## 3 CFD Simulation Results
### CO and CO\({}_{2}\) Emissions  
Figures 14 and 15 present the CO and CO\({}_{2}\) emissions formed inside the engine cylinder in the three cases of using various piston bowls, respectively. The simulation results showed significant reductions in CO formation during the combustion process of the engine in the re-entrant and \(\omega\)-type piston bowl cases in comparison to the U-type piston bowl case, in which the re-entrant piston bowl generated the lowest CO emission in the combustion process. The reason is because the turbulence intensities in the cylinder of the engine in the \(\omega\)-type, and especially in the re-entrant type of piston bowls, were higher than those in the U-type, as shown in Figure 6. CO formation is strongly influenced by the combustion quality which is strongly affected by the fuel-air mixing quality. Higher in-cylinder turbulences result in better fuel-air mixing quality leading to a higher combustion quality and lower CO formation. However, the CO emissions at the end of the expansion process of the engine in all three piston bowl cases were almost zero because they were oxidized by the residual oxygen inside the engine cylinder to form CO\({}_{2}\) emissions, the complete product of hydrocarbon fuel combustions. As is the same as in the soot characteristics of the engine, except for only a little difference in CO\({}_{2}\) formation during the combustion of the engine, the final generated amounts of CO\({}_{2}\) emissions when using the three piston bowl types were almost the same. This implies that changing the type of piston bowl has a negligible effect on the engine-out CO\({}_{2}\) emission characteristics of the engine. Figure 13: Illustration of soot formation in DIEs [58]. Observing the process of CO\({}_{2}\) formation closely, it can be seen that the CO\({}_{2}\) formation process in the re-entrant piston bowl case was slowest, followed by the \(\omega\)-type piston bowl case, and the U-type bowl case created the fastest CO\({}_{2}\) formation. This is due to differences in the in-cylinder temperature profile of the engine when using different piston bowl types. As has been known, CO\({}_{2}\) is the final product of a complete combustion of hydrocarbons. First, hydrocarbons are oxidized by O\({}_{2}\) to form carbon monoxide (CO). CO is then further oxidized to form CO\({}_{2}\) in high-temperature conditions by remaining O\({}_{2}\) inside engine cylinders. Higher in-cylinder temperatures will enhance the CO oxidation rate, or in another word, increase the CO\({}_{2}\) formation rate. This means that CO\({}_{2}\) formation is strongly influenced by the temperature and O\({}_{2}\) concentration inside engine cylinders. In ICEs, to ensure that all injected fuel is completely burned, the engines are designed to always be supplied with a larger amount of air than they need for the theoretical complete combustion of fuel. This amount of air that is more abundant than is chemically necessary for complete combustion is called air excess. The air excess ratio in diesel engines is typically about 1.6 to 2.2 [13]. This means that there will always be enough air for fuel oxidation during the combustion process of ICEs. Excluding the O\({}_{2}\) concentration factor, the remaining factor affecting the formation of CO\({}_{2}\) is the temperature in the cylinder. As can be seen from Figures 7 and 8, the in-cylinder temperatures, in the cases of using the \(\omega\)-type and re-entrant piston bowls, were lower than that when using the U-type bowl. This reduced the CO\({}_{2}\) formation rate in the cases of using the \(\omega\)-type and re-entrant piston bowls, and the \(\omega\)-type and re-entrant piston bowls, respectively. Figure 14: CO emissions diagrams for the three various piston types. Figure 15: CO\({}_{2}\) emissions diagrams for the three various piston types. bowls. Among these three piston bowl types, the re-entrant type had the slowest CO\({}_{2}\) formation rate due to its lowest in-cylinder temperature.