# 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  
To validate the reliability of the numerical analysis results of the combustion process, an engine operating cycle simulation was carried out using AVL BOOST software. The simulation was based on combustion analysis data (start of combustion, combustion duration, and combustion intensity shape parameter), as well as experimentally determined fuel consumption, turbocharger pressure, and other relevant parameters. Figure 10a,b presents the calculated in-cylinder pressure results, which are validated against the experimentally measured in-cylinder pressure under engine operating conditions at a load of BMEP = 0.4 MPa, with HEE = 0% and for both EGR off and on cases. Analysis of the  
Figure 8: Composition of the combustible mixture in the engine cylinders: (**a**) Oxygen concentration in the exhaust gases; (**b**) Change in the excess air ratio. Figure 9: Exhaust gas temperature: (**a**) Exhaust gas temperature in the exhaust pipe; (**b**) Temperature of recirculated exhaust gases returned to the cylinder. in-cylinder pressure results shows that the variation caused by intake air cooling and EGR conditions becomes evident during the compression and combustion phases. Higher HEE under EGR-off conditions increases compression pressure due to the higher density of cooled intake air, which improves volumetric efficiency. However, when EGR is on, the compression pressure slightly decreases with increasing HEE, which is attributed to a combination of factors: (1) the engine ECU monitors the intake air mass flow and responds to its increase by raising the EGR rate accordingly and (2) the recirculation of high-temperature exhaust gases into the cylinder limits the effect of intake air cooling. According to the engine ECU data, fuel injection starts at 2\({}^{\circ}\) crank angle (CA) before top dead center (TDC), and ignition occurs after a certain delay--approximately 4-5\({}^{\circ}\) CA after TDC. When the combustible mixture ignites under EGR-off conditions, the combustion process generates significantly higher pressure. This is driven by two main factors: higher pressure at the end of the compression stroke and a lower intake temperature, which prolongs the ignition delay and leads to increased heat release during the premixed combustion phase [11]. Based on the curves obtained during the experiment, it was determined that the maximum in-cylinder combustion pressure under EGR-off and 100% cooling conditions reached 7.5 MPa, which is \(\sim\)2.7% higher compared to HEE 0% (Figure 10a), and \(\sim\)10.6% higher compared to the case with EGR-on and HEE 100% (Figure 10b). The resulting elevated pressure imposes additional mechanical loads on the crank mechanism, making it essential for engine components to be resistant to withstand increased mechanical stresses. Under EGR-off conditions and at HEE 0%, the maximum pressure rise during combustion in the cylinder reaches approximately 0.2 MPa/\({}^{\circ}\)CA (Figure 11a). Increasing the cooling intensity to HEE 50% and HEE 100% results in a pressure rise increase of approximately 12% and 44%, respectively. This is caused by the higher air density and the extended ignition delay due to the lower intake air temperature. When EGR is on, the maximum pressure rise decreases by approximately 20% due to slower combustion caused by the recirculation of exhaust gases into the cylinder (Figure 11b). Increasing the heat exchanger effectiveness has little impact on the maximum pressure rise, as the engine ECU ratio is prescribed by the control algorithm, thereby preventing a significant increase in intake air mass. Figure 10: In-cylinder combustion pressure during the combustion process: (**a**) EGR off; (**b**) EGR on. The results of heat release rate (HRR) were calculated using the BURN subprogram of AVL BOOST, based on in-cylinder pressure data as well as experimentally measured fuel and air consumption and other relevant parameters.