Displacement on Demand question:

[Boyd]

If the manufacturers have the technology to shut down cylinders, why is it limited to V-8's, and why do they limit the maximum # of shut down cylinders to 4? It seems to me that they could shut down up to 6cyls on a V8, or 4cyl on a V6 - leaving one cylinder per bank for balance purposes. Mileage could be greatly increased on a V8 by shutting down 4 cylinders for city driving, then 6 cylinders for highway cruising.

And for us enthusiasts, there could be a "full-performance" switch that activates all 8 cylinders full-time.

[oestek]

I don't know if a V2 would push an Escalade... maybe the Mast guys know.

[dannyho]

Mitsubishi's mivec v6 used to shut down cylinders. I'm not sure if it applies to the ones now, but in the 90's they had em over in japan. not here

[sik68]

Chevrolet's variable displacement V8's go down to 4. Honda's v6's go down to 3!

http://en.wikipedia.org/wiki/Variable_Cylinder_Management

[Kenova]

When GM introduced their current DOD engines, I seem to remember them saying they by-passed the 6 cyl. mode because of balancing issues.
Since these engines (V8s) are used mostly in trucks, they need to generate enough torque to keep the truck in motion when the engine is in DOD mode. I believe that is why they don't have DOD in the 4.8s. I would think if these engines were used in a light weight car, they would spend more time in the DOD mode.

Ken

[68sixspeed]

I do have to say, the DOD works, I have a 07 z71 tahoe, if you drive nice (68mph) I've seen 21.8mpg; get on it to 72-75, it drops to 18. The DOD is smooth, you can't feel it come in/out, if it wasn't for the dash thing you'd never know it. It pretty much works on the level and downhill only though, any load and it must be more efficent in v8 mode.

[dstryr]

I do have to say, the DOD works, I have a 07 z71 tahoe, if you drive nice (68mph) I've seen 21.8mpg; get on it to 72-75, it drops to 18. The DOD is smooth, you can't feel it come in/out, if it wasn't for the dash thing you'd never know it. It pretty much works on the level and downhill only though, any load and it must be more efficent in v8 mode.

Displacement on demand and like a 6 speed automatic for cruising speeds you should be able to get 25-30mpg on the freeway.

[rb70383]

So do the "dead" cylinders drop fuel and spark, valves still operate? No damage to those cylinders over a long period of time?
Seems like a useful concept.

[tumper93]

The new 3.5l in the impalas do drop to 3 cylinders and runs an inline 3 cylinder basically cutting one bank completely. DOD works by controlling the lifters and the spark/fuel

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.
The cylinder deactivation system consists of the following components:

• The VLOM assembly
• Eight special valve lifters, 2 per deactivating cylinder
• The engine oil pressure regulator valve for cylinder deactivation operation
• Gen IV cylinder deactivation engine block
• The ECM