## 4 Conclusions  
In this study, analysis was implemented in detail on the advantages and disadvantages of both existing adjustment methods for threshing gap, i.e., concave adjustment type and threshing drum adjustment type. On the basis of the advantages and disadvantages, a variable-diameter threshing drum was designed and further used to design, based on the theory of Archimedes' spiral, a diameter adjustment device. The device was equipped with an electronic control self-locking device at the rear part of the drum and available for real-time adjustment of the drum diameter during the operation of the combine harvester, thus solving the difficulties encountered in the operation of threshing drum adjustment type, unavailability of real-time adjustment, etc. Comparative analysis through field experiment was implemented to identify the impacts brought to the grain-entrainment loss rate, damage rate, threshing efficiency, and threshing power consumption by both methods for adjustment. When the threshing gap is to be increased, the cross-section area of the threshing gap with threshing drum adjustment type is higher than that with concave adjustment type by 26.7%. While, if the threshing gap is to be decreased, the cross-section area of the threshing gap with threshing drum adjustment type is lower than that with concave adjustment type by 39.2%. As per the experiment results, under low and medium forward speeds (0.6-1.2 m/s), the grain-entrainment loss rate with threshing drum adjustment type is lower than that with concave adjustment type, i.e., lower than 0.8% in both cases; while the grain-entrainment loss rate with threshing drum adjustment type is higher than that with concave adjustment type under high speed (1.4-1.6 m/s). When the threshing gap is increased to 30 mm, the grain damage rate and the threshing efficiency with threshing drum adjustment type is lower than that with concave adjustment type. On the other hand, the concave adjustment type features relative large fluctuation amplitude of threshing power consumption, instable instantaneous threshing power consumption and irregular curve; while the threshing drum adjustment type features the threshing power consumption curve outlined in trapezoid, relatively stable curve fluctuation but higher average power consumption than that with concave adjustment type by 6.1 KW. When the threshing gap is decreased to 10 mm, the threshing drum adjustment type features higher grain damage rate than that with concave adjustment type, while the difference is small at low and medium speeds (0.6-1.2 m/s), but with far higher threshing efficiency in the meantime. Both the concave adjustment type and the threshing drum adjustment type feature relatively large fluctuation amplitude in terms of threshing power consumption; however, the fluctuation of the curve with threshing drum adjustment type is relatively stable and the average power consumption is higher than that with the concave adjustment type by 5.99 KW. The designed variable-diameter threshing drum was proven to have excellent threshing performance and stable operation efficiency. Based on this, successive studies should focus on how to decrease the mass of the variable-diameter drum, threshing power consumption, etc., as well as how to achieve to the self-adaptive control of variable-diameter drum during the harvestingprocess, thus improving the threshing performance and harvesting adaptability of the combine harvester effectively. Conceptualization, Z.S. ; methodology, Z.S. ; software, Z.S. ; validation, Z.S. and Y.L. ([PERSON]); formal analysis, Z.S. ; investigation, Z.S. ; resources, Z.S. ; data curation, Z.S. and Y.L. ([PERSON]); writing--original draft preparation, Z.S. ; writing--review and editing, Y.L. ([PERSON]) and Z.L. ; visualization, Y.L. ([PERSON]); supervision, Y.L. ([PERSON]); project administration, Y.L. ([PERSON]); funding acquisition, Y.L. ([PERSON]). All authors have read and agreed to the published version of the manuscript.