A model for diesel fuel additive lubricity

Abstract

Abstract Problems of diesel fuel reduced lubricity and associated injector pump wear rates are briefly reviewed to demonstrate the need for fuel lubricity additives and measurements of their effectiveness, the HFRR method. The general pattern of wear scar reduction using the HFRR test rig plotted against lubricity additive concentration has the form of an inverse sigmoidal curve. A general model is proposed for wear reduction by diesel fuel lubricity additives against their concentration. The model proposes an equilibrium between anti-wear additives solubilised in diesel fuel and an additive film of those additives adsorbed onto a metal surface undergoing rubbing wear, leading to three regimes. First, at low concentrations the model describes an initial, incomplete coverage of the metal surface by an additive film which is insufficient to support anti-wear action. Second, further increase in additive concentration gives a nearly complete and coherent film at the metal surfaces which is then able to separate the surfaces and support anti-wear action. Third, beyond this point further increase in fuel additive concentration gives no further decrease in wear rate because the adsorbed film is complete. The model therefore describes a reverse sigmoidal dependence of the metal surface wear rate upon additive lubricity concentration. It is further proposed that the interaction between a metal surface undergoing rubbing wear and anti-wear fuel additive species is in equilibrium between multiple additive species adsorbed in a non-specific manner at the metal surface and multiple additive species in the fuel. This interaction is not specific for any lubricity additive but may occur for a mixture of additive species but to a different extent for each separate molecular species. These separate molecular additive species may be adsorbed at the metal surface to form separate and coherent homogenous domain clusters at the metal surface due to entropy effects. Therefore, mixed lubricity additive concentration will act in a colligative manner with the contributions of one additive species concentration being either more or less effective compared to another additive species but adding together to give a joint effect. Examples of the model applied to diesel fuel lubricity wear rates are given for single additives together with examples of multiple additive effects on wear rates.