## 3 Results and Discussion
### Understanding of GCI Combustion
Regardless of the fuel, liquid penetration is similarly reduced when increasing the air density, and CN25 liquid penetration is less sensitive to the increase of air temperature. This is fully consistent with a more volatile fuel, as less energy is required for vaporization. On the other hand, the degree of vapor penetration is similar for both fuels at the conditions tested and only a slightly longer vapor penetration was measured with the CN25 fuel. In the first stage of the GCI program, each CN fuel was tested in different single cylinder engines (SCE) to design a GCI combustion model using low CN fuels. The selection of engine hardware was an Aramco pathway towards fuel and engine co-optimization (Figure 1). For example, CN35 fuel was tested in a SCE that represented conventional CI engine hardware (1.6 L Peugeot 308 engine in Table 2). Figure 4 shows the variable combustion modes in different parts of the engine map. At a low load (1500 rpm, 4 bar of IMEP), the fuel and air are fully premixed before combustion with the high impact of low cetane number fuel. In spite of the low injection pressure with CN35, particulate emissions are very low because the CN35 combustion is fully premixed and has a long ignition delay, and the combustion duration is short. The rapid combustion process of the premixed combustion mode without post injection enables a significant reduction in the exhaust losses with CN35, resulting in a notable fuel consumption reduction. At mid load (1500 rpm and 7.2 bar of IMEP), a partially premixed  
Figure 2: Average (250 cycles) in-cylinder pressure and heat release curves using different CN fuels in a 1.6 L CI engine at 10 bar of BMEP and 2650 rpm (with single injection). combustion (PPC) mode takes place with CN35. The short duration of the combustion with CN35 and the split fueling strategy allow a reduction of the heat losses compared to diesel. Even though the diluted combustion can dramatically improve the fuel efficiency and pollutant emissions, combustion noise has to be kept at a reasonable level. At a high load (2650 rpm and 11.4 bar of IMEP) with higher temperature and pressure inside cylinder, the ignition delay is mainly dominated by the mixing properties (local A/F ratio). The impact of the CN fuel becomes negligible. The combustion mode on the CN35 fuel is similar to conventional diesel combustion; a short, premixed combustion phase and a large diffusion- and mixing-controlled combustion phase are clearly identified. In general, the NO\({}_{\text{X}}\)/particulate trade-off for GCI combustion is relatively less at all of the operating conditions compared to diesel, and more EGR is able to reach the low NO\({}_{\text{X}}\) emission target. In the last stage of the GCI program, the calibration sets, which were generated with ICE\({}^{2}\) software, were then checked to validate the models and to assess the optimized targets based on the engine test. The final engine map was realized at the engine test cell. The sketches in Figure 5 are based on the Aramco projects for GCI studies on different engines (1.6-1.8 L and compression ratio of 15:1-16:1) without major hardware modifications. Based on the test results, the GCI engines with different CN fuels can be classified into several different zones and can highlight the benefits and risks in terms of the efficiency and emissions of the engine map. The figures illustrate the GCI engine map along with its benefit and challenge zones with three different CN fuels. The gray zone (diffusion combustion) is represented in a similar way to conventional diesel CI combustion. A short, premixed combustion phase and a large diffusion, mixing-controlled combustion phase are clearly identified in this zone. In that zone, GCI engine operation shows less benefits in terms of both emissions and efficiency, regardless of the fuel's auto-ignition quality. In the green zone (premixed combustion), the high dilution of the PPC provides maximum potential with strong improvement in efficiency and pollutant emissions.