3.1. Results of Soil Bin Tests
The design scheme and results of the orthogonal combination test are shown in
Table 4. By comparing the test data and observing the fluctuation changes in cutting torque and power, we can obtain the preliminary finding that both cutting torque and power increase with the increase in tillage depth and forward speed. The cutting torque decreases with the increase in forward speed, while the power increases with the increase in forward speed. Using Design-Expert to process the results of the test data in
Table 4, we were able to obtain the results of the analysis of variance for cutting torque and power (
Table 5 and
Table 6, respectively).
Quadratic multiple regression model equations for the coded independent variables of cutting torque and power consumption were established by fitting the data in
Table 5 and
Table 6 using the least squares method. The coded independent variable quadratic multiple regression model equation for cutting torque is shown in Equation (8) below, and the coded independent variable regression model equation for power consumption is shown in Equation (9) below.
where
Y1 is the cutting torque, N·m;
Y2 represents the power, kW.
The reliability of the cutting torque regression model equation was analyzed in conjunction with
Table 5. It was able to find
p < 0.0001 for the model, which means that the model is reasonable and significant. The model factors X
1, X
2, X
3, X
1 × 2, X
2 × 3, and X
22 are significant terms (
p < 0.05), and the rest of the terms are insignificant. The
p-value of the lack of fit is 0.2696, which is insignificant and indicates that the quadratic multiple regression model equation is acceptable.
It can be observed from
Table 6 that the
p value of the model is less than 0.0001, and the
p value of the misfit term is insignificant, which indicates that the regression equation of power is reasonable. The
p-value of X
1, X
2, X
3, X
1 × 2, X
1 × 3, X
2 × 3, X
12, and X
22 are less than 0.05, and they are significant terms, whereas the
p-value of X
32 is more than 0.05, and it is an insignificant term.
The response surface method was applied to analyze the effects of three influencing factors (rotary speed, tillage depth, and forward speed) and their interactions on the test indexes (cutting torque and power). During the analysis, one factor was fixed at zero level, and the effect of the remaining two factors was then analyzed and discussed.
When analyzing the effects of the three factors and their interactions on the cutting torque, the effects of tillage depth and rotary speed on the cutting torque at a fixed forward speed of 4 km·h
−1 can be expressed as follows:
When the tillage depth is fixed at 65 mm, the influence of forward speed and rotary speed on cutting torque can be expressed as follows:
When the rotary speed is fixed at 180 rpm, the influence of tillage depth and forward speed on cutting torque is as follows:
Figure 10 is the response surface of the influence of the three factors on cutting torque. Combining
Table 5,
Figure 10, and Equations (10)–(12), it can be seen that the three factors have a significant impact on the cutting torque. The tillage depth has an interaction with the forward speed and the rotary speed, respectively, but there is no interaction between the rotary speed and the forward speed. The cutting torque increases with the increase in tillage depth and forward speed and decreases with the increase in rotary speed. By observing the changing trend of cutting torque with the three factors in
Figure 10, it can be found from
Figure 10a that the change in cutting torque from “−1” to “1” with the level of factor D is greater than that from “−1” to “1” with the level of factor n, which shows the order of influence on cutting torque: tillage depth > rotary speed. Similarly, the order of influence on cutting torque can be found in
Figure 10b: rotary speed > forward speed. The order of influence on cutting torque can be found from
Figure 10c: tillage depth > forward speed. To summarize, the influence order of the three factors on cutting torque is as follows: tillage depth > rotary speed > forward speed.
When analyzing the impact of the three factors and their interaction on power, with a fixed forward speed of 4 km·h
−1, the impact of tillage depth and rotary speed on power can be expressed as follows:
The effect of forward speed and rotary speed on power at a fixed tillage depth of 65 mm can be expressed as follows:
The effect of tillage depth and forward speed on power at a fixed rotary speed of 180 rpm is as follows:
Observing the response surface plots of the effects of the three factors on power (
Figure 11), combined with
Table 6 and Equations (13)–(15), each factor can be demonstrated to have a significant effect on power. There are interactions between each of the three factors. With the increase in the three factors, the power increases. Observing the changing trend of power with the three factors in
Figure 11, it can be found from
Figure 11a that the amount of change in power with the level of factor D from “−1” to “1” is greater than the amount of change in power with the level of factor n from “−1” to “1”, which indicates the order of influence on power: tillage depth > rotary speed. Similarly, the order of influence on power can be found from
Figure 11b: rotary speed > forward speed. From
Figure 11c, the order of influence on power can be found as follows: tillage depth > forward speed. Therefore, the order of influence of the three factors on power is tillage depth > rotary speed > forward speed.
Using the regression equation of torque and power and the Design-expert software, the optimal solution for the operating parameters under the conditions of minimum torque and minimum power was found. Considering the actual size of the root–soil complex and in order to ensure the stubble-cutting effect, we choose the following operating parameters for practical application: a rotary speed of 240 rpm, a tillage depth of 90 mm, and a forward speed of 3 km·h−1.
