Zirconia a gem in fuel injector application: advances in materials technology play important role in the development of low-emissions engines

he use of advanced zirconia ceramics has significantly improved the performance, reliability and durability of the high-pressure fuel systems used in Cummins Inc. diesel engines. This application is a prime example of how advances in materials technology can play a key role in the development of low-emissions engines.

To reduce emissions and improve performance, Cummins fuel system developers have increased injection pressures to provide finer fuel atomization. The injector mechanical loads are increased and precision clearances must be maintained between the plungers and bores in order to maintain diesel fuel injection quality.

Meanwhile, the operating conditions that diesel fuel injectors must tolerate continue to be challenging. The reduction in diesel fuel sulfur levels, for example, also results in a decrease in fuel lubricity. During normal engine operation, the inadvertent contamination of fuel by water and other contaminants must be tolerated.

Cummins determined that zirconia ceramics solved most of the basic problems generated in this harsh tribological environment. This understanding came as a result of work partially funded by the Departments of Defense and Energy, working with U.S. suppliers in developing world-class zirconia materials capability in the U.S, One outcome of this work was the establishment of a joint-venture company, Enceratec Inc., to further develop and commercialize the use of advanced ceramic materials Cummins' tests with zirconia fuel injector plungers have demonstrated improved performance. With 1 percent water in the fuel, traditional tool steal plungers can seize in 10 hours or less. in contrast, zirconia plungers were operated more than 300 hours with 2 percent water in the fuel without seizing. Plungers of both materials were tested to determine the effects of side loading forces, overspeed and overpressure conditions and low lubricity. In these tests, zirconia plungers far exceeded the performance of the steel plungers.

Comparison tests for injector bore wear showed that zirconia plungers were superior to uncoated steel and titanium nitride coated steel Use of fuel systems with solid zirconia plungers has been increasing, since the ceramic material has been found to essentially eliminate plunger adhesion problems.

After Cummins' initial use of zirconia in its Celect fuel injector, the company incorporated zirconia plungers into its new Caps common raft fuel system.

With fuel system pressures rising, fuel lubricity declining and the continued problem with diesel fuel contamination, Cummins has developed a series of close-clearance, highly loaded components made from high-performance zirconia. Accordingly, the company has incorporated in its Celect fuel injector a metering plunger made from a re-engineered magnesia partially stabilized zirconia (Mg-PSZ) specifically tailored for the application. The Celect unit injector has been designed to pressurize and precisely control the amount of fuel fed into the fuel system.

The zirconia used for the timing plunger functioned well because its design requirements were less stringent than those for the metering plungers. However, more traditional zirconias did not have sufficient strength and fracture toughness for the more complex metering plunger. Auburn, Calif.-based Carpenter Advanced Ceramics, a subsidiary of specialty materials manufacturer Carpenter Technology Corp., developed a zirconia material with improved properties, which resulted in better retention of certain critical characteristics for the metering plunger.

Carpenter Advanced Ceramics is a vertically integrated manufacturer of technical ceramics, industrial ceramics and high-purity alumina- and zirconia-based structural ceramic components. The subsidiary of Carpenter Technology Corp. has facilities in California and Melbourne, Australia.

Cummins provided materials goals for Carpenter in the development of its new magnesia partially stabilized zirconia known as ZrX-GBP (grain boundary pinned) grade. Characterization of the new material was funded through a DOE program. This material, Carpenter reports, offers a combination of high strength, fracture toughness, wear resistance and corrosion resistance for automotive engine components and other applications requiring high performance and long-term reliability.

The new zirconia grade developed by Carpenter is considered an important advance in zirconia technology. It provides bending significantly higher than that of other commercially available Mg-PSZ materials, the company says. Its flexural strength is 850 MPa at 20 degrees C, well above that of 620 MPa for the TS (thermal shock) grade and 725 MPa for the MS (mechanical strength) grade at the same temperature. Both the TS and MS are grades of Carpenter Mg-PSZ