As engine management systems run powertrains closer to the edge for the greatest reductions in emissions and fuel use, the noise, vibration, and harshness burden the engine mounts have to bear increases. Conventional engine mounts are composed of rigid metal or plastic structures bonded to a rubber element. More costly hydraulic mounts add hydraulic fluid to the rubber element. In both cases the rubber acts as a connection between two parts while allowing a degree of movement, and suppressing noise and harshness. New alternatives from Trelleborg Automotive (Höhr- Grenzhausen, Germany; www.trelleborg.com) and Lord Corp. (Cary, NC; www.lord.com) promise the sophisticated response of a hydraulic mount with a cost closer to that of a conventional design.
It’s in the Air
Trelleborg’s new design does away with the hydraulic fluid and replaces it with air pockets in the rubber. The voids are designed to provide specific spring rate characteristics. This is achieved, in part, by forcing air out through one or more engineered exits holes designed to help dissipate energy in a controlled fashion. To make certain the mounts are optimized for each application, Trelleborg engineers developed a software tool that helps predict the best combination of properties (void size, exit hole size and shape, rubber durometer, etc.) that also takes into account the characteristics of the other bushings and mountings along the subframe (if fitted), engine, and gearbox. Development projects with General Motors, BMW, Porsche, Audi and Ford are currently underway.
Lord Corp., on the other hand, uses a glycol-based magnetorheological fluid that changes state from a liquid to a semi-solid in the presence of an electric current for its prototype engine mount design. It claims this allows the mount’s response to be tailored to terrain conditions and vehicle characteristics, as well as a reduction in parts count, complexity, package size, and weight when compared to hydraulic mounts. In addition, the fast-acting (10 ms) reversible fluid lessens the compromise between high- and low-displacement isolation, and responds better to transient situations. In addition, the technology can be adapted to suspension systems to improve handling and isolation.