Michael J. Readey

Insulating Structural Ceramics Program, Final Report

New materials and corresponding manufacturing processes are likely candidates for diesel engine components as society and customers demand lower emission engines without sacrificing power and fuel efficiency. Strategies for improving thermal efficiency directly compete with methodologies for reducing emissions, and so the technical challenge becomes an optimization of controlling parameters to achieve both goals. Approaches being considered to increase overall thermal efficiency are to insulate certain diesel engine components in the combustion chamber, thereby increasing the brake mean effective pressure ratings (BMEP). Achieving higher BMEP rating by insulating the combustion chamber, in turn, requires advances in material technologies for engine components such as pistons, port liners, valves, and cylinder heads. A series of characterization tests were performed to establish the material properties of ceramic powder. Mechanical chacterizations were also obtained from the selected materials as a function of temperature utilizing ASTM standards: fast fracture strength, fatique resistance, corrosion resistance, thermal shock, and fracture toughness. All ceramic materials examined showed excellent wear properties and resistance to the corrosive diesel engine environments. The study concluded that the ceramics examined did not meet all of the cylinder head insert structural design requirements. Therefore we do not recommend at this time their use for this application. The potential for increased stresses and temperatures in the hot section of the diesel engine combined with the highly corrosive combustion products and residues has driven the need for expanded materials capability for hot section engine components. Corrosion and strength requirements necessitate the examination of more advanced high temperture alloys. Alloy developments and the understanding of processing, structure, and properties of supperalloy materials have been driven, in large part, by the gas turbine community over the last fifty years. Characterization of these high temperature materials has, consequently, concentrated heavily upon application conditions similiar to to that encountered in the turbine engine environment. Significantly less work has been performed on hot corrosion degradation of these materials in a diesel engine environment. This report examines both the current high temperature alloy capability and examines the capability of advanced nickle-based alloys and methods to improve production costs. Microstructures, mechanical properties, and the oxidation/corrosion behavior of commercially available silicon nitride ceramics were investigated for diesel engine valve train applications. Contact, sliding, and scratch damage mechanisms of commercially available silicon nitride ceramics were investigated as a function of microstructure. The silicon nitrides with a course microstructure showed a higher material removal rate that agrees with a higher wear volume in the sliding contact tests. The overall objective of this program is to develop catalyst materials systems for an advanced Lean-NOx aftertreatment system that will provide high NOx reduction with minimum engine fuel efficiency penalty. With Government regulations on diesel engine NOx emissions increasingly becoming more restrictive, engine manufacturers are finding it difficult to meet the regulations solely with engine design strategies (i.e. improved combustion, retarded timing, exhaust gas recirculation, etc.). Aftertreatment is the logical technical approach that will be necessary to achieve the required emission levels while at the same time minimally impacting the engine design and its associated reliability and durability concerns.