## 4 Results and Analysis
### The Combustion Fit Results of Engine Nominal Operating Point
#### 4.2.1 Four-Cylinder Averaged Pressure Signals

Table 4: Variables and equivalence criteria in combustion fit functions. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & **Basic Seiliger Process** & & **Advanced Seiliger Process** \\ \hline \multirow{3}{*}{**Constant**} & \(n_{comp}\) & & & 1.36 & \\  & \(r_{c}\) & & & 13.107 & \\  & \(\Delta EO\) & & & 0 & \\ \hline \multirow{3}{*}{**Seiliger Variables**} & \multicolumn{2}{c}{**Value**} & **Heat Input Ratio** & **Value** & **Heat Input Ratio** \\ \cline{2-6}  & \(a\) & 1.331 & 22.11\% & 1.331 & 22.11\% \\ \cline{1-1}  & \(b\) & 1.339 & 41.87\% & 1.338 & 41.85\% \\ \cline{1-1}  & \(c\) & 2.428 & 36.02\% & 1.505 & 16.53\% \\ \cline{1-1}  & \(n_{cap}\) & 1.367 & 0 & 1.197 & 19.51\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Results of combustion fit of basic and advanced Seiliger process models. between pressure signals, due to relatively small scale, the difference among the three curves are not distinct. However, in the in-cylinder temperature diagram shown in Figure 10c, it is more likely to observe the trend and comparison of these curves, in which the basic and advanced Seiliger process before peak temperature completely coincide with each other and the duration of the isothermal process are different, and have a longer crank angle time with a larger \(c\) value in the basic Seiliger process model. The temperature at Exhaust Open (EO) in the basic Seiliger process is lower than that in the advanced Seiliger process model (around 150 K), which is caused by the late combustion occurring during expansion process in advanced Seiliger process model. Figure 10d shows the fitting results compared with measurement in temperature-entropy diagram, which is even more clear to reveal the differences between the two Seiliger process models. The same trend as the in-cylinder temperature in Figure 10c before peak temperature and the two fitting curves are slightly different from the measurement. Nevertheless, the discrepancy among them in the expansion process is especially evident. The vertical curve in the \(T\)-\(S\) diagram means adiabatic expansion and the negative slope indicated heat input to the work medium and vice versa. As a result, the advanced Seiliger process model fitted the same case as the measurement with heat input in the expansion process and in the basic Seiliger process model, the result is totally different, i.e., heat loss during expansion. Based on the fitting definition, when the basic Seiliger process model is used, the basic Seiliger process fitting shares the same equivalence criteria as the advanced Seiliger process model, but some of the heat enters the in-cylinder work medium in the isothermal process instead of the expansion process of advanced Seiliger process. From the fitting diagram, the advanced Seiliger process model fitting seems closer to the measurements, and when the ancient engine is operating with very late combustion, which is not expected, the advanced Seiliger process model is closer to the reality to describe the detailed phenomena of engine combustion. If the Seiliger process model fitting is used  
Figure 10: Comparison of basic and advanced Seiliger process models: (**a**) \(p\)-\(\varphi\) diagram; (**b**) \(p\)-\(V\) diagram; (**c**) \(T\)-\(\varphi\) diagram; (**d**) \(T\)-\(s\) diagram. in the modern engine without late combustion, the basic process model fitting is preferred to avoid making errors in the heat input analysis with relatively simple modelling.