Uneven cylinder wear with DOD???

[Dolomite]

With DOD, will cylinders and respective main bearings wear evenly over time?

[p71]

yes

[Aaron407]

With the DOD system I believe it still runs on all 8 cylinders for about a minute after 10 minutes in 4 cylinder mode to keep the other 4 wearing effectively. Plus, with the pistons in the inactive cylinders still moving while in 4 cylinder mode, I would think the wear effect would be pretty much the same on all 8.

[Sheffield Steve]

I don't think it makes much difference. I would expect the NONE DOD cylinders to wear slightly faster than the others.

[Steve]

The valves 'stop working' on those 4 , but the actual piston still moves up and down so as far as wear to the actual cylinder/piston/sleeve etc it'll be the same all around.

[supercharger]

The only parts that would get less wear are the valves and springs.

I wouldn't worry about it. My concern is the DOD valve/solenoid module that resides in the lifter valley. Don't know much about it but it's in there, and it can and will fail in some cars sooner or later.

If mine fails out of warranty it'll be my excuse for a non-dod cam and some headers :)

[Crazy Paul]

Here's a very old article which brushes on the question raised above.
http://www.popularhotrodding.com/features/0405phr_gm_engine_technology/index.html

Chris Meagher, assistant chief engineer of small-block truck engines at GM.
"Before our interview ended, we asked one final question of Meagher: With four cylinders working the entire life of the engine and four cylinders working for approximately half that time, is there any extra maintenance or any deviation of maintenance from a normal V-8? To that Meagher says: "The service life of the engine will be the same as normal current engines. There are a couple of reasons for not making the service requirement any different for these four cylinders. One would be to avoid any confusion; the second, quite frankly, is that it's not necessary."

[hartigan85]

last thing i heard about DoD is that when in 4 cylinder mode the computer cuts off Fuel to 4 cylinders, hence the fuel saving benefits. If the valves stopped opening then i would assume this would hurt fuel economy as the engine would be compressing the cylinders twice instead of once for every 4 strokes. here is example of how i think it works, first cylinders 1, 3, 5, 7 would fire then next cycle 2, 4, 6, 8 cylinders would fire this way none of the cylinders have uneven wear.

[todds87ss]

last thing i heard about DoD is that when in 4 cylinder mode the computer cuts off Fuel to 4 cylinders, hence the fuel saving benefits. If the valves stopped opening then i would assume this would hurt fuel economy as the engine would be compressing the cylinders twice instead of once for every 4 strokes. here is example of how i think it works, first cylinders 1, 3, 5, 7 would fire then next cycle 2, 4, 6, 8 cylinders would fire this way none of the cylinders have uneven wear.

ummm, no. If the valves continue to operate normally, you would be fighting compression and reducing effeciency. There is special DoD hardware (valves, lifters, etc) in this motor to accomodate this, not just a simple turning off of the fuel.
The OP makes a point, though, as the most severe stress on a motor is one at high RPM under no load (piston moving rapidly without opposition so-to-speak). With some pistons under load and some not, you would think that the stress component would change considerably, and wear would not be even. In higher RPM turbine applications, this type of phenomona does indeed cause radical stress and vibration, and can necessitate immediate removal from service.
But if GM says it's OK....

[Blackrider]

last thing i heard about DoD is that when in 4 cylinder mode the computer cuts off Fuel to 4 cylinders, hence the fuel saving benefits. If the valves stopped opening then i would assume this would hurt fuel economy as the engine would be compressing the cylinders twice instead of once for every 4 strokes. here is example of how i think it works, first cylinders 1, 3, 5, 7 would fire then next cycle 2, 4, 6, 8 cylinders would fire this way none of the cylinders have uneven wear.

No thats not how it works. There are only 4 cylinders fitting with the AFM hardware, 1,4,6,7 if I remember correctly. When AFM activates the valves for those 4 cylinders are disabled on the exhaust stroke keeping the exhaust from the last active burn in the cylinder, using it as an air spring of sorts. When AFM is deactivated its again on the exhaust stroke to force out the gas and start the 4 stroke cycle again.

I think in the end the wear will be a little different, sure the AFM cylinders are still moving in AFM mode but there is no combustion force acting on them. This will change how the pistons travel through the bore. Is it going to be noticeable enough for it to be a concern? I don't think so, and apparently nether does GM.

