## 5 Conclusions  
In this paper, firstly a pneumatic model of a mechanical ventilation system of double lungs with secretion clearance function is built. According to that pneumatic model a mathematical dimensionless model is constructed. To verify the mathematical dimensionless model, an experiment with secretion clearance function is designed. Finally, the coupling the working characters of the system and coupling effect between the lungs are obtained by the simulation model and orthogonal experiment.  
Therefore, the following conclusions can be drawn:  
(1) The output data of the experiment reached the parameters of the ventilator setting. In addition, the simulation curves are consistent with the experimental curves. So the experimental data and simulation data are reliable. In addition, the simulation model can be used in other studies of the respiratory system.  
(2) Changes in the dimensionless compliance (\(C\)*) and dimensionless air resistance (\(R\)*) will not only change the system's dimensionless respiratory cycle time, but also change the dimensionless suction duration.  
(3) The relationship between dimensionless compliance (\(C\)*) and dimensionless peak suction flow (\(q_{\text{s}}\)*) is not a simple linear relationship. There is a boundary when the dimensionless compliance of the right lung is equal to that of the left lung (\(C_{\text{r}}\)* = \(C_{\text{l}}\)*). In addition, the relationship between dimensionless air resistance (\(R\)*) and dimensionless peak suction flow (\(q_{\text{s}}\)*) can be regarded as an inversely proportional relationship. They are approximately negatively inversely proportional.  
(4) The relationship between dimensionless suction duration (\(T_{\text{sd}}\)*) and dimensionless compliance of right lung (\(C_{\text{r}}\)*) is roughly the same as the relationship between dimensionless suction duration (\(T_{\text{sd}}\)*) and dimensionless air resistance (\(R_{\text{r}}\)*). These two curves both raise first and then fall. In general, both the suction duration and time reference increase with the rise of compliance or air resistance. In the second part of these two curves, suction duration stays the same, and time reference increases. Therefore, the dimensionless suction duration of the two curves declines.  
(5) Compared with other parameters, dimensionless air resistance (\(R\)*) plays the most important role on both dimensionless suction duration (\(T_{\text{sd}}\)*) and dimensionless peak suction flow (\(q_{\text{s}}\)*). The effect of dimensionless compliance (\(C\)*) on dimensionless suction duration (\(T_{\text{sd}}\)*) is the larger than the other parameters, except for dimensionless air resistance (\(R\)*).  
This paper provides a novel research method for a secretion clearance mechanical ventilation system with double lungs, which promotes the development of mechanical ventilation technology and can be used in other mechanical ventilation studies as well as clinical medicine and treatment.  
[PERSON] and [PERSON] conceived and designed the experiments; [PERSON] performed the experiments; [PERSON] and [PERSON] analyzed the data, and [PERSON] wrote the paper.  
This work was supported by the National Natural Science Foundation of China (Grant No. 51575020), Open Foundation of the State Key Laboratory of Fluid Power and Mechatronic Systems and the Start-up Research Grant (SRG2016-00083-FHS) at University of Macau.  
The authors declare no conflict of interest.