*Model Validation*

A high fidelity engine model validated against experimental data for a conventional layout, in both steady-state and transients, has been applied in this study. The calibration of the combustion model (SI Wiebe) was performed by isolating Cylinder 1 of the engine and performing a Three Pressure Analysis (TPA). Figure 4 represents the comparison between measurements and simulation results for all engine cylinders at low loads.

**Figure 4.** Simulated cylinder pressure from Three Pressure Analysis (TPA) and measured pressure at low loads. (**a**) Cylinder 1; (**b**) Cylinder 2; (**c**) Cylinder 3; (**d**) Cylinder 4.

The small cylinder-to-cylinder variations observed in the measured data were difficult to capture in the 1D simulation. These variations may be a result of several factors, such as 3D air and gas flow behaviours at the intake and exhaust or different thermal conditions. Although there were slight variations, a good agreemen<sup>t</sup> between measured and simulated mass air flow and averaged indicated mean effective pressure (IMEP) (error of less than 2%) was observed for full load conditions, as shown in Table 2.

**Table 2.** Measured data and simulation results at full loads. IMEP, indicated mean effective pressure.


For part-load cases, the discrepancy between measured and simulated results was slightly larger, as can be found in Table 3. However, despite the 9.37% error in the averaged IMEP for the low-load case, the averaged absolute error in net IMEP was below 0.3 bar. Therefore, the modelling of the scavenging system and combustion system at low engine load is satisfactory.


**Table 3.** Summary of combustion model validation at 2000 rpm.

The overall shape of the port pressures at three operating points was captured. Figure 5 outlines the measured and simulated instantaneous pressure at the exhaust port of Cylinder 1 for medium engine loads. As can be seen from the figure, the magnitude of the pressure wave reflections was well predicted.

**Figure 5.** Instantaneous pressure at exhaust port of Cylinder 1, 2000 rpm medium-load.

The validation of the transient performance of the model was also performed against experimental data collected from transient tests performed with various wastegate positions. The calibration was performed for three different boost pressures and the comparison between measured and simulated mass air flow is presented in Figure 6. The experimental and simulation results show a good agreemen<sup>t</sup> with small variations (5%) that could be a result of the scavenging model in the 1D engine simulation.

**Figure 6.** Comparison between measurement and simulation results of transient performance for different boost targets with imposed turbocharger rotational speed.
