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air cleaner turbocharger 1 ratio 1:2

lair aftercooler turbocharger 2 ratio 1:2.3

1 air aftercooler air receiver

Figure 13. Turbocharging System for the AVL Single-Cylinder Engine.

rings and cylinder liners is also part of our program. In this section we highlight some of our progress to date. In particular, we emphasize our recent development of a low cost liquid lubricant, suitable for commercial diesel engine service, which considerably improves upon the performance of the best previous LHR engine lubricant.

At the beginning of our program, we formulated a new general concept for high-temperature diesel engine lubrication and selected lubricants and materials for evaluation within the concept (reference 3).

Briefly, our concept calls for three sources of in-cylinder engine lubrication. Primary engine lubrication is provided by an advanced synthetic lubricant. The advances in synthetic .liquid lubricant technology which we are pursuing are significant increases in stable life and significant decreases in deposit formation at high temperatures. We are evaluating both new base stocks and new additives. In general, improved additive technology offers the best hope for achieving improved lubricant performance.

Secondary engine lubrication is provided by a solid lubricant additive in the lubricant. The additive is included either as a particulate dispersion or as a soluble compound which decomposes to form a solid lubricant at elevated temperatures. This inclusion is necessary for engine lubricants which may need to function with short-term exposure to temperatures at or above 900 degrees F. At this temperature, not only would fluid viscosity be too low to effectively form a film separating sliding surfaces, but also the rate of decomposition of all known liquid lubricants is high. Solid lubricant additives function as antiwear additives at temperatures considerably higher than those which conventional soluble antiwear additives can survive.

Tertiary engine lubrication is provided by self-lubricating materials of construction. Piston rings and cylinder liners are made of, or coated with, materials which are not only wear-resistant, but which also impart relatively low friction.

This is necessary to prevent scuffing or seizure at the highest temperatures, at which temporary lubricant starvation may be experienced.

Our major breakthrough in the past year has been the development of a new polyol ester-based diesel engine lubricant, which we denote MRI-1. Polyol esters are the most oxidatively stable synthetic liquid lubricant basestocks which are available at a reasonable cost. As we describe in the following, this lubricant appears to be the most significant development in LHR engine lubrication in the past ten years.

The term "polyol esters" encompasses a wide variety of chemical compounds. The elements of polyol ester molecular structure which determine oxidative stability have been determined. Using these chemical structure/property relationships, and the physical properties necessary for an SAE grade 30 lubricant, we selected a basestock. We were able to theoretically predict that the polyol ester structure selected would have higher oxidative stability than any other possessing similar viscosity.

We then used high pressure differential scanning calorimetry (HPDSC) to experimentally determine the oxidation onset temperature of our selected base stock and other polyol esters of similar viscosity. The onset of oxidation for our selected base stock occurred at 421 degrees F. The onset of oxidation for 25 other polyol esters examined ranged from 347 to 424 degrees F.

We have therefore: proven, both theoretically, and experimentally, that the particular polyol .ester selected as,/the/kasestq^ is the most /oxidatively/ stable of, its :, / viscosity class/./.. Accordingly> there remains/ little moire that, ., can be done -to,-, optimize/basestock stability for any diesel engine lubricant currently available at a reasonable cost. The critical area requiring improvement is additive stability.

We next began a formulation program with an additive supplier. This high-temperature diesel engine additive development program had produced a second-generation high-temperature additive package at the time MRI-1 was formulated. The focus of this program was to produce a formulated fluid which produced minimum deposits at high temperatures.

Once formulated, we were able to compare the performance of MRI-1 with the LHR engine lubricant which has historically performed best. This previously best performing lubricant is a formulated polyol ester which has been used almost exclusively in LHR engine development programs for the past seven years.

The deposit-forming tendency of the lubricants was determined using a sliding ring deposits test. The results of this test have been shown to correlate extremely well with piston deposit ratings obtained from Caterpillar 1-H single-cylinder diesel engine tests, the standard engine test for assessing diesel engine lubricant performance. Deposit ratings were obtained at 750 degrees F.

