Cylinder deactivation: how it works

[Steven]

I've been reading a few articles and seen some clips on cylinder deactivation, but some things are still unclear to me.

From what I understand, there's always an even number of cylinders deactivated to maintain the balance in the engine. So if we have two cylinders deactivated, they should be always on different compression cycles (so one expanding while the other is compressing). I have seen some explanations where all valves of the cylinder and kept closed for the deactivated cylinders, maintaining exhaust gases within the cylinder until re-activation.

While I do understand that the expansion in one cylinder offsets the compression of exhaust gases in the other, I'm entirely unsure as to why this is done in road engines.

I have also read some f1 articles claiming that the moving camshaft is also how Renault deactivates its cylinders, but from what I have heard, that is untrue.

To me the most logical and simple implementation of cylinder activation would be to electronically cut ignition and also prevent fuel from being injected into the cylinder. That would be easier to do (especially in f1) and prevent unnecessary compression of exhaust gases while the cylinders are deactivated.

Any thoughts?

[radosav]

does compressed air from deactivated cylinders go through exhaust and makes some sort of cold blowing?

[xpensive]

Don't understand much about that really, but I can recall watching races live when traction-control was allowed and was governed by something similar, turning off ignition cylinder by cylinder, the sheer sound of it turned my stomach.

[flynfrog]

I think they are simply cutting the fuel injectors to the dead cylinders. Some displacement on demand road cars cut off cylinders by cutting oil to the hydraulic lifters causing them to collapse and keeping the valves closed to minimize pumping losses.

[xpensive]

I seem to remember that cutting off injection was way too slow for traction-control in F1, while Minardi actually had a system in 1993 of using the rear brakes which was waaaay too slow and pathetically ineffective?

[Steven]

I think they are simply cutting the fuel injectors to the dead cylinders. Some displacement on demand road cars cut off cylinders by cutting oil to the hydraulic lifters causing them to collapse and keeping the valves closed to minimize pumping losses.

Well I find that kind of strange, because I would intuitively think that allowing air to flow in and out of the cylinders would create less stress on the engine while also requiring almost zero lost power.

Here is btw Audi's explanation of how they do it on their engines:
[youtube]http://www.youtube.com/watch?v=KHWvaDQ-mb0[/youtube]

[xpensive]

In the perfect world with individual valve-timing you would simply keep the the deactivated cylinder's valves wide open?

[strad]

In the perfect world with individual valve-timing you would simply keep the the deactivated cylinder's valves wide open?

EEEK! Many of today's engines are zero tolerance and the valves would hit the pistons and would definitely happen in a race engine.

[Steven]

All valves open at the same time would have further complications that could be detrimental for an engine I think.

First of all, you'd get constantly changing pressures in the exhaust and intake manifolds. One other problem would be that exhaust gases could end up in the intake manifold, hence when ingested by another "active" cylinder you'd get less efficient burn.

[xpensive]

In the perfect world with individual valve-timing you would simply keep the the deactivated cylinder's valves wide open?

EEEK! Many of today's engines are zero tolerance and the valves would hit the pistons and would definitely happen in a race engine.

That...is a thought of course, but with electromagnetic valves I guess you could make the valves follow the piston's position?

[flynfrog]

I seem to remember that cutting off injection was way too slow for traction-control in F1, while Minardi actually had a system in 1993 of using the rear brakes which was waaaay too slow and pathetically ineffective?

How would cutting injection be to slow if you have individual injectors per Cyl? Wouldn't you just modify the pulse width for certain injectors. They maybe me cutting or retarding spark also.

My Old cars version of traction control was to use the rear brakes and retard ignition. After a few "brisk" Launches it would start to overheat the brakes.

[flynfrog]

In the perfect world with individual valve-timing you would simply keep the the deactivated cylinder's valves wide open?

I think you would get more losses from pumping than you would by holding the valves closed. Also the problems that Tomba mentioned.

[strad]

Yep..it would be breathing out through the intake.

[flynfrog]

From http://en.wikipedia.org/wiki/Active_Fuel_Management

The Pumping Loss

A so-called "pumping loss" is also cited as causing extra work for the engine to do under partial load conditions because the pistons have to work harder to suck in the fuel-air mixture when the throttle position results in low intake-manifold pressure. Under this scenario, deactivating some of the cylinders allows the remaining active ones to have less manifold vacuum to overcome during the intake stroke.
[edit]How it's done

In order to deactivate a cylinder, the exhaust valve is prevented from opening after the power stroke and the exhaust gas charge is retained in the cylinder and compressed during the exhaust stroke. Following the exhaust stroke, the intake valve is prevented from opening. The exhaust gas in the cylinder is expanded and compressed over and over again and acts like a gas spring. As multiple cylinders are shut off at a time (cylinders 1, 4, 6 and 7 for a V8), the power required for compression of the exhaust gas in one cylinder is countered by the decompression of retained exhaust gas in another. When more power is called for, the exhaust valve is reactivated and the old exhaust gas is expelled during the exhaust stroke. The intake valve is likewise reactivated and normal engine operation is resumed. The net effect of cylinder deactivation is an improvement in fuel economy and likewise a reduction in exhaust emissions. General Motors was the first to modify existing, production engines to enable cylinder deactivation, with the introduction of the Cadillac L62 "V8-6-4" in 1981.

[DaveKillens]

It's possible there could be more than one mode for cylinder deactivation. There could be one mode to lessen torque yet provide lots of exhaust gasses. But another mode may be for when the car is sitting and waiting, as in the scenario of the starting line, or waiting in line during qualifying. For those circumstances, cooling is an issue and deactivating cylinders can promote cooling and delay the inevitable melt-down long enough to get going again.

For the cars starting on the front row, they have to sit waiting for what to them is an eternity, and the longer you sit and wait, that's bad news to the engine guys. So if alternate cylinders go through the normal Otto cycle, then no spark, just cooling air pumped through the cylinders, that can keep the temperature from rising too quickly. Alternate and cycle through the cylinders, the cooling effect is functioning for all the cylinders.