ATV technology for efficient engine noise predictions
Author: Sound and Vibration

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 realized 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 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 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 1500 to 6750 RPM and frequencies from 100 to 2500 Hz for a new 3-liter 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 airborne transmission, engine mount vibration forces are calculated as part of the procedure, which is the first stage in structure-borne noise prediction.