There is a lot more to General Motors' new Gen-III Ecotec engine family than the mere 100-cm³ increase in displacement. The new 2.5-L naturally aspirated inline four is more compact and refined than the 2.4-L units (known internally as LEA and LE5) it is designed to replace. And it introduces (in the 2014 Chevrolet Impala) GM's first use of variable intake-valve lift, among many other significant changes. The 2.5 L is coded LCV in its 2013 Cadillac ATS and Chevrolet Malibu applications, and LKW in the Impala.
The larger displacement was created by increasing stroke by 3.0 mm, to 100.8 mm (3.97 in). The 88-mm (3.46-in) cylinder bore is carried over, as is the bore-center dimension, to save cost.
“We could have updated the 'old' Ecotec," which was already equipped with direct fuel injection, conceded Dr. Edward Groff, Assistant Chief Engineer at GM Powertrain. But developing a new DI engine made it easier to incorporate a long list of detail changes aimed at improving fuel efficiency, output, and manufacturability, while reducing NVH, he explained.
A new turbocharged 2.0-L variant also is part of the Gen-III Ecotec family. It carries over the "square" 86-mm (3.385-in) bore and stroke from the previous turbo engine. Compression ratio is 9.5:1.
The 2.5-L that serves as base engine in the ATS is SAE-certified to produce 202 hp (151 kW). The new 2.0-L version with twin-scroll turbocharger is SAE-rated at 272 hp (203 kW); it is expected to be the ATS's volume engine. Maximum boost pressure is 20 psi (140 kPa). The 3.6-L V6, SAE-rated at 303 hp (226 kW), also is available. The accompanying table shows other Ecotec engine ratings.
New combustion system and valvetrain
GM Powertrain engineers used the company's proprietary computational fluid dynamics (CFD) analysis tools to creature a new combustion system that enables a higher compression ratio (11.3:1) on the 2.5 L, which helps improve fuel efficiency and is more knock-resistant.
The 2.5 L is GM’s first domestic engine with variable-lift intake valve actuation. Known as IVLC (intake valve lift control), the system provides variable valve lift in addition to continuous variable timing.
The compact system has two roller followers built into the rocker arm, and a two-section cam lobe—one profile and follower for low lift, the others for high. A spring-loaded sliding piston, in an oil cylinder in the rocker arm, is operated by an oil pressure feed from an electronically regulated oil control valve, through a dual-path lash adjuster.
With oil pressure applied, the piston moves inward, latching only the low-lift follower to the rocker arm. With oil pressure relieved, the piston moves outward, locking the high-lift follower to the rocker. Low lift, activated for most operating conditions, provides 4.0 mm (0.157 in) valve lift. High lift, for hard acceleration and higher-speed operation, provides 10.5 mm (0.413 in).
The variable-valve timing system was upgraded, with each cam phaser given 5º additional authority, up to 65º on exhaust, 70º on intake.
The high-pressure returnless direct-injection system uses a camshaft-driven fuel pump to deliver 750 psi (52 bar) at idle and 2250 psi (155 bar) at full load. Engine redline is 7000 rpm.
The new modular I4 architecture enables virtually all of GM’s gasoline and diesel engines in this displacement range to be packaged in both front- and rear-drive vehicles. The 2.5 L is mounted longitudinally in the Cadillac ATS, and mounted transversely in the new 2014 Chevrolet Impala. However, a version of the incumbent 2.4 L (LUK code) equipped with GM's e-Assist stop-start is carried over from other models and will be available in the new Impala. Previously, only a 3.6-L V6 was available with the standard Impala.
Dr. Groff noted that the exhaust side of the new engine faces toward the front of the vehicle, similar to other GM fours used in transverse installations. He said there are arguments for and against both front- and rear-facing exhaust locations. The exhaust system design is simpler and shorter with the rear location, which also may help with emission controls. However, it puts more heat against the cowl and in particular with a turbocharger, the exhaust noise also has a shorter path to the cabin.
The front-facing exhaust helps cool the turbo and adds space for noise dissipation. “And with electric power steering and all-wheel drive on some cars, it can get crowded back there” on the cowl side of the engine, Dr Groff added. The alternator, however, was moved to the rear for commonality.
GM Powertrain engineers optimized the engine's strength without adding weight, which makes it inherently quieter. As a result, the engines have more upside performance, emissions, and fuel economy potential than the old units, beyond any effect from the 2.5-L's minor displacement increase.
Some reduction in engine width always is desirable, so the two balance shafts (one on each side of the block at the bottom of the cylinder bores) were relocated to the oil pan. The oil pump also was moved, from its crankshaft drive at the front to the rear of the oil pan, where it is gear-driven off the balance shafts. This design change shortened the engine 13 mm (0.5 in).
The new computer-controlled variable-displacement oil pump is a vane-type unit with two operating stages. The computer uses an algorithm based on coolant temperature and engine load to calculate oil temperature and adjust oil pressure accordingly. It replaces the gerotor type used on the 2.4 L.
Main and rod bearing lengths in the new Ecotec were “right-sized,” Dr. Groff said, meaning a better match for engine operation. The width of the mains' was reduced by 0.6 mm to 19.0 mm (0.748 in); the width of the rods' bearings was increased by 2.5 mm to 22.7 mm (0.893 in).
The camshaft chain drive was changed to the inverted tooth “silent” type, which permits a lower-impact mesh with the sprocket. The engine block bedplate has cast-iron inserts around each main bearing lower shell, providing greater rigidity for crankshaft support. And the plastic intake manifold has an insulating layer to reduce emission of intake noise. The fuel rail and injectors are isolated for noise reduction by rubber grommets.
The 2.5/2.4-L and 2.0-L turbo engines both carry over the oil-jet squirt systems, but each with a different objective. The 2.0-L turbo jet squirts oil into a target hole in a cooling passage in the piston. The 2.4/2.5-L jet is angled to spray oil onto the bore to reduce noise and vibration on cold starts. However, air movement from the reciprocating piston also directs some cooling oil spray from both jets onto the underside of the piston.