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.
Displacement
on Demand uses the oil pump system for hydraulic pressure to activate
the system. Four control valve solenoids then act together to achieve
seamless transition between four and eight cylinder operation.
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.
DOD will
make its way, eventually, into every small block V8 that General Motors
offers.
