Observation of Transient Oil Consumption with In-Cylinder Variables
Published October 1, 1996 by SAE International in United States
Annotation of this paper is available
Only a limited understanding of the oil consumption mechanism appears to exist, especially oil consumption under transient engine operating conditions. This is probably due to the difficulty in engine instrumentation for measuring not only oil consumption, but also for measuring the associated in-cylinder variables. Because of this difficulty, a relatively large number of experiments and tests are often necessary for the development of each engine design in order to achieve the target oil consumption that meets the requirements for particulate emissions standards, oil economy, and engine reliability and durability.
Increased understanding and logical approaches are believed to be necessary in developing the oil-consumption reduction technology that effectively and efficiently accomplishes the tasks of low oil-consumption engine development. Thus, detailed measurements along with analysis are necessary first to understand the oil consumption phenomena and mechanism, and second, to identify a key component design parameter(s) for finding a practical and effective means to reduce oil consumption.
A real-time oil consumption measurement system, using a sulfur dioxide tracer (SO2) technique, was used to measure oil consumption in a production heavy-duty diesel engine under step-transient conditions. Simultaneously, in-cylinder variables were measured by instrumenting both the piston and cylinder liner. Observation of piston-ring motion, inter-ring gas pressures, and piston temperatures along with oil consumption implied that an excess amount of oil existed in the area of an oil control ring, especially under low-load conditions. An oil control ring floats in its groove for a longer time under low-load than under full-load. Thus, a substantially higher amount of oil appears to flow through the clearance between the ring's upper surface and its groove during the piston downstroke than that between the ring running-face and the cylinder wall. The estimated amount of oil flow through the clearance above an oil control ring's upper surface was higher during the expansion stroke than during the intake stroke, and it was higher under low-load than under full-load.
Consequently, understanding the oil control-ring's performance and piston tilting effects were predicted to be the most important factors, when oil consumption needs to be reduced to extremely low levels with less variability. The results of both measurement and analysis are discussed to theorize oil transport and consumption under transient engine operating conditions.
OIL-CONSUMPTION REDUCTION TECHNOLOGY is expected to be increasingly important for achieving extremely low levels of oil-derived particulate emissions in future low-emissions engines. The current level of oil-derived particulates in a heavy-duty diesel engine is approximately 0.02 g/bhp-hr (only soluble organic fraction), measured under the U.S. FTP transient emissions test procedure. This is 25 percent of the engineering target typically aimed at in order to meet the 0.10 g/bhp-hr U.S. particulate standard. If the particulate standard is reduced to 0.05 g/bhp-hr (urban city bus and clean fuel fleets are already regulated at this level), the contribution of oil-derived particulate increases to 50 percent of the total particulate emission on the assumption of a 0.8 engineering target.
Thus, oil consumption will have to be maintained at a low level, and the deviation must be small for the long-term. The level of oil consumption varies depending on component design tolerance, production tolerance, engine operating conditions (particularly transient), and component wear and deformation over a long-term operation. To accomplish the above goals for each production engine family is time-consuming and costly, as numerous parametric engine tests are required to determine the right components for meeting the set targets.
Precious research results obtained by both industry and academia have been reported in the past to explain the in-cylinder oil consumption phenomena and mechanism. Furuhama, et al., explained that oil passed through a top ring is thrown off to the combustion chamber, due to inertial force and is then lost into exhaust gas*. Top-ring lifting due to inertial force during the compression and exhaust strokes also throws oil off into the combustion chamber, causing oil consumption to increase. Depending on the piston and ring design, as well as engine operating conditions, pressure of blowby gas below a top ring exceeds the cylinder pressure, causing blowby-flow to carry oil to above a top ring[1, 2, 3, 4, 5, 6 and 7]
Saito, et al., observed oil-flow through a piston ring pack in a transparent engine and pointed out oil-flow through a ring end gap due to pressure depression in-cylinder during intake stroke as a significant source of oil consumption. Noda, et al., investigated the relationship between the oil control ring's ability to conform and piston motion, and found that oil-scraping by an oil control ring could be poor due to the effect of side-to-side piston motion.
Hanaoka, et al., and Inoue, et al., theorized oil consumption under transient conditions by the time duration of light-load operation with the amount of oil accumulated in a gasoline engine under a step-transient condition. Izumi also determined the effect of light-load operation on transient oil consumption that substantially increased in a diesel engine. Ariga, et al., determined a similar effect on transient oil consumption in a gasoline engine.
Recently, Hitotsugi, et al., developed a model that calculates oil consumption due to cylinder bore distortion . The calculation was based on the assumption that oil flowed into the combustion chamber due to both inertial flow and static flow between the distorted cylinder wall and piston-ring running face. The trend of oil consumption agreed between calculation and measurement in a gasoline engine. Wahiduzzaman, et al., calculated oil consumption caused by thermal effects on oil on the cylinder wall in a diesel engine, and found that thermally-driven oil consumption contributes to only a fraction (about 5 percent) of the total oil consumption.
The above-referenced results explain the importance of understanding the effect of power-cylinder component dynamics, as well as pressure and temperature, on oil consumption and the significance of oil consumption increased by transient operation. However, oil consumption has not been measured simultaneously with in-cylinder variables, especially under transient operating conditions. It is believed the mechanism of oil consumption under steady-state conditions is quite different from that under transient conditions. Furthermore, it is believed to be important to understand the oil consumption mechanism under transient more than that under steady-state conditions, because an automotive engine is always operated under transient conditions.
Figure 1 shows the comparison of oil-consumption measurement under transient to simulated transient oil consumption. The steady-state oil consumption measurement results of 31 different speed and load conditions were interpolated, and an oil consumption value was obtained for a given speed and load condition to simulate oil consumption during transient operation . The test
engine was a V-6 medium-duty gasoline engine. Oil consumed (1.68 grams) over the entire transient operating period was similar to that (1.73 grams) obtained from the simulated transient oil consumption. However, real-time oil consumption measured at any incident of speed and load combinations during transient operation was quite different from the simulated transient oil consumption Therefore, it was suspected that an oil consumption mechanism theory based on steady-state oil consumption data may not be applicable in explaining oil consumption under transient conditions. Thus, refined component designs, as well as the refined oil properties, based on the steady-state oil consumption data, may not be an adequate solution to achieve low oil consumption with a small deviation in a production automotive engine.
Therefore, a production heavy-duty diesel engine was instrumented to measure in-cylinder variables such as ring motion, piston motion, inter-ring pressure, and temperature. A real-time, oil-consumption measurement technique, that used a sulfur dioxide (SO2) tracer method[17,18], measured oil consumption simultaneously with the measurement of in-cylinder variables, during a step-transient engine operating condition. The results were observed to understand the relationship between the in-cylinder variables and oil consumption under transient conditions, and to identify a key component that significantly contributed to oil consumption and its variability.
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