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Automotive Industry

In a family way: GM gives birth to a new generation of V-6 powerplants that promise to be world class

Automotive Industries, May, 2003 by John McCormick

Once the preserve of luxury car powerplants, variable valve timing is now spreading to more mainstream engines. Case in point: the arrival of General Motors' new 'high feature' global V-6 engine family, the first engine from GM to feature VVT on both intake and exhaust camshafts.

The first of this V-6 family, a 3.6L will make its debut in Cadillac's 2004 CTS, replacing its existing, U.K.-sourced 3.2L V-6. The new 255 hp CTS motor is one of several versions of the modular V-6, which will also come in 2.8L and 3.2L displacements, to be used in major markets worldwide. Engine size can be expanded to 3.8L, or as large as 4.0L when the cylinder liners are eliminated in favor of bore coatings.

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Versions of the engine will be both naturally aspirated and turbocharged and applications will include front-, rear- and all-wheel-drive, as well as hybrid vehicles.

One key to the new engine family's success, maintains GM, is that it will be competitive with the best in the V-6 class, such as the Honda/Acura 3.2L and Toyota/Lexus 3.0L units, yet be built at a lower cost All GM's V-6s will share the following features: aluminum construction, dual overhead camshafts, four-valve-per-cylinder valvetrain, roller-finger followers, continuously variable cam phasing, electronic throttle control, forged-steel crankshaft, piston-cooling oil jets, coolant-loss protection software, GM's oil-life system, 32 bit microprocessor and coil-on-plug ignition.

GM says the output of the most powerful variant of the new engine family will exceed 370 hp, with torque in excess of 350 lbs.-ft.

Work on the new V-S family started in 1999, and was completed in record time, says Tim Cyrus global V-S chief engineer.

"The goals for the engine were industry leading reliability, flexibility, package size, pleasibility, efficiency and value," Cyrus says. The engine can be easily integrated into most platforms, has industry leading NVH and performance with three discrete combustion systems MPFI, SIDI and turbo."

Importantly, the V-6 engine architecture was designed to incorporate different options, in order to create a broad range of configurations. As well as normally aspirated/sequential port fuel injection 'foundation' architecture, there are two potential major variants. One is a spark-ignition direct-injection (SIDI) V-6 of either 2.8L or 3.2L displacement. To account for differing market regulations and conditions, particularly in Europe, the engine design can accommodate both stratified-charge (lean-burn) and stoichiometric-charge SIDI architectures. Also in the pipeline are turbocharged 2.8L or 3.2L variants, with a variety of power and torque outputs depending on specific content.

As well as its adaptability, the key feature of GM's V-6 family is its use of variable valve timing. Moving to full WT eliminates the traditional compromise in engine design between power and torque outputs, and also improves fuel economy and lowers emissions. in the case of the 3.6L engine, the improvement in torque output is notable: 90 percent of the 252 lbs-ft. peak torque is available across a broad operating range, from 1,600 to 5,800 rpm.

Compared to an existing GM double overhead cam V-6, the 3.6L develops 20 percent more peak power, a 13 percent increase in peak torque--and a 24 percent increase in torque-integral, or the amount of torque available at most points throughout the rpm range.

"Flexibility was very important," says Bob Jacques, base engine design system engineer. "We insisted on going after high performance and high refinement at the same time."

The four-cam continuously variable cam phasing system is electronically controlled and hydraulically actuated. The phasers allow intake cam adjustment through 50 degrees of crankshaft rotation and 50 degrees for exhaust-cam adjustment.

An added benefit of the VVT system is that it allowed for the elimination of the exhaust gas recirculation system, thus reducing weight and complexity.

The V-6's intake system includes a dual-stage variable manifold. A valve in the manifold changes the plenum volume available for resonance tuning of the inlet flow path. When the valve is shut, the cylinders feed from two separate plenums. In this mode the system boosts cylinder charging in the low- to mid-speed range. At higher engine speeds, the valve opens and the cylinders all feed from a common larger plenum.

Another significant technical advance is the V-6's 32-bit micro-hybrid engine control unit, which GM claims is the most powerful currently used in the industry. The ECU design embeds all of the necessary electronic circuitry on a four-layer 'sandwich' substrate that reduces the size of the control unit. More robust and resistant to vibration than previous ECUs, the new design can be engine mounted. This move frees valuable space in the engine bay and eliminates attachment problems at the assembly plant

One function of the ECU is torque-based engine control strategy. Engine output for the driver's desired throttle opening is determined by the ECU. The torque-based strategy calculates optimal throttle position, variable intake manifold position, continuously variable cam phasing positions and various other operational inputs and then translates That information into an ideal throttle position. GM says the torque-based engine control strategy is superior to early electronically controlled throttle-based engine-management systems that relied only on the throttle position sensor to govern throttle opening.

Automotive Industry

In a family way: GM gives birth to a new generation of V-6 powerplants that promise to be world class

Automotive Industries, May, 2003 by John McCormick

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