Seaworthy Systems

Overview

The results of a 14,000-hour marine industry demonstration of thin ceramic coatings in diesel engine combustion components are truly encouraging. These superior ceramic coatings on the piston crowns, exhaust valves, and the heads have operated in everyday towboat operation for over two years and have demonstrated the durability needed for extended component lifetimes in commercial operation. These ceramic coated components have produced a smoother running engine, with a reduced cetane requirement and a reduced fuel consumption.

Results and Discussions The results of 14,000 hours of testing show all of the coatings in good condition on the cylinder head and on the piston crown. The exhaust valves were the most severe test for the thin ceramic coating, and the combination of thermal cycling and mechanical shock proved to be excessive for the magnesium zirconate and the flame alloyed, partially stabilized zirconia. The difficult environment of the exhaust valve proved to be more than the typical (or average) ceramic coating could survive. Only one ceramic*coating applied by a single supplier provided the unique combination of technical process controls and strict quality control so critical to the production of a superior ceramic coating.

Figure 1 shows valve faces after 14,000 hours of operation. Figure 2 shows a close-up view of valve face coating. The thermal barrier effect of these thin ceramic coatings reduces the thermal stresses in the pistons, cylinder heads, and exhaust valves. By reducing the metal temperature of piston rings and exhaust valves, the thermal stresses in these components are reduced; and the overhaul life of these critical components will be increased.

In this towboat, the maximum power of the engine was increased significantly with ceramic coated components. The diesel engine's maximum power would be measured by the shaft torque meter with the engine's fuel injector rack at maximum displacement.

The test engine, with ceramic coated components, performed with a smoother, more responsive operation as compared to the uncoated engine. Whereas this was reported by the tow-boat's captain and was subjective, it remains a demonstrated benefit of these ceramic coatings. The response of the diesel engine has been an important feature of towboat operation since frequent and large throttle movements are commonplace in safely navigating the inland river systems.

The thermal efficiency and mechanical efficiencies of the engine have been increased by the use of ceramic coatings and embedded ceramic cylinder liners. The ceramic coatings have produced the increase in thermal efficiency (see Figure 3), and the treated cylinder liners have reduced internal friction to increase mechanical efficiency. The combination of the increased thermal efficiency and the increased mechanical efficiency has resulted in a significant reduction in fuel consumption.

The reduction in specific fuel consumption has been analyzed from the performance data to be eleven percent (11%)• See Figure 4. This eleven percent (11%) reduction in specific fuel consumption has been considered very significant in the marine industry. Whenever a marine operator considers the use of an advanced technology, the operator considers the rate of pay back on the investment in ceramic coatings must be favorable and must pay back in less than one year. With these significant savings in fuel consumption, the pay back of investment would be less than three months.

In addition to savings in reduced fuel consumption, the lube oil consumption of the engine was reduced by about fifteen percent (15%). Also, the lube oil was noted to be "cleaner" than typical when engine inspections were made.

The internal cylinder performance testings provided a clear insight into the mechanism that results in the diesel performance improvements. These thin ceramic coatings reduced the ignition delay, reduced the rate of pressure increase (after ignition), slightly lowered and slightly delayed the pressure peak, and in creased the area under the pressure-volume (P-V) diagram. See Figures 5 and 6. These performance changes resulted in a diesel engine with less cetane sensitivity, with increased horsepower capability, and with reduced fuel consumption. With the reduced cetane requirement, an operator could order distressed diesel fuel or low cetane diesel fuel oil at a lower fuel cost for use in these ceramic coated engines. Where available, these low cetane diesel fuel oils could reduce fuel costs by another five percent (5%) or so.

Test Procedure

In order to demonstrate commercial ceramic coating durability and performance, it is essential to demonstrate in a commercial vessel, in everyday marine service, to provide the test environment and the duty cycle that could never be duplicated in a test stand, in a research institute, or in the engine builder's facilities. And, furthermore, the commercial vessel would be the only possible choice to operate and test for a 14,000-hour program. Test stand programs that run for fifty or even five hundred hour programs cannot begin to approach the value obtained by long-term, commercial operation. Commercial operators tend to favor commercial tests where the environment has not been "artificially" altered to suit the restrictions imposed by the testing company.

