## 5 Conclusions  
This article assessed a model-based calibration of the heat transfer coefficient during scavenging for a uniflow scavenged two-stroke engine. A 1D model, fluid-dynamically validated against experimental data, was used for this assessment. The experimental tests consisted of three engine speeds (2000, 2250, and 2500 rpm), with two different exhaust system configurations at full and partial loads conducted only at 2000 rpm. On observing satisfactory outputs for the 1D model, these results were used as initial and boundary conditions in CFD simulations to validate the scavenging process. CFD simulation results showed that the state-of-the-art heat transfer models embedded in a 1D engine model failed  
Figure 7: **(A)**: Effect on trapped mass varying coefficients \(a\) and \(b\) and (**B**): Difference in trapped mass (%) varying differences in coefficients \(a\) and \(b\) (%).  
to predict the in-cylinder temperature during scavenging. The temperature deviations also led to an error in predicting the trapped mass. A new heat transfer model was proposed, considering a pseudo-velocity that accounts for the flow motion inside the cylinder to calculate the Reynolds number. This calculation was based on the instantaneous mass flows in the exhaust and intake ports. The thermo-fluid dynamically validated 1D model was used for perspective analysis to showcase the potential of the heat transfer model for its robustness and performance, as follows:  
* Under identical initial conditions for all the heat transfer correlations, it was observed that the state of art correlations, which include Anand, Hohenberg, and Woschni heat transfer correlations, provide similar results during scavenging. The heat transfer coefficient during scavenging can be as low as 200 \(\frac{W}{m^{2}K}\), leading to a peak deviation of 85 \({}^{\circ}\)C in temperature predictions and up to 12% in deviation for trapped mass predictions. On the other hand, the proposed heat transfer model calculates a peak coefficient of 800 \(\frac{W}{m^{2}K}\) during scavenging, leading to a peak temperature deviation of 25 \({}^{\circ}\)C along with almost negligible error in trapped mass predictions.
* Under identical combustion settings and a similar peak heat transfer coefficient, a difference in trapped mass of up to 6% at the exhaust port closing was observed using the proposed heat transfer model. The gases inside the cylinder were cooled a further 85 \({}^{\circ}\)C, leading to a higher trapped mass. This translated to differences of up to 12% in the IMEP of the engine.
* A sensitivity analysis was performed to evaluate the impact of the model constants on the engine operation. These constants included both Woschni and the proposed heat transfer correlation during scavenging. In this analysis, coefficient \(b\) (correction coefficient of proposed heat transfer correlation) was changed by \(\pm\)50% from the mean value, and coefficient \(a\) (correction coefficient of Woschni heat transfer correlation) was changed by \(\pm\)42%. Results depicted that coefficient \(b\) had a more significant impact on trapped mass. A sweep along varying coefficient \(b\) and at constant coefficient \(a\) can lead to a 15% change in trapped mass. Conversely, a sweep along varying coefficient \(a\) and at constant coefficient \(b\) can lead to a maximum difference of 5% in trapped mass.  
H.C.: Resources, Supervision, Project administration, Conceptualization, Writing--original draft, Writing--review & editing. A.T.: Resources, Formal analysis, Writing--original draft, Writing--review & editing. J.G.-S.: Writing--review & editing, Methodology. A.D.: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Writing--original draft, Writing--review & editing. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.