Compositional Profiling of Carbonaceous Deposits From Failed Automotive Spark Plugs by Pyrolysis-Gas Chromatography...

Spark plug carbon deposits are closely related to ignition instability and engine malfunction. However, the detailed compositional profiles of deposits formed on failed spark plugs, especially their extractable and thermally releasable organic fractions, remain insufficiently characterized. In this study, carbonaceous deposits collected from failed spark plugs were investigated using pyrolysis-gas chromatography mass spectrometry (Py-GC-MS)-based compositional profiling.

Py-GC-MS was used as the core analytical method, together with scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) and GC-MS. The deposits showed clear heterogeneity in both morphology and composition. They mainly appeared as powder-like and block-like structures, with local fibrous features.

SEM-EDS showed that the deposits were mainly composed of C and O, with minor amounts of Si, P, S, Ca, Fe, Ni, Cu, and Zn. Cu was locally enriched in the fibrous regions. GC-MS analysis of the solvent-extractable fraction showed that the organic components were mainly distributed in the C10-C20 and C20-C30 ranges. Small amounts of esters, alkenes, and polycyclic aromatic hydrocarbons were also detected. These results suggest that the deposits were not simply residues of unburned fuel. Py-GC-MS further indicated the presence of nonvolatile and thermally stable precursor materials. Under thermal desorption conditions, medium and high-carbon number alkanes were the major released products. Under high-temperature pyrolysis conditions, the main products were carbon dioxide, cyclic siloxanes, alkenes, and minor aromatic compounds. This suggests that the deposits contained highly transformed organic matter as well as silicon-containing components. Comparative combustion simulation experiments with gasoline and lubricating oil provided further evidence for deposit origin. Gasoline mainly produced aromatic and condensed products. Lubricating oil mainly generated cracked long-chain hydrocarbons and oxygenated degradation products.

Overall, spark plug carbon deposits may be formed through the combined deposition and thermal evolution of fuel combustion products, lubricant-derived degradation products, and possible gum-like intermediates under high-temperature service conditions. This study demonstrates the value of analytical pyrolysis in revealing the chemical nature of spark plug deposits and provides compositional evidence for the interpretation of their possible formation characteristics.