## 4 Results and Discussion
### Suction Pressure Pulsation Influence on Compressor Operation for Ideal and Real Gas Models  
With the purpose to further verify the reliability of the developed methodology, in this section the effects of suction pressure pulsation on compressor operation with ideal and real gas models are examined. The investigated suction system is adjusted as three cases of no piping configurations, a 2.232 m long suction pipe and a 3.133 m long suction pipe for about 1 st harmonic resonance. For each case the discharge system is set to no piping configurations with constant pressure boundaries. It is worth pointing out that in Figure 4a the pressure fluctuation is about 46% of the mean pressure, which is much more than 8%. This indicates that current linear acoustic theory will not be accurate for predicting the pressure pulsations in this work since the large magnitude of pressure fluctuation violates the assumption on which the acoustic equation is based, i.e., acoustic pressure should be small compared to the mean pressure. The variation of (a) suction chamber pressure, (b) displacement of suction valve and (c) in-cylinder pressure for ideal and real gas model is shown in Figure 4. Generally, there is a clear influence of pressure pulsations in the suction chamber directly on the motion law of suction valve and the in-cylinder pressure oscillations, especially for the case of resonance. For these figures, the working cycle begins from Top Dead Center (TDC) where the cylinder volume is the same as the clearance volume. With the piston moving from TDC to BDC, the cylinder volume is increased and thus in-cylinder pressure decreases sharply with the curves, showing good agreement for all the cases until the suction valve is opened. The suction valve is opened slightly later for the hybrid model than the lumped model since the former suction chamber pressure is lower than the latter constant pressure. Once the suction valve is opened, the suction process begins. The pressure pulsation waves are subsequently transferred into cylinder, leading to the pressure oscillations in the cylinder. Also, the closing of the suction valve is strongly influenced. For instance, the pressure pulsations in the suction system lead to a delay in the closing of the valve in this work due to the suction pulsating pressure increasing at the end of the suction portion. Furthermore, the offset of pressure waves in the suction chamber between ideal gas and real gas models becomes larger for the resonance case than for that without resonance. \begin{table}
\begin{tabular}{l l l} \hline
**Specification** & **Value** & **Unit** \\ \hline Rotational speed & 1500 & rpm \\ Cylinder diameter & 120 & mm \\ Stroke & 90 & mm \\ Maximum valve lift & 2.5 & mm \\ Spring stiffness of suction valve & 20,000 & N \(\cdot\) m\({}^{-1}\) \\ Spring stiffness of discharge valve & 25,000 & N \(\cdot\) m\({}^{-1}\) \\ Area of valves & 2827 & mm\({}^{2}\) \\ Diameter of suction pipe & 60 & mm \\ Diameter of discharge pipe & 60 & mm \\ Volume of suction chamber & 0.0035 & m\({}^{3}\) \\ Volume of discharge chamber & 0.0035 & m\({}^{3}\) \\ Gas composition & Methane & - \\ Suction temperature & 285 & K \\ Suction pressure & 3 & MPa \\ Discharge pressure & 9 & MPa \\ \hline \end{tabular}
\end{table} 
Table 1: Main specifications of the natural gas compressor. Figure 5 depicts the effects of pressure pulsations on the dynamic in-cylinder temperature for real and ideal gas models under different suction configurations. With the similar variation of in-cylinder pressure illustrated previously in Figure 4, in-cylinder temperature declines strongly with the piston moving from TDC to BDC, following similar curves until the suction valve is opened. Also, the in-cylinder temperature oscillation during the suction phase is linked directly to the pulsating flow in the suction system.