Coking Phenomena in Nozzle Orifices of DI-Diesel Engines

Within a public founded project test cell investigations were undertaken to identify parameters which predominantly influence the development of critical deposits in injection nozzles. A medium-duty diesel engine was operated in two different coking cycles with a zinc-free lubricant. One of the cycles is dominated by rated power, while the second includes a wide area of the operation range. During the experiments the temperatures at the nozzle tip, the geometries of the nozzle orifice and fuel properties were varied. For a detailed analysis of the deposits methods of electron microscopy were deployed. In the course of the project optical access to all areas in the nozzle was achieved. The experiments were evaluated by means of the monitoring of power output and fuel flow at rated power. The usage of a SEM (scanning electron microscope) and a TEM (transmission electron microscope) revealed images of the deposits with a magnification of up to 160 000. The methods EDX (energy dispersive X-ray) and EELS (electron energy loss spectroscopy) delivered spectra of defined elements. The results of the project show enhanced nozzle fouling by increasing the temperature at the nozzle tip. Zinc marks a major parameter affecting the build-up of deposits. Above a critical concentration of zinc in fuel significant coking develops within a relative short period. These deposits are partly reversible by zinc-free operation. Zinc-derived deposits contain an inhomogeneous distribution of the metal. Nozzle coking based on zinc-free operation displayed morphology with a different texture in comparison to the coking developed in the presence of zinc. Cavitation inside the nozzle posed the major parameter to inhibit the formation of deposits.