The new ECU
is based on a Bosch 32-bit controller that is much more precise than the
previous Delphi unit in controlling the fuel system. Other improvements
to the new Duramax include:
thickness (by 4mm) in firing deck of the aluminum cylinder head
materials used in cylinder block
bearing and skirt areas of cylinder block strengthened
bore diameter increased
of turbocharger revised for quicker response
system re-tuned for quieter operation
also lowered the compression ratio from 17.5:1 to 16.8:1 with redesigned
pistons. The lower compression helps with emissions and reduces stress
on the engine with a lower peak cylinder firing pressure, which in turn
allows more turbo boost and more fuel to be burned. Lowering the compression
ratio can also mean trouble for cold-weather starts. So GM directed the
fuel spray directly over improved glow plugs that heat up faster and also
introduced a 1000-watt grid heater in the air intake.
On the transmission
side, the added gear is for highway cruising. First through Fifth gear
ratios are carried over from the previous Allison while the Sixth gear
is 0.61:1. At 60 mph, Fifth gear (0.71:1) engine speed is 1800 rpm while
the new Sixth gear is 1550 rpm. Tow-haul is carried over but now it’s
complemented by a tap-shift option. Drivers can pull the main shifter
down to M and then control gear selection manually using a thumb-activated
toggle on the shift lever. The range selection holds the gear of choice
but there are electronic safeguards to keep the engine from over revving.
Grade braking and cruise grade braking are available only when the tow-haul
mode is selected and the transmission is not in the range-selection mode.
A new low-traction mode is now standard on all vehicles equipped with
the Duramax. When the driver selects Second gear and wheel slip is detected,
then the transmission torque-manages the engine to limit tire slip. The
Allison also gets a new 32-bit controller, new solenoids for better torque
converter lockup and shift feel and quieter oil pump.
introduction of the new Duramax gave GM Powertrain engineers a chance
to show off emerging technologies that will further improve the diesel
engine. Most promising is implementing cylinder-pressure sensors in the
engine. These sensors would monitor pressure within the cylinder and help
the computer decide when to fire the injector. Research has shown there
are distinct temperature and pressure regions during the compression stroke
that allow emissions to form. But there is a small window of opportunity
that would significantly reduce the formation of soot and NOx because
combustion starts at a cooler temperature. The problem is that this window
moves around depending on the engine rpm and load. If sensors could be
tied to the engine-management computer, then a closed-loop system could
be developed that feeds fuel to the engine based on real-time information.
in conjunction with Chrysler, is also developing a two-mode full hybrid
system that could be available on 2009 Silverados. The system is based
on the GM hybrid busses already in operation. It uses two 60-killowatt
motors mounted in the transmission and a 300-volt battery pack. When mated
to a Vortec V8 engine that has Displacement on Demand, fuel savings could
be in the 25-percent range.
In a final
hint of the future, GM had a truck to show off the possibility of using
a urea solution to reduce NOx on diesel engines. Inquisitive journalists
sneaked a peek under the hood and found a 4.9-liter Isuzu-built diesel
engine that had all the earmarks of a V6 Duramax. We can only wish.