Brake thermal efficiency and BSFC of diesel engines: Mathematical modeling and comparison between diesel oil and biodiesel fueling

The objective of this work was to investigate the brake specific fuel consumption (BSFC) of the engine, installed in an agricultural tractor, fueled before with diesel oil (B0) and then with biodiesel (B100). This was done both vs. the engine speed, which vs. engine load. To understand the influence of the fuel heating value and the brake thermal efficiency (BTE), a mathematical modeling of the BTE vs. engine speed and engine torque was developed. The bench tests on the engine, fueled with the two fuels (B0 and B100), allowed to point out that on average, the BTE of the engine was unchanged in the comparison between the two fuels, while the average BSFC was 19% higher with the B100, consistent with the lower heating value of B100 (-17.2%). However, the power output of the engine with the B100 was not reduced by the same amount, but with an average value of 11%, as there was a partial recovery due to the slight increase in the fuel consumption rate. The fitting of the mathematical model to the engine, fuelled with the two fuels (B0 and B100), allowed to draw the diagrams of the calculated BTE and BSFC contours for both fuels, with a good accuracy represented by a mean relative error of 2.1±2.2% (B0) and 2.0±1.5% (B100). Finally, the comparison of the previous results, allowed to highlight the points of the engine speed-torque plane where the biodiesel BTE was different than that of diesel oil BTE. It emerged that the engine fuelled with B100 had a higher BTE at low load and speed, but lower than that of B0 at high speed due to reduced inclination to evaporate compared to diesel oil, which is critical when the speed increases and the time available for combustion is reduced.