I often hear the difference between a two-valve and a four-valve in a motorcycle engine. Many riders ask, what does this mean? Is it better to have two valves or four valves?
Motorcycle engines generally have two valves, three valves, four valves and even five valves. The difference between single-cylinder two-valve and four-valve is discussed here.
We all know that the valve is the part that manages the intake and exhaust when the engine is working. The engine works like a person running. The more valves, the smoother the breathing, the higher the efficiency of one-time inhalation and exhaust. Therefore, it also makes it easier for the engine to reach high speed, and the high speed is the only way to increase the power data of the engine. Therefore, the engine adopts a four-valve design, and its high-speed performance tends to be better than the design of the two-valve.
However, compared with the two valves, the design of the four-valve is not very helpful for the low-torque output at low and medium speeds. With the same displacement engine, the two-valve design will be smoother and more powerful at low and medium speeds. For example, the engine of GW250 adopts the design of two valves per cylinder. The dynamic characteristics pay more attention to the low-torque output. The maximum power is not large, only 18KW, and the high-speed performance is general.
But not all engines with four-valve design will be better than two-valve engines. This is not absolute. The engine's power size is generally closely related to displacement, cylinder number, cylinder-to-bore ratio, compression ratio, and maximum power output speed. The main factor of the valve is the last factor - the maximum power output speed. The four-valve design makes the engine more efficient in intake and exhaust, making it easier to achieve high speeds.
Simply say it! The four-valve design engine tends to breathe smoothly, and is more suitable for the needs of high-speed engines. The high-speed performance is better, but the valve noise is often larger, the structure is more complicated, the cylinder head is larger, and the cost is higher. The two-valve design engine has a low-torque response in a timely manner, which can provide better power feedback at low and medium speeds, and the engine noise is relatively small, the fuel consumption is relatively low, the structure is simple and the cost is low, but the high-speed performance is generally inferior to the four-valve. engine.
Some people may ask, since the two-valve engine is so good, it is only weak under high-speed conditions. Why not increase the diameter of the atmospheric door, so that it can not play the role of high efficiency of intake and exhaust of the four-valve? Well, the idea is good, but this will not work, the valve diameter can be increased, but at the same time the valve weight is also up, the valve is a reciprocating motion, the vibration and noise will increase, you have to find a way to solve the vibration and noise If the problem is too big, it won't pay off. The design of the engine is a ring-and-loop. It needs an overall balance. It won't be lost because it highlights a certain performance. Therefore, some high-performance engines often use light weight. Titanium alloy valves, this is the reason.
Therefore, the motorcycle engine uses several valves, which is completely the result of the comprehensive measurement of the engineer's performance according to the performance of the engine. Each has its own advantages and disadvantages. It cannot be said who is more advanced and who is behind, mainly on what model. For small-displacement vehicles or cruise princes that do not pay much attention to high-speed performance, a single-cylinder two-valve engine is sufficient. For high-performance street cars, imitation races, off-road vehicles, etc., four-valve engines are preferred.