This new specific-output leader is more efficient and refined.
The Ward’s 10 Best Engines competition has recognized outstanding powertrain achievement for 19 years. In this installment of the 2013 Behind the 10 Best Engines series, WardsAuto looks at the development of GM’s all-new turbocharged 2.0L I-4.
Today's most common new engine displacement seems to be 2.0L. Neither too small for decent performance nor too large for good fuel efficiency, it also sneaks under the bar for heavy taxation in some global markets because the displacement number typically is rounded up from 1,998 cc or so.
Turbocharged 2.0L 4-cyl. engines have become near-ubiquitous as smaller, lighter, more fuel-efficient stand-ins for naturally-aspirated V-6s.
So it is no surprise that three of Ward's 2013 10 Best Engines are 2.0L turbos: One from, another from , and this outstanding new example from . It trumps those two – and all other I-4s – in SAE-certified output with 272 hp and 260 lb.-ft. (353 Nm) of torque. Yet it still delivers 21/31 mpg (11.2-7.6 L/100 km) city/highway in the rear-wheel-drive ’13 Cadillac ATS.
The 2.0L turbocharged DI I-4 in the ’12 front-wheel-drive Buick Regal GS was a Ward's 10-Best winner last year, but this engine is different. As a member of GM's all-new Ecotec I-4 family, it shares next to nothing with its same-displacement predecessor. In this application, it generates two more ponies but 35 fewer lb.-ft. (47 Nm) of torque. However, the torque band is broader for better overall low-end response.
WardAuto editors averaged 23 mpg (10.2 L/100 km) in mixed city driving, and one logged nearly 27 mpg. "It's a great new engine in a great new car," commented Steve Finlay. "Strongest of all the 2.0L turbos we tested this year," added Drew Winter. "Sounds great at full throttle."
Mike Anderson, global chief engineer-program manager for large gasoline I-4 engines confirms the engine is totally new other than some fasteners and corporate-common roller finger followers. The block, head, crank, pistons, rods, bearings, pumps; fuel system and twin-scroll turbochargers are all new.
“It's a mirror image of the previous engine, with exhaust ports on the other side, so very little can stay common when you do that," he says.
Intake and exhaust positions were reversed for good reasons, including commonization of GM global I-4 architectures and under-hood heat management in transverse FWD versions.
Previous U.S. FWD Ecotec I-4s had their exhausts running down the back of the engine, under the cowl, while the new architecture puts all the hot stuff facing forward, where it's easier to cool. In the RWD ATS, where the engine faces north-south, the intake is on the passenger side and the turbo on the driver’s side.
Anderson says efficiency was the top design priority, followed closely by improved refinement. Maintaining GM's lead in specific output, now 136 hp/L, also was key.
“We design our engines from the inside out, starting with the combustion system,” he says. “At the time we were doing this engine, we had new proprietary tools – computational fluid dynamics, finite element analysis, and more – and new methods to use them to achieve an optimized design.”
Redesigning a combustion system means rethinking every element: Valve sizes, valve angles, the top shape of the piston and size of the bowl, fuel injector angles, director plate and spray pattern, while optimizing compression ratio, mixture and turbulence.
Cold start and particulate emissions also are concerns, Anderson adds. “It's a highly dimensional space in terms of design and desired output, and these tools enabled us to explore a lot more of that space to find a much more optimized design.
"After we did it virtually, we proved it out on a single-cylinder engine and it delivered exactly what we thought it would from our virtual work. Then we moved into the multi-cylinder world. This is just a better combustion system. It's got a faster burn early, takes off quicker, has shorter burn duration and is more knock resistant.
“One of the games there is staying stoichiometric (keeping the perfect air/fuel mixture), making sure you're not needing to enrich, and keeping your spark as advanced as you can for thermodynamic efficiency."
To minimize friction, the team addressed every bearing width and clearance to "thrift" every amount of oil flow throughout the engine and added GM's first actively controlled, 2-stage variable-displacement oil pump, which delivers just enough pressure and flow to protect the engine but no more than needed for any given condition.
To ensure durability of this turbocharged version, the block is sand-cast with cast-in-place iron cylinder liners and pistons are jet-spray cooled. The head is head rotocast and exhaust valves are sodium-filled.
The engine’s lower peak torque is considered appropriate for the light, agile ATS, though some ’14 versions of this new 2.0L turbo 4-cyl. will match the 295 lb.-ft. (400 Nm) of the ’12 Regal GS. All versions will feature broader torque bands that move it further down the rev scale.
"That does translate into better transient response at the lower end for part-throttle tip-ins," Anderson says. “But it's hard for any turbo engine to get into the upper-left corner of the torque curve. It takes seconds for things to settle out to achieve peak torque levels at low rpm, so about the only time you can do it is uphill with a manual transmission while intentionally bogging it in fifth or sixth gear.
“Some turbos are quoting 1,500, even 1,250 rpm, where they hit their torque peaks, but it's very difficult for any turbo to operate in that upper-left corner.”
One advantage of this engine's dual variable valve-timing system is "increased authority" cam phasing, meaning 70 degrees of range, or 10 more degrees than previously on each side, relative to the crankshaft.
"That's a big knob to turn," Anderson says. "When you start cold and need the catalyst to light off, you want the cams to be at one position to generate a lot of heat. When you want a lot of efficiency, or performance, you want them in different places. With more authority, you have to make less compromise among efficiency, emissions and performance.”
A substantial number of measures, including major architectural changes, were taken on the refinement side of the equation to make it good enough to be Cadillac’s first 4-banger in decades.
For example, all new GM I-4s share balance shafts in a module with an integrated oil pump in the oil pan; a strong, stiff forged-steel crankshaft; inverted-tooth chains for both camshaft and balance-shaft drives; a unique 2-piece oil pan design with an aluminum upper and a stamped steel lower section; a (patent pending) cast-aluminum bedplate with cast iron bearing cap inserts; structural front engine and cam covers and a lightweight composite acoustic intake manifold cover.
"There's a source of a noise, a path and a receiver," Anderson explains. "The engine owns the lion’s share of source; the path is owned by the vehicle, and how you balance that is an architectural choice. We put a ton of effort into source reduction, and there is more opportunity there, so we'll keep working on it."
Opportunity for efficiency improvement also exists down the road. "We are engineers, we're always polishing and there is always room," Anderson says. "We will continue to work on efficiency, and we have things in the works to do that in a number of applications. There is probably room for a little more output as well."