Stephens explains fate of GM’s stillborn 4.5-L V8 diesel

  • 20-Jul-2009 01:34 EDT

A 4.5-L V8 in its Gamma development phase, showing preproduction cast aluminum camshaft covers. Much of the innovative engine’s learnings will be passed to other future GM engine programs, according to Stephens.

G­eneral Motors­­’ mu­ch anticipated 4.5-L Duramax diesel V8 program slated for MY2010 light-duty trucks broke new ground in design, packaging, performance, and program management. But the program fell victim to the prospects of even tighter future global emissions regulations, explained GM Vice Chairman for Global Product Development Tom Stephens.­

­“As the regulations get more stringent, the cost of certifying diesels for passenger-vehicle applications at reasonable cost, compared with other alternatives, makes it difficult to justify moving forward,” Stephens told AEI in a mid-July interview. He described the 4.5-L diesel’s status as “more than just on temporary hold.”

As a former powertrain engineer and an “engine guy” at heart, Stephens said he deeply understands the disappointment of the 4.5-L V8’s engineers led by Director of Diesel Engineering Charlie Freese. Along with Chief Engineer Gary Arvan, they whisked the program literally from sketches on napkins to production-ready status in record time during 2006-2008.

The team used genetic algorithms to optimize the combustion process before reaching the single-cylinder test stage—a key to getting the first development engines up and running quickly.

“Diesels are fuel-efficient, yes, but in the long view I’ve got to get to something that will really help get us off of petroleum,” Stephens explained. “It’s not OK to be 96% dependent on petroleum as we are today.” He said GM currently has more powertrain technology options on the shelf than can be implemented at the moment, many of them focused on advanced gasoline engine solutions mated with various types of hybridization. Stephens also indicated many learnings from the 4.5-L program, including cylinder block and head design, software controls, and packaging, will be passed onto other engine programs.

The new light-duty DOHC V8 met the very stringent U.S. Tier 2 Bin 5 emissions standards. Packed with innovation, its narrow 72º cylinder angle was designed to fit the same ultracompact box volume as GM’s small-block gasoline V8s. The engine’s innovative cylinder heads featured inboard exhaust ports and internal intake ports.

In this configuration, the exhaust ports face toward the valley where the single turbocharger and EGR coolers are located. The intake ports, located inside the two-tiered head, are fed pressurized charge directly through the tops of the intake camshaft covers. The design allows the exhaust manifolds to be integrated into the aluminum head casting. (See AEI October 2007.) It also helped Arvan’s team dramatically reduce parts count compared with other V-type diesels.

Besides the unique cylinder-head arrangement, the new Duramax boasted a compacted-graphite-iron (CGI) cylinder block, piezo-type common-rail fuel injectors operating at 2000 bar (29 ksi), and a selective catalytic reduction (SCR) aftertreatment system for reducing NOx (oxides of nitrogen) to meet the Bin 5 emissions levels. The engine was also package-protected for closed-loop cylinder pressure monitoring.

Rated output was claimed to be more than 310 hp (231 kW) for 68 hp/L (51 kW/L) and 520 lb·ft (705 N·m).

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