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The stability of Diesel/Biodiesel blends can play an important role in deposits formation inside the fuel injection system (FIS). The impact of the stability of FAME/Diesel fuel blends on lacquer deposits formation and on the behavior and reliability of the FIS was investigated using blends of Rapeseed and Soybean methyl esters (RME, SME) and conventional Diesel fuel (volume fractions of RME and SME range from 0 to 20%v/v). Fuels were aged under accelerated conditions and tested on an injection test rig according to an operating cycle developed to provoke injector needle blocking. The soaking duration was found to affect injector fouling. A relationship between the injector fouling tendency and the fuel stability was established. Under current test condition, injectors fouling increased with fuel oxidation measured with Total-Acid-Number.

Due to the increasing use of Fatty Acid Methyl Esters (FAME) in diesel fuel blends and the sensitivity of the latest Diesel engine technology (higher pressures and temperatures), a reliable method for predicting the thermal stability of biodiesel fuels is required. A laboratory bench test, called MicroCoking test, which usually qualifies engine oils, has been modified in order to be used for biodiesel blended fuels. Experimental conditions, and especially temperature range, were optimized in order to lead to a sensitive and discriminating method to qualify pure biodiesel fuels and Diesel blends containing FAME.