New 4-cylinder 2.5L Ecotec for Malibu marks debut of new Ecotec engine family; enhanced power, efficiency and refinement
|New 4-cylinder, 2.5L Ecotec. Click to enlarge.|
Chevrolet revealed more details about the new Ecotec 2.5L four-cylinder engine that will be the standard engine in the 2013 Malibu. The new 2.5L—which GM says is more efficient, refined and powerful than its 2.4L predecessor—will be available next summer, marking the debut of the all-new Ecotec engine family.
GM expects the 2.5L to deliver an estimated 190 horsepower (140 kW) and 180 lb-ft of torque (250 N·m)—about 12% more horsepower and 16% more torque than the current Ecotec 2.4L, while offering estimated highway fuel economy of more than 30 mpg (7.84 L/100km) (final fuel economy numbers are pending). The current MY2012 Malibu with 2.4L engine carries an EPA rating of 33 mpg (7.13 L/100km) highway.
Increased efficiency was the top development priority, achieved in part through lower engine friction. It was reduced by an average of 16% across the entire speed range, using new technologies such as a variable-displacement oil pump and an actively controlled thermostat. GM proprietary computational fluid dynamics (CFD) analysis techniques were used to develop an all-new combustion system with a higher compression ratio, which also helped boost efficiency.
The new combustion system features improved knock resistance and higher-flowing intake and exhaust ports in the cylinder head, which help increase efficiency, power and torque. The new Ecotec also has increased-authority cam phasing to minimize any compromise between efficiency, performance, emissions and driveability.
Like the current generation of Ecotec engines, the new 2.5L also features a high-pressure, direct-injection fuel system, dual overhead camshafts with continuously variable valve timing, electronic throttle control and pistons with jet-spray oil cooling.
GM also expects the new engine to be one of the quietest and most refined engines in the segment. Specifically, the engine’s noise frequency signature was targeted, with the aim of pushing radiated noises into a higher frequency range well above 2,000 hertz, which is more pleasing to the ear—particularly in the high-load operating ranges where engine sound is the most intensive.
The noise intensity is 40 percent less than our 2.4L direct-injected engine, which was named one of Ward’s 10 Best Engines. Engineers also tuned the 2.5L to deliver more of its torque at lower rpm, giving the Malibu a stronger feel at launch and during on-demand maneuvers, such as passing or accelerating on a freeway entrance ramp.—Mike Anderson, global chief engineer
GM engineers implemented 10 key elements fundamental to giving the 2.5L its refinement, ranging from major items such as rotating and reciprocating components within the engine to items as minor as the front cover bolts:
Relocated Balance Shafts. The 2.5L’s balance shafts— which are commonly used in four-cylinder engines to reduce vibration—are located in a cassette in the oil pan. It’s a move from previous Ecotec engines’ cylinder block-mounted shafts, which helps reduce noise through three key design features: a shorter, quieter drive chain, precision shaft-to-shaft reversing gears and light drag torque from driving the oil pump.
The short drive chain eliminates the previous long, winding “bushed” chain that included driving the water pump. It uses a premium inverted tooth chain design instead of a conventional roller-type chain, for quieter performance. The shaft-to-shaft reversing gear set allows the drive gears of the shafts to mesh directly, eliminating the need for a chain to “back drive” the second shaft, which must rotate in the opposite direction of the first shaft. The second shaft also drives the oil pump, providing a light drag torque to pre-load the reversing gear teeth for smooth, rattle-free and quiet operation.
In-Pan Oil Pump Assembly. Another significant change from previous Ecotec engines is the relocation of the oil pump assembly from the front of the crankshaft to within the oil pan, where it is driven by the second balance shaft. This reduces noise from the front cover area—an aluminum-intensive area that radiates noise—and provides a small drag torque to ensure quiet balance shaft gear operation. Also, the oil-sump location minimizes the potential for pump cavitation noise.
Chain drive. Click to enlarge.
