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Oil consumption has been identified as a contributor to diesel particulate exhaust emissions. The strict requirements for reduced diesel exhaust particulates in 1991 and in 1994 call for a much greater understanding of the level of oil consumption during transient as well as steady-state operations. A system has been assembled to continuously measure diesel engine oil consumption through on-line detection using sulfur as a tracer. The system requires a sulfur-free fuel, a high-sulfur diesel engine oil, an exhaust sampling system, and a sensitive detector. A series of engine tests were conducted using the sulfur-trace oil consumption measurement system. Variations in oil consumption at steady state and during transient operations have been measured. Quantitative measurement of oil consumption rates were confirmed by two separate calibration checks.

The contribution of lubricating oil to diesel engine particulate emissions is of concern not only because of stringent particulate emissions standards but also because of engine-to-engine variability. Unburned oil contributes directly to the particulate soluble organic fraction. A real-time oil consumption measurement technique previously developed was further refined to also measure real-time unburned oil consumption. The technique uses high sulfur oil, low sulfur fuel, and fast response, sensitive SO2 detection instrumentation. Total and unburned oil consumption maps over the engine operating range are presented. Results show that both total and unburned oil consumption generally increase as speed and load are increased. Unburned oil consumption shows some peaks at intermediate speed, high-load conditions. Oil consumption from individual cylinders was measured and shown to be approximately equal.

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.

A unique combustion system using an energy cell was designed during the course of this two-stroke, spark-assisted DI diesel engine development program. The system generated high gas turbulence and significantly increased fuel-air mixing rate during combustion. Using the spark-assisted diesel engine concept, this system allowed modification of a production, loop-scavenged, two-stroke gasoline engine to increase fuel tolerance, decrease fuel consumption to levels close to diesel engines, and produce a power-to-weight ratio comparable to a gasoline engine. The experimental engine was constructed and developed by Southwest Research Institute and the project was funded by Sanshin Industries. This paper summarizes the results of the project.

A loop-scavenged, two-stroke, spark-assisted DI diesel engine was developed by modifying an outboard marine gasoline engine to operate on diesel fuel with high fuel efficiency similar to a diesel engine, yet retain the two-stroke engine advantages of low cost, light weight, and high power-to-weight ratio. Engine modification was concentrated in the area of the combustion system, including transfer port design to generate air swirl in the cylinder, and combustion chamber design to generate air squish and turbulence. Bore and stroke (84 × 72 mm) remained the same as those of the base engine. The experimental engine used the production engine's piston, crankshaft, connecting rod, bearings, and cylinder block. The transfer port design was optimized using a flow test bench for best swirl and air flow pattern with a simple flow visualization technique. The best combustion chamber geometry, compression ratio, and fuel injection spray pattern were determined through engine experiments.