## 3 Results and Discussion
### Engine Exhaust Emissions  
#### 3.3.1 NOx Emissions  
NOx consists of nitrogen monoxide (NO) and nitrogen dioxide (NO\({}_{2}\)) [35]. NOx is highly dependent on the combustion temperature (primary dependence), local concentration of oxygen and the combustion duration [6; 35; 38]. Therefore, as the combustion temperature increases, the NOx emissions increase [21]. When the injection is advanced, a large percentage of the combustion occurs in the premixed stage, which increases the NOx formation [35; 38]. However, as the injection is retarded, it reduces the cylinder temperature as combustion occurs late, thereby reducing NOx [6; 21; 38]. In the current study, the PEMS instrument measures all of the oxides of nitrogen (NO, NO\({}_{2}\) and N\({}_{2}\)O) without differentiating between them, the indicated specific NOx (ISNOx) values reported here are therefore a measure of all of the oxides of nitrogen. The reason indicated specific (rather than brake specific) emissions are used in this study was to ensure that the results were tied to combustion rather than mechanical friction, particularly during cold start where the friction losses are significant, as seen in Section 3.2.1. Figure 15a,b show the ISNOx emissions at each load in all stages of the engine warm up in the custom cycle. During Stage 1, in Segments 1 and 2, the ISNOx emissions were high due to the advance injection strategy used during the cold start, which has been shown to increase the ISNOx and particle emissions [6; 34; 37; 40; 41]. However, as the engine coolant temperature increased to approximately 50 \({}^{\circ}\)C to 60 \({}^{\circ}\)C in Segment 2 from 25 \({}^{\circ}\)C to 45 \({}^{\circ}\)C in Segment 1, the ISNOx emissions increased further by approximately 15% to 30%. During Stage 2, in Segment 3, when the coolant temperature reached greater than 70 \({}^{\circ}\)C and the injection was retarded, the ISNOx emissions decreased by approximately 30% to 40% and 25% to 45%, at 1500 and 2000 rpm, respectively. During Stage 3, in Segments 4 and 5, an increase of approximately 4% to 5% in ISNOx emissions was observed, as the coolant temperature reached greater than 90 \({}^{\circ}\)C and the oil temperature was between 70 \({}^{\circ}\)C and 90 \({}^{\circ}\)C. During Stage 4, in Segments 6, 7, and 8, when both the coolant and oil temperatures were greater than 90 \({}^{\circ}\)C, with all engine components completely warmed up, the ISNOx became stable and varied between \(\pm\)2%. Figure 15: NOx emissions (g/kWh) at each load in each segment at (**a**) 1500 and (**b**) 2000 rpm. The blue circle represents 100% load, the red diamond 50% load, the green pentagon 75% load and the pink hexagon 25% load. In the current study, the advanced injection strategy led to higher combustion temperatures and an increase in engine coolant temperature in Stage 1, which led to decreased BSFC by approximately 2% to 10%. It is reported in the literature that the ISNOx increases due to an increase in temperature [21] and decreases with a decrease in fuel consumption [8]. However, the effects of higher combustion temperature seem to be more pronounced than that of lower BSFC, as an increase of approximately 15% to 30% ISNOx emissions were observed in Stage 1 (from Segments 1 to 2), due to the high in-cylinder temperatures produced [6; 21]. Therefore, the advanced injection strategy, which was successful in achieving a faster warm up and reducing fuel consumption in Segments 1 and 2, resulted in increased ISNOx emissions. It was also observed that the ISNOx in Stage 1 increased as the engine run segment progressed from one load to another, with the maximum values observed in the last load (25%) of Segment 1. The most likely cause of this could be attributed to the temperature increase from the start to the end of Segment 1 (100% to 25% load in the 120 s of the engine run segment). Therefore, it seems that in Stage 1, while the engine was cold, the temperature generally had more influence on the ISNOx rather than the load or fuel consumption.