# Impact of Heat Exchanger Effectiveness and EGR on Energy and Emission Performance of a CI Engine
## 3 Results and Discussion
### Numerical Analysis of the Combustion Process
It must be confirmed that, in all analyzed cases, combustion took place under consistent engine load, speed, and start of fuel injection conditions. A single-injection strategy was applied; however, the injector is equipped with a dual-spring needle design. This specific needle design contributes to the appearance of two distinct HRR peaks, corresponding to the premixed and controlled combustion phases. As shown in the graphs (Figure 11(a)), under EGR-off conditions, the HRR during the premixed combustion phase (at constant volume) reaches 31.5 J/\({}^{\circ}\)CA without heat exchanger cooling. When HEE is increased to 50% and 100%, the HRR rises by approximately 11% and 28%, respectively. The more intensively the intake air is cooled, the longer the ignition delay period becomes, allowing more unburned fuel to accumulate in the cylinder, which then burns at the onset of intense combustion together with the subsequently injected fuel. The second HRR peak is generated during the controlled combustion phase (at constant pressure) and is responsible for producing the required torque. Due to the increased HRR during the premixed combustion phase when HEE is raised, the heat release during the controlled combustion phase slightly decreases. With EGR activated, the HRR peak during the premixed combustion phase decreases by approximately 7% due to reduced combustion intensity. As the HEE increases to 100%, the HRR approaches the level observed under EGR-off conditions. The HRR peak during the controlled combustion phase also decreases by 9%, indicating a more intense later combustion phase (Figure 11(b)). In the case of EGR off and HEE 0%, the temperature at the end of compression reaches 1017 K, while for HEE 50% and HEE 100% it decreases by approximately 28 K and 52 K, respectively, as cooling intensity increases (Figure 12(a)). This correlates with the maximum combustion temperature, which reaches 1489 K at HEE 0% and decreases by approximately 4% and 6%, respectively. When EGR on and HEE 0%, the compressed air temperature reaches approximately 933 K (Figure 12(b)) due to a significant increase in intake gas temperature and a corresponding decrease in its density. Increasing HEE to 100% reduces the end-of-compression temperature to 823 K as a result of lower intake air temperature. Figure 11: In-cylinder pressure rise during the combustion process: (**a**) EGR off; (**b**) EGR on. The maximum combustion temperature under EGR on and HEE 0% conditions reaches 1349 K, which is approximately 16% lower compared to the EGR off case. With HEE increased to 100%, the maximum combustion temperature further decreases to 1229 K--representing an approximate 18% reduction compared to the EGR off and HEE 0% case. This variation in combustion temperature is primarily determined by the initial intake air temperature and the reduced combustion intensity caused by the suppressive effect of the EGR gases. Lower combustion temperatures result in reduced NO\({}_{\text{x}}\) emissions; however, they may lead to increased smoke emissions [44]. The maximum temperature rise under EGR-off and HEE 0% conditions reaches 50 K/\({}^{\circ}\)CA (Figure 14a). However, when air cooling is intensified to HEE 50% and HEE 100%, the temperature rise starts later due to a lower compression temperature and a longer ignition delay, but the temperature increases more steeply--by approximately 10% and 24%, respectively. When EGR is on and HEE is 0%, the maximum temperature rise decreases by approximately 8% compared to the EGR-off case due to slower combustion (Figure 14b). However, as HEE intensity increases, although the ignition delay becomes longer, the temperature rise remains nearly unchanged because the engine ECU maintains the predetermined EGR ratio. Figure 12: Heat release rate in the cylinder during the combustion process: (**a**) EGR off; (**b**) EGR on. Figure 13: In-cylinder temperature during the combustion process: (**a**) EGR off; (**b**) EGR on.