All-New Ecotec 2.5L Powers Chevrolet Malibu’s Quest for Quietness

The 2.5L development team reduced noise intensity by 40-percent of the Ecotec 2.4L engine, which was already a Ward’s 10 Best Engines award winner. It also passed subjective evaluations of what sounds good as the engine climbs through its rpm band. To Malibu’s passengers, that is expected to mean a quieter driving experience and a more refined sound as the engine revs to its 7,000-rpm peak.

The refinement and quietness of the 2.5L are impressive,” said Tom Slopsema, Noise and Vibration engineer. “No fastener, cover or internal engine part was left unexamined in our quest to make this an engine that surpasses the benchmarks in the industry.”

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.

“Think of it as the difference between low-frequency, course noise, such as a vacuum cleaner, versus a higher frequency, precision noise, such as a sewing machine,” said Slopsema. “We focused on reducing the overall engine noise level and placing the remaining noise in a higher frequency range.”

The refinement-enhancing changes and improvements over previous Ecotec engines ranged from the comparatively simple – such as integrating a sound-absorbing cover into the intake manifold and specifying quieter drive chains – to more fundamental architecture items, such as relocating the balance shafts from the cylinder block to a cassette within the oil pan.

Engineers worked around core elements of the engine that they simply couldn’t change, namely its aluminum-intensive architecture and the use of other lightweight, composite plastics.

“Both aluminum and plastic are very effective radiators of noise, but their mass-saving advantages are a must for fuel economy,” said Slopsema. “That meant we had to be more creative in how we approached noise reduction, addressing individual components’ effects on overall N&V and how to counter them.”

So, engineers implemented 10 key elements fundamental to giving the 2.5L its segment-challenging refinement, ranging from major items such as rotating and reciprocating components within the engine to items as minor as the front cover bolts.

Here’s a look at them:

1. 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.

2. 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.

3. 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.

4. 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.

5. 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, thanks 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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. It is available in LS, LT, ECO and LTZ models in North America. Malibu 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.