The struggle in automotive design between performance and passenger comfort never ends. Comfort and performance sell cars. Fuel economy satisfies increasingly stringent government regulations. Caught in the middle, engineers work to strike effective compromises between them.
A classic example in automotive design is the need to balance fuel efficiency with noise, vibration, and harshness (NVH) limits. Given the complexity of the two major systems involved -- the drivetrain and the engine -- it is also one of the hardest challenges.
The engine and the drivetrain work at near 100% efficiency and deliver maximum fuel economy when a device called a lock-up clutch engages and provides a direct physical connection between the engine and the transmission. Clutches most often lock up when a vehicle is cruising at a constant speed, usually on flat stretches of highway.
However, clutch lockup can transmit high levels of vibration through the vehicle. It also aggravates a condition called “lugging,” which causes the vehicle to bump and lurch when it’s cruising in a high gear and the driver steps on the accelerator.
Bring in the Torque Converter
Engineers can reduce vibration by adjusting the torque converter, the component that contains the lockup clutch, to “slip,” or provide less than a 100 % locked-up connection between the engine and the drivetrain. Slip reduces the amount of vibration transmitted through the steering wheel and seats.
That improved NVH performance comes at a price, however. Slipping decreases the drivetrain’s efficiency because of the energy lost to friction and “fluid coupling,” which is when the liquid in the transmission provides the connection between the engine and the drivetrain. Fluid coupling is less efficient than physical coupling, so it delivers lower fuel economy.
Recognizing that some slip is necessary for passenger comfort, automotive engineers have sought the optimal amount of slip for years. Until recently, the only way to test slip was to wait for a prototype vehicle. The problem is that if the engineers weren’t correct on their slip calculations on the first try, it was nearly impossible to do anything about it. The design is essentially frozen at the prototype stage, which makes changes expensive and potentially delays production.
Ford wanted a way to simulate the effects of different torque converter designs so engineers could make informed trade-offs between slip and lockup. The company took advantage of an open standard for co-simulation called Functional Mock-Up Interface (FMI). FMI enables engineers to create a virtual product from a set of models.
Bring in the Simulation
Ford used FMI to create models of the drivetrain and the full vehicle in MSC Software’s Adams multi-body analysis software. It enabled engineers to create separate models of the vehicle and the drivetrain and then test them in the same simulation.
Ford built 3D vehicle and drivetrain models in MSC Software’s Adams multi-body dynamics software, which supports FMI, and combined them with a 1D model of the slip controller created in AMESim software. Their objective was to optimize the slip in the torque converter to meet NVH targets while at the same time maximizing fuel economy.
The engineering staff ran the model for different values of desired slip. The simulation results showed that a slip of 30 revolutions per minute (RPM) would fail to meet the NVH target while a slip of 40 RPM or greater would meet the target. The simulation demonstrated that 40 RPM slip was the optimal trade-off between NVH and fuel economy.
Engineers also studied related assemblies to gauge the effects of slipping. They found that slipping reduced vibration through the steering wheels and seat tracks. In the future, Ford plans to validate the models with physical testing results, then integrate simulation into the design process so the torque converter design can be optimized early in the development cycle. This will help develop vehicles that deliver the comfort and performance required to appeal to customers and the efficiency to meet increasingly stringent fuel economy standards.
Yijun Fan is a product marketing manager and Wulong Sun is a technical support engineer at MSC Software Corp.
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