## 3 Results and Discussion
### Engine Performance Parameters
#### 3.2.2 Specific Fuel Consumption and Combustion Energy  
A useful parameter to measure the engine's performance is the specific fuel consumption (SFC), defined as the total fuel flow rate per unit power output [6]. The ISFC is defined as the total fuel flow  
Figure 9: Normalised FMEP (by IMEP) 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. rate per unit indicated power (IP) output, whereas the BSFC is defined as the total fuel flow rate per unit brake power (BP) output, as shown in Equation (3) [6]. \[ISFC=\frac{\dot{m}}{IP}\text{ and }BSFC=\frac{\dot{m}}{BP}\text{ }(\text{kg/kWh}) \tag{3}\]  
where \(\dot{m}\) is the mass flow rate of the fuel (kg/h), \(IP\) and \(BP\) are the indicated power and brake power, respectively (kW). The rate of fuel energy (E) supplied, which can be released by combustion is given by the product of the mass flow rate of the fuel supplied to the engine per cycle and the lower heating value (LHV) of the fuel, as shown in Equation (4) [6]. \[\dot{E}=\dot{m}_{c}xLHV\text{ }(\text{kJ/kWh}) \tag{4}\]  
where \(\dot{m}_{c}\) is the mass flow rate of the fuel per cycle (kg/kWh) and \(LHV\) is the lower heating value of the fuel (kJ/kg). The rate of fuel energy (\(\dot{E}\)) in Equation (4) was integrated to calculate the total fuel energy (kJ) at each load in each segment. This section analyses the ISFC, BSFC, and the fuel energy released from combustion during the various stages of engine warm up. The influence of the different stages of engine warm up on the ISFC and BSFC are discussed in this section, shown in Figures 10 and 11, respectively, at each load in all stages of engine warm up in the custom cycle, at both engine speeds. During Stage 1 (engine cold start--Segments 1 and 2), an improvement of approximately 1% in the ISFC and 2% to 10% in BSFC was observed from Segments 1 to 2, due to the increase in engine temperature from 25 \({}^{\circ}\)C to 45 \({}^{\circ}\)C in Segment 1 to 50 \({}^{\circ}\)C to 60 \({}^{\circ}\)C in Segment 2. It can be understood that lower values of ISFC during Stage 1 were due to higher friction (higher IP). It has also been observed that the fuel flow rate was restricted at 100% load in Segments 1 and 2 at 1500 rpm, however, they were not restricted at 2000 rpm. Increased BSFC during cold start has been reported by a number of studies in the literature, such as [7; 22; 40], as observed in this stage. Figure 10: Indicated specific fuel consumption (ISFC) 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 2 (intermediate start--I--Segment 3), the injection was retarded (refer to Figure 12a,b), when the coolant temperature reached 70 \({}^{\circ}\)C, leading to an increase in the ISFC and BSFC by approximately 4% to 6% and 3% to 7%, respectively. The coolant temperature during this stage was greater than 70 \({}^{\circ}\)C, and therefore, it seems that the injection retard had a greater influence in increasing the BSFC, dominating over the effects of reduced friction and increased engine temperature, which would have tend to decrease BSFC. A decrease in the ISFC and BSFC of approximately 0.5% to 1% and 1% to 3%, respectively, was observed from Stage 2 to 3 (Segments 3 to 4), which can be attributed to the increased coolant and oil temperature. During Stage 3 (intermediate start--II--Segments 4 and 5), the engine coolant and oil temperatures were greater than 90 \({}^{\circ}\)C and 70 \({}^{\circ}\)C, respectively, in Segment 4, while in Segment 5, the oil temperature also reached greater than 90 \({}^{\circ}\)C. Moreover, during this stage, the ISFC and BSFC varied approximately between \(\pm\)2% and \(\pm\)1%, respectively, when Segments 4 and 5 are compared.