# Impact of Heat Exchanger Effectiveness and EGR on Energy and Emission Performance of a CI Engine
## 3 Results and Discussion
### Investigation of Engine Environmental Performance Indicators  
As engine load increases, smoke emissions also rise due to a decreasing excess air ratio. In the case of EGR on, smoke emissions increase significantly, as the recirculated exhaust gases--characterized by low oxygen concentration but elevated CO2 levels--further reduce the air excess, suppress combustion, and lower the combustion temperature. Within the BMEP range of 0.2 MPa, 0.4 MPa, and 0.6 MPa, the activation of EGR results in an increase in smoke levels by approximately 4.3-fold, 4.8-fold, and 6-fold, respectively (from 0.09 m\({}^{-1}\) to 0.39 m\({}^{-1}\), from 0.11 m\({}^{-1}\) to 0.53 m\({}^{-1}\), and from 0.13 m\({}^{-1}\) to 0.81 m\({}^{-1}\), respectively) (Figure 16). This indicates that within the tested range, the increase in smoke opacity is linear under EGR-off conditions, whereas it follows an exponential trend when EGR is active. In the case of EGR off, increasing the HEE intensity to 100% reduces smoke emissions by 15-18% due to an increase in \(\lambda\). However, the effect is not particularly significant, as the combustion temperature is also reduced. In the case of EGR on, increasing HEE intensity up to 100% reduces smoke emissions by up to 14% (to 0.34 m\({}^{-1}\)) at a load BMEP = 0.6 MPa; however, at 0.6 MPa, smoke emissions even increase by 9% (to 0.88 m\({}^{-1}\)) compared to the HEE 0% case. This is caused by the reduction in intake mixture temperature, which lowers the combustion temperature, without an increase in excess air. NO\({}_{\mathrm{x}}\) formation is directly dependent on the in-cylinder temperature and the excess air ratio. High temperatures are reached when the engine operates under higher load. However, it is important to note that this study analyses specific pollutant emissions [g/kWh], which depend not only on the volumetric concentration of pollutants, but also on the total emitted pollutant mass, the molar mass of the pollutants, and the engine brake power (Equation (3)). Figure 16: Effect of engine load and heat exchanger effectiveness on smoke opacity variation. When comparing the cases of EGR off and EGR on, the specific NO\({}_{\rm x}\) emission at BMEP levels of 0.2 MPa, 0.4 MPa, and 0.6 MPa decreased from 3.57 g/kWh to 1.95 g/kWh, from 4.40 g/kWh to 3.127 g/kWh, and from 3.94 g/kWh to 2.56 g/kWh, respectively--corresponding to reductions of approximately 45%, 30%, and 35%. During testing with EGR off and HEE increased to 100%, at an engine load of 0.6 MPa, the specific NO\({}_{\rm x}\) emission decreased from 3.94 g/kWh to 2.30 g/kWh, representing a relative reduction of around 40% (Figure 17). Under the same conditions but with EGR on, the specific NO\({}_{\rm x}\) emission decreased from 2.56 g/kWh to 0.917 g/kWh, a relative reduction of approximately 60%. Compared to the case with EGR off and no intake air cooling, at BMEP = 0.6 MPa, the overall NO\({}_{\rm x}\) reduction achieved with EGR on and 100% HEE reached approximately 76%. This reduction is attributed to the recirculation of a portion of exhaust gases into the cylinder, which displaces part of the intake air and lowers the combustion temperature. Furthermore, the intake of cooler air contributes additionally to the temperature reduction within the cylinder. The concentration of hydrocarbons (HC) in the exhaust gases indicates incomplete combustion. Experimental results showed that as engine load increases, specific HC emissions decrease due to improved combustion quality at higher combustion temperatures; increasing engine load also reduces the calculated specific emission. In the case of EGR on, the specific HC emission decreased by approximately 12% (from 0.077 g/kWh to 0.068 g/kWh) under low load conditions (0.4 MPa), which is attributed to the increased temperature of the air-fuel mixture and improved fuel evaporation (Figure 18).