# Dual-Fuel Combustion for Future Clean and Efficient Compression Ignition Engines
## 3 Paths for Increasing RCCI Efficiency
### Fuel Autoignition Qualities Modification  
The third path to maximize RCCI efficiency is based on modifying the fuel properties in order to look for a suitable combination of high reactivity fuel (HRF) and low reactivity fuel (LRF) that improves RCCI combustion. Considering the mandatory presence of biofuels in the future context of road transport [40], the ability of ethanol to be blended with gasoline [41], and the main conclusions extracted from RCCI literature regarding ethanol [42], the low reactivity fuels selected to perform this study are E10-95, E10-98, and E20-95. In addition, a diesel fuel containing the maximum biodiesel percentage currently allowed to be distributed as a regular fuel grade in Europe, 7% by volume, has been used as high reactivity fuel during all the study. This will be referred to as diesel B7. The main characteristics of the four fuels are listed in Table 6. All the properties were obtained following the American Society for Testing and Materials (ASTM) standards. As can be seen in Table 7, the lower heating value (LHV) of E20-95 is lower than the other two LRF. This is because of the greater ethanol content in the blend. To take into account this fact during the comparison, the premixed energy ratio (PER) is presented in Equation (3). The PER is defined as the energy ratio of the LRF versus the total delivered energy, so that it ensures that during the tests all the LRF are compared at equal conditions in terms of energy delivered to the engine. \[\text{PER}[\%]=\frac{m_{\text{LRF}}\cdot\text{LHV}_{\text{LRF}}}{m_{\text{HRF }}\cdot\text{LHV}_{\text{HRF}}+\ m_{\text{LRF}}\cdot\text{LHV}_{\text{LRF}}} \tag{3}\] \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Fuels**} \\ \hline Properties & Diesel B7 & E10-95 & E20-95 & E10-98 \\ Density [kg/m\({}^{3}\)] (T = 15 \({}^{\circ}\)C) & 837.9 & 739 & 745 & 755 \\ Viscosity [mm\({}^{2}\)/s] (T = 40 \({}^{\circ}\)C) & 2.67 & - & - & - \\ RON [-] & - & 98.8 & 99.1 & 103 \\ MON [-] & - & 85.2 & 85.6 & 90 \\ Ethanol content by volume [\%] & - & 9.7 & 19.7 & 9.7 \\ Cetane number [-] & 54 & - & - & - \\ Lower heating value [M]/kg] & 42.61 & 41.32 & 40.05 & 41.29 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Physical and chemical properties of the fuels used along the study. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{4}{c}{**7.7 bar IMEP**} \\ \hline PER [\%] & Diesel SOI\({}_{\text{Pilot}}\) & Diesel SOI\({}_{\text{main}}\) & Energy/cycle [J] \\ \hline
65 & & \(-20\) to \(-35\) & \\
70 & \(-60\) & \(-20\) to \(-35\) & \\
75 & & \(-20\) to \(-30\) & \\
80 & & \(-20\) & \\ \hline  & \multicolumn{4}{c}{13.5 bar IMEP} \\ \hline
60 & & & \\
65 & \(-60\) & \(-9\) to \(-21\) & 4900 \\
70 & & & \\
75 & & & \\ \hline  & \multicolumn{4}{c}{18 bar IMEP} \\ \hline
50 & & & \\
55 & - & \(-9\) to \(+3\) & 7200 \\
60 & & & \\
65 & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Summary of all the tests performed to evaluate the three combinations of HRF + LRF. As done in the previous section, to evaluate the influence of the LRF properties on RCCI combustion, a batch of parametric studies varying the diesel injection timing and PER were proposed at low, medium, and high loads at 1200 rpm. The different settings studied are depicted in Table 7.  
Figure 8 synthetizes the results of the best tests extracted from the merit function. As it can be seen, all the fuels allow operating under the NOx and soot emissions limits. It is interesting to remark that, at 7.7 bar gross IMEP, soot emissions were under the detection limit of the smoke meter for all the fuels. This is due to the high PER used (low diesel amount injected) and the large advance of the diesel injection timing [43], which provides enough mixing time to avoid soot formation [44]. Focusing on CO and HC emissions, it is possible to state that, as a general trend, the operation with biofuels results in higher levels of unburned products than with regular diesel and gasoline fuels.