GM's diesel trio: on its way to the hydrogen highway, GM makes a stop in diesel town.

Cadillac's New Diesel

European versions of the 2009 Cadillac CTS will have an engine option their North American counterparts won't: a 2.9-liter V6 diesel. Jointly developed with Italian engine design and manufacturing house VM Motori (Cento, Italy, www.vmmotori.it), the engine is based around what Charlie Freese, executive director, GM Powertrain Diesel Engineering, calls "a flexible premium diesel architecture." That means it can be mounted longitudinally or transversally and has low noise, vibration, and harshness (NVH) levels.

The engine marks GM's first application of closed-loop combustion in a production motor. "Piezo-resistive pressure sensors integrated into the engine glow plugs measure the pressure against it and assign a number to this electrical impulse," says Freese. "That allows us to run high levels--65%--of cooled EGR that enables highly pre-mixed combustion without the combustion stability issues you have with an open-loop system, and lets us achieve a smooth transition between combustion modes." The pressure sensors allow the engine controller to adjust injection timing and quantity to compensate. This brings the response much closer to the fueling target, and avoids the fuel consumption penalty inherent to NOx reduction efforts made with open-loop systems. "In addition," says Freese, "it can help compensate for differences in fuel quality arising from variances in the fuel's cetane rating." It will be adapted to other GM diesels in the future.

Because GM is in the process of developing a number of new engines, it made the decision to leverage the design, development, and production capabilities of VM Motori rather than go it alone. GM also utilized a proprietary combustion optimization program that allows its engineers to do iterative investigations of combustion chamber designs. This software can compare designs, run projections, combine features of competing designs, and rank the outcomes based on various criteria to determine a "winner." "That has allowed us to investigate a number of designs that would have taken months to develop otherwise," says Freese.

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The common-rail injection system uses piezo injectors operating at 2,000-bar max. pressure, and capable of eight injections per cycle. It works in concert with an electronically controlled variable-geometry turbocharger, and a 16.5:1 compression ratio. A close-coupled NOx catalyst and particulate filter make up the external emissions package. Compacted graphite iron (CGI) is used for both the 60[degrees] cylinder block and bedplate, while the heads are DOHC aluminum units with four valves per cylinder. With a bore of 83 mm and a stroke of 90.4 mm, the V6 displaces 2935 cc.

Duramax Heavy and Light

At the other end of the scale sits GM's revised Duramax 6.6-liter V8. Found in the 2007 Chevy Silverado and GMC Sierra heavy duty pickups, this turbocharged V8 produces 356 hp @ 3,200 rpm and 660 lb-ft @ 1,600 rpm. Upgrades to the injection system and the addition of a particulate filter mean a 90% reduction in particulate matter and 50% less NOx. "We increased the EGR cooling capacity, improved the turbo efficiency, and revised the injector nozzle flow, angle, and hole quality," says Freese.

By 2010, GM will introduce a new diesel for its light-duty trucks and SUVs. A so-called "premium high-tech V8," the new motor will share the Caddy V6's CGI block/aluminum construction, and promises NVH levels approaching gasoline engines. The DOHC heads have four valves per cylinder and are fed by an electronically controlled variable-geometry turbo. GM claims the new design is capable of meeting Tier 2 Bin 5 emission levels. Expected to displace between 4.0- and 5.0-liters, the light-duty diesel's box volume is equal to that of GM's small block gasoline V8, which means it will package in any rear-drive application that engine can.

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by Christopher A. Sawyer

EXECUTIVE EDITOR