I have a lot of useful material on my site regarding the engine timing system, from the belt or timing chain to why you need to adjust the valves. Read the articles really useful. But recently I was asked such a question - and what is a valve closure? How is it regulated and is it possible to set it all up? What I think is a very interesting topic.
I've talked a lot about valve closure, although in this article I've also talked about phase-regulators. And in fact, this concept is very simple.
What is a valve closure?
It is a process where both valves are open for a very short period of time. The inlet opens earlier and the outlet closes later. This is usually the case when the piston is in the PMT (upper dead center).
This is mainly done to ensure that the engine cylinders are better filled at medium to high rpm (this effect is not so pronounced at low rpm). When the revolutions are high then and the flow of fuel-air is much higher, it needs to be somehow pushed into the cylinders, but the exhaust gases also need to be removed more quickly.
That's what the overlap is for - when the exhaust gases come out into the exhaust manifold (i.e. the exhaust valves are open), a strong discharge in the cylinders is created, and the inlet valve begins to open slightly closer to the upper dead center (TDC). The discharge in the cylinders begins to "suck in" the fresh fuel-air mixture. Thus, the filling process is much better, i.e. the air-fuel mixture is fed into the engine more, which has a better effect on the power.
Balance of gas distribution phases
Now a lot of people can say, "Well, cool, we need to do more valve closure. Why a short interval? After all, the blowing is better, the filling rate is higher, the capacity is growing.
But not everything is that simple. If you take ordinary cars with old technologies, where there are no phase-regulators, then the average values are made here - both for high and low revolutions.
Look what the point is:
If you make a large valve closure. That is, the inlet valves will open much larger and earlier than the actual outlet valves. This means that at low rpm the engine will run unstable or even deaf. But why? It's simple - the exhaust gases will be able to enter the inlet and mix with the new fuel mixture, impoverishing it, because there is no big flow! Thus, the engine will work at the bottom will be worse. However, at high speeds, blowing is really better.
However, if you have phase shifters, either one (usually at the inlet) or two (inlet and outlet). Then you can change the phases according to your needs.
In simple words:
When the speed is low. There is no overlapping at all, or it is minimal because there is no need to digest large amounts of fuel-air mixture
When the revolutions are medium or even higher. Then the "phase shifters" can change the angle, making the phases bigger, and the overlap is also bigger. Then the blowing and filling will be better.
As you can see everything is very simple.
Nowadays modern foreign cars come with at least one at the entrance. This motor at high revolutions turns out more powerful and often more economical.
Otto - Miller cycle
Now my attentive readers can say - yes, it is necessary only to change cycles of Otto and Miller (on high revolutions and on low ones).
However, it is not quite so. Indeed, "Phasic" regulates various cycles now it is normal, almost on all modern engines. When there is a cycle of Miller at low revolutions and a high cycle of OTTO (go to the top of the link and read the article, and watch the video - everything will become clear).
But also for overlapping of valves "Rotators," it is simply a necessary thing, and automatic which works very well.
And what about other cars?
As I wrote from above, the old motors, had just an ordinary star, which was dressed either chain or belt GRM. Now we are not talking about new ones, but about old ones.
As you can guess, they had average phase values (and overlaps).
The factory method of phase adjustment was practically not adjusted, I am now silent about the valves adjustment. Also, the sports camshafts will not be affected here (this is a bit different).
However, our craftsmen, in the garage and various tuning studios, put the so-called "splitting gears of camshafts". What is this? It is a pinion, which is made up of two parts: the inner part is connected to the camshaft.
The outer part - which is connected to the chain - is connected to the timing belt.
They are bolted together with each other, the holes for these bolts have a small stroke. That is, these parts can rotate slightly (by a small angle) relative to each other. In this way, it was possible to change the angle and to set the required power, flow rate and motor operation by the selection method.