GM’s new twin-turbo V6 boasts BMW-beating output, brilliant packaging

  • 21-Mar-2013 05:46 EDT
2014 GM twin turbo 3.6L V6.jpg

No, you can't see the twin turbochargers in this view of GM's new LF3 V6 because they're tucked in tight against the block, behind the alternator and accessory pump. Note the minimal height of the intercooler module on top of the engine—a triumph of systems engineering.

GM’s new twin-turbocharged 3.6-L V6, developed for longitudinal and transverse vehicle installations, will debut in the rear-drive 2014 Cadillac CTS. The engine will also see duty in the 2014 XTS, which offers front-drive and AWD layouts.

Recently certified by SAE International at 420 hp (313 kW) and 430 lb·ft (583 N·m), the engine (internally coded LF3) is claimed to be GM’s most powerful production V6, with the highest specific output (118 hp/L; 88 kW/L) of any six-cylinder engine in the midsize-luxury segment.

According to Rich Bartlett, GM’s Assistant Powertrain Chief Engineer, the twin-turbo V6 handily beats two benchmark engines—BMW’s 4.4-L V8 (N63B) and 3.0-L turbocharged inline six, both available in the 5-Series—in peak output and power density. Approximately 90% of GM engine’s peak torque is available from 2500 to 5500 rpm; redline is 6500 rpm, Bartlett said. Electronically controlled vacuum-actuated wastegates keep maximum boost pressure to slightly above 12 psi (80 kPa).

GM initially is pairing the turbo V6 with Aisin’s TL-80SN 8-speed planetary automatic, which will serve CTS and other GM vehicles until GM’s next-generation automatics enter production. (See 8-speed article: http://www.sae.org/mags/sve/11942/.)

The powertrain is capable of accelerating the new CTS from 0-60 mph (0-97 km/h) in an estimated 4.6 s and achieve an estimated top speed of 170 mph (274 km/h).

The turbocharging system is worth an additional 99 hp (74 kW) beyond the 321 hp (239 kW) projected for the 2014 CTS's naturally aspirated standard 3.6-L. The 2014 CTS will also offer a 2.0-L turbocharged engine rated at 272 hp (203 kW). According to Dave Leone, GM’s Executive Chief Engineer for Luxury Performance Vehicles, an even higher-performance V-Sport model will be added to the CTS range.

GM engineers expect the twin-turbo V6/8-speed powertrain to deliver 17 mpg city/25 mpg highway when EPA testing concludes. By comparison, the 2.0-L RWD model is expected to achieve 19/30 mpg, and 18/28 mpg for the AWD version. The non-boosted 3.6-L will deliver 19/28 mpg in RWD form and 18/27 mpg for the AWD, the engineers said.

Diesel piston technology used

For a video animation showing assembly of the 3.6-L twin-turbo V6, and its internal componentry, go to http://youtu.be/8LKB6fBYVaw.

Bartlett’s development team did a masterful job of efficiently packaging the pair of Mitsubishi Heavy Industries (MHI) turbochargers (tucked in tightly on each side of the cylinder block) and GM’s patent-pending, twin-brick charge-air cooler module that is mounted above the cylinder heads. He credits a close collaboration with MHI engineers in tailoring the turbo machines (which use off-the-shelf compressors) to this application. Hitachi developed the new direct injection system featuring 6-hole injectors.

The boosted engine is a significant upgrade of GM’s naturally aspirated 60° aluminum, direct-injected V6 with continuously variable cam phasing that is offered across GM’s brandscape. Bartlett said 90% of the turbocharged engine’s bill-of-material is unique, including an all-new cylinder block casting. It includes cast-in provisions for turbocharger coolant and oil connections, as well as PCV passages. While the turbo engine uses the stock steel crankshaft, the main bearing caps are nodular iron for greater strength in coping with the turbo engine’s higher cylinder pressures.

The Rotocast cylinder heads are unique to the twin-turbo version. Upper and lower coolant jackets were added to improve thermal performance. A unique high-tumble inlet port design “was developed with lots of computational fluid dynamics (CFD) work; we ran hundreds of simulations to develop the ports, piston crowns, and optimal injector spray geometries,” Bartlett told AEI.

Valve diameters are 38.3 mm (1.5 in) intake and 30.6 mm (1.2 in) for the sodium-cooled exhaust valves. The latter use AR20 hardened-steel seats for increased thermal robustness. A special multilayer steel head gasket is also employed.

Stronger steel connecting rods carry new pistons featuring a unique dome topology. The pistons incorporate a steel carrier for the top ring land that is derived from diesel piston experience. The camshaft covers are aluminum on the turbo version, to reduce noise and provide greater PCV volume needed for the turbocharger set-up.

Extensive CFD flow animation and wide-open throttle (WOT) testing on the engine-tilt rig at GM’s Milford Proving Ground helped engineers design special oil-pan baffling for the turbo engine, which was designed to endure high-g track day competition.

Bartlett noted that the engine’s dual-independent cam phasing system assists the internal exhaust gas recirculation (EGR), as well as promotes airflow scavenging for quicker turbocharger response.

Compression ratio is 10.2:1, requiring 93-octane gasoline. Two knock sensors are located in the cylinder-block valley.

Novel cooler and wastegates

The high-capacity integrated charge-air cooler is a brilliant piece of systems engineering, influenced by the configuration used on GM’s supercharged 6.2-L LSA V8. Its ultra-squat dimensions are dictated by a significantly lower hoodline on the CTS. Within the cooler housing, a single, centrally located throttle body is bracketed by two radiator “bricks.”

“With this compact design, we’ve kept the distance from the turbo compressors to the throttle body and intercooler very short and direct,” Bartlett explained. The system development consumed hundreds of hours of CAE. He added that “extensive use of CFD analysis enabled his team to take 60% of the compressed air volume out of the system’s plumbing, vs. design alternatives that used a remote-mounted heat exchanger.

He said the twin bricks lower the air-charge temperature by more than 74°C (130°F). The intercooler system achieves better than 80% efficiency with only about 1 psi (7 kPa) flow restriction at peak power, for fast torque production, Bartlett noted.

The turbo V6’s extensive sensor array—dual mass-airflow sensors, an integral inlet air temperature/humidity sensor, a dual-compressor inlet pressure sensor, and dual manifold pressure sensors—caused one GM engineer to quip: “With the right algorithms I think we can predict a cold front coming in!”

Each MHI turbo machine is fitted with a vacuum-actuated wastegate using electronic vacuum-actuated recirculation valves. Compared with conventional pressure-actuated wastegates, GM’s vacuum-actuated system gives more consistent boost control, particularly at lower rpm, for greater low-rpm torque, Bartlett claimed.

The wastegates are independently controlled on each cylinder bank to balance the compressors’ output, providing more-precise boost-pressure response. The recirculation valves eliminate co-surge from the turbos; Bartlett said this condition can result in dynamic flow reversal when the throttle closes abruptly. He said the overall system integration contributes to the engine’s smoother, more-consistent power delivery.

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