## 3 Results and Discussion
### Engine Performance Parameters
#### 3.2.2 Specific Fuel Consumption and Combustion Energy
Figure 11: Brake specific fuel consumption (BSFC) 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. Figure 12: Start of injection 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. During Stage 4 (engine hot start--Segments 6, 7 and 8), although the engine coolant and oil temperatures were greater than 90 \({}^{\circ}\)C and all the engine components were completely warmed up, the ISFC and BSFC varied approximately between \(\pm\)2%, with higher variations observed at 1500 rpm. Similar variations of approximately \(\pm\)2% were also observed between Stages 3 and 4 in the ISFC and BSFC, with higher variations observed at 1500 rpm, compared to 2000 rpm. The higher variations in the ISFC and BSFC at 1500 rpm could be attributed to a relatively more stable combustion at 2000 rpm, compared to 1500 rpm. Moreover, as the friction was reduced (IP decreased), the ISFC was observed to have increased by approximately 5% in Segment 8. The ISFC has been observed to be higher in all stages at 1500 rpm, compared to 2000 rpm. The likely cause of that could be attributed to higher IP, due to higher friction at 2000 rpm, compared to 1500 rpm [6], (ISFC being inversely proportional to IP). It has been observed that there was a smaller difference between the ISFC and BSFC at higher loads (100% and 75% loads), while a higher difference was observed at lower loads (50% and 25% loads). This is because of the FMEP is largely related the engine speed, rather than the load, which is also indicative of a higher mechanical efficiency at higher loads, as also shown by the normalised FMEP in Figure 9a,b. The cumulative energy released from the combustion of fuel was observed to be higher at 2000 rpm, as the BSFC at 2000 rpm was higher, compared to 1500 rpm, except at 100% load in all stages, where higher BSFC values were observed at 1500 rpm. However, due to the higher engine speed and higher number of cycles at 2000 rpm (12.5 cycles and 16.66 cycles in a second at 1500 and 2000 rpm, respectively), the cumulative energy produced from combustion was always higher at 2000 rpm compared to 1500 rpm. The mean values of the cumulative energy released from the combustion of fuel at each load in each segment, at 1500 and 2000 rpm, respectively, are shown in Figure 13a,b. The higher cumulative energy produced from combustion at 2000 rpm seemed to be responsible for the faster heating of the coolant at 2000 rpm, where the temperature at the end of Segment 2 at 2000 rpm was approximately 5 \({}^{\circ}\)C higher than at 1500 rpm, as shown in Figure 4a,b. The cumulative energy produced from the combustion is representative of the rise in the coolant temperature, with the energy transferred to the cylinder walls as heat, with more heat accumulated due to a higher number of cycles at any particular load at 2000 rpm, compared to 1500 rpm. Therefore, the BSFC and combustion energy are important engine performance parameters, which influence the engine warm up, rather than the operating conditions of speed and load alone [6]. Figure 13: Cumulative energy released from combustion 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.