Coating Durability Study

In 1985, the AAR Alternative Fuels Research Program joined with EMD and the U.S. Maritime Administration (MARAD) in a study to investigate the use of ceramic coatings in medium-speed diesel engines. EMD furnished coated engine components; AAR tested the coated components in the twelve-cylinder EMD engine; SwRI and Lawrence Berkeley Laboratory provided (under contract to AAR) detailed analysis of the coatings; and MARAD, in conjunction with EMD, arranged for a long term (9,000 hour) in- service test of an identical set of coated components. The long term test took place in a tugboat.

The objective of the AAR Coating Durability Study was to determine if:

o the ceramic coatings would adhere to the engine component for the duration of a 500 hour engine test o engine efficiency would increase as a result of using the ceramic coatings

Coatings Two different coatings (designated herein as Coatings A and B) were selected for the test. Coating A was a plasma spray deposited nickel-ciiramium-aluminum-yttria (NiCrAlY) bond coat with a Cr intermediate layer and a magnesia stabilized zirconia top coat. Coating B was comprised of a yttria stabilized zirconia on a NiCrAlY bond coat. Coating A was designed to provide maximum protection against corrosion/erosion while coating B was intended to provide maximum thermal protection.

EMD furnished twelve new power assemblies (liners, pistons, valves, and heads) for the test program. Four sets of pistons, heads, and valves had coating A applied, four sets had coating B, and the remaining four sets (designated as Q were used as control and were not coated. EMD machined the piston crowns and heads by an amount equal to the thickness of the coatings, thereby maintaining the proper clearance within the combustion chamber.

Coatings were applied only to the tcps of the pistons. The sides of the pistons were masked during spraying to prevent any of the material from being deposited there. Similarly, for the head, only the fireface was coated. However, when coating A was applied, a 5/16 inch wide ring next to the head gasket was left uncoated while the heads with coating B were covered up to the gasket.

The valves were not machined prior to coating. Coating A was applied only to the face of the valve. Coating B was applied to the face of the valve as well as on the exhaust side tulip and a portion of the valve stem. The valve seats were not coated as heat flow through this area is the primary mechanism for valve cooling.

Engine Testing To evaluate the abili ty of the ceramic coatings to withstand the combustion forces generated in a medium-speed diesel engine, they were installed in the AAR/SwRI twelve (12) cylinder 645E3 EMD two-stroke diesel engine and a 500 hour engine test was conducted. Wear was determined by measuring the components before and after the 500 hour test.

All twelve power assemblies were dismantled and measured prior to installation in the engine. Assemblies with coating A were installed in cylinders 2,5,9 and 12 while those with coating B were installed in cylinders 3,6,7,and 10 and the uncoated assemblies were installed in cylinders 1,4,8, and 11 (see Figure 2 ). Through this arrangement cylinder interaction was minimized as no similar power assemblies were installed across from or adjacent to each other.

Final preparation of the engine was completed by filling the crank case with a new charge of DELCO 6000 oil. Instrumentation to monitor and record the information necessary to calculate engine performance parameters was installed. Engine performance parameters monitored included brake horsepower (Bhp), brake thermal efficiency (BTE), brake specific fuel consumption (BSFC), inlet and outlet cylinder water temperature, and cylinder exhaust leg temperatures. Special instrumentation to measure cylinder pressure, injection timing, and engine crank angle were installed only during the two performance tests

Coating Cylinder Cylinder Coating
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