LMS Virtual.Lab Engine Acoustics Enables More Efficient and Faster Noise Predictions
The time needed to predict the radiated noise for a new engine design has been reduced at Cosworth Technology Ltd. following a collaborative project with Ove Arup & Partners (Arup), a leading UK-based engineering consultancy firm. Arup and Cosworth Technology, a global integrated powertrain solution provider, succeeded in assessing the acoustic performance of a new engine for a luxury automobile using just a few days of processing time. These significant time savings were realised in two steps. First, a dedicated acoustic mesh was rapidly created within LMS Virtual.Lab Pre-acoustics. Thereafter, the innovative Acoustic Transfer Vector (ATV) method was applied in LMS Virtual.Lab Engine Acoustics, allowing fast predictions of the noise levels over the full rpm range of the engine. This approach yielded detailed results, giving the confidence and assurance that potential problems could be accurately traced and subsequently fixed.
Using the conventional Boundary Element (BE) approach, calculations must be repeated for every load condition and engine speed, so that a limited number of frequencies and engine speeds can be evaluated within the time constraints of modern engine design programs. This new approach reduces the process time by exploiting the ATV technology. An ATV is the relationship between the unit vibration velocity at each surface point and the acoustic pressure, independent of engine speed or vibration response, at some point in the radiated noise field. The ATVs are then multiplied by the structural vibration, under the desired loading and engine speed condition, to deliver the noise prediction. Using this approach, engineers from Cosworth Technology and Arup calculated engine sound power levels for all combinations of speeds from 1,500 to 6,750 rpm and frequencies from 100 to 2,500 Hertz for a new 3-litre engine design. The result of having detailed acoustic information early in the design process makes it possible to optimize the design of the external ribs, sump and ladder frame bolting arrangement and bell housing before the base engine design is frozen ahead of prototype build.
In the increasingly competitive automobile market, consumers expect engines that are not simply more powerful and fuel-efficient, but also more refined and, in particular, quieter. Traditional methods of reducing noise, such as building stiffer and larger structures, incorporating isolating and damping materials, conflict with other goals such as reducing weight and fuel consumption. Stringent engineering goals together with an increasingly discriminating consumer raise the importance of acoustic engineering in the design process. Target-setting strategies mean that noise and vibration levels are defined as part of the overall vehicle-performance goals and are then cascaded down to the power-train level. Noise is transmitted from the engine to the vehicle interior both through the air and through the structure. Although the main focus of the process described here is air-borne transmission, engine mount vibration forces are calculated as part of the procedure, which is the first stage in structure-borne noise prediction.
The time needed to predict the radiated noise for a new engine design has been reduced at Cosworth Technology Ltd. following a collaborative project with Ove Arup & Partners (Arup), a leading UK-based engineering consultancy firm. Arup and Cosworth Technology, a global integrated powertrain solution provider, succeeded in assessing the acoustic performance of a new engine for a luxury automobile using just a few days of processing time. These significant time savings were realised in two steps. First, a dedicated acoustic mesh was rapidly created within LMS Virtual.Lab Pre-acoustics. Thereafter, the innovative Acoustic Transfer Vector (ATV) method was applied in LMS Virtual.Lab Engine Acoustics, allowing fast predictions of the noise levels over the full rpm range of the engine. This approach yielded detailed results, giving the confidence and assurance that potential problems could be accurately traced and subsequently fixed.Using the conventional Boundary Element (BE) approach, calculations must be repeated for every load condition and engine speed, so that a limited number of frequencies and engine speeds can be evaluated within the time constraints of modern engine design programs. This new approach reduces the process time by exploiting the ATV technology. An ATV is the relationship between the unit vibration velocity at each surface point and the acoustic pressure, independent of engine speed or vibration response, at some point in the radiated noise field. The ATVs are then multiplied by the structural vibration, under the desired loading and engine speed condition, to deliver the noise prediction. Using this approach, engineers from Cosworth Technology and Arup calculated engine sound power levels for all combinations of speeds from 1,500 to 6,750 rpm and frequencies from 100 to 2,500 Hertz for a new 3-litre engine design. The result of having detailed acoustic information early in the design process makes it possible to optimize the design of the external ribs, sump and ladder frame bolting arrangement and bell housing before the base engine design is frozen ahead of prototype build.In the increasingly competitive automobile market, consumers expect engines that are not simply more powerful and fuel-efficient, but also more refined and, in particular, quieter. Traditional methods of reducing noise, such as building stiffer and larger structures, incorporating isolating and damping materials, conflict with other goals such as reducing weight and fuel consumption. Stringent engineering goals together with an increasingly discriminating consumer raise the importance of acoustic engineering in the design process. Target-setting strategies mean that noise and vibration levels are defined as part of the overall vehicle-performance goals and are then cascaded down to the power-train level. Noise is transmitted from the engine to the vehicle interior both through the air and through the structure. Although the main focus of the process described here is air-borne transmission, engine mount vibration forces are calculated as part of the procedure, which is the first stage in structure-borne noise prediction.Traditional BE approach not practical within modern project timescales
Engine designers begin to address noise and vibration targets immediately after the end of the design intent phase, when major components such as the block and head, crankshaft, camshaft, and cooling jacket are defined. Completion of this phase enables a detailed structural Finite Element (FE) model of the engine to be created and used for various calculations, including thermo-mechanical, fatigue, dynamics and vibration response. Since prototype testing is not completed until later in the development cycle, engineers need to begin calculating the noise radiated by the engine. The conventional BE approach derives a BE model from the structural model and uses the vibrations of the exterior surfaces to define the boundary conditions for the acoustic radiation problem. The drawback of this approach is that boundary conditions vary with frequency and engine speed, which means that the BE solution must be computed for each set of loading conditions. Engineers need to evaluate all possible conditions so that all potential problems can be resolved. Acoustic problems that are not discovered until prototypes are tested and when design changes are more expensive, will cause delays and add cost to the programme.In the design of the new V6 engine, Cosworth Technology and Arup engineers established a process that aimed to provide a complete acoustic evaluation in a fraction of the time required by the conventional approach.This then paved the way to an analysis process that would fit well within the design program timescales.
