## 3 Results and Discussion
### Engine Performance Parameters  
2.1 Indicated Mean Effective Pressure (IMEP), Brake Mean Effective Pressure (BMEP), Friction Mean Effective Pressure (FMEP)  
While the engine torque is a measure of the engine's ability to do work, it depends on the engine size. Therefore, a more useful engine performance measure can be the IMEP, which is obtained by dividing the indicated work (IW) per cycle by the cylinder volume, as given in Equation (1) [6]. \[IMEP=\frac{IW}{V_{d}}\ (\mathrm{kPa}) \tag{1}\]  
where \(IW\) is the indicated work per cycle and \(V_{d}\) is the swept volume. Typically, the indicated work done per cycle is determined by integrating across all four strokes of the engine and can be written as Equation (2):  
\[IW=\oint\ pdV \tag{2}\]The FMEP is indicative of the mechanical work lost due to the friction generated from the moving components (friction losses), running the engine accessories (auxiliary losses) and pumping losses [6]. The FMEP is defined as the difference between the IMEP and BMEP, where the BMEP is calculated from the engine torque measured by the dynamometer. The influence of the different stages of engine warm up on the IMEP, BMEP, and FMEP are discussed in this section. Figures 6-8 show the IMEP, BMEP, and FMEP values, respectively, at each load in all stages of the engine warm up in the custom cycle, at both engine speeds. During Stage 1 (engine cold start--Segments 1 and 2), the mean values of the IMEP were observed to be the highest in Segment 1, due to increased friction during cold start. In Segment 2, as the coolant temperature increased and the friction was reduced, the BMEP was observed to increase by approximately 1% to 2%, while the IMEP varied within \(\pm\)0.5% from Segments 1 to 2. It was also observed that the FMEP values showed a reduction of approximately 35% to 45% from Segments 1 to 2, when the coolant temperature increased from ranging approximately between 25 \({}^{\circ}\)C and 45 \({}^{\circ}\)C (Segment 1) to 50 \({}^{\circ}\)C to 60 \({}^{\circ}\)C (Segment 2). Due to the ECU restrictions applied in Segments 1 and 2 at 1500 rpm to achieve 100% load, the IMEP in Segment 1 at 1500 rpm recorded a lower value. During Stage 2 (intermediate start--I--Segment 3), the IMEP and BMEP values decreased by approximately 2% to 3%. Although in this stage, the BSFC increased by approximately 3% to 7%, due to the retarded injection, the most likely reason for the decrease in the IMEP and BMEP could have been due to lower cylinder temperatures. Moreover, a reduction of approximately 60% in the FMEP from Segments 1 to 3 (Stage 1 to Stage 2) was observed. During Stage 3 (intermediate start--II--Segments 4 and 5), as the engine coolant and oil temperatures reached greater than 90 \({}^{\circ}\)C in Segment 4, an increase in the IMEP and BMEP by approximately 4% to 8% was observed. Moreover, the largest reduction in FMEP was observed at 100% load, approximately 45% and 15% from Segment 3 to Segment 4 (Stage 2 to 3), at 1500 and 2000 rpm, respectively. The significant increase in the IMEP and BMEP (4% to 8%) in this stage indicate that these parameters are influenced by the engine temperature rather than the fuel consumption, because the BSFC was observed to have improved by only approximately 1% to 3% in this stage. During Stage 4 (engine hot start--Segments 6, 7, and 8), the IMEP and BMEP remained consistently varying between \(\pm\)0.5% to \(\pm\)1% (similar to the BSFC). Moreover, it has been observed that at 25% load, the BMEP increased by approximately 15% and 20% at 1500 and 2000 rpm, respectively, from Segments 1 to 6 (Stage 1 to 4). The most likely reason for this significant improvement in the BMEP can be attributed to the high air-to-fuel ratio at 25% load, resulting in significant efficiency improvements [6; 38].