# Dual-Fuel Combustion for Future Clean and Efficient Compression Ignition Engines
## 3 Paths for Increasing RCCI Efficiency
### Piston Bowl Geometry Optimization  
As a second path to increase RCCI efficiency, it was considered to modify the piston geometry for reducing the in-cylinder heat transfer losses. For this purpose, two new piston bowl geometries were defined. The volumes of the designed combustion bowl geometries were matched to keep the same geometric compression ratio as with the stock piston, which is 14.4:1. The three geometries are illustrated in the cross-sectional views presented in Figure 6.  
The first new piston was called 'tapered'. This bowl shape maintains the same central geometry than the stock piston, with slightly higher height necessary to keep the same compression ratio. The major change versus the stock piston is the tapered shape of the piston crown, which has two main purposes. The first one is to limit the heat transfer in this region through the heat transfer coefficient reduction due to the lower squish flow velocities [37]. Second, the tapered shape is intended to improve the penetration of high temperature gas into the squish and near-liner regions, where a great amount of gasoline gets trapped [22]. Moreover, this geometry resulted in near 6% less piston surface area than the stock piston which also contributes to the heat transfer reduction. The second geometry was called 'bathtub' as it follows some of the design guidelines provided by [PERSON] et al. [38; 39], which suggested that the efficiency of RCCI improves as the piston bowl radius increases and the bowl depth decreases. The application of these findings to the piston blanks, resulted in a piston bowl geometry with near 16% less piston surface area than the stock piston. This large reduction in surface area, in combination with the more quiescent combustion chamber created by the resulting flatter bowl geometry, should significantly reduce the heat transfer losses. To evaluate the influence of the piston bowl geometry on RCCI combustion, a batch of parametric studies of the key variables governing the fuel reactivity stratification (diesel injection timing and GF) were performed from low to high load at 1200 rpm. Table 5 summarizes all the engine settings tested. Note that the effective compression ratio (CR\({}_{\text{eff}}\)) was reduced down to 11:1 in the case of 18 bar IMEP to avoid the excessive knocking levels provoked by the sudden ignition of the high amount of homogeneously mixed gasoline. The CR reduction was done by shortening the intake event duration (early Miller cycle), through the VVA system. The bar chart shown in Figure 7, in which all bars have the same baseline value, summarizes the best merit function results for each piston and load. The tests at 7.7 and 13.5 bar correspond to double injection, while single injection was found to be more suitable at 18 bar. As it can be seen EURO VI  
Figure 6: Cross-sectional views of the stock (left), tapered (middle), and bathtub (right) piston bowl geometries. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**7.7 bar IMEP**} \\ \hline GF [\%] & Diesel \(\text{SOI}_{\text{pilot}}\) & Diesel \(\text{SOI}_{\text{main}}\) & CR\({}_{\text{eff}}\) [-] \\
60 to 85 & \(-60\) to \(-40\) & \(-40\) to \(-15\) & 14.4:1 \\ \hline \hline \multicolumn{4}{c}{13.5 bar IMEP} \\ \hline
60 to 70 & \(-60\) to \(-40\) & \(-40\) to \(-9\) & 14.4:1 \\ \hline \multicolumn{4}{c}{18 bar IMEP} \\ \hline
50 to 75 & \(-60\) to \(-40\) & \(-16\) to \(-4\) & 11:1 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of all the tests performed to evaluate the three piston geometries. NOx emissions levels are reached with all the pistons. However, only the stock and tapered geometries allow working in the region under 0.01 g/kWh of soot emissions. From the second subfigure, it can be inferred that the bathtub piston has a higher sooting tendency than the other two geometries. This is thought to be related to the less prominent bowl, with reduces the turbulence near the top dead center (TDC) and worsens the air-fuel mixing process.