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 and Cogeneration Purposes

The partial replacement of conventional fuels with “bio-based” fuels represents a viable energy strategy in the medium-short term for cleaner distributed-power generation (agricultural and co-generative units). Although the internal combustion engine is a traditional and well-established technology, it will continue to play a crucial role in this energy revolution thanks to its flexibility of use and reliability. Considering compression-ignition engines, the fuel change is particularly simple to carry out and requires no particular modification. Yet some critical issues related to the variation of the fuel viscosity may arise. For these reasons, the kinematic viscosity of many fuel blends (diesel oil-biodiesel-bioethanol) was inquired at the two typical temperatures of 40 °C and 100 °C, referred by standards. On the basis of the collected data and of the mathematical model obtained through the Response Surface Methodology, it was possible to observe that: (a) in order to have a blend with the same viscosity of diesel oil at 40 °C (3.03 mm2 s-1), the fuel-feed system should work at higher temperatures (about +28 °C) and hence a preheater is required to ensure that the fuel pump operates at its nominal flowrate; (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% for each percentage point of bioethanol in the blend. Also, a set of trials on a farm tractor fuelled with some of these blends allowed: (a) to obtain another numerical model using the fuel viscosity as statistically-significant estimation parameter, (b) to observe that it is possible to have a torque increment of about 49.3 Nm per each an increment of 1 mm2 s-1 of the blend viscosity (regardless of whether the cause is the viscosity itself or other properties that change concomitantly with it). These results have been used to propose an alternative fuelling for the farm tractor, having observed that the best increase of the torque can be obtained when the used ternary fuel blend has a viscosity greater than 3.33 mm2 s-1, whatever the composition within the validity ranges of the models (0 ≤ biodiesel ≤ 100, 0 ≤ bioethanol ≤3).