I have a large amount of useful material concerning the timing system of the engine, ranging from belt or chain timing, ending — why do we need to adjust the valve. Article really useful read. BUT I was recently asked this question – what is valve overlap? How is it regulated and is it possible to somehow all it to put? I think a very interesting topic. As usual there will be a text version of the video at the end (for those who don't want to read).
THE CONTENT OF THE ARTICLE:
- What is valve overlap?
- Balance valve timing
- OTTO cycle – MILLER
- And what about our WHA?
About the valve I was talking a lot, but here in this article and video about the phase-shifters. Yes, and in fact this concept is very simple.
What is valve overlap? This process, when both valves are open for a very short period of time. The intake opens earlier and closes the exhaust later. Usually this happens when the piston is at TDC (top dead center).
This is mostly done to the engine cylinders is better filled at medium to high rpm (low this effect is not so strongly expressed). When the revs are high then the flow of air-fuel mixture is much more it as something you need to push in the cylinders and to deflect the exhaust gases need also faster.
The overlap of the valve
Are made to overlap when the exhaust gases go into the exhaust manifold (i.e. exhaust valve open), creates a strong vacuum in the cylinders, closer to top dead center (TDC) starting to open the intake valve. Vacuum, which is in the cylinder begins to "suck" the fresh air / fuel mixture. Thus, the content is much better, that is, the air-fuel mixture supplied to the engine more, which is better for power.
Balance valve timing Now many people can say — Oh cool, need to do longer valve overlap. Why the short period? After all, the purge is better the filling the more power is growing.
But not so simple. If you take ordinary cars with old technology, where there is no phase-shifters, doing here the average values for both high and low speed.
See what's the point
If you make a large valve overlap. That is, the intake valve will be open a lot more and sooner as the actual prom. On lower rpm this engine will operate unstably or even will stall. BUT why? Yes, everything is simple — the exhaust gases can go into the intake and there mixed with the new fuel mixture, impoverishing it, because the large flow no! Thus to run the motor on the bottom will be worse. However, at high rpm scavenging will do better.
gases can get into the intake
However, if you have shifters, or one (usually the intake) or two (inlet and outlet). Then you can change phases based on your needs.
In simple words:
When the revs are low. The overlap did not exist or it is minimal, because there is no need to digest the large amount of air-fuel mixture When the rpm average or higher. Then the "shifters" can change the angle, making the phase more and more overlap also. Then purging and filling will be better. Shifters
As you can see it's very simple.
Now modern cars are at least one pathocratic (intake). This motor at high revs it turns out is more powerful and often more economical.
OTTO cycle – MILLER Now, my attentive readers, I can say – Yes, shifters, you only need to change the cycles of OTTO and MILLER (high speed and low).
However, it is not so. Really "TAZIKI" regulate the various cycles now it's OK on almost all modern engines. When at low revs there is a MILLER cycle and high cycle OTTO (go top the link and read the article and view the video all will become clear)
OTTO cycle - MILLER
And what about our WHA? As I wrote above, old engines (and Mercedes is no exception), had just an ordinary star, which was dressed either a chain or a timing belt. Now we are not talking about new and old.
As you might imagine, they were averaged values of the phases (and respectively overlap).
Production phase practically does not set up, I'm not talking about adjusting the valves. Also here will not affect sports camshafts (that's a different topic).
However our craftsmen, the garage and various tuning houses, put so-called "split gears of the camshafts". What is it? This gear, which is of two parts: the Inner part is connected with the camshaft.
Split shafts
The outer part — which is connected with a chain timing belt.
With each other they are pulled together by bolts, the holes for these bolts have a small stroke. That is, these parts may slightly rotate (a small angle) relative to each other. Thus it was possible to change the angle and the method of selection to set your desired power consumption and the operation of the engine.
BUT this is an exception to the rule, and now modern motors say, LADA VESTA already have the shifters (so that with the right valve overlap is no problem).