## 3 Results and Discussion
### Engine Exhaust Emissions
#### 3.3.2 Hydrocarbon Emissions  
HC emissions in diesel engines, consisting of decomposed fuel molecules, as a consequence of incomplete combustion of the hydrocarbon fuel, are a useful measure of the combustion efficiency [6]. In a diesel engine, the heterogeneous mixture consist of pockets of rich (producing HC due to incomplete combustion) and lean mixtures (producing HC due to too lean to auto-ignite), becoming the primary source of unburned HC emissions. However, sources such as lubricating oil (especially when cold), cylinder crevices, and the fuel nozzle sac (which leaves the injector nozzle at low speed, late in the combustion process) also become secondary sources of unburned HC emissions [6; 35; 38]. Generally, engine idling and low load operation and a sudden load demand, produce high HC emissions [6; 38]. Similarly, advancing the injection reduces HC emissions, with a NOx and particle emissions penalty, while retarding the injection increases HC emissions [6; 38]. In the current study, the mean values of the indicated specific hydrocarbon (ISHC) emissions in (g/kWh) at each load were used to analyse the effects of the various stages of engine warm up on ISHC emissions. Figure 16a,b show the ISHC emissions at each load in all stages in the custom cycle at 1500 and 2000 rpm. During Stage 1, in Segments 1 and 2, the ISHC emissions were higher due to the cold cylinder. A reduction in the ISHC emissions of approximately 2% to 7% was observed from Segments 1 to 2, as the engine temperature increased to approximately 50 \({}^{\circ}\)C to 60 \({}^{\circ}\)C in Segment 2 from approximately 25 \({}^{\circ}\)C to 45 \({}^{\circ}\)C in Segment 1. The cylinder temperature being low during Stage 1 could have hindered the atomisation of the vapourised fuel, resulting in the incomplete combustion of the fuel and consequently higher HC emissions. Moreover, the ECU employed an advanced injection strategy during Stage 1 to heat up the engine faster. This strategy could have been helpful in reducing the ISHC emissions, however, it led to increased NOx, as discussed in Section 3.3.1 and has also been shown in other studies, such as [6; 38; 41; 42; 35]. During Stage 2, in Segment 3, the ISHC emissions increased by approximately 12% to 18%, at each load, due to the engine switching to a retarded injection strategy, when the coolant temperature reached 65 \({}^{\circ}\)C. Moreover, an exceptional increase in the ISHC of approximately 50% was observed at 25% load at 2000 rpm, which could be an effect of deteriorated combustion at lower load [6]. Although the combustion process settled, as the engine was intermediately warmed up in Stage 2, the BSFC increased by approximately 3% to 7%, with a retarded injection, consequently leading to an increase in the ISHC emissions in this Stage. During Stage 3, in Segments 4 and 5, the ISHC emissions have been observed to increase by approximately 6% to 10%. Although the engine warm up improved in this stage, with the engine coolant temperature greater than 90 \({}^{\circ}\)C and the oil temperature between 70 \({}^{\circ}\)C and 90 \({}^{\circ}\)C, the increase in the ISHC emissions could have been due to the increase in the BSFC by approximately 1% to 3% in this stage. During Stage 4, in Segment 6, 7, and 8, the ISHC emissions have been observed to increase by approximately by 3% to 7%. Although, the engine was completely warmed up in this stage, with both the coolant and oil temperatures greater than 90 \({}^{\circ}\)C, the increase in the ISHC emissions can be attributed to two main reasons: firstly, it could have been due to the change in the crevice volume with the engine warm up [6]; and secondly, it could have been due to the presence of lubricating oil on the piston and combustion chamber walls, causing the ISHC emissions to increase [6]. It has been observed in the current experimental study that the ISHC emissions increased with a decrease in load, in all stages of engine warm up (cold, intermediate--I and II and hot start).