*4.1. Validation*

Following the guidelines depicted during the methodology description in Section 3, a unique engine cycle was calculated and validated prior to run the multiple parallel executions with the modified initial conditions. The target of this preliminary step is to obtain a numerical solution that reproduces the mean behaviour of the experiments.

In this way, experimental and simulated cylinder pressure profiles of the baseline simulation are compared in Figure 2. As it can be seen, the CFD model correctly predicts the in-cylinder pressure. Indeed, the predicted pressure shows a similar deviation as measurements dispersion. Although RoHR traces show that the combustion phasing is slightly delayed and its peak value is minimally overestimated, they do not affect the maximum peak pressure in a significant way. Given that the calculation of this parameter depends on material deformations and blow-by losses that the model does not take into account, this effect may be attributed to a minor underestimation of the effective compression ratio.

**Figure 2.** Comparison between measured and CFD calculated results of baseline test, measurements dispersion is represented by their standard deviation (SD).

Once that the baseline simulation can be considered representative of the real operating conditions, ten parallel executions were performed with randomly distributed variations of the IVC conditions. In Figure 3, the same comparison previously done in Figure 2 can be seen but including now the CCV spread of this latter simulations. As can be seen from the graph, the standard deviation of simulations is similar to that observed in the measurements proving the validity of the methodology in this particular case of study.

**Figure 3.** Comparison between measured and CFD calculated in-cylinder pressure traces of baseline test, dispersion due to CCV is represented by their standard deviation value.

However, the interest of the analysis is not only to check the performance of the methodology to reproduce a realistic CCV but also to examine the effects of this variability on the traditional combustion/knock parameters. In order to accomplish this target, the maximum pressure rise rate and the Maximum Amplitude Pressure Oscillation (MAPO) are plotted at each measured and simulated engine cycle in Figure 4. The standard deviation is also included to evaluate the capability of the methodology to reproduce CCV effects. Again, comparable results were obtained in terms of mean and dispersion levels for both parameters.

**Figure 4.** Comparison between measured and CFD calculated results of baseline test, dispersion due to CCV is represented by their standard deviation value. The maximum pressure rise rate and MAPO are plotted to compare experimental and simulation results.
