Understanding Polyisobutylene Succinimides (PIBSI) and Internal Diesel Injector Deposits

Deposit control additives have been added to diesel fuel for over 30 years to prevent the build-up of deposits in the fuel injection system and thereby ensure optimum engine operation. As the complexity of modern diesel fuel injection systems has increased their tolerance to deposit formation has decreased and therefore there is now an even greater need for high quality deposit control additives. The predominant and most effective chemistry for diesel deposit control additives used over the past 30 years is polyisobutylene succinimides (PIBSI). The design and manufacture of these additives is critical to their performance in the field in terms of functioning to control deposits but also from a no-harm standpoint.

Recent work has identified low molecular weight PIBSI as a cause of internal diesel injector deposits (IDID) associated with injector sticking. Low molecular weight PIBSI would not normally be associated with a high quality deposit control additive and the term PIBSI can cover a multitude of chemical structures. This paper presents new work carried out to further understand the relative effects of a range of PIBSI molecules and more precisely understand which low molecular weight PIBSI species can lead to IDID.

Laboratory bench tests have been carried out to test a range of PIBSI to identify those species responsible for IDID. The effects of fuel components and refinery additives have also been investigated. Industry Standard and modified engine tests have been used to demonstrate the effect of low molecular weight PIBSI species. They have also been used to show that a PIBSI specifically designed and manufactured to function as a diesel deposit control additive does not cause internal diesel injector deposits and will also function effectively to prevent their formation.