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Figure 4. Thermal Barrier-Coated Piston No. 1

3. A thin thermal barrier coating on the capped piston design reduced the top ring groove temperature by 200-300 degrees F.

4. The thermal barrier-coated iron piston provided a higher level of insulation than the average capped piston design.

5. The thermal barrier-coated titanium alloy piston provided a significantly greater level of insulation than any of the other designs.

6. The titanium piston ring groove temperature was significantly higher than that predicted for the iron piston.

From the analytical work it was decided that the capped piston design to be investigated in the exploratory development phase would use the insulating disk plus an air-gap, no seal at the cap-to-base air-gap, and a thin coating on the combustion surface.

It was concluded that for the AIPS engine application the insulating disk could be dropped in favor of a simplified design. In addition, an alternative attachment design was proposed which would provide a spring to improve the control and level of preload maintained on the cap shank. This would reduce the tendency for the cap to become loose or overly tight due to thermal growth differences. This design would result in improvements in fatigue resistance. The final capped design proposal is illustrated in Figure 5.

The most promising thermal barrier-coated design concept was considered to be the titanium alloy piston. The most significant advantage for a titanium alloy piston was considered to be its light weight. In addition, due to the low thermal conductivity of the alloy, a thinner thermal barrier coating would be required. The greatest potential disadvantage was considered to be the high ring groove temperatures. A decision was made to explore both the iron and titanium pistons and consider design optimization as appropriately determined.

Figure 5. Capped Air-Gap-Insulated Piston

Exploratory Development: First priority has been placed on the thermal barrier coating concept for insulating in-cylinder components because this approach offers the lowest risk, least failure damage levels, greatest flexibility and ease of application, shortest potential delivery schedule, and analytically offers the same level of insulation effectiveness as more complex capped designs. Therefore, the greatest emphasis is being placed on thermal barrier coatings during the first stages of Phase II.

In addition to specific component design issues and analyses which are being addressed in Phase II, material and design bench test screening is included. Two major bench test rigs were adopted; 1) simple coupon thermal fatigue rig, and 2) small bore test engine.

The thermal/shock/fatigue rig is illustrated in Figure 6. This rig is composed of a natural gas flame which heats the coated coupon face (1.25 inch diameter) for a determined amount of time and then quenches the entire coupon in a water bath. This test is considered an accelerated version of the engine cycle which simulates transient heating (combustion) and quenching (intake and compression of air). The types of coating crack failures observed on the rig are very similar to those observed on full-sized diesel engines. Coupon samples have been coated by selected thermal barrier suppliers. Almost all coatings have been deposited using the plasma spray process. Test results are included in Figure 7. Two failure categories have been noted. The first is craze (mud) cracking and the second is separation or loss of coating. Generally the first failure to occur is the craze cracking.

Figure 6. Thermal Shock Fatigue Rig Illustration.
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