The Influence of Friction Modifiers in Fully Formulated Motorcycle Engine Oils

Globally, emissions legislation placed on motorcycles is becoming ever more stringent [1]. One way of meeting these new regulations is to use friction modifiers (FMs) in the engine oil to reduce frictional losses in the engine. This is, however, complicated by the fact that many motorcycles use a common oil sump for both the engine and a lubricated clutch. It is often the case that if a FM reduces friction in a steel/steel contact it will also reduce friction in a steel/friction material contact. Therefore, it is usually viewed that there will be a necessary compromise between maximizing engine efficiency and maintaining efficient clutch performance. In this paper we examine the effect of a range of organic FMs on commercial fully formulated motorcycle engine oils (MCOs) using benchtop tribotests and full-scale rig tests (SAE #2 clutch test machine). The results show that by careful selection of appropriate FM chemistry it is possible to reduce steel/steel friction whilst maintaining clutch performance. To obtain a deeper understanding of the effect of FM chemical structure on the clutch friction performance the friction-speed (μ-v) behaviour of a commercial MCO formulation is investigated. It is found, in agreement with previous studies, that a lower amount of branching in the alkyl tail of an organic FM corresponds to a positive correlation between friction and speed of the type usually desired in an automatic transmission fluid (ATF). This contrasts with the μ-v behaviour of a commercial MCO intended for use in motorcycles with a wet clutch, which shows almost no correlation between friction and speed. This insight could be used to help rational design of new OFMs for motorcycle oils.