Optimizing the acoustic performance of variable transmission systems at P+Z

Over recent years, Continuous Variable Transmissions (CVTs) have made tremendous technical advances, providing increased convenience along with lower fuel consumption and better performance. However, the broadband noise excitations of these stepless transmissions introduce specific acoustic engineering challenges. In development consultancy projects for leading automotive OEMs, P+Z Engineering effectively optimized the acoustic performance of new CVT designs without adding extra weight or increasing production cost. The secret of the success is a dedicated virtual simulation process, which P+Z Engineering developed to accurately qualify the acoustic performance of designs early in the development process. LMS SYSNOISE, one of the key components in this process, supports the P+Z team in performing acoustic radiation simulations with unprecedented speed, accuracy and flexibility.

Over recent years, P+Z Engineering, a leading German engineering consultancy company, contributed to various CVT development projects for leading automotive OEMs. During these assignments, P+Z Engineering focused on optimizing the acoustic performance of new CVT designs, which typically imply different acoustic challenges when compared to regular gearboxes. Whereas manual or automatic gearboxes with sets of toothed gearwheels mainly emanate noise peaks at fixed frequencies, stepless gearboxes typically generate vibrations in a broadband frequency spectrum. Gisela Quintenz, Project Manager CAE Gearbox at P+Z Engineering in Munich, comments, “To avoid the application of damping material, which causes higher production costs and reduced thermal conduction, we established a dedicated acoustic simulation workflow that can be effectively applied from the early concept stages onwards. Acoustic simulation performed on early design concepts enables us to identify and eliminate the root causes of major acoustic issues by implementing global gearbox design modifications, such as adjusting bearing positions or modifying CVT housing design.”

The virtual simulation process starts with the creation of a structural Finite-Element (FE) model of the CVT assembly. In addition to the housing, P+Z engineers carefully model all the interior parts, including variator pulleys, chain, shafts and bearings. If possible, the structure of the engine is also modeled. In this regard, it is important to correctly define its original weight, center of gravity and flange design. After completing the model, P+Z engineers run initial dynamic analyses to reveal the characteristic natural frequencies of the assembled model. For the subsequent calculation of operational vibrations, they start with the definition of the most critical operating conditions. The related bearing excitation forces are imported either from prototype measurements on the test rig or from multibody simulation. P+Z engineers perform frequency-response analyses in the frequency range between 0 and 4 Kilohertz, with a frequency step size of 1 or 10 Hertz, which typically results in an overabundance of operational vibrations. The challenge is then to focus on the acoustically relevant phenomena.

The intensity of the noise radiated by vibrating CVT housing surfaces and internal components is investigated using acoustic simulation. Starting from the assembly CAD or FE model, P+Z engineers create an acoustic Boundary-Element Model (BEM). The BEM model of a CVT typically consists of over 15,000 elements with an average element size of approximately 10 millimeters, allowing accurate acoustic predictions to be performed up to 4 Kilohertz. P+Z uses LMS SYSNOISE to compute the Sound Pressure Level (SPL) at microphone positions correlated with future acoustic tests, or at points on an ISO-hemisphere generated at a distance of 1 meter from the object. Together with such a fieldpoint mesh, LMS SYSNOISE takes the BEM model of the CVT as input for its Indirect Uncoupled BEM simulation approach, which calculates a matrix of Acoustic Transfer Vectors (ATVs). Gisela Quintenz explains, “LMS SYSNOISE generates the ATVs and combines them with normal surface velocities imported from traditional frequency response analyses. Crystal-clear sound pressure graphs allow us to concentrate on critical resonance phenomena, and by evaluating element contribution plots, we are able to retrieve the related acoustic hotspots and to select the most appropriate design modifications. The LMS ATV method is an essential part of this approach, since it provides relentless simulation speed throughout the acoustic evaluation process of candidate design modifications.”

To optimize the acoustic performance of early CVT designs, P+Z engineers evaluate an arsenal of design modifications, which potentially include the positions of bearings, the attachment of the assembly to the chassis, the geometry of the shafts, the casting materials and the housing geometry. Specific attention is paid to the flange design, which sometimes contributes to significant noise reductions. During acoustic simulations, the possibility to reuse certain predecessor parts is systematically taken into account. At this stage in the development, the P+Z engineering team typically achieves local sound pressure reductions of 6 decibel or more.

The subsequent development stage consists of an acoustic simulation refinement loop to optimize function, assembly and casting as well as to reduce costs and weight. It is only at the end of this stage that the first functioning physical prototype of the assembly becomes available for acoustic measurements. Although this assembly is not up-to-date with the latest design status, it offers the possibility to calibrate the virtual model and further increase its reliability for future simulation runs. Gisela Quintenz highlights a number of typical engineering challenges and explains how they are tackled during the refinement stage, “Virtual simulation on CVT gearbox designs performed at this stage typically include the geometry optimization of the housing and its internal brackets. Ribs or grooves are applied to avoid excessive local membrane vibrations on the housing surface. Besides simulating the effect of shape modifications, LMS SYSNOISE supports the evaluation of wall thickness variations of molded housing surfaces. Stiffer designs, for example with corrugation or ribs, generally shift specific resonances to higher frequencies and lower the amplitudes of the noise that is radiated locally. 

However, in exceptional cases where stiffening measures enlarge the area of the vibrating surface, the sound pressure levels may increase. In such cases, an arched housing design may be a valid alternative, which, besides shifting the resonance frequency, also shows the tendency to reduce sound pressure levels. In certain cases, rather unusual countermeasures are applied to solve specific acoustic problems, as it occurred in the case of an internal bearing bracket. Where stiffness and weight variations did not yield sufficient sound pressure level reduction, the splitting of the internal bearing bracket into a decoupled arrangement of two separate parts with slightly modified fixations provided the targeted levels.” Before freezing the gearbox design, engineers also finish the integration of the electronics module, tune the overall gearbox stiffness and assess the impact of the gearbox oil filling.

Next to in-depth expertise and extensive experience, the execution of complicated acoustic engineering projects calls upon high-performance and reliable engineering solutions. Gisela Quintenz concludes, “LMS SYSNOISE has proven to be particularly valuable in mastering broadband CVT excitations, which require computational analysis over a wide frequency range with small frequency steps. Its fast and flexible acoustic simulation capabilities support P+Z Engineering in identifying cost-effective and production-feasible design modifications that optimize the acoustic performance of CVT gearbox systems. Overall, the success of these acoustic optimization projects relies on the tight cooperation between experienced CAD and test engineers together with the simulation-driven approach that P+Z Engineering deploys from the very early start of the development process.”