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The heightened interest level in Fuel Economy for Heavy Duty Diesel Engines the industry has seen over the last few years continues to be high, and is not likely to change. Lowering the fuel consumption of all internal combustion engines remains a priority for years to come, driven by economic, legislative, and environmental reasons. While it is generally assumed that lower viscosity grade lubricants offer fuel economy benefits, there is a lot of confusion about exactly what drives the fuel economy benefits. Fuel Economy claims in trade literature vary over a broad range and it is difficult for the end user to determine what to expect when a change in lubricant viscosity is adopted for a fleet of vehicles in a certain type of operation. This publication makes an attempt at clarifying a number of these uncertainties with the help of additional engine test data, and more extensive data analysis.

Over the last decades, heavy duty diesel engines have experienced many changes in design and operation. More stringent emission legislation has been a driver for changes in the design of heavy duty diesel engines since the 1980s. Optimization of the combustion process and the introduction of exhaust gas recirculation allowed for significant reductions of exhaust emission levels over the years, but the thermal loading of the engine and its lubricant has increased. In the coming years, diesel engines will have to meet even more stringent requirements for particulate matter and nitrogen oxide emissions. These low emission diesel engines are expected to be equipped with exhaust gas after-treatment systems.

Emission legislation for diesel engines is becoming more stringent over time. While the exhaust gas composition requirements for prior iterations of emission legislation could be met with improvements in the engine's combustion process, the next issue of emission limits may require more rigorous measures. In order to meet severe particulate matter limitations, such as those mandated in Japan's new long-term (2005-) emission regulation and in Euro-4 emission legislation, the use of exhaust gas after-treatment systems seems unavoidable. Many engine builders seem to have selected the Diesel Particulate Filter (DPF) as an integral part of their selected after-treatment strategy. It has been stated that engine lubricants may hurt the durability of such after-treatment systems when oil consumption results in deposit build-up on the DPF walls. This paper describes how a study was performed on the possible impact of engine lubricant composition on DPF plugging.

Historically, the characterization of fresh and used diesel engine lubricants has been based on a limited number of analytical techniques. One of the most important analyses has always been the total base number (TBN) measurement. Although the TBN measurements are informative, easy, and quick, it can be misleading to base the judgment of an oil's performance solely on one criterion. This paper offers observations from a field test, showing that some detergent approaches gave unacceptable performance even though the TBNs were at an acceptable level. It is hypothesized that some detergents do not effectively neutralize all acidic species present in the lubricant, thereby reserving their own base while in fact the oil may no longer provide sufficient protection against bearing corrosion. This hypothesis is supported with bench and engine test data. It is recommended that, at a minimum, total acid number (TAN) measurements be included in any analysis.

Oil consumption control in internal combustion engines is affected by both engine design and lubricant related factors. In an attempt to study the lubricant related factors, an oil consumption measurement engine test, using the Mercedes-Benz OM 364A engine was developed. The engine test data confirm the effects of viscosity and volatility. In addition, results indicate that the selection of base oil and viscosity index modifier may have an effect on oil consumption, which would not be predicted by the rheological properties of the oil. From various different engine tests, indications were obtained about a possible relationship between oil consumption and viscosity increase. A data search yielded results that confirm this correlation.

Soot-related oil thickening problems have been reported over the years by multiple OEMs in Europe, Japan, and in the U.S.A (1,2,3). The earliest problems, from the late 1970s, were often attributed to adverse changes in operation [lower engine speeds, heavy loads and low air/fuel ratio, or severe operation such as stop-and-go service (3)] which led to a high soot generation rate. In the late 1980s, the emission legislation became more stringent and soot-related oil thickening concerns resurfaced. It appeared that even engines that produced a relatively low level of soot in the exhaust gas showed a high level of soot contamination in the lubricant (4). For the oil and oil additive industry, the Mack T-7 engine test offered a useful tool to evaluate the ability of oils to disperse soot, but it has been noted that the industry remains without a test based on a European engine to adequately evaluate an oil's ability to disperse diesel engine soot.