Displacement on Demand (DOD)
OK, before you fall off your chair laughing (or crying) about the previous attempt Cadillac made back in the 1980s with its V4-6-8 engine, you really need to hear about the new Displacement on Demand engine expected to debut in 2004.
Since many folks aren't familiar with Displacement on Demand engines, or tried to bury those painful memories from the past, here's a quick backgrounder. Displacement on Demand switches off those engine cylinders that aren't needed when a truck is in motion, such as when cruising at highway speeds. When more power is needed, for acceleration or towing, all cylinders are activated. See the diagram below for a cutaway of the process.
The latest version skips the six cylinder mode all together, because of its inherently unbalanced nature, and switches only between four and eight cylinders.
We drove a Sierra equipped with the new DOD technology and were challenged to determine when the vehicle was using four or eight cylinders, and to identify exactly when the engine made the change. If it wasn't for the green indicator light on the dash we probably would not have guessed. The switches were imperceptible.
With a design goal of 8% fuel economy savings, up to a claimed 25% depending on driving conditions, DOD may finally have arrived when it will be most readily accepted in the market.
Using a lifter mechanism for the valvetrain, activated by and Eaton designed solenoid control, GMC implements this sensible approach to saving fuel with a very simple configuration, and a minimum of supplemental equipment. As the diagram shows, the lifter is something that sits within the normal valvetrain spaces, and is capable of locking down the valvetrain for economy operation with a minimum of fuss.
In our driving we substantially underestimated the number of shifts from 4 to 8 and back to 4 cylinders, showing the clear progress that GM has made with this technology. Changes occur within .05 seconds and over the course of our 11 mile drive the DOD engine changed cylinder modes a remarkable 76 times and was in V4 mode for just over half the trip!
It was also clear the truck was no compromise when it came to power. It was in the 'normal' V8 mode every time we stepped on the accelerator. Like the Parallel Hybrid Sierra, GMC is taking an approach to fuel savings that tries to minimize, if completely remove, the disadvantages of compromise when it comes to power, towing or flexibility.
make its way, eventually, into every small block V8 that General Motors