Spatially Resolved Molecular Compositions of Insoluble Multilayer Deposits Responsible for Increased Pollution from Internal Combustion Engines

Abstract

Internal combustion engines are used heavily in diverse applications worldwide. Achieving the most efficient operation is key to improving air quality as society moves to a decarbonized energy system. Insoluble deposits that form within internal combustion engine components including fuel injectors and filters negatively impact CO2 and pollutant emissions. Understanding the composition, origins, and formation mechanisms of these complex materials will be key to their mitigation however, previous attempts only afforded nondiagnostic chemical assignments and limited knowledge toward this. Here, we uncover the identity and spatial distribution of molecular species from a gasoline direct injector, diesel injector, and filter deposit in situ using a new hyphenation of secondary ion mass spectrometry and the state-of-the-art Orbitrap mass analyzer (3D OrbiSIMS) and elemental analysis. Through a high mass resolving power and tandem MS we unambiguously uncovered the identity, distribution, and origin of species including alkylbenzyl sulfonates and provide evidence of deposit formation mechanisms including formation of longer chain sulfonates at the gasoline deposit’s surface as well as aromatization to form polycyclic aromatic hydrocarbons up to C66H20, which were prevalent in the lower depth of this deposit. Inorganic salts contributed significantly to the diesel injector deposit throughout its depth, suggesting contamination over multiple fueling cycles. Findings will enable several strategies to mitigate these insoluble materials such as implementing stricter worldwide fuel specifications, modifying additives with adverse reactivity, and synthesizing new fuel additives to solubilize deposits in the engine, thereby leading to less polluting vehicles.

Molecular Formula Prediction for Chemical Filtering of 3D OrbiSIMS Datasets

“…Understanding the extent of this reactivity by visualizing that maximum mass of ions is important in unraveling key deposit formation mechanisms. 15 The filtered DBE plot also highlighted a unique series in injector tip 2 with a DBE of 1 and 2 which we putatively assign as alkyl sulfonates which have not been identified in these systems before. Their low intensity compared to benzyl sulfonates meant that without the visualization approach they were unidentified in our previous analysis of these sample types.…”

“…Their low intensity compared to benzyl sulfonates meant that without the visualization approach they were unidentified in our previous analysis of these sample types. 15 In total 410, 353 and 35 sulfated ions were detected in injector tip 1, injector tip 2 and injector needle 1, respectively. The comprehensive nature of this categorization of species is evidently a useful feature and is shown in the tabulated output of oxygenated ions found in all samples in Supporting Information Table S6 .…”

“…In addition, they are heterogeneous samples and previous work found that most species had different depth profiling behavior. 15 We employed MFP and DBE filtering to depth profiling data from several samples to interrogate these complex datasets and focused on key classes of compounds, followed by MVA on the filtered data. The first type is sulfated species, which can be indicative of contamination of lubricating oil in engines.…”

“…The first type is sulfated species, which can be indicative of contamination of lubricating oil in engines. 15 Filtered spectra and DBE plots from the engine deposit samples after searching for species with compositions of C n H n S 1 O 3 – are shown in Figure 3 .…”

“… 19 , 20 An intense SIMS-induced fragment ion (C 8 H 7 SO 3 – , DBE = 5) was also in all filtered spectra and DBE plots ( Figure 3 a–c), which has been confirmed previously using 3D OrbiSIMS and MS/MS. 15 Benzyl sulfonates with masses higher than the cited parent ion were present in both injector tip deposits, termed “high mass benzyl sulfonates” ( Figure 3 a,b), which extended up to much higher masses in injector tip 1 (C 78 ) but were not present in the injector needle deposit ( Figure 3 c). High mass sulfonates have been identified and confirmed with MS/MS in previous work and are attributed to the product of reactivity of the deposited residue in the engine itself.…”