## 1 Literature Review
### Components of the Exhaust Outlets of Internal Combustion Engines  
Various studies have attempted to identify the components of exhaust gases from internal combustion engines. A previous work [23] reported that the constituents of internal combustion engine exhaust gases include major constituents that are greater than 1% (water, H\({}_{2}\)O, carbon dioxide, CO\({}_{2}\), nitrogen, N\({}_{2}\), oxygen, O\({}_{2}\), carbon monoxide, CO, and hydrogen, H\({}_{2}\)) and minor constitutes (less than 1%) that include oxides of sulfur, SO\({}_{2}\), SO\({}_{3}\), hydrocarbons, C\({}_{n}\)H\({}_{m}\), carbon monoxide, hydrogen and smoke, oxides of nitrogen, NO, NO\({}_{2}\), aldehydes, HCHO\({}_{2}\), organic acids, HCOOH, alcohols, CH\({}_{3}\)OH, etc., The study presented threshold limits for continuous 8-h exposure for carbon dioxide, carbon monoxide, sulfur oxides, formaldehyde, and nitrogen oxides. The study also presented the results of a study in which exhaust-gas samples were obtained from gasoline, diesel, and propane-powered motor coaches of similar passenger capacity under idling, accelerating, cruising, and decelerating driving conditions. The samples were analyzed for oxides of nitrogen, carbon monoxide, formaldehyde, and hydrocarbons. The authors reported that the difference in emissions for oxides of nitrogen, formaldehyde, and hydrocarbons by the three types of coaches that were evaluated was relatively small. Carbon monoxide and Hydrogen were reported as the primary constituents in spark ignition engines and minor constituents in diesel engines. The international agency for research on cancer (IARC) [24] mentioned that \"the major products of the complete combustion of petroleum-based fuels in an internal combustion engine are carbon dioxide (13%) and water (13%), with nitrogen from air comprising most (73%) of the remaining exhaust\". A small portion of the nitrogen is said to be converted to nitrogen oxides and some nitrated hydrocarbons. Depending on the operating conditions, some excess oxygen may also be emitted. On the other hand, incomplete combustion results in the emission of unburnt fuel and lubricating oil, carbon monoxide, thousands of chemical components in gaseous and particulate phases, etc. Both gasoline and diesel are hydrocarbons; however, they contain some sulfur compounds too. Aerosol precursor gases that are primarily emitted from diesel engines include SO\({}_{2}\), SO\({}_{3}\), H\({}_{2}\)SO\({}_{4}\), NO\({}_{x}\), H\({}_{2}\)O, soot particles, and low and semi-volatile organic species. A large fraction of hydrocarbons is removed by oxidation catalysts, boosting SO\({}_{2}\) to SO\({}_{3}\). SO\({}_{3}\) reacts quickly with exhaust water (H\({}_{2}\)O) to form sulphuric acid [25]. Combustion of the fuel in the air also allows for the formation of chemical compounds with other components of the air. The IARC [24] cited a previous work that indicated that \"gasoline engines are designed to operate at a nearly stoichiometric ratio (an air: fuel ratio of approximately 14.6:1) and diesel engines operate with excess air (an air: fuel ratio of approximately 25-30:1)\". Sulfur compounds from fuels such as diesel and gasoline cause air pollution. To address this issue, large-scale oil refinery processes remove the majority of sulfur from fuel down to a government-mandated level [26]. Some researchers [27] developed a technique that has the potential to further reduce sulfur content to a fraction of the amount that is mandated by government regulations. The scholars claimed that \"a robust potassium (K) alkoxide (O)/hydrosilane (Si)-based ('KOSi') system efficiently desulfurizes refractory sulfur heterocycles. Subjecting sulfur-rich diesel (that is, [S]\(\sim\)10,000 ppm) to KOSi conditions results in a fuel with [S]\(\sim\)2 ppm, surpassing ambitious future governmental regulatory goals set for fuel sulfur content in all countries\".