What is continuously variable valve timing?

Honda is the first to introduce a VTEC engine that can change the valve stroke and timing. As for how it changed the itinerary, I won't talk about it here for the time being. However, if the timing and performance of valves can be changed automatically as needed, it can just make up for the deficiency of multi-valve engine at low speed. It is easy to imagine that when the engine is at a low speed, the intake area can be reduced by using a small valve stroke, which is equivalent to the state of calm breathing and can improve the intake and exhaust efficiency at a low speed; When the engine is at a high speed, if a large valve stroke is adopted, it is equivalent to increasing the intake area, which is equivalent to increasing the intake flow at a high speed, just like people need to breathe with their mouths when they are exercising vigorously. Therefore, by changing the valve stroke, we can obtain good performance at both high speed and low speed. Now domestic engines of Guangben and Dongfeng Honda are equipped with VTEC system. However, VTEC is not absolutely good.

Due to the limitation of structure, it is difficult to realize stepless adjustment of valve stroke. The early VTEC had only two levels of adjustment, and now the latest VTEC has only three levels of adjustment. However, when the valve is switched from one stroke to another, the output of the engine is not linear, that is, it is not soft, because the intake air flow suddenly increases, and the output of the engine also suddenly increases. The VTEC engine often feels sudden back thrust when accelerating, which greatly improves the driving pleasure, but it is not suitable for a stable and comfortable RV.

Of course, if we want to realize linear power output, we need to make greater efforts in technology to realize stepless adjustment of valves. But this was achieved by BMW. BMW's VALTRONIC is a system with stepless adjustment of valve stroke. Because the stroke can be infinitely adjusted, this means that the engine can maintain the best intake and exhaust efficiency at any speed. Moreover, after stepless adjustment of intake air volume is realized, the setting of throttle valve can be cancelled. Because of the ordinary engine, there is a butterfly throttle valve in the intake pipe, which is connected with the throttle valve to adjust the intake air, thus controlling the engine speed. Because all the intake air flows through this throttle valve, the throttle valve will produce certain air resistance and reduce the intake efficiency. Therefore, the biggest benefit brought by canceling the throttle is to further improve the intake efficiency. It can be seen that BMW's VALTRONIC is the highest level of variable valve technology at present.

Because VTEC changes the valve stroke by selecting different cams, the cam at low speed not only has a short stroke, but also has a short valve opening time (related to the shape of the cam). On the contrary, the shape of the high-speed cam can make the valve open for a longer time, thus achieving the purpose of changing the valve timing, that is, changing the valve timing. However, just like the variable stroke of VTEC, it can only realize piecewise adjustable timing. Because its timing changes depend on the use of different cams, at present VTEC can only switch three groups of cams at most, that is, it can be adjusted in three stages. However, in contrast, Toyota's approach is more sensible. Toyota VVT-I is a set of mechanism with continuously variable valve timing function, which can realize stepless adjustment of valve timing. Then the benefits are self-evident, it can perfectly match the valve timing with the engine speed and working conditions. This makes the power output of VVT-I engine very linear and not as abrupt as VTEC. The comfort and stability of engine operation are also greatly improved.

In addition, there is a final means to improve the intake efficiency, which is to use intake pipes with different lengths to obtain more intake energy. This system is not complicated, it is called variable intake manifold system. The familiar engines of Mercedes-Benz, BMW and Audi are all equipped with this system, and the domestic Fox engine also has this function. So how does it improve the intake efficiency? In fact, the reason is very simple, that is, using the principle of * * * vibration.