The kinematic viscosity of conventional and bio-based fuel blends as a key parameter to indirectly estimate the performance of compression-ignition engines for agricultural purposes

The partial replacement of conventional fuels with “bio-based” fuels represents a viable energy strategy for cleaner distributed-power generation (agricultural/co-generative units). Although internal combustion engines represent a well-established technology, they will continue to play a crucial role in this energy revolution thanks to their flexibility of use and reliability. Considering compression-ignition engines, the fuel change is simple and requires no modification. Yet some critical issues related to different fuel viscosity may arise. The aim of this study is, therefore, to investigate with a mixed experimental–numerical approach: (a) the kinematic viscosity of many fuel blends (diesel oil-biodiesel-bioethanol) at the standard temperatures of 40 °C and 100 °C, and (b) the effects of the fuel viscosity on engine performance. The data and the mathematical model obtained through the Response Surface Methodology allowed observing that: (a) the fuel-feed system should include a preheater to obtain the same fuel viscosity in blends as in pump diesel oil and avoid issues in the fuel feed; (b) the viscosity at 40 °C progressively increases by 38% (from 3.03 to 4.18 mm2 s−1) as the biodiesel percentage in the blend spans from 0 to 100%; (c) bioethanol fluidizes the blends, reducing the viscosity by about 2% per percentage point of bioethanol in the blend. Also, some trials on a farm tractor fuelled with some of these blends allowed to identify that ternary blends with a viscosity > 3.33 mm2 s−1, whatever the composition within the validity ranges of models (0 ≤ biodiesel ≤ 100, 0 ≤ bioethanol ≤ 3), give rise to the maximum torque increment.