In this demonstration project, a twin engine powered towboat, the M/V BILL GEE, owned by Ingram Barge Company, was used as the test vessel. The starboard (stbd) 8-EMD-645 engine was rebuilt using new EMD pistons, exhaust valves, and heads that were ceramic coated using three different combinations of ceramic coatings.

To determine the long-term performance of these thin ceramic coatings, the starboard engine was instrumented; and a computer-controlled data conditioning and acquisition system was installed to monitor and record data on twin (floppy) disks for later data reduction and analysis.

To prevent testing errors when comparing two different engines, the starboard engine was run for 8,990 hours with the 100% ceramic coated components; and then the ceramics were moved to the port engine to continue to operate and accumulate operating experience.

Back to the starboard engine -the engine was then rebuilt with the same type, new but uncoated, components to permit performance data to be recorded for an uncoated baseline to provide a comparison to the ceramic coated data. Due to the long duration of this test work, many thousands of data points were recorded and evaluated to produce the longest recorded ceramic coated diesel test ever run. Additionally, engine injection rack position was recorded to provide a backup analysis to the torque meter and other instrumentation.

In addition to the long-term, computer controlled, and recorded data, additional internal cylinder analysis and realtime, steady-state engine data were recorded at equally spaced intervals during the ceramic coated testing and during the uncoated baseline testing. Furthermore, the internal cylinder analysis was used to document the-differences between the combustion characteristics of the ceramic coated combustion components and the uncoated, standard engine combustion components.

Ceramic Coating Descriptions

The starboard test engine had all eight cylinders coated with three different configurations of thin (0.015" total) coating thickness. The three different ceramic coatings are a magnesium zirconate, a flame alloyed, partially stabilized zirconia, and a pre-alloyed, partially stabilized zirconia.

The partially stabilized magnesium zirconate was 22% MgO-ZrC>2 sprayed over a FeCrAlY or NiCrAlY metallic bond coat transition layer. In some components, a third chromium layer was applied over the metallic bond coat, but before the ceramic was applied.

The partially stabilized zir-conias (PSZ) both were 8% Y2O3-Zr02 over NiCrAlY or NiCoCrAlY metallic bond coat transition layers. One of the PSZ's used conventional flame zone alloying of the yttria and the zirconia where the other PSZ used pre-alloyed powders. The purpose of this long endurance testing was to determine the durability (and stability) of thin ceramic coatings as well as the differences between these coating systems.

Lastly, the cylinder liners used in this ceramic coated component testing had silicon carbide (SiC) particles embedded into the liners' running surfaces. The purpose was to test a modified running surface to reduce the bore friction between the piston rings, the piston, and the liner.

Upon completion of the testing, detailed, distinctive analyses were performed on the thin ceramic coatings. These analyses were compared to original ceramic analyses performed on the sample ceramic coatings before diesel operation. These before and after comparative analyses were the basis for the durability of these ceramic coatings.after 14,000 hours and over two years of commercial towboat operation.

Ceramic Coating Performance Of the three ceramic coating systems evaluated in this 14,000-hour endurance testing, only one system, applied by only one vendor, provided outstanding durability and performance on pistons, cylinder heads, and exhaust valves. The pre-alloyed, partially stabilized zirconia (PSZ) coating provided the chemical homogeneity and phase stability to result in "as new" condition. The NDT analysis indicated that, after 14,000 hours of operation, these superior ceramic coatings were within 0.001" thickness of the "as sprayed" coating. See Figures 7 and 8.