Deposits in the sliding ring test are evaluated in the same way as deposits from engines, i.e., by means of a visual demerit rating from 1 to 10, with 1 representing the most deposits and 10 representing the least deposits. The previously best formulated polyol ester LHR engine lubricant produced a rating of 1. Deposit formation is-known to be a serious problem in high-temperaturerengine tests of this lubricant .,, ,MRI-1, on the other hand, produced a rating of 9.1 Lubricantsywith ^ratings of 9 and- 10 are suitable for . commercial use. v .,.; ,

We have.therefore proven that, with proper additive selection, deposit formation by polyol ester-based diesel engine lubricants can be controlled to acceptable levels at temperatures at least as high as 750 degrees F. Comparative high-temperature single-cylinder diesel engine tests are currently in progress. Preliminary results appear to confirm our laboratory results on deposit formation.

MRI-1 contains antioxidant additives, which raise the oxidation onset temperature to 590 degrees F (compared to 421 degrees F for the basestock without oxidation inhibitors), as determined by HPDSC. This temperature is not much different from that for the previously best polyol ester LHR engine lubricant. What is different about MRI-1 is its significantly longer stable life at high temperatures, i.e., its much slower rate of oxidation. Comparative studies of oxidation kinetics are in progress. The much longer stable life of MRI-1, however, is easily seen from the results of friction and wear experiments conducted at 500 degrees F.

The results of friction and wear experiments conducted on this program are presented in Table 4. The first two lines compare the performance of MRI-1 with that of the previously best LHR engine lubricant, lubricating steel sliding against steel at ambient temperature and 500 degrees F. These are bulk oil experiments, with 0.6 milliliters of lubricant immersing the sliding contacts in an open cup. When lubricant is completely depleted, by any mechanism, friction rises rapidly to unlubricated values and the test is terminated. The "% Completion" column of Table 4-therefore provides a relative measure of lubricant useful life at the specified bulk oil? temperature under dynamic conditions. '' ■'"■■■■'•''■•■-'■> . ■;- -' = ■-'<■'■■■'.

The stable life of MRI-1*in this friction and' wear test is approximately five times the ; stable rifé of the previously best LHR engine lubricant> both at ambient temperature and at 500 degrees F. In addition, use of. MRI-1 in place of the previously best lubricant lowers the wear rate of the steel specimen by approximately an order of magnitude (more at high temperature) and cuts the friction coefficient by one third at 500 degrees F. These dramatic improvements in performance are real. Each experiment was repeated three times and the standard deviation of the reported results is quite low.

Our lubricant candidates other than MRI-1 are even more stable than MRI-1. Two of several other advanced synthetic liquid lubricants being evaluated on our program are listed in Table 4, a polyphenyl ether (PPE) and a perfluoroalkylpolyether (PFPE). These are products representative of the two liquid lubricant classes known to possess the greatest high-temperature stability. The oxidation onset temperature, determined by HPDSC, is 825 degrees F for the PPE and 900 degrees F for the PFPE, compared to 590 degrees F for an oxidation-inhibited polyol ester.

The stable life of these extremely high-temperature liquid lubricants is, of course, much longer than that of the polyol ester lubricants at any temperature and their friction and wear performance is excellent. The PFPE lubricant is a particularly outstanding boundary lubricant and forms no deposits at all. The high current cost due to limited availability of these lubricants precludes their use in near-term commercial diesel engine fleets. We plan to conduct the first high-temperature single-cylinder diesel engine tests of these lubricants. Development of these lubricants for longer term military diesel applications will be pursued if warranted by their s performance. ,. << . ■-„, ,

The first steps!fbr the sdiid lubricant suspension assessment '" have been initiated^ '"T£fcie°4 ' ;,'"Vi includes preliminary friction' -wear results using; a MRI-i ■ ' f polyolester basestock which has no diesel'additives. Tests show that the addition of P-l, a '<■■ < ■ proprietary inorganic solid lubricant, lowers the wear rate