[p71]

last thing i heard about DoD is that when in 4 cylinder mode the computer cuts off Fuel to 4 cylinders, hence the fuel saving benefits. If the valves stopped opening then i would assume this would hurt fuel economy as the engine would be compressing the cylinders twice instead of once for every 4 strokes. here is example of how i think it works, first cylinders 1, 3, 5, 7 would fire then next cycle 2, 4, 6, 8 cylinders would fire this way none of the cylinders have uneven wear.

comes in makes one wildly inaccurate comment and leaves....

typical

[Slizzo]

Just because it makes sense logically, doesn't mean it makes sense engineering wise. :)

[hartigan85]

Sorry about my "wildly inaccurate comment" i guess i was thinking of it too logically. I've never done any real research on it, but i guess that's why you guys are here.

[DRCUSTOMPARTS]

Cylinder Deactivation (Active Fuel Management) System Description

To provide maximum fuel economy under light load driving conditions, the engine control module (ECM) will command the cylinder deactivation system ON to deactivate engine cylinders 1 and 7 on the left bank, and cylinders 4 and 6 on the right bank, switching to a V4 mode. The engine will operate on 8 cylinders, or V8 mode, during engine starting, engine idling, and medium to heavy throttle applications.

When commanded ON, the ECM will determine what cylinder is firing, and begin deactivation on the next closest deactivated cylinder in firing order sequence. The Gen IV engine has a firing order of 1-8-7-2-6-5-4-3. If cylinder number 1 is on its combustion event when cylinder deactivation is commanded ON, the next cylinder in the firing order sequence that can be deactivated is cylinder number 7. If cylinder number 5 is on its combustion event when cylinder deactivation is commanded ON, then the next cylinder in the firing order sequence that can be deactivated is cylinder number 4.

Cylinder deactivation is accomplished by not allowing the intake and exhaust valves to open on the selected cylinders by using special valve lifters. The deactivation lifters contain spring loaded locking pins that connect the internal pin housing of the lifter to the outer housing. The pin housing contains the lifter plunger and pushrod seat which interfaces with the pushrod. The outer housing contacts the camshaft lobe through a roller. During V8 mode, the locking pins are pushed outward by spring force, locking the pin housing and outer housing together causing the lifter to function as a normal lifter. When V4 mode is commanded ON, the locking pins are pushed inward with engine oil pressure directed from the valve lifter oil manifold (VLOM) assembly solenoids. When the lifter pin housing is unlocked from the outer housing, the internal pin housing will remain stationary, while the outer housing will move with the profile of the camshaft lobe, which results in the valve remaining closed. One VLOM solenoid controls both the intake and exhaust valves for each deactivating cylinder. There are 2 distinct oil passages going to each cylinder deactivation lifter bore, one for the hydraulic lash-adjusting feature of the lifter, and one for controlling the locking pins used for cylinder deactivation.

Although both intake and exhaust valve lifters are controlled by the same solenoid in the VLOM, the intake and exhaust valves do not become deactivated at the same time. Cylinder deactivation is timed so that the cylinder is on an intake event. During an intake event, the intake cam lobe is pushing the valve lifter upwards to open the intake valve against the force of the valve spring. The force exerted by the valve spring is acting on the side of the lifter locking pins, preventing them from moving until the intake valve has closed. When the intake valve lifter reaches the base circle of the camshaft lobe, the valve spring force is reduced, allowing the locking pins to move, deactivating the intake valve. However, when cylinder deactivation is commanded ON, the exhaust valve for the deactivated cylinder is in the closed position, allowing the locking pins on the valve lifter to move immediately, and deactivate the exhaust valve.

By deactivating the exhaust valve first, this allows the capture of a burnt air/fuel charge or exhaust gas charge in the combustion chamber. The capture of exhaust gases in the combustion chamber will contribute to a reduction in oil consumption, noise and vibration levels, and exhaust emissions when operating in V4 mode. During the transition from V8 to V4 mode, the fuel injectors will be turned OFF on the deactivated cylinders. The ignition system secondary voltage or spark is still present across the spark plug electrodes on the deactivated cylinders. If all enabling conditions are met and maintained for cylinder deactivation operation, the ECM calibrations will limit cylinder deactivation to a cycle time of 10 minutes in V4 mode, and then return to V8 mode for 1 minute.

Switching between V8 and V4 mode is accomplished in less than 250 milliseconds, making the transitions seamless and transparent to the vehicle operator. The 250 milliseconds includes the time for the ECM to sequence the transitions, the response time for the VLOM solenoids to energize, and the time for the valve lifters to deactivate, all within 2 revolutions of the engine crankshaft.

[hartigan85]

Thanks for the explanation. The only thing that seems weird to me is how both valves are fully closed so its like having a 4 cylinder with another 4 cylinders being compressed. i would think i would makes more sense to have the exhaust valve open all the time on the deactivated cylinders to decrease the drag on the engine making it more efficient. Then again I'm sure there is a reason why they didn't do it this way.