The flame stabilized, partially stabilized zirconia (PSZ) coatings were not suitable for use on exhaust valves since ,a near complete loss of ceramic coating had occurred in less than 8,990 hours. The flame stabilized PSZ had transformed some of the tetragonal and cubic Zr02 to the monoclinic phase during the 8,990 hours. This lack of phase stability was believed to be unacceptable for long-time engine exposure.

The magnesium zirconate ceramic coatings were not suitable for use on exhaust valves since a near complete loss of ceramic coating had occurred in less than 8,990 hours. The magnesium stabilizer was believed to be removed by the reaction with sulfur from the diesel fuel oil, thus removing the critical stabilizing function of the magnesium in the zirconate.

Once the stabilizer was removed, a major transformation to monoclinic Zr02 had occurred ; and the volume expansion associated with this transformation was, most prob ably, the cause of the ceramic coating failure.

The silicon carbide (Sic) embedded particles in the cylinder liner running surfaces resulted in a mixed result after 14,000 hours of engine exposure. This treatment resulted in a "plateau honed" running surface thought to be responsible for the reduced bore friction resulting in an increased mechanical efficiency. This is a positive benefit and has been responsible for a portion of the recorded reduced specific fuel consumption.

The negative benefit was the embedded (Sic) particles that started to migrate out of the liner and to embed in the piston skirt. After the release of the SiC particles from the liner, mild scuffing occurred between the liner and the piston skirt. This scuffing was considered unacceptable, and this process would not be recommended for EMD engines.

It is possible to plateau hone these liners without embedding particles in the liner walls. This modified honing process would be recommended to achieve the benefits of reduced bore friction and increased mechanical efficiency without the negative aspects of embedded particles.

Conclusions

1. The pre-alloyed, partially stabilized (PSZ) zirconia ceramic coatings of one vendor demonstrated vast superiority, with no noticeable degradation, for 14,000 hours in an EMD engine in commercial towboat service. This ceramic coating's superiority was without peers and would be the only coating recommended for future use.

Due to the variations in process, powders, rigorous application, quality control, and numerous other factors, only this one ceramic coating can be recommended. A caution should be noted as well; unless proven by long, long, commercial testing, with continuous instrumentation, other ceramic coatings cannot be considered as effective!

The long, stable operation of the pre-alloyed (PSZ) ceramic coating demonstrated the ability to protect base metals from the peak temperatures that reduce useful component life. Therefore, with thermal stress reduced, the overhaul life of critical components (ceramic coated) should be greater than the same uncoated components. The longer overhaul life of critical components can reduce the cost of operating the engine.

The magnesium zirconate ceramic coatings are not suitable for long-term operation in diesel engines due to the loss of magnesium stabilizer, by chemical reaction with the sulfur from the fuel oil, and the subsequent phase change to mono-clinic and the resultant spallation of the ceramic coating.

References:

M. F. Winkler, "Experience With Ceramic Fire Deck Coatings in Diesel Engines," Institute of Marine Engineers, Eastern U.S.A. Branch, March 5, 1987.

A. V. Levy, S. MacAdam, "Performance Analysis of Ceramic Barrier Coatings After 9,000 Hours Service In A Marine Diesel Engine," ASME 87-ICE-14.

I. Kvernes, "In-Service Performance of Ceramic and Metallic Coatings in Diesel Engines," SAE 860888.

A. V. Levy, S. MacAdam, "The Behavior of Ceramic Thermal Barrier Coatings on Diesel Engine Combustion Zone Components," 13th International Conference on Metallurgical Coatings, San Diego, California, April 7, 1986.

F. X. Critelli, J. Fairbanks, A. V. Levy, "Application of Ceramic and Metallic Coatings To The Combustion Components of Marine Diesels," Marine Port Engineers, San Francisco, California, September 26, 1985.

C. C. J. French, "Ceramics in Reciprocating Internal Combustion Engines," S.A.E. 841135.

G. W. Goward, "Thermal Barrier Coatings For G.T.'s - A Review of Development & Production Capability," N.A.T.O. Advanced Workshop - "Coatings For Heat Engines," Italy, April 1984.

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