DieselTech: Zirconia a Gem in Fuel Injector Application

The 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.

A cutaway of Cummins’ Celect fuel injector shows the white metering plunger made from Carpenter’s ZrX-GBP zirconia, and the longer white timing plunger constructed from a conventional magnesia partially stabilized zirconia (Mg-PSZ).

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 steel 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 rail 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. 

Two finished metering plungers made from Carpenter’s new ZrX-GBP zirconia.

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 strength 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. Carpenter says its new ZrX-GBP zirconia has flexural strength approaching that of traditional Y-TZP, but with higher fracture toughness and without the thermodynamic instability and high-temperature creep problems associated with that grade.

ZrX-GBP zirconia can be matched to a very small plunger-bore clearance (single-digit microns). This property allows close tolerance, high-pressure applications with metal components.

The Carpenter zirconia has provided such high-contact stress resistance that it is very well suited for the complex geometry of the metering plunger. How that result relates to performance goes back to the component and injector design.

Carpenter provides Cummins with partially ground zirconia blanks with a drain hole in one end that intersects a cross drilling. Precisely located edges and other features are precision ground at the other end by Cummins. Fuel flows under pressure through ports in the injector bore, then passes through the holes in the zirconia plunger as it moves in and out of position in the bore. The features that control fuel metering on the plunger must remain perfect throughout the lifetime of the injector so that the fuel flow is started and stopped precisely.

This is all a matter of extreme dimensional precision. To obtain desired injector efficiency, timing control of fuel flow is measured in milliseconds. The slightest deviation or interruption in flow can result in performance variation.

Carpenter follows a production process strictly controlled by QS9000 procedures in preparing its ZrX-GBP zirconia blanks for Cummins. Carpenter grinds blanks and grinds the outside diameter and taper to tolerances as close as the low single-digit micron size (0.00000X meters) for cylindricity. The finished blanks undergo extensive testing and inspection for quality control before shipping.

After it receives the ground zirconia metering plunger blanks from Carpenter, Cummins completes the finishing of the o.d. surface, spring post and groove on specialized infeed and throughfeed centerless grinders. Machine operators achieve submicron tolerances on cylindricity, which is then match-fit to similar diametrical tolerances. After surface finishing, the plungers are assembled in Celect diesel fuel injectors.

All Cummins design requirements for strength and toughness have been met by ZrX-GBP in the new metering plunger, although it is likely that still higher levels will be required for newer, more demanding applications in the future. Scuffing and adhesion problems frequently encountered with mating metal components have been eliminated with zirconia, Carpenter says. A monolithic metering plunger of the newly developed zirconia has proved to be a very costeffective alternative to coating, which can be expensive and is typically only a few microns thick.

Carpenter reports that the mean coefficient of thermal expansion for its new zirconia over a temperature range of 25 to 400 degrees C is 10×10-6/degrees C. This is comparable to that of metal, and distinctly higher than the 6×10- 6/degrees C for most ceramics.

The flexural strength of the new ZrX-GBP zirconia is said to be higher than that of any of the known Mg-PSZ materials due to development of a highly engineered microstructure. The very fine average grain size attained, together with a new technology for inhibiting grain growth and boundary movement, contribute greatly to the grade’s high strength and impact resistance, Carpenter says.

Average grain size of the zirconia used for this fuel injector application is significantly smaller than the 40 microns average grain size for the MS grade, and 45 microns average for the TS grade.

Carpenter’s ZrX-GBP zirconia has exhibited the same strong corrosion resistance and overall stability of its MS and TS grades of Mg- PSZ zirconia. The new grade also has excellent resistance to galling, scuffing and wear in sliding contact with steels and other metals, the company says.