# Dual-Fuel Combustion for Future Clean and Efficient Compression Ignition Engines
## 4 Conclusions  
The present work has investigated the effectiveness of different strategies to maximize the efficiency of RCCI combustion in a single-cylinder heavy-duty diesel engine. The solutions explored consisted of the optimization of the engine settings, modification of the piston bowl geometry, and  
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.  
variation of the low reactivity fuel properties. The major findings from this study are summarized as follows:  
* The study regarding the engine settings combination revealed that both strategies (EGR + GF and T + GF) improve as unburned products (CO and HC) as the GIE without increasing NOx and soot emissions with respect to baseline condition. In addition, the results suggested that the EGR + GF strategy provides more efficient operation than the T + GF strategy, allowing GIE peaks near 49%.
* Maintaining equal compression ratio (14.4:1), different piston geometries were tested. The results showed that the bowl shape is a key parameter on the mixing process when delayed diesel injection timings are used, i.e., when moving towards high loads. Indeed, only two of these geometries, tapered and stock, were able to work below 0.01 g/kWh of soot levels at 18 bar IMEP, whereas the bathtub geometry produces excessively high levels of soot due to the absence of a prominent bowl shape, which reduces the turbulence at TDC. The stock piston produced lower unburned HC than the other geometries. However, the two new geometries showed improved thermal efficiency versus the stock piston, findings that are in line with the heat transfer reduction due to the lower bowl surface area.
* Following the path to improve the RCCI concept, fuels have been studied in order to establish criteria about the types of HRF and LRF which enhance the RCCI combustion. The study has been performed with ethanol-added gasoline and B7 biodiesel fuel. Results demonstrate that this type of fuel can promote more efficient operation than regular diesel and gasoline, achieving up to 3% greater GIE with the B7 + E10-95 fuel combination. On the other hand, excessively higher unburned HC and CO emissions were obtained. This fact should be further studied in the future, since it can compromise the efficiency of the aftertreatment systems.  
The authors acknowledge VOLVO Group Trucks Technology for supporting this research and express their gratitude to the Spanish economy and competitiveness ministry for partially funding this investigation under the project HiReCo (TRA2014-58870-R). The author [PERSON] thanks the Universitat Politecnica de Valencia for his predoctoral contract (FPI-S2-2015-1531), which is included within the framework of Programa de Apoyo para la Investigacion y Desarrollo (PAID).  
[PERSON] and [PERSON] conceived and designed the experiments; [PERSON] performed the experiments; [PERSON] and [PERSON] analyzed the data and wrote the paper. All authors discussed the results and commented on the manuscript at all stages.  
The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.