*3.3. 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 tab:applsci-07-00036-t006. All the properties were obtained following the American Society for Testing and Materials (ASTM) standards.



As can be seen in tab:applsci-07-00036-t007, 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.

**Table 7.** Summary of all the tests performed to evaluate the three combinations of HRF + LRF.


$$\text{PER}[\%] = \frac{m\_{\text{LRF}} \cdot \text{LHV}\_{\text{LRF}}}{m\_{\text{HRF}} \cdot \text{LHV}\_{\text{HRF}} + m\_{\text{LRF}} \cdot \text{LHV}\_{\text{LRF}}} \tag{3}$$

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 tab:applsci-07-00036-t007.

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. Moreover, it is seen that the higher levels of both emissions are produced with E20-95 fuel, which has a large ethanol content. Considering this, the higher CO and HC levels are thought to be related with the greater enthalpy of vaporization of ethanol, which results in a cooling effect in the intake manifold and therefore lower temperature peak at TDC.

**Figure 8.** Best merit function results for each fuel combination at the different engine loads at 1200 rpm. Note that all bars have the same baseline value.

The maximum PRR increases as load increases. In this case, even using a single injection pattern at 18 bar, values near the 15 bar/CAD limit are observed. This is because the combustion phasing (CA50) has been reduced substantially as compared to the results shown in the previous section. On the other hand, following the same reasoning as the previous section, the combustion duration (CA90-CA10) reduces first, and later increases. Regarding engine performance, the figure shows that all the biofuels investigated allow more efficient operation than regular fuels (note that the GIE values for E20-95 and E10-98 at low load are equal). In addition, it is clear that B7 + E10-95 performs better than the rest of the fuels in all the load range, promoting the highest increase of GIE at low load (3% higher GIE than regular fuels).
