valve crossing

valve crossing

Surely you have heard of the valve crossing, or maybe you haven't and are curious to know what it is. In this article we explain everything you need to know about this very interesting concept that occurs in internal combustion engines. In addition, you will learn what it is for, the levels in which it is produced, the duration, etc.

What is valve crossing

Electronic injection system

El valve crossing, or overlapping, in the intake and exhaust valve openings, consists of the space, measured in sexagesimal degrees of crankshaft rotation, in which the exhaust and intake valves are open simultaneously. This happens at the time the piston is on its upstroke, before the intake valve is fully closed and the piston reaches TDC.

To calculate the valve crossover, add the degrees of the Admission Opening Advance (AAA) and the degrees of the Exhaust Closing Delay (RCE). In addition, it is important to remember that the greater the crossover of the valves, the better the breathing at high rpm. In other words, the gas outlet is used to generate a certain suction in the combustion chamber to help admission.

Glossary of terms

As you can see, a series of terms appear that you may not know. So I do a kind of glossary to explain them all and thus be clear about what is being talked about:

  • PMS: In reciprocating internal combustion engines that use plunger and piston, there are two dead centers, the upper one or PMS and the lower one or PMI. That is, they mark the beginning and the end of the piston's travel. TDC is therefore the dead point where the piston reaches the top of the combustion chamber.
  • AAA (Advance Opening Advance)- On four-stroke engines, the best time to open the intake valve is when the piston is at the highest point of travel or TDC. However, as the air-fuel mixture is in motion, opening the valve before TDC allows more time to enter the cylinder, achieving a better filling.
  • RCE (Delayed Closing Exhaust): Due to the inertia of the exhaust gases, they continue to exit through the exhaust valve even when the piston reaches TDC and begins the downward stroke. For this reason, the closing of the exhaust valve is delayed. That is to say, there will be a moment, as we have seen before, when the intake and exhaust (valve crossing) are open.

What is the valve crossing for?

As I mentioned in the previous paragraph, a crossover valve allows for better breathing. If it's wide, will allow the engine to reach a higher RPM, although running at low speeds will drastically reduce engine performance. That is, at low RPM it will deliver less power and torque. This can be interesting for competition vehicles that tend to work at high RPM most of the time.

On the contrary, when the valve crossing is short, it wins performance at low speeds, but loses it at high RPM (approx. from 3500 or 4000 RPM, depending on engine design). This can be interesting for vehicles that are working more constantly at low RPM and don't usually get as high RPM.

Between what two levels does the crossing of valves take place?

intake and exhaust valve, valve crossover

Said crossing of valves occurs between two known dimensions. It occurs between the AAA (Advance to the Admission Opening) and the RCE (Delay to the Closing of the Exhaust). That is the exact moment when the so-called overlap occurs.

crossing vs. duration on one camshaft

Valve overlapping necessitates a very precious camshaft design, i.e. a duration control on a camshaft so that the intake and exhaust valve are synchronized and remain open simultaneously at one point. In addition, a balance must be found between harnessing the energy of the engine and reducing emissions, something very important today.


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