## 3 Results and Discussion  
This section analyses the effects of engine start during different stages of warm up on the performance and emissions of a diesel engine, over a custom designed cold and intermediate start drive cycle. The drive cycle was designed to facilitate discretising the engine warm up period into different stages of engine start (cold, intermediate, and hot start). As the custom drive cycle includes an engine stop/start period between the engine run segments, it ensured the discretisation of the engine run segments into various stages of engine warm up, based on the engine coolant and oil temperatures. This will be discussed in the first part of this section (Section 3.1), by analysing the engine coolant and oil temperatures at different stages. The second part of this section (Section 3.2), is a comparative analysis of engine performance parameters such as the indicated specific fuel consumption (ISFC), brake specific fuel consumption (BSFC), energy produced from combustion, IMEP, brake mean effective pressure (BMEP), friction mean effective pressure (FMEP), and exhaust temperature. Finally, in the last part of this section (Section 3.3), engine emissions such as NOx and HC emissions have been investigated.  
The experimental data at each load in each segment, at both engine speeds, was analysed by calculating the mean of the data. Approximately 5 s of the data at the start of each load in each segment was excluded from the analysis because it was highly transient and unstable, owing to turbocharger lag. The turbocharger lag caused the engine to be highly transient during the first few seconds of each 30 s time period at each load in each segment. Since mean values were used in the analysis of the data at each load, the initial 5 s of data being highly transient and unstable would have caused a significant offset in the results. This was particularly critical while analysing the cold start emissions. The turbine rotor's acceleration is a key contributor to the phenomenon of turbocharger lag, as the engine crank shaft and the turbine shaft are not mechanically connected to each other and hence are not in a synchronous rotation. The mismatch between the rotational speeds produces a subsequent lag in the turbocharger [22; 35].  
It was observed that due to the engine starting cold, the engine control unit (ECU) restricted the maximum engine load at 1500 rpm (engine speed for rated torque). The most likely reason for this strategy was to safeguard the engine from any potential failure. It was further observed that these engine restrictions were not applied at 2000 rpm (engine speed for rated power). It was also observed that the ECU followed an advanced injection strategy in Stage 1 (Segments 1 and 2) of the custom cycle,  
Figure 2: Mean values of indicated mean effective pressure (IMEP) (**k**Pa) (**a**) and NOx (g/kWh) (**b**) at full load in all segments for experiments 1, 2 and 3 at 2000 rpm. The blue circle represents Experiment 1, the red diamond Experiment 2 and the green pentagon Experiment 3.  
when the engine was cold, while by the end of Segment 2, when the coolant temperature reached 65 \({}^{\circ}\)C, the ECU switched to a retarded injection strategy at both engine speeds. The most likely reason for the advanced injection strategy during cold start (Segments 1 and 2) was to achieve higher in-cylinder pressure and temperatures to warm up the engine faster, while a retarded injection strategy in an intermediately warmed up engine was helpful in reducing NOx.