and the better the engine’s performance. The intake is sucked by vacuum, and the exhaust is squeezed to push out the exhaust gas. Therefore, it is easier to exhaust than to intake. In order to get more fresh air to participate in the combustion, therefore, more intake air is required. For this reason, the intake valve needs to be larger, and the overall size has not changed, the exhaust valve can only be made smaller. Therefore, the intake valve is generally designed to be larger than the exhaust valve to reduce the difficulty of intake and lift the intake air volume. Some simply design one more intake valve, which leads to a 3-valve (2 in  1 ex) and 5-valve (3 in 2 ex) design.

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Intake Valve

 

Three Factors That Affect Valve Size:

(1) Influence of Airflow Resistance:

Theoretically, the amount of charge and exhaust are equal, and the opening and closing time of the exhaust valve and the opening and closing time of the intake valve are also equal.
But this is not the case in fact, because the throttle valve directs the speed, and the valve opens and closes big or small. When the inhalation is finished, the cylinder should be filled with a mixture equal to the atmospheric pressure (about 1 kg/square centimeter). However, when the gas flows through the intake pipe and the intake valve, it is subject to resistance. In order to overcome these resistances and make the mixture flow at a certain speed, a part of the pressure must be consumed, so the air pressure in the cylinder is always lower than the atmospheric pressure. The density of the gas changes with the pressure, and the density of the low pressure decreases, and the weight of the actual inhaled mixture decreases. To solve and reduce the influence of airflow resistance, it is necessary to increase the cross-sectional area of ​​the airflow passage at the intake valve, that is, to increase the area of ​​the air valve. In the case of thin air in the plateau area, the area of ​​the intake passage should be enlarged, otherwise the engine will not reach its rated power at all in these areas.

(2) Influence of engine speed:

Under the condition that the size of the original airway remains unchanged, the suction pressure also changes according to the speed and load. Speed ​​varies with load while the throttle valve is held wide open. Assuming that the inflation weight is 1, the pressure decreases from 0.95 to 0.80 when the speed is increased from 700 rpm to 2,100 rpm, which means that the air density in the cylinder is reduced, and the air flow cannot keep up with the needs of the inflation volume. Because when the rotation speed is increased three times, the piston movement speed is also accelerated three times, and the airflow speed must be accelerated three times to ensure that the same weight of gas breaks into the cylinder. However, due to the increased force in the airway at high speed, the airflow cannot be increased in the same proportion. That is to say, the inflation effect is better when decelerating, and the average effective pressure and torque are also larger (the torque is proportional to the average effective pressure).

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(3) Speed ​​comparison Factor of Intake and Exhaust Flow:

The mixed gas entering the cylinder enters when the piston stroke volume forms a vacuum (that is, the air pressure difference) when the piston moves from the top dead center to the bottom dead center. faster. Of course, it is good to have more fresh air mixture entering the cylinder, but the speed of the airflow cannot keep up with the speed of the piston, which creates conditions for the intake air and increases the intake valve to meet the needs of the air charge.Exhaust is the exhaust gas discharged from the cylinder. The self-discharge begins at about 120′ down the working stroke of the piston, 180. The rear piston starts to move upwards. At this time, the exhaust gas is behind the front piston. The exhaust gas discharge speed depends on the upward speed of the piston, and the exhaust gas is forced out. It can be said that the upward speed of the piston and the exhaust speed are equal, which is the ratio of the exhaust valve to the intake. The valve is small. Generally, the exhaust diameter is two-thirds of the intake diameter. The caliber of the exhaust valve is designed without affecting the exhaust.

In this way, it can be known that the larger the intake valve is to meet the needs of the engine air volume passing through within a reliable range, and the small exhaust valve is to be reduced as much as possible without affecting the exhaust volume.
In addition, the exhaust passage is superior to the intake passage condition, so the intake and exhaust valves appear to be much larger than the exhaust valves. In some special engines, such as the piston aircraft engine that prevailed in the 1940s, its intake valve is more than twice the size of the exhaust valve.

In this way, it can be known that the larger the intake valve is to meet the needs of the engine air volume passing through within a reliable range, and the small exhaust valve is to be reduced as much as possible without affecting the exhaust volume.
In addition, the exhaust passage is superior to the intake passage condition, so the intake and exhaust valves appear to be much larger than the exhaust valves. In some special engines, such as the piston aircraft engine that prevailed in the 1940s, its intake valve is more than twice the size of the exhaust valve.

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