The present paper describes an analysis made with two methods for the emissions estimation of an engine, when using Diesel fuel and Biodiesel, based on mathematical interpolation and approximation functions. Results are validated against experimental data obtained on an instrumented test-bed comprising a tractor Diesel engine (UTB 240555 Brasov-type) and by simulations applying the AMESIM code simulation tool. Further studying activities corresponding to the use of the models applying split-injection strategies should highlight a reliable manner to decrease NO_x emissions by preserving engines performances.