Advanced Tribological Systems

Cummins has considerable expertise in rig and engine testing of advanced tribological systems through in-house efforts and government-funded programs.

A variety of plasma sprayed, detonation-gun and other ceramic coatings have been evaluated in both engine and bench rig tests. Figure 5 shows wear data as a function of temperature for several ring and liner combinations in dry sliding and with a commercially available polyol ester based synthetic lubricant. It may be seen that dry wear rates are typically one to two orders of magnitude higher than could be tolerated in a commercial engine. The best ceramic material identified in these tests was a plasma sprayed chromium oxide, which was also identified from uncooled engine tests as being an excellent wear material. Figure 6 shows the mirror-smooth surfaces generated with this material running dry at 400°C. Conversely, a TiC-CaF2-Ni-Cr plasma sprayed ring running against a D-gun C^C^-Ni-Cr liner was found to be a poor wear combination in both engine and unlubricated rig tests.

Lubricated wear tests confirmed the superiority of the chromium oxide material, and suggest that substantial improvements in elevated temperature wear rates compared to conventional systems may be obtained by using ceramic wear coatings and synthetic lubricants (Figure 5). Figure 7 shows the minimal wear which occurred on a Cr203 liner sample during a 3 hr Cameron Plint test at 420°C using the polyol ester based lubricant.

Referring to Figure 5. it may be seen that the distribution of wear between the ring and liner surfaces may be altered by suitable choice of coatings. For example. with the

TiC-CaF2-Ni-Cr ring versus

Cr3-C2-Ni-Cr liner combination, the wear rate of the coated liner is improved by about an order of magnitude compared to the wear rate of the standard grey iron material in the conventional Cr plate/grey iron ring: liner system. On the other hand, the wear rate of the coated ring shows no improvement over the baseline.

In conventional engines, the wear rate of the rings is usually designed to be much lower than that of the liner (for the materials in Fig. 2. the rings were found to be 10-100 times more wear resistant than the liners). The reason for this is that sophisticated piston ring profiles (eg. barrel face, taper face. etc) are required to provide the necessary gas sealing pressures and to minimize blowby and oil consumption. When original ring profiles are lost due to wear. oil consumption increases dramatically, and the engine has to be overhauled. Thus. for the ring: liner tribology system, very low ring wear coefficients are important in addition to low total wear rates (ring + liner).

Unfortunately. increasing the wear resistance of one of the sliding surfaces often causes an increase in the wear rate of the counterface. Thus. the wear properties of both the ring and liner surfaces must be matched to achieve an optimum tribology system.

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