The effect of biofuel blends on the engine performance and emissions of agricultural machines can be extremely complex to predict even if the properties and the effects of the pure substances in the blends can be sourced from the literature. Indeed, on the one hand, internal combustion engines (ICEs) have a high intrinsic operational complexity; on the other hand, biofuels show antithetic effects on engine performance and present positive or negative interactions that are difficult to determine a priori. This study applies the Response Surface Methodology (RSM), a numerical method typically applied in other disciplines (e.g., industrial engineering) and for other purposes (e.g., set-up of production machines), to analyse a large set of experimental data regarding the mechanical and environmental performances of an ICE used to power a farm tractor. The aim is twofold: i) to demonstrate the effectiveness of RSM in quantitatively assessing the effects of biofuels on a complex system like an ICE; ii) to supply easy-to-use correlations for the users to predict the effect of biofuel blends on performance and emissions of tractor engines. The methodology showed good prediction capabilities and yielded interesting outcomes. The effects of biofuel blends and physical fuel parameters were adopted to study the engine performance. Among all possible parameters depending on the fuel mixture, the viscosity of a fuel blend demonstrated a high statistical significance on some system responses directly related to the engine mechanical performances. This parameter can constitute an interesting indirect estimator of the mechanical performances of an engine fuelled with such blend, while it showed poor accuracy in predicting the emissions of the ICE (NOx, CO concentration and opacity of the exhaust gases) due to a higher influence of the chemical composition of the fuel blend on these parameters; rather, the blend composition showed a much higher accuracy in the assessment of the mechanical performance of the ICE.

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As opposed to gasoline, diesel oil has a lower limit temperature for its use in internal combustion engines (ICEs), which ranges between -7 ºC and -20 ºC for the summer/winter formulation: it is therefore close to the average winter temperature typical of some European countries. In approaching such temperature, the formation of paraffins alters the physical characteristics of diesel oil (viscosity in particular) and makes it impossible to be used in ICEs. The same, aggravated problem is presented also by biodiesel and diesel-biodiesel blends, which are very interesting given their benefits in terms of performance and pollutant reduction in conventional compression-ignition engines (e.g., agricultural and operating machinery, cogeneration systems). Some varieties of biodiesel, such as biodiesel from palm oil, solidify at 13 ºC, whilst others, more suitable for winter temperatures, solidify at -10 ºC (biodiesel from canola seeds), which is nevertheless a higher temperature than that of diesel oil. In this work, a simple industrial freezer set at -21 ºC was used to assess the freezing temperatures of many solutions of multiple components with different freezing points (diesel-biodiesel blends). The illustrated procedure use slow-cost and simple equipment thus allowing to reproduce similar experiments in industrial environments. The elaborations carried out have included the use of polynomial functions to fit the data and the identification of temperature tangency traits. Although the results are not in the form of the usual significant temperatures indicated by standards as cold-flow properties for fuels (i.e., pour point, cloud point, cold filter plugging point), they are substantially aligned with the literature data. However, the outcomes in the form of upper and lower liquid-to-solid temperatures are very interesting and useful to give the experimenters/users practical indications about the opportunity of using diesel-biodiesel blends with different compositions.