## 3 Results and Discussion  
### Understanding of GCI Combustion  
In conventional diesel engines, it is difficult to control particulates and NO\({}_{\chi}\) emissions. In practice, diesel fuels are significantly less volatile than gasoline fuels [23]. Auto-ignition is determined by chemical kinetics [24], which depend on the pressure and temperature development, mixture strength, and chemical composition of the fuel. Figure 2 shows the filtered in-cylinder pressure and heat release curves using different reactivity fuels (different CN fuels) on a CI engine while maintaining a similar combustion phasing with a single injection. Diesel fuel auto-ignites very quickly before the fuel and air mix sufficiently in a long diffusion and mixing-controlled combustion. If the mixing is accelerated or if the chemical reaction is slowed down such as in the case of low CN fuels, then auto-ignition can be made to occur after the fuel and air are better mixed and when the diffusion duration can be shortened. Moreover, NO\({}_{\chi}\) levels can be reduced by reducing the combustion temperature by either running lean, pre-mixed EGR (Exhaust Gas Recirculation). In summary, fuels that are more resistant to auto-ignition allow mixing time before combustion occurs and help to reduce NO\({}_{\chi}\) and smoke in CI engines. If the combustion phasing and delay are matched for any two fuels at a given condition, then their emissions behavior is also matched the auto-ignition resistance of the fuel is the most important fuel property when the injection is completed before the combustion starts [11; 12; 13]. However, a high peak of the maximum in-cylinder pressure and maximum pressure rise rate (MPRR) will be problematic at higher loads, and the combustion stability, hydrocarbon (HC), and carbon monoxide (CO) emissions can create a low operating load limitation because combustion is more likely to take place in lean mixture packets with better mixing caused by higher ignition delays. These problems can be significantly alleviated by managing the mixing by reducing fuel injection pressure in common rail (upstream of fuel injector) and injection strategies, e.g., multiple injections. Injector design is also likely to be important--larger hole diameter is better for gasoline fuel. The resistance of a fuel is far to auto-ignition is more important than its volatility in this type of combustion, a finding that is in line with previous observations [25; 26; 27]. As previously mentioned, the injector design is also important to adapt low CN fuels in CI engines. The main concern of the nozzle design for CN fuels is ensuring that the same injection duration and similar energy density is delivered for low CN fuels compared to diesel. This result indicates that no additional timing will be necessary at a full load to inject the same energy quantity with low CN fuels. At this stage, this might be considered to be a positive result to mitigate the risks related to soot post oxidation, as this chemical/physical process is closely related to the injection end. Figure 3 illustrates an example of the dynamic energy flow quantity versus the energizing duration and fuel injection pressure of diesel and CN25 fuel and spray penetration in the liquid and vapor phase. The lower density of CN25 fuel (\(\rho\_CN25\) = 0.736 kg/L, \(\rho\_ diesel\) = 0.834 kg/L) along with its higher lower heating value LHV (\(LHV\_CN25\) = 43,287 kJ/kg, \(LHV\_ diesel\) = 42,660 kJ/kg) can now be considered, thereby allowing us to look at the energy that was introduced over the energizing duration. A final nozzle with 340 cc/30 s 10 MPa hydraulic flow was selected for the CN25 fuel instead of the original nozzle with a 290 flow-number for diesel fuel. Regarding spray penetration, the liquid penetration of CN25 fuel is much lower than that of diesel fuel due to its high volatility and its tendency to vaporize more quickly.