*2.1. Experimental Fuels*

In the experiments, diesel and LPG fuels were used as test fuels. LPG content was 70% propane and 30% butane. The properties of the test fuels are given in Table 1. Diesel fuel meets requirements of EN590 for cetane index, density, and viscosity. The test engine was operated with four different fuel types as D-100 (pure diesel), LPG-30 (%70 diesel + %30 LPG) LPG-50 (%50 diesel + %50 LPG), and LPG-70 (%30 diesel + %70 LPG). The pilot diesel quantity was changed by varying the flow rate of LPG for each load condition at 3000 rpm constant speed. LPG flow rate was adjusted by changing Gasoline Direct Injection (GDI) injector trigger pulse duration via the designed ECU. The starting of GDI injector trigger pulse was set to 250 Crank Angle (CA) towards the end of the intake stroke. Thus, it was prevented that the mixing of evaporated LPG with intake air and reducing the amount of intake air. The width of the trigger pulse of the GDI injector for D-100 fuel was set to zero. In this case, it was ensured that the fuel sent into the cylinder was only diesel fuel. For LPG-30 fuel the duration of the GDI injector trigger pulse was gradually increased starting from 1 ms. The mass flows of the diesel and LPG fuels within a certain time interval were measured for each GDI injector trigger pulse duration. Thus, the fuel sent into the cylinder was set 70% diesel + 30% LPG. The same procedure was repeated for the LPG-50 and LPG-70 fuel ratios respectively. The LPG ratio in total fuel is calculated by using the following equation.

$$\text{LPG}\_{\text{ratio}} = \frac{\text{m}\_{\text{LPG}}}{\text{m}\_{\text{diesel}} + \text{m}\_{\text{LPG}}} \ast \% 100 \tag{1}$$


**Table 1.** Diesel and Liquefied Petroleum Gas (LPG) fuel properties [34,35].
