By 2025, regulators from the Environmental Protection Agency (EPA) project 90% of cars sold in the United States could have small, turbocharged engines. At roughly a quarter of the engines sold today, turbocharging has allowed automakers to cut engine size, reduce weight, and improve fuel economy, but those gains come at a cost. Downsized, boosted engines are noisier and harsher than the naturally aspirated models they replace.
The problem is inherent in how turbocharging works. Exhaust gases turn a turbine that compresses incoming air, effectively forcing more oxygen into engine combustion cylinders. That allows the engines to burn more gasoline, create more-forceful explosions, and generate more power.
“The more you downsize the system, the more violent each combustion event gets. So these smaller engines can create a lot of vibration and noise,” says Jeff Hemphill, chief technical officer for supplier Schaeffler’s Americas group. “As the number of cylinders goes down, the vibration gets worse.”
As each combustion event gets more violent, the torque spikes from the engine are carried into the transmission and driveline, so vibration can create annoying shaking or odd noises in the vehicle.
Manufacturers set acceptable noise, vibration, and harshness (NVH) levels in cars depending on their price points – from strict limits on luxury vehicles to annoying-but-livable amounts on budget cars. Suppliers then come in with systems to absorb vibration, dampen sound, or other methods to make the driving experience more pleasant.
Steve McKinley, vice president of engineering for the Americas at Honeywell Transportation Systems, says as the number of turbocharged engines increases, automakers have become more sophisticated in how they handle NVH problems.
“With a lot of the engines 20 years ago, when you’d stick a turbo on, you kind of got what you got,” McKinley says. “With turbos being such a big part of the strategy going forward, they’re figuring out how to design the engines so they can handle anything thrown at it.”
He adds that automakers are changing designs of cylinder heads and other engine components to contain more of the shock of higher-powered combustion events, but there’s only so much that can be done on the engine blocks themselves.
One Schaeffler method for reducing NVH is a torsion damper that rests between the engine and the transmission. As turbocharging creates higher torque spikes, that extra torsional energy needs to be stored as the engine comes up through its compression cycle.
The stored energy can then be returned to the driveline, smoothing the vibration. The damper uses springs and centrifugal pulleys to absorb those spikes, smoothing out the engine’s performance as it shifts from non-boosted to boosted operation.
“Our customers were looking for simple solutions that can handle multiple engine states,” Hemphill says.
Dampers that absorb some engine vibration can mitigate NVH close to the engine, but extra vibration still exists and can travel throughout the powertrain and exhaust systems.
“Companies tend to tune their turbocharged engines to operate at lower rpms. So there’s lots of low-frequency energy and vibration,” says Andrew Pontius, director of product design and development for supplier Faurecia’s Emissions Control Technologies group. “Engine noises have a specific frequency, and exhaust pipes can reinforce that frequency, amplifying sound levels.”
When the exhaust system amplifies turbo sounds, the traditional response has been to use resonators - devices that absorb sound waves in the exhaust system. When turbos were only used on high-end luxury and sports cars, adding sound-absorbing components was less of an issue. However, Pontius says that as turbocharging has moved into inexpensive, mainstream vehicles, automakers have been less enthusiastic about adding more components and costs.
One Faurecia solution has been to break up sound waves before they can reinforce NVH levels from the engine. Engineers study wave patterns in the exhaust system and cut holes in exhaust pipes to stop wave forms from propagating across the length of the system.
“We can place the hole strategically at the location where the wave wants to peak,” Pontius says.
Faurecia engineers then apply a mesh material that covers the holes but prevents sound waves from reverberating. The mesh keeps exhaust gases in the system but arrests much of the vibration. Eliminating the resonator and using exhaust pipes that are thinner but don’t resonate can save 4 lb to 11 lb per vehicle, Pontius adds.
McKinley says Honeywell and other turbo suppliers constantly work with automakers to find new NVH solutions. In some cases, minor design changes can improve airflow into turbochargers to eliminate pulsation noises before they occur.
“Turbos are going something like 200,000rpm to 250,000rpm. It’s a device that’s going much faster than anything else under the hood. That in itself is going to bring some unique noise-generation opportunities,” McKinley says. “Every turbo that you see built into an OEMs product today goes through a high-speed balance and correction mechanism before it gets to the OEM.”
In some ways, increased NVH from more-powerful combustion is more noticeable because turbo engineers have eliminated much of the noise that came from the turbocharger itself, he adds.
“Our goal in every program with the OEMs is to not annoy the customer, to not let them know what’s going on under the hood,” McKinley says. “We want them to feel as good about using a downsized, boosted engine as they would with a larger, naturally aspirated engine.”
In talking to automakers, McKinley adds that NVH tends to be the No. 3 concern, behind cost and performance, for turbochargers. For Honeywell, the bigger concern is ramping up production to add capacity for as many as 5 million more turbochargers per year by 2025 when fuel economy regulations will require automakers to hit 54.5mpg.
Schaeffler Group North America
Honeywell Transportation Systems
Faurecia Emissions Control Technologies
About the author: Robert Schoenberger is the editor of TMV and can be reached at 216.393.0271 or email@example.com.