Effects of engine operating conditions on the size and nanostructure of diesel particles

Diesel particulate is composed of spherical or nearly spherical primary particles. In this study, the primary particle size and nanostructure were investigated at five engine operating conditions for a medium-duty direct injection diesel engine, using a high-resolution transmission electron microscope (TEM). The diesel particles were collected from diluted exhaust gas and an ultrasonic extraction method was used for obtaining the TEM samples. The TEM images reveal that primary particle size and nanostructure depend on the engine operating conditions. The mean diameter of the primary particles is in the range 23.8–28.5 nm, depending on the engine speed and engine load. The nanostructure could reflect the particle formation processes. The larger primary particles are formed at low engine speed and high engine load. They have irregular shape and distinguishable trace of pre-formed particle, indicating the combined effect of particle coalescence and surface growth. Primary particles with core–shell structure are common at high engine load with different engine speeds. The particles exhibit disorder structure at low engine load and high engine speed due to lower in-cylinder gas temperature and shorter combustion duration. Quantitative analysis of the nanostructures indicates that graphitization of primary particles is affected by engine load but not by engine speed.