## 2 Materials and Methods  
### Experimental Setup  
The experimental test rig employed a prototype SCE integrating the main combustion system components of a modern CI ignition engine. The fuel system was refurbished to permit the use of LPG/diesel blends. Some critical aspects were evidenced during the experimental test campaign, such as the blend temperature increase during the engine run and the need to modify the fuel system to ensure the liquid phase on supply and return fuel lines. It is worth underlining that the petroleum gas was liquid, which is LPG, at a pressure above the 5 bar pressure at ambient temperature. To this aim, the injection return line was modified to ensure sealing at higher pressures. For this reason, LPG could not be directly mixed in the diesel fuel tank, and for the storage of the mixture, a proper pressurized LPG-fuel tank was used. The tank made of anti-corrosion metal material must guarantee the absence of fluid leaks, and must also withstand pressures of 20 bar, and above the nominal working pressure of 8 bar, for safety reasons. In the LPG configuration, as illustrated in Figure 1, the fuel filter was not installed to prevent the fuel phase separation (liquid to gaseous) due to the concentrated pressure losses. The fuel tanks and system described needed to be filled every time before starting the test campaign. However, to have a more flexible system for real engine applications, further developments of the storage and fuel lines are under consideration. The injection system, fuelled by diesel-LPG, a closed pressure circuit (8 bar), was realized to avoid vaporization by reducing the risk of cavitation at the inlet of the high-pressure pump [4]. The fuelling system was modified in a way it could exploit the original diesel injection system composed of a conventional common rail solenoid injector with a 7-hole nozzle characterized by a 0.14 mm diameter and a cone angle of 148 degrees. The other engine components were properly designed, manufactured, and assembled at the CNR-Istituto Motori laboratories. A system capable of monitoring and controlling the engine control parameters, pressure, temperature signals, and pollutant emissions was developed on Labview. Combustion metrics (indicating) were measured and calculated on the base of a piezo quartz pressure sensor. The average cycle was estimated by 128 consecutive cycles at the sampling rate of 0.1 degree and used for detailed thermodynamic analysis. For a comprehensive thermod  
Figure 1: Detailed scheme of the fuel-injection system [4]. the engine system, the entire test rig was equipped with pressure and temperature sensors. An exhaust gas-analyzer model AVL CEB II and AVL315s were used to measure gaseous and smoke emission composition at the exhaust, respectively. A detailed scheme of the test cell layout is depicted in Figure 2.  
Fuel consumption evaluation, as the LPG-diesel blend system was designed, was not possible through a conventional gravimetric balance. Therefore, to determine the fuel consumption, an indirect estimate of fuel consumption was obtained by equating the carbon mass in the exhaust to the carbon concentration of the fuel, estimated from the exhaust emissions (CO\({}_{2}\), CO, and HC). The detailed procedure for the indirect calculation of fuel consumption is given in Appendix A. The applied method was preliminarily validated by comparing the fuel consumption of diesel, using the conventional open-circuit fuelling system, measured by the gravimetric balance and the calculation methods. The maximum detected error was around 3%. The employed nozzle characteristics are listed in Table 1.  
It is well known that the combustion process and its associated emissions are significantly affected by the injection process, as well as the fuel properties. To better identify their effects on the combustion process, a specific injector test campaign hydraulic characterization was associated with the engine tests.