The combustion of the paraffin oil, which is a highly refined fossil fuel product similar to kerosene composed of C14-C16 hydrocarbons, resulted in the accumulation of black soot on the glass cylinder of you Stirling engine. When hydrocarbons are burned, some of the hydrocarbons do not fully react with the oxygen, and a complex range of compounds including PAH's (polycyclic aromatic hydrocarbons) are created.

There is a strong correlation between the length of the hydrocarbon chain, and the amount of soot produced during burning. For instance, burning fuels with long chain hydrocarbons such as heavy fuel oil or diesel fuel produces a lot more soot than burning fuels with shorter chain hydrocarbons such as gasoline or propane.

Use molecular diagrams to explain this phenomenon.

Paraffins or Alkanes which have a general formula of CnH2n+2 are saturated hydrocarbons (which means the carbons present have all their carbons with the maximum amount of hydrogen or there's only the presence of C-C) whole Olefins or Alkenes (with a general formula of CnH2n) and Alkynes or acetylenes (CnH2n-2) are unsaturated hydrocarbons (that os the presence of C-C and other double and triple bonds of C-C)

Sooty flames are caused by the incomplete combustion of an unsaturated hydrocarbon. When a fire is ignited to a paraffin, it burns in oxygen with a blue luminous flame which does not produce any smoke or soot but an unsaturated hydrocarbon is burned in the presence of oxygen; a yellow non-luminous flame is formed which produces a lot of smoke and soot.

Examples of paraffins are: butane (C4H10), propane (C3H8); Alkenes: ethene (C2H4), butene (C4H8) and Alkyne: hexyne (C6H10), butyne (C4H6)