*3.1. 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 keep the combustion phasing at the desired value.

tab:applsci-07-00036-t004 summarizes the engine settings tested with both strategies as well as those for the baseline operating condition.


**Table 4.** Summary of all the tests performed at 7.5 bar to evaluate the two optimization strategies.

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).

**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.

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 ◦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.

**Figure 4.** Comparison of the emissions and efficiency obtained with the two strategies versus the baseline condition at 7.5 bar IMEP and 1200 rpm. The merit function values for each case are shown in the second subplot.

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 ◦C). The RoHR peak of the EGR + GF strategy is 50 J/CAD greater than the other two cases, which results in higher bulk gas temperature, even using an intake temperature 10 ◦C lower than T + GF.

**Figure 5.** RoHR traces and bulk gas temperature for the best tests of both strategies and that for the baseline condition at 7.5 bar IMEP and 1200 rpm.

The reduction of the unburned products found in Figure 4 with the two strategies, should be related with the faster expansion period as compared to the baseline case. On the other hand, CO emissions are greatly influenced by the in-cylinder temperature. As it can be seen, the bulk gas temperate exceeds 1400 K in all cases, which is the threshold temperature to accelerate the CO oxidation. Since the bulk gas temperature of EGR+GF and T + GF is higher than that of the baseline operating condition, it is expected that a greater part of the in-cylinder charge experiences temperatures greater than 1400 K, explaining the CO reduction observed with the two strategies.

In the light of the results, it is possible to conclude that to achieve low emissions and high efficiency under the operating conditions tested, the EGR rate should be in the range of 43%–45%, GF 69%–71%, and intake temperature 40–50 ◦C. Moreover, the most efficient strategy seems to be achieved with slightly lower GF and higher oxygen concentration than the others. This knowledge will be used to define the boundary conditions in the next studies.
