Figure 1 - Stribeck Diagram illustrating Lubrication Regimes (Courtesy Compu-Tec)

These liquid lubrication regimes are shown in figure 1 related to their relative coefficient of friction. The hydrodynamic regime is characterized by a film thickness greater than 2.5 times the average surface roughness. In boundary lubrication, metal to metal contact through the thin film of oil occurs at the highest asperities. These asperities are depicted in figure 2. Increasing contact between asperities increases friction and eventually leads to seiz.ure. A low friction anti-wear additive in the liquid lubricant which coats the asperities could reduce friction.

Figure 2 - Ring and Bore Surface Roughness (Courtesy Compu-Tec)

The lubricating oil provides a seal between the piston ring and the Uner to minimize "blow by" of combustion gas around the periphery of the piston during the power stroke. This sealing function minimizes oil leaking around the piston into the combustion chamber. This sealing is of increasing importance as mechanical and thermal loading distort the cylinder liner from a true circle. This cylinder distortion will be more severe at higher pressures and higher temperatures. This sealing problem is depicted in Figure 3.

Figure 3 - Cylinder Bore Distortion and Liquid Lube Oil Sealing

Lubricating oil acts as a cleaning agent for attracting, capturing and neutralizing degradation products

Figure 1 - Stribeck Diagram illustrating Lubrication Regimes (Courtesy Compu-Tec)

and holds them in suspension. The alternative is to have these products form sludge and deposits on pistons, piston rings, valve stems and seals. The particles in suspension can be removed with filters or with the oil change.

Lubricating oils are used to cool the piston rings as well as the internal piston structure in the crown and ring groove area where oil is sprayed through nozzles directing spray at the internal surfaces adjacent to these areas. This is referred to as "gallery cooling." Heat is transferred from the oil as it is collected and circulated through an oil cooler.

Current diesel engine lubricating oils are either natural or synthetic oils with additives to improve operational service. One of the most frequently used additives is zinc dithiophosphate (ZDP) which acts in an anti-wear role as well as an oxidation inhibitor. Although the anti-wear action of ZDP is most beneficial, it breaks down as a function of service life and temperature to form zinc sulfates, phosphates or oxides which are Insoluble in the basestock and can deposit as a wear increasing ash. All fuels currently contain some level of sulfur as it would be exorbitantly expensive to totally remove it. The sulfur level in fuel is dependent on the origin of the crude oil as well as subsequent refinement. The ASTM standard for No. 2 diesel fuel allows up to 0.5 percent sulfur by weight. The national average is about 0.29 percent although some areas such as the Rocky Mountain region average about 0.37 percent sulfur. Sulfates in the exhaust make up 5 to 10 percent of the direct diesel particulate emissions and the S03 gas reacts in the atmosphere to form additional sulfate particles.

Sulfur in diesel fuel is a major cause of engine wear. Reduction in sulfur levels from 0.29 to 0.05 could extend engine life and time between overhauls by 30 percent* (6). This low sulfur content can be achieved by hydrodesulfuriz.ation but would add several cents per gallon. There are several groups concerned with air quality that are strongly encouraging that this sulfur level reduction be accomplished. Caterpillar indicates a 0.03 sulfur level can reduce particulates by 13 to 19 percent in quiescent combustion system engines (6). The problem is that water is also a combustion product that can react with the fuel bond sulfur forming sulfur containing acid which can attack engine components. The antidote to the fuel sulfur content is to include an overbase in the additive pack to neutralize the sulfurous acid. The level of overbase additives are often referred to as the total base number (TBN). Typically magnesium or calcium, sulfate or phenate are use to obtain the desired TBN. However these overbase additives can result in oxides or phosphates of calcium or magnesium that eventually degrade to undesirable ashes which reduce the oil lubricating capability and also deposit on engine parts.

DOE/NASA's High Temperature Liquid Lubricant (HTLL) program is directed at developing a lubricant which will accomodate TRR temperatures of 932 F (500 C) as well as having viscosity to allow operation to -65 F (-85°C). This is now being developed by molecular engineering techniques. The total base number (TBN), or the ph, is controlled by an external source. The proprietary anti-wear additives do not leave ash residue. They must be compatible with the wear surfaces. Early work suggests that the additive packs successful with one pair of surfaces may not work as well with all pairs. Feasibility testing in pre-proposal in-house work by the Cummins-Stauffer team has demonstrated 350 hours of cyclic testing up to 800 F (427 C) TRR temperatures and 300 hours at 850°F (454°C) TRR temperature with virtually no ash deposition. The HTLL is projected to sell retail in the $20-25 gallon range and allow a 3 to 5x extension of the drain interval. It might also be compatible as a transmission fluid. The HTLL should be ready for commercial-iz.ation in 3 to 5 years (7).

The major Europeanj Japanese and US engine manufacturers are virtually in agreement that liquid lubrication will be used for production transportation diesel engines well into the 21st Century. The competitive solid phase and gas phase lubricants cannot perform all of the functions of the liquid lubricant, have significantly higher friction factors, and their introduction and replenishment involve greater engine complexities. To help place the rapid development of high temperature liquid lubricants in perspective, three years ago a majority of the lubrication community did not believe 800 F (427 C) TRR temperatures were achievable unless the exorbitantly expensive lubricants (i.e., greater than $1200/gal) such as the polyphenylethers and the perfluor-oalkyl-ethers were used. The Cummins-Stauffer breakthrough is stimulating broad development in liquid lubrication thereby solidifying the role of liquid lubricants in the diesel engine.

One area that could challenge the liquid lubricants is the gas bearing concept achieved with very tight tolerances between the piston and liner which forces a thin film of air or combustion gas between the piston and liner. The thermal expansion of materials suggests that the low expansion ceramics are primary candidates. There are a few moderately supported efforts along these lines in the US, West Germany and Ireland. Early work indicates about a 10 percent leakage with ringless pistons installed with about a 0.0015" clearance. Perhaps higher engine speed or a ring could help. Most investigators think the side loadings on the piston should be removed. This implies that translational piston-connecting rod movement needs to be converted to rotary output through a scotch yoke or similar mechanism. A typical engine scheme of this type is shown as figure 4. This implies that a translational piston-connecting rod.

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