3211Diesel Engine Components

The development of low heat rejection diesel engine technology, for both stationary and mobile power plants, will require new in-cylinder materials capable of withstanding the higher temperatures produced from insulating the combustion chamber and nearby components. One of those applications involves the evaluation of CFCCs as a self-lubricating valve guide.

Tests demonstrate continuous carbon fiber-reinforced silicon nitride provides a low-friction surface, resulting in decreased wear of the valve stem. Since they require no oil for lubrication, CFCC valve guides will save fuel, increase engine efficiency 30%, increase reliability, and reduce particulate emissions by 25%.

Current cast-iron valve guides operate for 20,000 h. Commercial liquid lubricants are fed into the guide-valve stem interface. At temperatures above 300°C (572°F), the cast-iron has insufficient yield strength to support valve stem side loads. The resulting deformation allows the valve to contact the cylinder liner and not seat properly in the valve seat, decreasing its performance and life. This problem is intensified in new designs that operate at higher temperatures. At 500° C (932°F), lubricants cannot withstand the severe thermo-oxidative environments without deposit formation and wear. These deposits cause valves to stick and form particles that accelerate wear. The CFCC valve guide will be used initially in stationary diesel engines and applications may extend to internal combustion engines in general.

The top piston ring in a diesel engine is exposed to very severe thermal, mechanical, and chemical conditions. Each cycle of the engine exposes the top of the piston and the first piston ring to a burst of high-pressure and high-temperature combustion gases. They force the piston to move through the cylinder, resulting in high mechanical stresses imposed upon the ring at both the interface with the piston and the cylinder liner. The life of the rings is limited by wear and cyclic fatigue.

Lubricant must be supplied to minimize these mechanical stresses. Lubricant leaks past the ring, burns, and exits as pollution. Ceramic rings have the potential for lower wear, greater resistance to cyclic fatigue, and higher temperature capability. They have less tendency to distort, resulting in less lubricant leakage. Their thermal stability will permit more complete combustion and less pollution.

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