The all-aluminium engine was required to meet strict noise targets, cascaded down from the vehicle refinement targets for the luxury car in which it is used. LMS Virtual.Lab Engine Acoustics provides an environment for predicting acoustic performance within a timeframe that makes it possible to optimize the design prior to prototyping. This environment includes tools that substantially reduce the amount of time needed to produce a BE model based on the structural model. Most importantly of all, the new Modal ATV technology eliminates the need to generate a BE solution for each speed-frequency combination and is the key factor in the success of this process.
Streamlining radiating-surface meshing
LMS Virtual.Lab Pre-Acoustics meshing tools were used to create the BE mesh of the radiating surface. The meshing process was often awkward in the past because the surface data upon which the structural FE model is created is far too detailed for BE meshing and at the very least a lot of re-meshing work is necessary. The Pre-Acoustics program removes this process bottleneck by automatically creating an envelope mesh on the outer surfaces of the structural mesh, using a wrapper technique. Special tools assist the user in identifying and filling holes, removing unwanted details such as ribs, and bridging gaps between components, such as the engine block and valve cover. These tools make it possible to create a high-quality mesh in just a few hours.
Vibratory modes driving rapid acoustic radiation predictions
A detailed FE model of the engine structure was created and an eigenvalue solution yielded some 750 modes in the frequency range below 3,100 Hz. The structural model was loaded with cylinder pressures and inertial forces/moments. These excitations were applied at the cylinder and main bearing locations. The mode shapes generated from the structural model were used to calculate Modal ATVs (MATVs) at the desired resolution in the frequency range 100 to 2,500 Hz using LMS Virtual.Lab Engine Acoustics. The MATVs are based on ATVs and define how each mode shape might contribute to radiated noise. MATVs are fundamental to the entire process as they form a link between an acoustic problem and a structural modification that might reduce noise.
The MATVs were calculated at an array of locations, analogous to test cell microphone positions, based on an ISO3744 sound-power mesh comprising 19 field points. The software provides a computation process for ATVs and acoustic results that interpolates between ATV values computed at master frequencies and a limited number of field points. The MATVs were in turn multiplied by the corresponding modal participation factors to determine the acoustic radiation. LMS Virtual.Lab Engine Acoustics simplifies the data handling by providing a software environment that provides access to a number of disparate data sources and this, coupled with an efficient process management facility, saves a significant amount of time.
Significant time savings
Such time savings were achieved because Arup and Cosworth Technology engineers were able to derive the acoustic radiation results from any operating condition with just a single BE solution. The BE solution takes a few hours in total with no need to re-compute when the structural modes are changed. Once all frequencies are computed color maps make it possible to display both order-related and resonance-related phenomena. Having all this information in one diagram makes it easy to relate the acoustic results to the different structural modes and the features of the design that excite them. The first execution of the complete process, from receiving the structural mesh to viewing the color maps of radiated noise was completed using two to three days elapsed computation time. This is significantly less than what would have been required in the past to obtain information at a limited number of frequencies and speeds.
Cosworth Technology and Arup engineers have successfully established a new process, which provides a much more efficient method of predicting engine-radiated noise for the full range of operating speeds and loading conditions. Importantly, the process can be completed within the tight timeframes that exist in a modern engine design program. The MATV technology, fundamental to the entire process, has made it possible for problems with radiated noise and sound power to be traced back to the structural dynamic behavior and for design changes to be implemented and rapidly evaluated. The net result is that engineers have the opportunity to optimize the engine design and incorporate those structural design features necessary for target conformance.
