**4. Conclusion**

Biodiesel from waste cooking oil was tested to investigate the impact of WCO percentage in blends with ultralow-sulfur diesel oil in the performance and emission characteristics of an up-to-date light and compact common rail diesel engine, whose main application is in microcars and in urban vehicles.

The engine performance (in terms of available torque, brake specific fuel consumption, brake thermal efficiency) with biodiesel blends was found quite similar for B20 and diesel fuel. B40 suffered for the lower caloric value of the blend in comparison with ULSD.

For what concerns the exhaust emissions, a reduction in CO and HC was observed for biodiesel blends, which was more significant with the increase in WCO in the fuel.

A reduction in particulate emissions was attained for WCO blends, along with a corresponding increase in NO<sup>x</sup> , according to the well-known trade-off between NO<sup>x</sup> and particulate. Particle size distributions were characterized by a bimodal distribution, in which accumulation mode dominated. A slight reduction in the particulate number concentration was observed as compared to diesel oil. The reduction was more evident as the WCO content in the fuel increased. The mean size of particles in B20 and B40 was smaller than that obtained with ULSD. For all fuel, engine load and speed conditions affected the particle size distribution: the increase in engine speed was responsible for a reduction in particles' diameters; the increase in load led to a reduction in the number of smaller particles.
