VE at high RPMs

The VE peaks as the RPM climbs and then drops off again at high RPM. Why does the VE drop at high RPMs? Considering you're also at WOT at high RPMs, the engine cycle would only be faster but same amount of air is available as long as you're at WOT, no?

Hello Christian,

Cylinder filling is different at various engine speed. Theoratically, yes, you have the same amount of air available, but with increasing RPMs you give less time for the mixture to enter the cylinder. Therefore, air velocity is greater and there could be "choke" points in the engine (valves, intake runners, air filter, turbo sizing...) that could affect engine efficiency as well as other factor like valve overlap for exemple.

Finally, OEM stock engines are designed with compromise so the engine can work at a broad rev and load range. Camshafts design, variable valve lift and timing are great tools for them to increase the range of operation.

This one is very easy to realise. For example let's say you have an engine with 0.5 L cylinders which you need to fill out with air. Each cylinder has X amount of total diameter from intake valve seats throats for air to go through. At WOT the absolute air pressure would be close to 1 Bar for NA engine. Now, at 3000 RPM through that X total diameter of valve seats throats with 1 Bar pressure you have 0.04 sec between cycles to fill out 0.5 L cylinder with air. At 6000 RPM you have only 0.02 sec between cycles to fill out the same 0.5L cylinder through the same valve seats throats diameter and the same 1 Bar pressure. So at what RPM the efficiency of cylinder filling(VE) is better - in which case you can put more air in that cylinder?

I think the answer is obvious. In reality the efficiency of cylinder filling at high RPM is also affected by air inertia as it takes some time for the column of air to start moving so it will hurt VE as well since the time available between cycles is very short - any delay in cylinder filling will result in less air getting inside the cylinder.

If you compare the torque curve of an engine with the VE table, you will see that they typically follow the same shape. This is as Shota said, Peak torque happens when the engine is getting the peak filling of the cylinder, and then tails off before and after that point.