Few things in the automotive space are more soul-stirring than the sound of a V8 engine – at wide-open throttle, or top-gear cruising, or at any combination of engine speed and load in between. For Chevrolet Corvette enthusiasts, engine sound quality is Holy Grail stuff. But the Vette’s classic roar is just one piece of an aural signature that has grown increasingly sophisticated in the car’s last two (C7 and C8) generations.
“Exhaust note is part of the Corvette’s DNA, but the car also must meet pass-by noise laws and various global-market sound regulations,” observed Dmitri Konson, VP of global clean air engineering at Tenneco, which supplies the complete cold-end exhaust (from the catalytic converter to the exhaust tips) for the 2020 C8 Stingray and components for the LT2 V8. However, acoustics engineering isn’t only about numbers and curves, Konson noted. Engineers “must listen together, talk about solutions and decide what’s right,” he said.
Only through close collaboration could Tenneco and GM engineers deliver such a sweet-sounding result for the C8. Konson and Tenneco engineering manager Megan Wikaryasz, who had day-to-day responsibility on the C8 program, spoke with SAE’s Automotive Engineering about the challenges and achievements in this marquee program.
“This was quite an integration exercise that ultimately involves cabin comfort,” Konson said. “It wasn’t a ‘sprint’ program; it was very methodical and well-coordinated between our partners.” Compared with the front-engine C7 Corvette, the C8’s new mid-engine architecture imposes many different boundary conditions in terms of managing various aspects of vehicle performance, acoustics and emissions, within a much tighter physical package than was available on C7. The mid-engine layout enables C8’s exhaust system to be shorter overall, for greater engine efficiency and performance. But it generates more heat for the given amount of engine-compartment volume, creating thermal-management challenges.
Adding to design complexity, the C8 “active” exhaust system also is electronically integrated with the car’s driver-selectable driving modes as well as with the V8’s Active Fuel Management (cylinder deactivation) controls. “The C8 and C7 systems have similar components – valves, passive acoustic devices, pipes, heat shields – but they are very different systems,” Konson said. “Simply combining similar pieces does not mean you’ll get performance at a similar level.”
Downstream of the Corvette’s tubular headers, the exhaust system uses two electronic control valves (one per cylinder bank) to limit exhaust flow when the V8 is operating in four-cylinder mode. Developed at Tenneco Clean Air’s Edenkoben, Germany, engineering facility, the valves are used to manage backpressure and effectively “neck down” the area within the tube to improve exhaust velocity. And according to Wikaryasz, the Tenneco e-valves also play a role in warming up the catalysts during cold starts.
C8 models fitted with the optional performance exhaust package include two additional Tenneco valves to further open-up exhaust flow. Chevrolet claims this is good for an extra five horsepower. “There’s only so much you can do in a tight space with muffler volume,” Konson asserted. “Combining the sound management with the valves and back-pressure management with the cylinder-deactivation technology – while ensuring that the driver doesn’t feel there is anything going on that he’s not expecting – is a big challenge,” he said.
Wikaryasz describes the C8 exhaust as “the most complex system among all of our light-vehicle programs.” Development was “very analytical-heavy,” requiring use of every analysis tool in the Tenneco toolbox. “Our active power-flow software, along with CFD, was used to look at the flow dynamics within the system. To achieve the tonal-quality target, her team packaged similar tuning elements to what they used on the C7 Corvette, but in a much tighter packaging space.
“We had to tune exhaust pipe diameters, hole diameters and locations to achieve that desired characteristic sound,” she said. “We did that using both our GT-Power tools, in which we did the iterative up-front development. For the on-vehicle development we worked collaboratively with GM [at its Milford, Mich. and Arizona proving grounds] for the fine tuning.” Supporting the effort was a team of acoustics experts at Tenneco’s Grass Lake, Mich., technical center who handled myriad ride-and-drives during the critical fine-tuning process.
FEA and thermal analysis was used to understand and find solutions for managing heat across the system. The C8 cold-side exhaust uses 18 different heat shields, both multiple-layer and single-layer designs depending on location and thermal requirements. The exhaust valve electronics also had to be overly robust, for thermal survivability.
“Thermal challenge in C8 is a big one, but we understood the customer’s requirements and challenged our development team,” Konson explained. Tenneco was design-responsible for the large and geometrically complex heat shields. “We worked closely with our supplier partners to develop the shields so they were manufacturable, so we could move the metal without issues such as cracking,” Wikaryasz noted. Validation of the system’s thermal performance was done collaboratively with GM.
To protect the electronic valves, Tenneco’s valve experts in Germany developed a decoupler mechanism that enables the valve actuators to be remotely placed, away from the hottest surfaces. For muffler thermal-shielding, the team “also had to consider the impact on radiated-noise mitigation, and on cool-down noise reduction,” Konson said. “It required optimizing for multiple variables. Doing it through a trial-and-error method of ‘build, test and repeat’ is not going to work under the timeframe of the program.”
Tenneco’s Smithville, Tenn., plant handles production. “It’s a complex system to manufacture,” Wikaryasz said. “For our whole team at Tenneco, the greatest achievement of this program was achieving the PPAP [GM’s product part approval process] of the system. To get it set up in our plant, to meet the run rates and our customer’s requirements, was the pinnacle for our team.”
Added Konson: “This program advanced our capabilities in the valves and development of the heat-shielding solution,” he said. “There is a lot that can be done in the exhaust system to enable technologies that are really implemented in the engine, including cylinder deactivation and emissions control. It’s the nature of today’s world in powertrain.”Continue reading »