[p71]

All the energy that goes into compressing the air in the cylinders on the upstroke is returned on the downstroke.

GM described it as an "air spring".

The thing that cylinder deactivation is stopping is "pumping losses"... the pumping of air into and out of the cylinders. Leaving the exhaust valves over would have the engine pump exhaust in and out of the cylinders that were deactivated though that valve... there would still be pumping losses, there would be no air spring and you would have cylinders full of exhaust that would require two (I believe) extra crank rotations before they can start working again.

[hartigan85]

I've never thought of the cylinder being used as an air spring, makes more sense now. thanks

[p71]

De nada...

trust me when I was first reading up on it I thought the same thing as you... once I saw air spring it clicked.

[R.Penguin]

Cylinder Deactivation (Active Fuel Management) System Description

To provide maximum fuel economy under light load driving conditions, the engine control module (ECM) will command the cylinder deactivation system ON to deactivate engine cylinders 1 and 7 on the left bank, and cylinders 4 and 6 on the right bank, switching to a V4 mode. The engine will operate on 8 cylinders, or V8 mode, during engine starting, engine idling, and medium to heavy throttle applications.

When commanded ON, the ECM will determine what cylinder is firing, and begin deactivation on the next closest deactivated cylinder in firing order sequence. The Gen IV engine has a firing order of 1-8-7-2-6-5-4-3. If cylinder number 1 is on its combustion event when cylinder deactivation is commanded ON, the next cylinder in the firing order sequence that can be deactivated is cylinder number 7. If cylinder number 5 is on its combustion event when cylinder deactivation is commanded ON, then the next cylinder in the firing order sequence that can be deactivated is cylinder number 4.

Cylinder deactivation is accomplished by not allowing the intake and exhaust valves to open on the selected cylinders by using special valve lifters. The deactivation lifters contain spring loaded locking pins that connect the internal pin housing of the lifter to the outer housing. The pin housing contains the lifter plunger and pushrod seat which interfaces with the pushrod. The outer housing contacts the camshaft lobe through a roller. During V8 mode, the locking pins are pushed outward by spring force, locking the pin housing and outer housing together causing the lifter to function as a normal lifter. When V4 mode is commanded ON, the locking pins are pushed inward with engine oil pressure directed from the valve lifter oil manifold (VLOM) assembly solenoids. When the lifter pin housing is unlocked from the outer housing, the internal pin housing will remain stationary, while the outer housing will move with the profile of the camshaft lobe, which results in the valve remaining closed. One VLOM solenoid controls both the intake and exhaust valves for each deactivating cylinder. There are 2 distinct oil passages going to each cylinder deactivation lifter bore, one for the hydraulic lash-adjusting feature of the lifter, and one for controlling the locking pins used for cylinder deactivation.

Although both intake and exhaust valve lifters are controlled by the same solenoid in the VLOM, the intake and exhaust valves do not become deactivated at the same time. Cylinder deactivation is timed so that the cylinder is on an intake event. During an intake event, the intake cam lobe is pushing the valve lifter upwards to open the intake valve against the force of the valve spring. The force exerted by the valve spring is acting on the side of the lifter locking pins, preventing them from moving until the intake valve has closed. When the intake valve lifter reaches the base circle of the camshaft lobe, the valve spring force is reduced, allowing the locking pins to move, deactivating the intake valve. However, when cylinder deactivation is commanded ON, the exhaust valve for the deactivated cylinder is in the closed position, allowing the locking pins on the valve lifter to move immediately, and deactivate the exhaust valve.

By deactivating the exhaust valve first, this allows the capture of a burnt air/fuel charge or exhaust gas charge in the combustion chamber. The capture of exhaust gases in the combustion chamber will contribute to a reduction in oil consumption, noise and vibration levels, and exhaust emissions when operating in V4 mode. During the transition from V8 to V4 mode, the fuel injectors will be turned OFF on the deactivated cylinders. The ignition system secondary voltage or spark is still present across the spark plug electrodes on the deactivated cylinders. If all enabling conditions are met and maintained for cylinder deactivation operation, the ECM calibrations will limit cylinder deactivation to a cycle time of 10 minutes in V4 mode, and then return to V8 mode for 1 minute.

Switching between V8 and V4 mode is accomplished in less than 250 milliseconds, making the transitions seamless and transparent to the vehicle operator. The 250 milliseconds includes the time for the ECM to sequence the transitions, the response time for the VLOM solenoids to energize, and the time for the valve lifters to deactivate, all within 2 revolutions of the engine crankshaft.
Exactly........................................... .................................................. :p