Old question. Sorry to beat a dead horse... (DIY cylinder deactivation)

[whatmaycome14]

I've read through no less than 10 threads about cyl deletion and also googled a mess of websites. I haven't quite gotten an answer I'm looking for yet, so here goes with my question:

IF you block off the air going into and out of a cyl, (thus trapping it and it would be compressed then decompressed as the piston moves) while disconnecting the fuel injector and spark plug, what happens? I understand that the engine runs "rougher", but I haven't found a reason why this would be bad.

It's late and I'm tired, and maybe I've read so much that I've confused myself.
Can anyone explain the effects of doing this in a simple, easy to understand, way?

[Frank Lee]

The cylinder would no longer be making power so yes the engine will seem "rougher" without that power pulse(s) and especially so because of the uneven or further apart "spacing" of the remaining pulses. However with no air flow in and out of that cylinder it incurrs no pumping losses so it being a dead cylinder isn't such a drag on the engine as a normal failed or weak cylinder i.e. one that has no/poor compression, spark, or fuel.

Deactivating cylinders reduces pumping losses two ways: 1) there are fewer working cylinders moving air around, and 2) the remaining cylinders- each now working harder to produce the same total output- require the throttle plate(s) to be open more. The more open a throttle plate is, the less the engine is working to overcome the vacuum behind it.

The reduced pumping losses from fewer working cylinders are but one component of why deactivation works; because the remaining cylinders are each working more at their capacity- getting a fuller air/fuel charge- the combustion pressures and tumble/swirl velocities are much higher and thus more efficient, in the same way we know that any ICE is more efficient at near wide open throttle than at near idle.

I don't know what your intent is with this question- just seeking to understand cylinder deactivation or perhaps you have a deac scheme in mind to try on your car? "Permanent" deac- that you can't do on the go- wouldn't be bad unless you deactivate so many cylinders that it won't start or that it loses so much power as to become hard to drive. As an experiment I deactivated two on my four cylinder and it wouldn't start, although I think it would work if I could start it on all 4 then switch 2 off at speed, as all other deac systems do (and/or have a heavier flywheel).

I then tried deactivating only one cylinder and it did start, run, and drive but the exhaust note and vibes from the missing power pulse were very, very objectionable.

Deactivating 2 or 3 out of 6, or 2 or 4 out of 8 cylinders leaves enough powered cylinders to keep things going well and reasonably in balance.

There are 4 cylinder engines with 2 deactivateable cylinders but they are far less common than 6s and 8s; I'd imagine there is a diminishing returns thing going on there with each cylinder in a 4 typically being more heavily loaded anyway.

The potential savings I've read about range from 7% "city" to 30% highway. Since I personally haven't had much experience driving a vehicle with this feature, I'm imagining that the deac protocol is much more conservative than a hypermiler would desire- we would be more content to putter around on fewer cylinders more of the time than the manufacturers dare to foist on the general, horsepower hungry public.

[ChazInMT]

Here's my understanding of cylinder deletion in a nut shell.

We delete/deactivate cylinders to reduce pumping losses. The pumping loss is reduced because we open the throttle more and less energy is required to drag air into the engine.

Only 4% of the engine loss is due to pumping loss (See This) so it isn't like you'll double your mileage by eliminating it.

So now you gotta ask yourself a few questions.

Can I live with less horsepower if I delete cylinders?

Can I delete the cylinders effectively and maintain optimal performance in the remaining cylinders? Doesn't do you much good to delete only to have your engine management system try and compensate for a running condition it was never programmed to operate under, consequently yielding worse mpg. I really believe this elephant sitting in your sandbox is greatly overlooked or never considered by peeps talking cylinder deletion.

How much time effort and energy are you going to expend doing this for what payoff? Ya really gotta consider the cost benefit analysis of all this.

Factory attempts to deactivate are better and more sophisticated, but these require deactivation on the run, which I pray you are not wanting to attempt. Even still the benefit is not huge by doing this.

I know it is sort of a stretch, but variable valve timing is actually a back door way to limit the displacement of the engine, you sort of deactivate a percentage of all the cylinders in order to reduce pumping loss. Again, factory engineered schemes here not meant for the common man to attempt to modify on your own car.

Anyway, all this is just thought exercise, I'm 99% certain no one has ever done a homemade cylinder deactivation and gotten outstanding results from it, power decreases immensely, the engine management system would hate it and need to be modified to the point of causing madness, and you may not see any gain in fuel efficiency.

Hope this helps you understand, or maybe give you other things to look into.

[Cobb]

If you do this on a newer engine you will get a check engine light for misfire. When that happens it causes the computer to do other things that is against fuel economy. On the insight it screws with the auto stop and egr usage which is suppose to reduce displacement on demand as well.

[pgfpro]

I have cylinder deactivation on my 2013 GMC Sierra Z71.
Instant readings 18 mpg disable, and 24 mpg enable.

I wish GM would of made it more usable in a broader range then they did. It's only comes in at very very light load. When activated it does help with fuel mileage. I could see it working great in areas where the terrain is very flat.

[Frank Lee]

^ +33%! That's a lot better than the OEMs dare to claim!

[Cobb]

One would think there is a way to activate that mode by a switch.

Honda has something on the v6 in the mini van that cuts off half the cylinders, but unsure of its benefits or mpg gains.

[whatmaycome14]

Originally Posted by ChazInMT

Can I live with less horsepower if I delete cylinders?

Yes, most certainly. I haven't used more than 60hp in this car as it is. That wouldn't be my concern.

Originally Posted by Cobb

If you do this on a newer engine you will get a check engine light for misfire.

Why would a check engine light be a problem? Would there actually BE a problem, or would the computer just recognize that something is "different?"

[t vago]

Originally Posted by ChazInMT

Anyway, all this is just thought exercise, I'm 99% certain no one has ever done a homemade cylinder deactivation and gotten outstanding results from it, power decreases immensely, the engine management system would hate it and need to be modified to the point of causing madness, and you may not see any gain in fuel efficiency.

Effective cylinder deactivation requires that the intake and exhaust valves be disabled. Homebrew methods involve physically removing parts of the valvetrain. There are a few members here who have actually done so, with reasonably good results, in terms of fuel economy gains.

I looked into cylinder deactivation as a method of saving fuel economy, and have come to the conclusion that it may be simpler to merely fool the engine computer into running leaner than normal, by feeding it a modified O2 sensor signal that makes it appear that the engine is running richer than it really is, and causing the engine computer to lean out the mixture to compensate.

[Frank Lee]

I ignore the CEL when I KNOW what it is- for example, running straight E85 in a non-flex fuel vehicle can trigger it. It comes and goes, depending on load, and when I next fill on E10 it goes out.