by Jim Kerr

It’s finally here. Nearly three years after GM started talking about Displacement on Demand, the first vehicles are hitting the streets with this fuel saving technology. Chrysler actually beat GM to the punch by offering their MDS (Multi-Displacement System) in the Hemi-powered Chrysler 300C and Dodge Magnum RT. Although not interchangeable, both the GM and Chrysler systems utilize components manufactured by Eaton Corporation to cut out four cylinders on the V8 engine and optimize fuel economy.

2005 GMC Envoy XL. Click image to enlarge

While Chrysler has started with the passenger car market, GM’s first production vehicle with Displacement on Demand (DOD) is the 2005 GMC Envoy XL equipped with the 5.3 litre V8 engine. DOD systems will appear in other GM SUVs and pickups soon. Initial plans are to produce more than 150,000 DOD V8’s in the first year and increase production to nearly 1.5 million DOD engines in 2007. GM estimates fuel economy savings of 8% when driven on the standard fuel economy test routine and up to 25% increased economy for some driving conditions. Multiply that by 1.5 million vehicles and the fuel savings are gigantic.

So how does it work and how good is it? DOD uses the power of today’s 32-bit engine computer to control valve deactivation, throttle control, injection control and spark advance in an integrated sequence so that the switch between four and eight cylinders is smooth. While GM tried a simplified DOD system on the 1981 Cadillac 4-6-8 engine, the computing power simply wasn’t enough at the time. The current engine computer is roughly 25 times faster, has 50 times the computing power and 100 times the memory of the 1981 system.

To switch the engine from 8 cylinder to 4 cylinder operation, the computer operates four solenoids that control oil flow to special hydraulic lifters for the intake and exhaust valves for cylinders number 1, 7 4 and 6. Developed by Eaton Corporation, the lifter is designed so that one section can collapse, or telescope, into the other section. The two sections can be either coupled or uncoupled to each other by means of a locking pin. When coupled, the lifter can transfer the lift of the camshaft to the rest of the valve train. When uncoupled, the lifter acts like a spring and the valve train doesn’t move, stopping that cylinder from producing power.

Displacement on Demand. Click image to enlarge

Hydraulic oil pressure, supplied by the engine oil pump and controlled by computer-activated solenoids, is used to dislodge the locking pin and collapse the lifter, thus closing the valve. In reactivation mode, removing hydraulic pressure causes the locking pin to return to its latched position to restore the lifter’s normal function. The computer stops valve operation for all four of the cylinders within one engine cycle or two revolutions of the crankshaft.

Synchronizing the throttle opening, fuel injector control and spark advance with the valve deactivation is the difficult part and GM has mastered the programming for this critical sequence. I could not tell when the engine was switching between 8 cylinder and 4 cylinder operation. There are no indicators on the GMC Envoy to indicate it has the DOD system nor any visual indication to the driver that the engine is running or 4 or 8 cylinders. I did notice that the average fuel economy readouts on the trip computer did keep improving as I was cruising or coasting. Sit at a stoplight or accelerate and economy decreased.

During start-up and idle, the Vortec V8 runs on all 8 cylinders for smooth operation. During acceleration, all 8 cylinders provide power but the system switches to 4 cylinder operation during light throttle cruise or deceleration. I hooked up a computer scan tool that GM technicians use for diagnostics and checked out the readings as the system switched. In a short 10 minute city drive route, the system switched between 8 and 4 cylinder operation over 40 times. The only way I could tell this was by looking at the computer data. There was no roughness, hesitations, r.p.m. changes or any other sign of the DOD system operation other than my scan tool readouts.

My experience with the DOD system was very positive. It is totally transparent in operation. There is nothing visible under the hood that has changed – the mechanical changes are internal to the engine. The Vortec V8 engine was smooth and powerful yet fuel economy was very good. I experienced about 25 m.p.g. fuel economy during city driving and over 30 m.p.g. for some highway travel. From studying the scan tool readouts, I found the system is very sensitive to engine load so hills, higher speeds, wind direction and wind speed could all cause a significant difference in fuel economy. Even so, GM’s claims of up to 25%
improvement in fuel economy seem to be well founded.