The use of biodiesel is one of the alternatives to reduce oil dependence in the transport sector and to reduce greenhouse gas emissions. One of the most common engines in Europe was subjected to some tests, aiming to discover the efficiency effects and the emission characteristics when consuming a fuel containing 20% of biodiesel and 80% of diesel (B20), and comparing the results with the use of 100% diesel (B0). Using an engine test bench, several working points of the engine were chosen considering different engine rotation from idle speed to 3500 rpm and from residual torque to 120 Nm, covering the great majority of the normal running operation of this kind of engines when installed in light vehicles. The results revealed a non-proportional effect for fuel consumption for different engine regimes where the energetic differences were, in some operation regimes, totally compensated with efficiency increase. The NOx emission analysis allows to admit that the use of biodiesel in the fuel leads to a consequence on emissions increase that is not always obvious, since in some regimes that increase is noticeable, but for other regimes a slight decrease or no significant change was detected.