What is the effect of engine deposits?
A new engine may only require a fuel of 6-9 octane numbers lower than the same engine after 25,000 km. This Octane Requirement Increase (ORI) is due to the formation of a mixture of organic and inorganic deposits resulting from both the fuel and the lubricant. They reach an equilibrium amount because of flaking, however dramatic changes in driving styles can also result in dramatic changes of the equilibrium position. When the engine starts to burn more oil, the octane requirement can increase again. ORIs up to 12 are not uncommon, depending on driving style [17,19]. The deposits produce the ORI by several mechanisms:- – they reduce the combustion chamber volume, effectively increasing the compression ratio. – they also reduce thermal conductivity, thus increasing the combustion chamber temperatures. – they catalyse undesirable pre-flame reactions that produce end gases with low autoignition temperatures.
