A vehicle's exhaust system is more than just a tailpipe and a muffler. In fact, it is one of the most complex systems on your car. Exhaust systems act like finely tuned musical instruments. It emits different sound frequencies and harmonics based on exhaust gas pulses and how they travel through the pipes. In order to achieve a viable exhaust note, Karma engineers and Magneti Marelli (our Italian exhaust supplier) both took a novel approach in developing the Revero GT exhaust system for our BMW TwinPower™ Turbo three-cylinder on-board generator.
Developing the Revero GT exhaust system posed unique challenges. Conventional vehicles typically use a front-to-back system. Our unique powertrain consisting of an on-board generator, battery pack and dual electic motors, necessitated the need for a horizontally opposed exhaust system.
“A loud, droning exhaust can cause mental fatigue over a long highway drive. Our goal was ‘tuning’ harmonic frequencies to create a pleasant-sounding exhaust system. Delivering a quiet and comfortable driving experience were core objectives throughout the entire development process,” mentioned Melody Vo, Karma Automotive lead engineer for powertrain mechanical.
The resulting three-piece stainless-steel exhaust system uses dual mufflers and a center resonator. The resonator was designed with an internal chamber that uses an S-shaped tube and baffles to suppress sound. Additionally, a variable electronic valve system was incorporated on the driver side muffler. This electronic motor opens and closes a butterfly valve to manipulate the amount of exhaust flow which coincides with the increase or decrease in tone.
“The exhaust valve is variable so it’s not simply an open or closed scenario. The valve is controlled by a hybrid control module that interprets information according to the engine behavior during [all] three driving modes (Stealth, Sustain, and Sport), [as well as] Launch Mode. The resulting change in exhaust tones and decibel levels were quite dramatic,” says Vo.
From the first prototype Revero GT exhaust to final production, testing revealed that sound frequencies were dramatically reduced by 5 to 10 dB at the tailpipe.
At low-speeds including both “Stealth” and “Sustain” mode, the valve remains closed to divert exhaust gases to the passenger side muffler, minimizing sound. In “Sport” mode, when the accelerator pedal is depressed past 80 percent, the valve begins to open to increase flow. When Launch Mode is initiated, the valve immediately opens to provide a direct path for exhaust gases to exit the tailpipe for optimal performance.
“We’ve noted a number of inquiries about the unique side exit exhaust design and the possibility of exhaust fumes wafting into the interior cabin. We performed multiple CFD simulations to monitor exhaust flow patterns and confirmed that obnoxious fumes will not flow into the cabin or circulate under the vehicle.”
“Our team is proud to have met all of our objective targets while creating an exhaust system that delivers a unique personality of sound. We’ve received positive feedback from consumers and media outlets alike. They really enjoy the distinct exhaust tones and perfromance the Revero GT delivered when driven in the different modes.”
This post was edited 10/22/2019 12:41:57