Definition of the optimal content of used cooking oil methyl ester in blends fuelling a turbocharged diesel engine.

An extended investigation on the application of blends between diesel oil and used cooking oil methyl esters (UCOME) was developed on an automotive turbocharged diesel engine. Two commercial diesel oils were considered as reference fuels, the first with the conventional content of biodiesel (7%), the second with an amount of 15%. Then, increasing levels of UCOME were added, on a volumetric basis (20%, 40%, 55 or 60%), to analyse the influence of biodiesel content on engine behaviour. Three part load engine operating conditions were selected for tests, measuring average engine parameters and indicated pressure diagrams. Their processing allowed to calculate the most important operating, energy and environmental engine quantities, rate of heat release and related combustion parameters. A proper management of the exhaust gas recirculation circuit, variable nozzle turbine and injection system was applied thanks to the availability of an open electronic control unit. An optimal UCOME content (40%) was identified, leading to highest values of brake thermal efficiency (with maximum increase around 5% referring to baseline fuels) and lowest levels of NOX emissions. For this blend, the analysis of combustion parameters outlined that the use of the renewable fuel corresponded to an enhanced premixed combustion phase, while reducing combustion duration. Lower local temperature levels, related to the decrease of Lower Heating Value, other local effects and biodiesel degree of unsaturation justify NOX reduction. Furthermore, soot emissions were always reduced when increasing biodiesel content, while combustion stability was not affected. All these outcomes were observed with and without EGR.