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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:
GM Powertrain 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.
The press 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.
GM, working 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.
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