Silicone making autos quieter than ever.

One of the hallmarks of a quality automobile is quiet operation. Increasingly, consumers are demanding cars that are quiet when they come from the showroom and that stay that way over tens of thousands of miles through all kinds of driving conditions. From the automaker's point of view, longer and more inclusive warranties call for materials that will help maintain vehicle quality and owner satisfaction over longer periods of time.

While the choice of automotive materials is governed by many factors, the ability to contain and reduce noise has become a major consideration.

Noise is vibration and automotive engineers are constantly looking for materials that serve to dampen vibration and hence reduce noise. Where noise cannot be significantly reduced or eliminated, materials are sought that will acoustically insulate the passenger compartment.

A family of materials that have both vibration reducing and acoustical insulating properties, silicones are found in numerous automotive applications where noise is a concern. Silicone gaskets and bushings help to reduce noise within the engine by providing long-lasting vibration absorption. These gaskets also help to contain noise produced by timing chain mechanisms, valves and other enclosed moving engine parts.

Gaskets molded of silicone have an excellent long-term performance record because of that material's unique chemical and physical properties. Silicone performs well at extremes of heat and cold, and throughout repeated thermal cycles. Silicone gasket materials exhibit excellent compression-set resistance. Silicone gaskets recover quickly and fully from a compressed position, even when exposed to a variety of compression pressures and durations.

Engineers working on gasket design now have at their disposal a new method for evaluating and specifying sealing materials. Developed by scientists at GE Silicones, compression stress relaxation testing measures the actual response of elastomeric materials and allows for age-testing in oils and solvents to determine how the materials react to prolonged contact, simulating actual sealing conditions.

Compression stress relaxation testing not only allows engineers to measure the response of a material under actual operating conditions, but it enables them to incorporate improved stress relaxation properties in new generations of silicone elastomers. As a result, the reliability of engine gaskets is increased. A gasketing material that loses its sealing function also contributes to increased vibration and engine noise. Hence, elastomers with improved stress relaxation properties contribute to quiet engine operation.

Ford's new 4.6 liter modular V-8 engines provide an example of how silicone gaskets contribute to a powerplant that produces far less noise than its predecessors. According to engineers at Ford, the design of the new engine resulted in a 50% reduction in noise level when compared to the previous 5.0 liter V-8.

Vibration in the Ford engine was reduced, in part, by the use of silicone bushings made from material supplied by GE Silicones. The 16 bolts that attach the cam covers to the cylinder head pass through these bushings to reduce vibration and noise. The excellent mechanical properties of silicones mean that the bushings will continue to reduce vibration and noise throughout die life of the engine.

Silicone gaskets are also specified throughout the Ford modular engines. While the primary function of most gaskets is to provide a durable, leak-proof seal, silicone gaskets also contribute to long-term quiet operation. The durability of silicone gasketing material helps to ensure that gaskets in the Ford 4.6 liter V-8 engine will retain their sealing properties and noise reduction capabilities for the life of the engine.

General Motors' new 32-valve Northstar engine, which powers the Cadillac Seville and Eldorado sedans, also makes significant strides in noise reduction. The basic engine design - one that is more commonly found in racing engines - incorporates an aluminum upper block, a ladder design lower block and an aluminum oil manifold plate. The design contributes to the overall strength and rigidity of the engine, factors which help to lessen vibration and reduce noise. Silicone rubber gaskets provide the seal between these critical components.

While some gaskets are essential to controlling vibrations, others serve to control noise generated by engine components. The Northstar engine has an innovative timing chain cover that incorporates silicone rubber gaskets between two steel plates to isolate and dampen the noise generated by the chains and sprockets. The cam covers also feature silicone gaskets for sealing and noise control.

The Northstar's air and fuel delivery systems incorporate special silicone gaskets to help reduce noise. The traditional intake manifold is replaced by a fuel management system that is contained in a magnesium housing. The unit includes a silicone carrier gasket module for the fuel rail and fuel injectors, distribution plate gaskets and spark plug gaskets. The fuel injection system is positioned within the lower part of the housing, dampening the clicking sound often associated with fuel injected engines. These design improvements have resulted in an engine that is so quiet that an ignition interlock has been added to prevent the driver from inadvertently turning the key when the engine is already running. The ability of silicone materials to perform multiple functions, including sound dampening, is demonstrated in another automotive application where noise reduction is a consideration. On its new LH platform, Chrysler uses a special silicone coating system, developed by GE Silicones, as a coating on EPDM weatherstripping for windows and doors. The silicone improves the long-term weatherability and anti-squeak properties of the rubber. Because silicones remain flexible at very low temperatures, the weatherstrip coating prevents squeaks even in sub-zero temperatures. Versatile silicone materials are used in dozens of automotive applications. Their primary function may be sealing, heat-shielding or weatherproofing, but in the majority of applications, silicones also play a big role in helping today's cars to run quietly and do so over the long run.

Donald L. Finney is manager of advanced automotive concepts at GE Silicones in Waterford, NY.