## 3 Results and Discussion
### Engine Coolant and Oil Temperature
#### 3.1.1 Stage 1--Engine Cold Start (Coolant and Oil \(<\) 70 \({}^{\circ}\)C)  
Stage 1 can be defined as 'engine cold start period', in which the engine coolant and oil temperatures were less than 70 \({}^{\circ}\)C. This period is called cold start because according to the EU commission 2012/46/EU [13], the cold start period ends when the engine coolant temperature reaches 70 \({}^{\circ}\)C. The temperature at the end of Segment 2 in Stage 1 was approximately 70 \({}^{\circ}\)C. The temperature increased approximately 5 \({}^{\circ}\)C during the engine stop period between Segments 2 and 3, due to continued heat transfer from the metal wall to the coolant, even when the coolant flow was stopped. Figures 3-5 show that Stage 1 (engine cold start) occurred in Segments 1 and 2 of the custom cycle. 1.2 Stage 2--Engine Intermediate Start--I (70 \({}^{\circ}\)C \(<\) Coolant \(<\) 90 \({}^{\circ}\)C and Oil \(<\) 70 \({}^{\circ}\)C)  
Stage 2 can be defined as the 'engine intermediate start period--I', in which the engine coolant temperature was between 70 \({}^{\circ}\)C and 90 \({}^{\circ}\)C and the oil temperature was less than 70 \({}^{\circ}\)C. In this stage, the coolant and oil temperatures continued to increase and therefore, the engine was not thermally stabilised. Figures 3-5 show that Stage 2 (engine intermediate start--I) occurred in Segment 3 of the custom cycle. Although the coolant temperature reached 95 \({}^{\circ}\)C, the oil temperature reached up to 70 \({}^{\circ}\)C at the end of Stage 2 (Segment 3), at both engine speeds. The thermostat opened and allowed the coolant to circulate through the radiator during the engine stop period at the end of Segment 3. The coolant temperature dropped to approximately 70 \({}^{\circ}\)C, with the opening of the thermostat and thereafter, it was cyclic. At the end of Stage 2, although the coolant temperature dropped due to the mixing of the fresh coolant from the radiator, there was no heat transfer from the oil to the cylinder wall (it was rather the opposite) and the oil temperature continued to rise, as shown in Figures 3 and 5. This is indicative of the oil continuing to warm up and reaching approximately 90 \({}^{\circ}\)C in Segments 4 and 5 and Segment 4 at 1500 and 2000 rpm, respectively. 1.3 Stage 3--Engine Intermediate Start--II (Coolant \(>\) 90 \({}^{\circ}\)C and 70 \({}^{\circ}\)C \(<\) Oil \(<\) 90 \({}^{\circ}\)C)  
Stage 3 can be defined as the 'engine intermediate start--II', in which the engine coolant temperature was greater than 90 \({}^{\circ}\)C, and the oil temperature was between 70 \({}^{\circ}\)C and 90 \({}^{\circ}\)C. In this  
Figure 5: Lubricating oil temperature 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. stage, the engine coolant reached an optimal temperature, while the oil temperature continued to increase and therefore, the engine was still not thermally stable. The lubricating oil temperature reached approximately 90 \({}^{\circ}\)C, at the end of Segments 5 and 4 at 1500 and 2000 rpm, respectively. The oil warm up also followed the same trend as the coolant and it was observed to have warmed up faster at 2000 rpm, compared to 1500 rpm, due to the higher cumulative energy produced at 2000 rpm. Moreover, the oil temperature gradient experienced a lag of approximately 10 \({}^{\circ}\)C as compared to the coolant temperature. Figures 3-5 show that Stage 3 (engine intermediate start--II) occurred in Segments 4 and 5 and Segment 4 of the custom cycle, at 1500 and 2000 rpm, respectively. In the current experimental study, although the engine was considered to be warmed up when the coolant temperature reached 70 \({}^{\circ}\)C the engine performance did not stabilise, due to sub-optimal oil temperature. This has also been reported in the literature by [18; 19].