Fuel saving technologies

Displacement On Demand   

Mark Lemay

GM pioneered the technology of deactivating cylinders with the 1981 Cadillac V8 engine (4.6 L). The system used electrical solenoids to disengage the rocker arm assemblies, having the engine operating on 8 cyl mode to accelerate, 6 cyl on light accelerations and 4 cyl mode for cruising. It was very erratic and unreliable, and this is why it disappeared the year after.

With modern technology and computers that outperform 100 times the ones in the 80s,  it’s a completely different game. Today’s DOD system provides seamless operation thanks to engine management systems like throttle by wire systems and electronically controlled transmissions, to name a few.

How it works

The DOD system deactivates cylinders while the electronic throttle is adjusted to maintain the current torque levels and road speed. In some cases an instrument warning light tells the driver that the system is active, otherwise it would go unnoticed. GM, Honda and Chrysler are using it, and the terminology may vary from one manufacturer to another. Of course the direct benefits are better fuel economy (8 to 20%) available on large engines.

Getting technical

On pushrod engines special valve lifters are used while on overhead cam ones the design of the rocker arms is modified. In fact, the valve train modification allows the valve action to be disengaged.

In illustration no. 1, under normal conditions, the locking pins in red are spring loaded to lock the pin housing and the outer housing in blue together, acting as a normal lifter. In illustration no. 2, when activated by an electric solenoid, oil pressure forces the locking pins into the center of the pin housing and the outer housing follows the camshaft, but the pin housing and lifter don’t move. The top of the pin housing holds a normal lash adjuster style lifter.

In illustration no. 3 we have a partial view of the oil galleries and oil flow. On GM DOD systems, the firing order is different (1-8-7-2-6-5-4-3), and companion cylinders are deactivated one after the other (1,7) and (4,6) at very precise times.

Deactivation mode

Deactivation always occurs at the beginning of the compression stroke in order to trap a spent charge in the combustion chamber, thus reducing oil consumption. To accomplish this, both lifters are activated at the same time through the solenoid, but the valves are deactivated at different times mechanically when locking pins match together.

When the conditions are suitable, the PCM will switch the engine over to V4 mode, shutting off cylinders not as a group, but rather one at a time following the firing order. At the same time, the correspondent injector is turned off at the intake stroke while the ignition system remains on.

Disabling DOD

The DOD will be disabled under certain conditions and safety considerations. Low manifold vacuum, APP errors, low ignition voltage, oil temperature or pressure or RPM out of range and many many more.

Critical maintenance

Oil pressure and oil cleanliness are essential for correct system operation. For this reason, oil pressure to the intake
oil manifold is monitored for pressure and aeration, and any fault is reported to the PCM. This means that regular maintenance is critical for an appropriate system operation.


Monitoring DOD systems

The DOD system must be able to detect faults if deactivation or reactivation fails. In this case both the misfire and O2 monitors are used to determine if the system is working. The DOD is also monitored for electrical faults.

Here (illustration 4), we can compare graphics between a normal deflection (top) in the CKP signal and an engine with disabled cylinders (bottom).

On illustration 5, we can see both front O2 sensors. When the system goes active there is a slight change in O2 operation. The change only lasts for a second and then the O2 sensors return to normal operation.
 
Diagnostics

To test DOD circuits and trouble shoot any problem, we can use a combination of instruments according to the defect encountered. Multimeters are useful to check voltage and current values at different locations, and a good scope or engine analyser will help greatly to go deeper into mode cycling and patterns, retrieving code faults to guide you the source of the problem.

The Control Module uses a comparator circuit to determine if there is a circuit fault, and then sets the appropriate code that can be read with the san tool or analyser.

Mechanical problems

The intake manifold on DOD systems has a special design, and if it is found defective or you need to remove it to access a bad lifter, read carefully the manufacturer’s manual first. In many cases special testers are needed also, and this goes also for any suspicious oil pressure problem. You should also consider regular training to get acquainted with these systems, because there are more on the way as you certainly know.