ACT Cylinder Shutdown: Run
Engine unit,  Machine operation

ACT Cylinder Shutdown: Run

ACT Cylinder Shutdown: Run

Active cylinder deactivation, known primarily as the hood of Volkswagen vehicles (referred to as ACT in TSI), is becoming more common among competitors due to environmental restrictions that are becoming very difficult to maintain. So this is another trick, which may be a bit like Stop and Start, to avoid stopping losses. Here we are not wasting when we need a little power (a bit like a lean / stratified mixture), i.e. at relatively low speeds (1500 to 4000 rpm at 1.4 / 1.5 TSI ACT) and when the accelerator pedal is lightly loaded (light loads). Note that this range of use is about two-thirds of the route of the old NEDC cycle, so we can understand why this was interesting for the brand ... In real life, we will not enjoy it as much, except the drivers are extremely peaceful.

Cylinder shutdown principle

You will understand, the story is that some of the cylinders are no longer used to limit the need for fuel. If we feed half as much, it will only benefit!

Therefore, we, in principle, will no longer refuel some of them. But if it sounds simple, it's actually more complicated.

Indeed, then we get two cylinders that pump air at the inlet and spit it out at the outlet? We will lose performance because we will have pumping ... In addition, the deactivated cylinders will receive a dose of intake air, however, reserved for the cylinders that are running.

In short, just turning off the injection and ignition on some cylinders does not work at all, we must go further. This is when the variable cam system comes into play to change the behavior of the intake and exhaust valves. If the cylinders are no longer activated (no more ignition and no more injection), you must also remember to lock the valves so that they remain in the closed position.

ACT Cylinder Shutdown: Run

Finally, it is also necessary that the deactivated cylinders do not cause imbalance in the engine. Because if only one of the 4 masses (in the case of L4, therefore) is no longer animated (hence only one cylinder), there will be a logical appearance of vibrations.

Therefore, for this it is necessary to cut an even number of cylinders, and the cylinders, which, moreover, have symmetrically opposite cycles (when one compresses, the other relaxes, there is no need to cut two cylinders that have similar cycles). In short, the two deactivated cylinders were not chosen by chance by the engineers and it goes without saying. Volkswagen with TSI has two cylinders in the middle (out of 4 cylinders in rows 1.4 and 1.5), because they have completely opposite duty cycles.

And the last, very important thing, we cannot close the valves at random and at any time ... Indeed, if I close, for example, immediately after the intake (immediately after filling the cylinder with air), I will have a piston full of air, which will be very difficult to reassemble : causes resistance to the piston, making it very difficult to crush it to reassemble.

So the strategy is as follows: we close the valves when the cylinder is in the middle of the exhaust phase (when we expel the exhaust gases through the valve).

Thus, we will have a cylinder that is half filled with gas (so it is not too difficult to squeeze), and the valves of which will be closed. Thus, the deactivated cylinders mix the exhaust gases in their chamber.

Obviously, the two shutdown cylinders are not in this phase at the same time, so the shutdown will occur in two stages: the valves are blocked from the moment the cylinder is halfway through the exhaust phase (when it spits out half of the gas contained in them).

The cam pushes the valve, which is a classic action like any car. I didn't put the swing, but we basically don't give a damn, we forget them.

Deactivating the cylinder halfway through the exhaust gas:

Here the cam is biased to the left, so it no longer pushes the valve down to open it. We now have a cylinder that will spend its time compressing and expanding the trapped exhaust gases.

ACT Cylinder Shutdown: Run

Here in real life at TSI. Below we see two actuators and "guides" for moving the cam to the left or right.

Cylinder shutdown operation

In fact (TSI ACT motor) we have an electrical system with an actuator that deflects the valve cams (see here to understand) so that they no longer open.

When the actuator is activated, the cam is no longer in front of the valve and therefore the latter no longer lowers. Another competing system is to disable the rocker arms (intermediate part between the camshaft and the valves). Thus, this adjustable device is located only above the respective cylinders, others have a completely normal and passive "end of the camshaft" above the heads.

This way the cylinders never shut off, cylinders 2 and 3 in our example will see their valves block only from the moment they are in the exhaust phase (halfway through as above). All this is controlled by the electronics thanks to the data provided by the sensors.

ACT Cylinder Shutdown: Run

The drive (right in blue) disables one of the cylinders. The other waits for it to enter the release phase to close the valve.

ACT Cylinder Shutdown: Run

Find 4 active cylinders in the opposite direction. Here you can clearly see that the cam (highlighted in green) is offset to the left of the rocker arm. The operation here is to bring it forward to the right.

So, cutting cylinders consists of shut-off valves at the certain time, don't light (ignition of the spark plug), no more injecting fuel et modulate butterfly opening take the air needed for 2 cylinders, not 4.

Greater fuel savings?

By cutting off half the cylinders, we could hope for big savings (without hesitation, we can even say 40% at half stops). Unfortunately, no, we are in the region of 0.5 liters per 100 km ... Two disabled cylinders are still traveling back and forth, and this requires energy. The range of use of the device is also quite limited: low torque (anemic driving). In short, especially in the NEDC (or even WLTP) cycle, which requires little power, we will see the greatest savings. This will actually be less impressive, although it will largely depend on the type of use of your vehicle.

Reliability?

If the device is not really a problem yet, it should still be noted that this complexity logically leads to the possibility of additional failures. If the actuator no longer works, it can be a concern, and since nothing lasts forever ...

All comments and reactions

Dernier comment posted:

AL (Date: 2021, 05:18:10)

Hi,

I have a LEON 3, 150 hp. ACT from 2016, 80000 km and I am very happy with this system. Indeed, as was said in the previous comment, the change is almost imperceptible. There is little interest in the city or in the mountains. The 2-cylinder passage is made, in particular, by means of a liquid line. This is especially interesting on main highways or motorways and we keep 130 km / h without problems with only two cylinders. In terms of consumption, you can feel it is much more than the 2L / 0.5 that you indicated, I think. The only negative is a rattling noise at low speeds. In 100th or 3rd gear, when changing from 4 to 4 cylinders at low speeds, noise is heard, as if the engine was running at low speeds, with annoying clicking sounds. My mechanic doesn't seem to care. Can other users confirm that they have the same phenomenon?

cordially

Il I. 1 reaction (s) to this comment:

  • Administrator SITE ADMINISTRATOR (2021-05-19 11:55:47): Thanks for your feedback, which I would also like to see here.
    The noise is probably due to pumping down the output cylinders (valves closed) like in a big bicycle pump, so ... So that would be quite normal.

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