The Ward's 10 Best Engines competition celebrates 14 years of recognizing outstanding powertrain development. In the second of our 2008 series, Ward's looks at the design philosophy behind GM's 3.6L high-feature V-6 in the Cadillac CTS.
Regular Fuel, Spectacular Engine
New engine designed to burn regular from the get-go. Hybrid version coming.
In just a few short years, gasoline direct fuel injection has gone from being rare, cutting-edge technology to almost commonplace.
Long used to improve the efficiency and performance of diesel engines, it now is almost a requirement on premium and turbocharged high-performance gasoline powertrains. Direct injection is employed on no less than six of this year's Ward's 10 Best Engines.
ButCorp. added a couple of new twists when it began development of its all- aluminum DOHC 24-valve, dual-VVT “high-feature” V-6 available in the '08 Cadillac CTS.
It was designed to run on regular, instead of premium fuel, and it incorporates fuel injectors with a unique design to isolate and minimize the noise high-pressure injection systems are prone to create.
The new engine was designed to burn regular fuel from the start, says Tim Cyrus, chief engineer for GM's high-feature V-6 and Northstar premium V-8.
“The development team did not waiver; the focus was detailed combustion development,” Cyrus says.
However, the focus on regular fuel did lengthen development time, because more iterations of ports, pistons and injection components were required. The extra time appears well spent now, as many competitive (and some lesser) engines still require premium.
As its name implies, direct injection sprays fuel directly into each cylinder, instead of into the intake manifold ports. As the fuel mixes with air and vaporizes in the cylinder, the air/fuel mixture is cooled, which improves combustion efficiency and enables a higher compression ratio (11.3:1 vs. 10.2:1) in the port-injected 3.6L.
In the version used in the CTS, DIG boosts peak power by 41 eager horses from 263 to 304 hp and peak torque from 253 lb.-ft. (343 Nm) to 273 lb.-ft. (370 Nm), compared with the standard port-injected version.
DIG also can be controlled to create a richer mixture around the spark plug during cold starts, making it easier to ignite. This smoothes the combustion event and reduces cold-start hydrocarbon emissions by about 25%.
But DIG has had its issues — primarily injector noise and (in earlier applications) intolerance of certain impurities and additives in gasoline. As the first auto makers that brought the concept to the U.S. learned, less-than-perfect gas progressively could plug the injectors. Because this was good for neither customer satisfaction nor warranty cost, these early adopters temporarily backed away from introducing the technology to the U.S. and other markets with questionable fuel quality.
Now GM, Audi AG,AG, Motor Corp., Motor Corp. and others have resolved the plugging issue and — as this spectacular V-6 illustrates — are winning the war on noise as well.
One key to the latter is GM's industry-first and U.S-market exclusive isolated injector system.
“That's where this engine differentiates itself from the competition,” GM's Cyrus says. “It differs from the rest of its family by its cylinder heads and injectors, high-pressure fuel pump and considerable noise treatment. Because the pump and injectors are under extremely high pressures, a lot of ticking noises can be telegraphed through the engine block.”
“It's all about isolating the fuel-injection system. We are the only ones right now with injectors that are isolated instead of rigid-mounted into the cylinder head. Each injector sits down in a pocket on what looks like a little lip seal with a metallic spring and an elastomer, and that also serves as a secondary seal.” Nearly a decade ago, GM decided to step up to its urgent need for an all-new, high-tech V-6 architecture. A small project center was formed in February 1999 at GM Powertrain in Warren, MI. Cyrus came on board that September as chief engineer.
“We brought in people from Australia, because we knew we were going to manufacture and sell it there,” he says. “Sweden, because we knew we were going to be there, and Germany and the U.S. Bob Jacques, our design-systems engineer and the base-engine designer, led the team until I got there.
“From the beginning, we had the vision it would be a world engine that would package in all GM architectures and would be turbocharged (in versions such as the 2.8L Saab engine) and direct injected. The design comprehended all of those elements up front. It would go into production in 2003, and we would bring out a turbocharged version and the DI not long after that.”
“For North America, the noise was our biggest concern, and when we decided to roll it into the Cadillac, we knew it had to be very, very quiet,” Cyrus says. “Because direct injection in diesels is essentially the same technology, people assumed these things were going to sound like diesels. Some competitors already had rolled out direct-injection engines, and you could hear them coming: ‘tick-tick-tick-tick-tick.’
“We took the approach the DI had to be as quiet as, or quieter than, the MPFI. My counterparts at the platform also stepped up. They wanted the advantages of this technology in fuel economy and performance. So they worked to make the vehicle quieter, in case we were not so successful in quieting the engine.”
As a result, the DIG engine is quieter than the MPFI, he says.
The second major concern was injector plugging.
“We ran fleets here in the U.S., where we know we have lousy fuel, and abroad, including China, where we also have lousy fuel. And we ended up in very good shape, thanks to design features from our injector supplier,(Robert Bosch GmbH) and geometric features in the combustion chamber.”
Beyond its four cams, 24 valves and variable timing (aka cam phasing) on both intake and exhaust valves, GM's high-tech, high-feature DIG V-6 boasts cast-aluminum pistons with floating wrist pins, polymer-coated skirts and lubricating/cooling oil squirters; a forged steel crankshaft; sinter-forged connecting rods and a structural cast-aluminum oil pan with steel baffles.
GM intends to produce some 200,000 DIG vehicles globally this year, and one in six GM vehicles should have it by 2010, including some in much lower price ranges.
“There is a lot of pull from customers already,” Cyrus says. The extra cost of the DIG technology is primarily related to the higher-pressure injectors, with bigger coils to open them, and an extra mechanical fuel pump.
As Corporate Average Fuel Economy rules increase to 35 mpg (6.7 L/100 km) in the next dozen years, Cyrus says there is significant room for improvement, beginning with the next-generation DIG V-6 due in 2010.
“The beauty of combining four cam phasers with direct injection is that it can get you both performance and fuel economy,” he says.
“We will continue to work the combustion process and the injection system to improve both. And we can downsize the engine with more (turbo) boost to get more power out of smaller displacement with better fuel economy. You'll probably also see this high-feature DI V-6 with hybrid machinery on it,” Cyrus adds, noting GM's 2-mode hybrid system.
Since GM has suspended development of the next-generation premium V-8 that would have replaced its able but aging Northstar V-8, the DIG high-feature V-6 could be a suitable substitute down the road.
“We expected to sell quite a few V-8s and smaller numbers of V-6s, but sales have gone the other way around, probably 80% V-6 and 20% V-8 in the STS. For the time being, we'll continue to offer both and let the customer decide.”
For more details on Ward's 10-Best engines: http://wardsauto.com/reports/2007/tenbest
3.6L DOHC V-6
Displacement (cc): 3,564
Block/head material: aluminum/aluminum
Bore × stroke (mm): 94.0 × 85.6
Horsepower (SAE net): 304 @ 6,400 rpm
Torque: 273 lb.-ft. (370 Nm) @ 5,200 rpm
Specific output: 84 hp/L
Compression ratio: 11.3:1
Assembly site: Flint, Michigan
Application tested: Cadillac CTS
City/highway (mpg): 17/26