Diesel Engines and Their Particle Signatures

Abstract

Instrumentation is described and sampling and analytical techniques are discussed for the physical and chemical characterization of diesel engine exhaust particulates and wear particles found in diesel engine lubrication oil. In order to collect typical particle populations, an inertial impactor divided the exhaust sample into eight particle size ranges and collected the smaller sizes (80% to 90% of the sample) and organic matter on successive back-up filters. The particle size distribution was strongly dependent on exhaust temperature below 300 degrees C and independent above this temperature, the shift interpreted as the result of liquid hydrocarbon aerosols. Organic matter increased at lower exhaust temperatures. A scanning electron microscope (SEM) was used for physical characterization of the particulate matter and a transmission electron microscope for the study of individual particles. After separating particles according to size by passing them through a magnetic field, it was possible to obtain a visual idea of the particle size distribution from a ferrogram slide by means of optical microscope analysis. A second level of analysis was achieved by using a bichromatic microscope to distinguish between free metal particles and oxides or other compounds. At a third level of analysis, the SEM provided greater magnification for studying particle origin or wear mechanism via particle morphology. With a heated ferrogram analysis technique, the source or type of material in the wear particles was identified. Various component materials take on different thicknesses of oxide layers upon a given exposure to an elevated temperature. Minimum wear was correlated with an inlet oil temperature below 77 degrees C, and an outlet temperature below 82 degrees C.