# Dual-Fuel Combustion for Future Clean and Efficient Compression Ignition Engines
## 3 Paths for Increasing RCCI Efficiency
### Engine Settings Combination  
The first path for increasing the RCCI efficiency relies on the engine settings combination. To do this, it was decided to control three variables that govern the in-cylinder reactivity. Two of them were the EGR rate and intake gas temperature (T), which define the gas charge thermodynamic properties. The third variable studied was the gasoline fraction (GF), defined as the mass ratio of gasoline versus the total fuel injected, because it defines the in-cylinder fuel reactivity. The tests were done at 1200 rpm and 7.5 bar IMEP, which corresponds to 25% load in this engine platform. This load was selected as representative as it is the minimum load considered in the World Harmonized Stationary Cycle (WHSC), proposed by the EURO VI regulation. The combustion phasing (CA50) was kept constant at +5 CAD ATDC, because this combustion phasing was found to offer a good compromise between engine-out emissions and performance. The injection strategy for diesel fuel was fixed at \(-60/-50\) CAD ATDC, and the pair of values EGR + GF and T + GF were adjusted to  
Figure 2: Typical apportionment of the fuel energy among the different outgoing energy paths during RCCI combustion. keep the combustion phasing at the desired value. Table 4 summarizes the engine settings tested with both strategies as well as those for the baseline operating condition. Figure 3 shows the RoHR traces for the two strategies, EGR + GF (left) and T + GF (right). Comparing both graphs, it is confirmed that EGR + GF strategy leads to higher maximum RoHR peaks than T + GF. In the case of EGR + GF, the start of the low temperature heat release (LTHR) and the LTHR peaks are equal for the four conditions tested. By contrast, the tests of T + GF denote a clear dependence of the LTHR onset on the intake temperature, showing earlier onset as temperature rises. In addition, it can be seen that the magnitude of the LTHR increases as diesel fuel mass increases (GF is reduced). In order to select the best tests for each strategy, the merit function defined in Section 2.3 was applied to the complete batch of tests. Figure 4 represents the engine-out emissions and efficiency for the best tests of the two strategies as well as for the baseline condition. It is worth noting that the tests selected for both strategies are those that have near 70% of GF (EGR = 43% and T = 50 \({}^{\circ}\)C). The results show that both methods allow the decrease of unburned products and improve the GIE without penalizing NOx and soot emissions as compared to the baseline condition. Note that soot emissions are not depicted in the graph because all the tests provided soot levels under the detection limit of the smoke meter. Moreover, it is clear that EGR+GF provides higher gross indicated efficiency (GIE) than the T + GF strategy. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **EGR [\%]** & **Intake T [\({}^{\circ}\)C]** & **Intake P [bar]** & **Gasoline Fraction [\%]** & **Diesel SOI [CAD ATDC]** \\ \hline Baseline & 45 & & & 75 & \\  & 50 & & & 50 & \\ Strategy 1 (EGR + GF) & 46.5 & 40 & & 61 & \\  & 43 & & & 69 & \\  & 38 & & 1.35 & 78 & \(-\)60/\(-\)50 \\  & & 30 & & 50 & \\ Strategy 2 (T + GF) & 45 & 40 & & 61 & \\  & & 50 & & 71 & \\  & & 60 & & 79 & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of all the tests performed at 7.5 bar to evaluate the two optimization strategies. Figure 3: RoHR traces for the experiments with both strategies: EGR + GF (left) and T + GF (right) at 7.5 bar IMEP and 1200 rpm. The results of Figure 4 can be explained looking at Figure 5, where the RoHR traces and bulk gas temperature for the three cases are represented. As seen from Figure 5, the RoHR profiles of baseline and T + GF conditions are very similar, with only slightly earlier LTHR and HTHR onset in the case of T + GF due to the higher intake temperature (+10 \({}^{\circ}\)C).