Wednesday, June 13, 2012

Large particles in modern diesel engine exhaust

Summary: During research on diesel engine EGR cooler fouling a test stand giving visual access to the building deposit layer has been developed. Initial experiments reveal the presence of large particles in the exhaust. While conventional wisdom is that diesel particulates typically have log-normal size distributions ranging approximately 10-200 nm, the tests reported here observe small numbers of particles with sizes on the order of tens of micron. Such particles are not generally reported in the literature because exhaust particle sizing instruments typically have inertial separators to remove particles larger than ~1 micron in order to avoid fouling of the nanoparticle measurement system.

In order to study these particles further, an exhaust sample was passed over a fiberglass filter, and the resulting filtered particles were analyzed. Samples were taken at the engine EGR passage, and also in the test stand tubing just before the visualization fixture. The resulting images indicate that the particles are not artifacts of the test system, but rather are present in engine exhaust.

MATLAB routines were developed to analyze the filter images taken on the microscope camera. Particles were identified, counted, and sized by the software.

It is not possible to take isokinetic samples and give quantitative measurement of the number and size distribution of the particles because the particles are large enough that inertial and gravitational effects will cause them to at least partially settle out of the flows. Nonetheless, the presence of particles tens of μm is documented.

Such particles are probably the result of in-cylinder and exhaust pipe deposits flaking. While these larger particles would be captured by the diesel particulate filter (DPF), they can affect intake and exhaust valve seating, EGR cooler fouling, EGR valve sealing, and other factors.

Impact: The existence of large particles in diesel exhaust is previously unreported, and can have effects on engine components.

Conference: Presented at the ASME 2012 Internal Combustion Engine Division Spring Technical Conference on May 6, 2012. ASME proceedings ICES2012-81232.

Submitted by John Hoard, Associate Research Scientist, Mechanical Engineering, College of Engineering.

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