The formation of deposits within diesel injectors leads to reduced engine performance, lower fuel economy and increased emissions. Internal diesel injector deposits, first identified in 2009, differ from other engine deposits as they form from hot fuel on hot engine surfaces. Despite the prevalence of these deposits, a mechanistic understanding of how they form, and specifically the role of iron in deposition, is poorly defined. This work aims to develop a mechanistic understanding of these systems to facilitate the development of fuel formulations which prevent deposition and therefore maintain optimal engine performance for the duration of the lifetime of a vehicle.

A model system was formulated in collaboration with an industrial partner BP, to elucidate key parameters in deposition; hexadecane was used as a model diesel compound due to its chemical and physical properties. Using secondary ion mass spectrometry, the injector steel Fe-13Cr was characterised, over engine relevant temperatures (RT-400oC), to be predominantly iron-based. As the chemical state of the iron within the injector is undefined, three model iron surfaces, with differing chemistries, were used to reveal key factors in the deposition mechanism.

Deposits were simulated by refluxing hexadecane with the model surfaces at 200oC and subsequently characterised using X-ray photoelectron spectroscopy, scanning electron microscopy and secondary ion mass spectrometry. The level of hexadecane oxidation in solution was analysed by electrospray ionisation mass spectrometry. Compared to silicon surfaces, iron surfaces were found to promote oxidation and deposition of hexadecane. The iron surface chemistry was found to affect the amount and morphology of deposition, with the interplay between oxidation and solubility determining deposition levels.

Comparisons between the various surface chemistries of iron demonstrate that i) iron surfaces promote the degradation of hexadecane, ii) regardless of the original surface chemistry, an iron (III) oxide (and/or hydroxide) surface is formed and iii) further fuel degradation can occur with this surface when the system is cooled to room temperature.

A rig and cell were developed to facilitate in situ measurements of the transitions occurring at the iron surface. Grazing incidence wide angle X-ray scattering and X-ray absorption fine structure were used to investigate the structural and chemical transitions of iron; these findings further support the formation of an iron oxide hydroxide.