Camshaft Drive with Inverted-Tooth Chain. Like the drive chain for the balance shafts, the camshaft drive chain uses a premium, inverted-tooth design that is significantly quieter than a roller-type chain. As its name implies, an inverted-tooth chain has teeth on its links—two-pin rolling pivot joints—that essentially wrap around the gear sprocket to take up virtually all the tension. This allows for smoother meshing of the chain links to the sprocket teeth, the cause of most noise in chain drive systems. The chain-to-sprocket tooth impact is greatly reduced with the inverted-tooth design (also known as a silent chain drive), which virtually eliminates noise and enhances durability.
Two-Piece Oil Pan. When it came to the oil pan, engineers faced a conundrum: Aluminum provides stiffening structure to an engine, but it radiates noise. Stamped steel, on the other hand, radiates less noise, but doesn’t offer the structural benefits needed for a stiff powertrain assembly. Their solution was to combine the materials to create a unique, two-piece oil pan that features a stiff aluminum upper section to support the engine’s structure—maintaining the Ecotec engine’s signature full-perimeter transmission mounting surface—and a stamped steel lower section to provide greater overall sound performance.
Structural Camshaft Cover. As a cast-aluminum part mounted on the very top of the engine assembly, the camshaft cover can be a significant source of noise. That’s not the case with the 2.5L, due to a new, structural cover design that is stiffer and mounts more rigidly to the engine. It features increased ribbing and additional attachment bolts down the center, all of which increase the cover’s stiffness to help push the engine’s sound frequency above 2,000 hertz. It also enables excellent oil sealing for valvetrain oil control passages integrated within the cover.
Acoustic Intake Manifold Cover. Like many engines in the segment, the 2.5L uses a composite plastic lightweight intake manifold. But plastic conducts noise, so engineers wrapped the intake with a clamshell-like isolating cover. It has a sound-absorbing “blanket” on the inside that snugs against the intake to provide isolation, plus the cover has a visually clean outer layer, which works as a noise barrier.
Forged Steel Crankshaft. Engineers selected a forged steel crankshaft for the 2.5L because, along with its strength and durability, it is stiffer than a conventional cast iron crankshaft. That reduces noise and vibration at mid- and high-rpm levels, enhancing the engine’s smoothness.
Iron Main Bearing Cap Inserts. Iron inserts are cast into the 2.5L’s aluminum cylinder block bedplate, enhancing the structure at the main bearings, for greater smoothness and quietness. The bedplate provides stiffness to the bottom of the cylinder block and incorporates the main bearing caps—components used to secure the crankshaft within the block. The iron insert material ensures close main bearing tolerances over a wide range of engine operating temperatures, for quieter engine lower-end noise.
Isolated Fuel Rail. Although not new to the 2.5L, its isolated fuel rail nonetheless helps achieve overall quietness. Like the Ecotec 2.4L and Ecotec 2.0L turbo, the 2.5L features direct injection, which employs a very-high pressure fuel system, including an engine-mounted fuel pump and complementing fuel injectors that “fire” with very high pressures directly into the combustion chambers. This can be a source of noise. The fuel rail is a tube-like component that supplies gasoline to the injectors. To reduce the noise associated with this efficiency-enhancing system, the injectors are suspended and the fuel rail is attached to the cylinder head with rubber-isolated, compression-limiting mounting provisions.
Structural Front Cover. Similar to the structural camshaft cover described above, the 2.5L’s front cover, which covers both the camshaft drive system and balancer drive systems, was designed with extra ribbing and secured with extra fasteners—including a new row of attachments down the middle of the cover. Like the camshaft cover, the result is a stiffer, more rigid, quieter cover that contributes to lower engine noise.
The new Malibu will be sold in nearly 100 countries on six continents and will be built in multiple locations around the globe, including the Fairfax, Kan., and Detroit-Hamtramck assembly plants in the United States. Pricing will be announced later this year.