.2% .5% 1% T 10W EP LR&O TCP AT 370°C LIQUID LUBE AT 75°C


Figure 9. Comparative A Wear Values at 10 Kg Load for TCP Vapor at 370°C and conventional Liquid Lubricants at 75 C in the four-Ball Wear Tester erated wear at the piston ring-liner interface poses the greatest threat to advanced low heat rejection LHR engine durability. The upper ring reversal temperatures predicted for such an engine, 550 C, cause conventional and synthetic lubricants to decompose and form large quantities of solid deposits around the piston ring. These deposits in turn act as wear generating debris. As a result, the components wear rate, engine oil consumption and blowby values become all excessive. The solution of the high temperature ring-liner tribology problem appears to be a prerequisite for further development of the commercial LHR diesel engine. A promising approach for solving the ring-liner tribology problems is the VP lubrication concept.

As indicated earlier, VP lubrication involves the delivery of a lubricant vaporized in a carrier gas to the hot cylinder liner-ring interface in quantities that are just sufficient to replace the lubricant removed. By metering the VP lubricant and tightly controlling oil leakage from the crankcase, deposit formation at the piston ring location can be eliminated. Because the VP lubricant is continuously replaced, this concept has the potential for providing the wear rates necessary to meet a 5000 hour engine durability target. A proof of concept of VP lubrication is now being pursued using the Cylinder Kit Tribology Test Fixture [CKTTF] and an LHR single-cylinder diesel engine. Three phosphate ester fluids which compare favorably in cost to synthetic commercial lubricants are being considered as lubricants. The results of this research will provide basic information on the effects of vapor delivery rates, carrier gas composition, liner and ring materials, and lubricant types, on friction and wear rates in a VP lubricated advanced diesel.

VP DELIVERY METHODS - Four basic VP delivery concepts can conceivably be pursued for application to advanced diesels. Figure 10 shows the proof of concept approach that is currently being used.1 It utilizes a separated ring belt piston concept, and vapor delivery behind the compression ring. Figures 11 and 12

Figure 10. Currently Pursued VP Delivery

Concept. Vapor Delivery is Behind the Compression Ring of a Separated Ring Belt Piston

Figure 10. Currently Pursued VP Delivery

Concept. Vapor Delivery is Behind the Compression Ring of a Separated Ring Belt Piston


Figure 11.

VP Lubricant Injection into the Charge Air

Figure 12. VP Lubricant Injection as a Fuel Additive describe approaches where the lubricant is either atomized into the incoming charge air or placed directly in the fuel as an additive, respectively. These two schemes would distribute the lubricant vapor directly to the most critical location, top ring reversal, and provide valve stem lubrication. The fourth possible VP concept is where the oil control rings in the engine would be metered to allow the correct amount of crankcase lube to pass and vaporize in the hot in-cylinder area. While simplest in concept, this method may be the most difficult to implement. The goal for all VP systems should be to provide low wear rates and friction in the advanced engine while reducing lubricant consumption to levels below that of current commercial heavy-duty diesels.

STRATEGY - As mentioned earlier, the current strategy is to use the VP engine delivery method shown in Figure 10. The Separated Ring Belt [SRB] concept, depicted in Figure 13, has been designed to separate the hot section components as an isolated tribological system. The laboratory tests described earlier in this paper provide the trends and a data base for the ultimate proof of concept testing in a SRB fired engine. Before engine tests, however, the VP concept requires evaluation in a rig simulation that bridges the gap between the basic tests and real life engine environments.

Figure 13. The Separated Ring Belt (SRB) Piston Concept

Figure 12. VP Lubricant Injection as a Fuel Additive

Figure 13. The Separated Ring Belt (SRB) Piston Concept

RIG SIMULATION - An accurate rig simulation means that the tests should be controllable and with measurable parameters. It should also closely simulate engine boundary and operating conditions while maintaining the ability of correlation to the basic tests. This will likely result in duplicating the engine wear mechanisms and will provide quantitative information for engine tests.

The Cylinder Kit Tribology Test Fixture [CKTTFJ is designed to investigate wear and friction of actual engine hardware under controlled conditions. It is a dual test chamber, opposed cylinder arrangement with a 10 mm stroke crankshaft. The fixture speed range is 600 to 4200 r/min. Engine piston rings are mounted into a piston duplicating the ring belt area of the engine. The piston reciprocates in the bore of an actual cylinder liner segment. Each CKTTF chamber contains dual piston rings, as shown in Figure 14. Normal ring force can be applied via mechanical and/or gas loadings.

Figure 14. Cross Section Through a CKTTF Chamber

The CKTTF was originally designed for conventional engine cylinder kit tribological investigations. Thermal finite element analysis was performed to determine the insulation strategies required for running an LHR ring-liner interface simulation while maintaining the integrity of the basic fixture structure. This resulted into insulated test chambers, with heat isolated ring-liner interfaces. The fixture crankshaft and cross-head link bushings are lubricated by an independent temperature controlled pressurized oil system. High temperature heat sources on the outside diameter of the liner and inside diameter of the piston allow simulation of engine thermal loads at the interface. The crosshead link is strain gaged for friction measurements. Conventional or real time in-situ wear measurements can be undertaken. The objectives of the planned CKTTF tests are to extend the basic laboratory test data to the actual hardware under simulated environments, and to provide preliminary values of the VP parameters for engine proof of concept running conditions.

VP DELIVERY SYSTEM - In order to adopt CKTTF for the VP lubrication concept, a vapor generator, shown in Figure 15, has been fabricated. It consists of an in-line heater for the lubricant carrier gas, a hypodermic needle driven by a syringe pump for introduction of a liquid lubricant, and a quartz vaporization

Figure 15. Lube System Vapor Generator
Figure 16. Vapor Generator Mounted on CKTTF

chamber packed with glass rings. The vapor generator tower is mounted on the CKTTF test chamber, as shown in Figure 16. A stainless steel transfer line connects the generator to the CKTTF test chamber.

In operation, a nitrogen carrier gas is heated to 260 C. A predetermined small amount of lubricant is introduced to the gas stream via the hypodermic needle. Full evaporization is secured by having the nitrogen-lubricant mixture pass through the heated glass rings. The setup is currently operating as a flow through system without recirculation.

CANDIDATE LUBRICANTS - It is planned to test phosphate ester fluids as the LHR engine high temperature lubricant. The basic tribology data base is available, as reported earlier in the paper. An important factor is that the required concentration levels are comparable to today's engine oil consumption values. Further, phosphate esters have an attractive cost potential which is equivalent to the synthetic lubricants available in the market at the present time.

TEST PLANS - The CKTTF test plan consists of comparative (B versus C type) proof of concept simulation. This will be followed by a parametric study to possibly determine the primary influential factors in VP cylinder kit tribology. Since we are also investigating solid film lubrication for the LHR hot section, VP-solid film hybrid tribological system tests may be added to the rig simulation test plan.

The engine test plan will examine the friction and wear rate levels for reasonable lubricant consumption rates. It is intended to accumulate significant fired engine test time to indicate the trends. Later follow up phases of this research effort will likely pursue more detailed characterization of the VP concept as a new approach for LHR and advanced engines lubrication.

RESULTS - Only preliminary results from a single CKTTF test run have been available at the time of this workshop. The test compared a non-lubricated CKTTF chamber to the opposed chamber having a 0.12 mole percent concentration of TCP lubricant. The wear scar on the lubricated liner showed uniform radial and axial wear patterns similar to those observed in conventional oil lubricated engines. As expected, the dry liner experienced severe scuffing.

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