Table 4. Friction and Wear of Candidate Lubricants

Relative

Wear Rate Friction Coeff. % Completion Lubricant 75 ° F 500° F 75° F 500° F 75 ° F 500 0 E

formulated polyol ester

formulated polyol ester

Polyphenyl Ether 4.41 0.57 0.09 0.15 90 100

Perfluoroalkyl- 0.15 0.33 0.04 0.06 100 100

polyether

MRI-1 polyol ester 5.12 8.90 0.15 0.22 85 100

basestock Only ("B")

P-l inorganic solid lubricant additive

carbon fluoride solid lubricant additive

1. Conditions: Flat pins (440-C steel) on flat disks (C-1018 steel). Oscillatory motion, 35°, 1000 cpm. Load=55.5 lb=2010 psi. Velocity=14.7 m/min. Complete test=200,000 cycles=2.932 km sliding distance. Sliding interface immersed in 0.6 ml lubricant. All values are averages for two or three experiments.

2. Wear rates for disk wear only. Reported wear rates are in units of mm /Nm and are multiplied by 10 , i.e., entry "27.2" means wear rate=27.2 x 10 mm /Nm. Friction coefficients are steady state values. Experiments terminated if friction coefficient began to abruptly increase above steady state value and reach 0.4.

and friction by about one half. The use of carbon fluoride solid lubricant lowers the wear rate by about one third. The follow-on work will include the addition of the solid lubricant to the additive package formulated polyolester lubricant.

Conclusion

An overview and current status of the TACOM-sponsored Tribology and In-cyUnder Components Program were presented. Analytical techniques have been used to screen potential high-temperature concepts to meet the aggressive low-heat-rejection targets desired by TACOM. The program has identified two major technology-transfer time frames: Before 1990 and After. 1990. The technology development activities have been targeted for providing tribological and i^cylindef component insulating tiechhology for the TACOM-sponsored Cummins Engine Company/AIPS program.

Lubricants and additive packages for before 1990 have been identified through laboratory bench testing. The results show lower levels of deposits which are characterized by reduced hardness and abrasive properties. In addition, chrome oxide surface coatings have been identified which reduce the degree of lubricant deposit formation. These systems have been assessed scientifically through laboratory bench testing and characterization and are ready for engine test assessment. Advanced tribological systems have been identified for higher liner bore temperatures for technology needs beyond 1990. These are currently nearing laboratory assessment through high temperature friction-wear bench testing.

Thermal barrier coatings have been identified as the primary concept for providing in-cylinder component insulation, for the before 1990 technology needs. Capped air-gap insulated concepts are being developed in parallel with thermal barrier coatings to provide a back-up approach while advanced cast-in composite concepts are being investigated and developed for beyond 1990.

Thermal coupon tests of the most prominent plasma coatings have been nearly completed with advances being made in chemical coatings. A small bore engine assessment of these coatings is now underway. Plans to establish a correlation between the thermal bench rig test and the engine test is progressing. In addition, titanium alloy 6242 has been identified as the key piston material candidate due to its favorable properties and is currently being engine-tested.

The full-size self-sustained AVL-SCE system is nearly complete with plans to have the engine operating well ahead of schedule. All indications to daté point toward significant advances, being established for high-temperature low-heat-rej ection and adiabatic engine technology requirements. It is believed that the contributions made in this program will significantly advance the state of the art adiabatic engine technology and provide support which will one day soon allow full realization of the adiabatic engine advantages.

References

1. Woods, M., Glance, P., Schwarz, E., "In-cylinder Components for High-Temperature Diesel," SAE technical paper 870159, 1987.

2. Carr, J., Jones, J., "Post-densified Chrome Oxide Coatings for Adiabatic Engine," SAE technical paper 840432, 1987

3. Sutor, P., Bryzik, W., "Tribological Systems for High-Temperature Diesel Engines, SAE technical paper 870157, 1987

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