*3.2. 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.

**Figure 6.** Cross-sectional views of the stock (left), tapered (middle), and bathtub (right) piston bowl geometries.

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 grea<sup>t</sup> 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 Splitter 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.

tab:applsci-07-00036-t005 summarizes all the engine settings tested. Note that the effective compression ratio (CReff) 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.


**Table 5.** Summary of all the tests performed to evaluate the three piston geometries.

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 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. This hypothesis is in line with the results, since the sooting tendency is more evident at high loads, where near TDC single injections are used and therefore the bowl shape plays a key role on the mixing process. In terms of CO and HC emissions, all the pistons are far from EURO VI levels. As it can be seen, the stock piston leads to less unburned products than the new geometries at low and medium loads. The inversion of this trend at high load is thought to be related with the change from double to single injection strategy. In this sense, the more prominent bowl of the stock piston confines the diesel injection and avoids increasing the reactivity in the crevice zone, which worsens the burning of the gasoline trapped in this region.

**Figure 7.** Best merit function results for each piston geometry at the different engine loads at 1200 rpm. Note that all bars have the same baseline value.

Maximum PRR is reduced with the two new geometries, providing a grea<sup>t</sup> margin to the limit at medium load conditions. The combustion duration (CA90-SOC) decreases when moving from 7.7 to 13.5 bar, and later increases. This occurs because the more reactive in-cylinder conditions at 13.5 than 7.7 bar allow introducing greater amount of gasoline, which becomes homogeneously-mixed and promotes much faster heat release, even having similar combustion phasing (CA50). At 18 bar IMEP, the injection strategy follows a single pattern to reduce the knocking levels. This leads to some diffusion combustion period, which provokes an increase of both the combustion duration and combustion phasing. Finally, as it can be inferred from the figure, the GIE has an inverse trend with the piston surface area, i.e., higher efficiency as bowl surface area reduced. This fact suggests that heat transfer reduction is contributing to the efficiency gain with the two new geometries. However, considering the excessive sooting tendency of the bathtub piston at high load and that the tapered piston does not provide a notable GIE increase versus the stock geometry, it was decided to keep the stock piston mounted in the engine for the next studies.