The results of the soil bin comparison tests are shown in
Figure 12. From
Figure 12a,b, we are able to observe that the torque of the BSCD and the TPSB decreases with the increase in rotary speed, while the power increases with the increase in rotary speed, which is similar to the research results of Matin and Yang et al. [
34,
35]. At the rotary speed of 240 rpm, the torque is the smallest, while the power is the largest. The torque of the BSCD and the TPSB are 31.89 N m and 37.85 N m, and the power is 0.844 kW and 0.951 Kw, respectively. The torque and power of the BSCD is always less than that of the TPSB. Under the three working conditions of 120 rpm, 180 rpm, and 240 rpm, the cutting torque of the BSCD is reduced by 16.4%, 15.2%, and 15.7%, respectively, and power consumption is reduced by 9.9%, 10.1%, and 11.3%, respectively, compared with the TPSB. This indicates that the BSCD has a better resistance reduction and energy-saving performance.
3.2. Results of Field Experiments
The cutting torque results of the field comparison tests can be observed in
Figure 13. It is not difficult to find that the changing trends of the BSCD and the TPSB are similar, and their cutting torque gradually increases with the increase in forward speed. This change is consistent with the results of previous studies [
18,
36,
37]. The reason for this changing trend is because the bite length of the blades increases with the increase in forward speed, resulting in an increase in cutting torque. The torque of the BSCD is always lower than that of the TPSB under operating conditions of 3 km·h
−1, 4 km·h
−1, and 5 km·h
−1. Compared with the TPSB, the cutting torque of the BSCD is reduced by 15.4%, 15.8%, and 16.1%, respectively, which means that the bionic cutting tool device has better drag reduction.
Figure 14 shows the power results of the BSCD and TPSB in the field comparison tests. The power is, at minimum, 3 km·h
−1 and, at maximum, 5 km·h
−1. The trend of power at different forward speeds is similar to the trend of power at different rotary speeds in the soil bin test. The power of the TPSB is always greater than that of the BSCD for the different forward speed operating conditions (3 km·h
−1, 4 km·h
−1 and 5 km·h
−1), and the power of the BSCD is reduced by 11%, 10.2%, and 9.2%, respectively, compared to the TPSB. This indicates that the BSCD has better energy-saving performance.
The results of the stubble-cutting rate of the BSCD and TPSB in the field comparison test are shown in
Figure 15. The stubble-cutting rate decreases with increasing speed. This is because, at a given rotational speed and unit cutting length, the greater the forward speed, the fewer times the blade cuts the stubble, and the cutting effect becomes worse. Under the operating conditions of 3 km/h, the stubble-cutting rate of the BSCD reached 97.2%, and the stubble-cutting rate of the TPSB was 93.4%. The stubble-cutting rate of the BSCD is always greater than that of the TPSB during the test, which indicates that the BSCD has a better stubble-cutting performance.
3.3. Discussion on the Mechanism of Drag Reduction and Energy Saving of the BSCD
The results of the above-mentioned soil tank comparison test and field comparison test have proven that the BSCD has a better operating performance than the TPSB under different operating conditions of rotary speed and forward speed. The increased performance of the BSCD reflects two factors: the cutting-edge structure of the blade and the way the blades cut the root–soil complex.
The cutting edge of the BSCD is a designed curve based on the multi-toothed contour structure of the leaf-cutting ant’s mandibles, while the cutting edge of the TPSB is a smooth straight line. When they cut the root–soil complex at the same depth, the effective contact area between the cutting edge of the BSCD and the root–soil complex is smaller than that between the cutting edge of the TPSB and the root–soil complex, and the length of the bionic cutting edge is larger than that of the ordinary cutting edge, which results in the resistance of the root–soil complex to the BSCD, being less than the resistance to the TPSB (see
Figure 16a,b). The cutting edge of the BSCD is multi-toothed. Compared with the cutting edge of the TPSB, the multi-toothed cutting edge has stronger penetration performance [
38]. When the BSCD cuts the root–soil complex, the tooth-shaped structure penetrates the root–soil complex, which can cause local damage to the penetrated part and produce cracks, thereby reducing the overall strength of the root–soil complex and reducing cutting resistance. In addition, the outer contour curve of each tooth-shaped structure has sliding/cutting performance, which can further reduce resistance to cutting the root–soil complex.
The BSCD is a double disc cutting device composed of power cutting blades and passive cutting blades. When the BSCD cuts the root–soil complex, the movement direction of the power cutting blades and the passive cutting blades is opposite to each other, and the cutting speed of the power cutting blades is much higher than the cutting speed of the passive cutting blades, which can produce a large relative motion speed between them. On the basis of the multi-toothed structure piercing the root–soil complex, the power cutting blades and the passive cutting blades rely on relative motion speed to shear the root–soil complex, which reduces the slip distance of the root–soil complex and greatly reduces cutting torque and power consumption (
Figure 17a). The cutting edge of the TPSB is a smooth straight line without a piercing and slip cutting effect. It cuts the root–soil complex in the form of smooth chopping, and the root–soil complex may slip when being cut, requiring greater torque and energy consumption to complete the cutting operation (
Figure 17b).
In summary, the excellent performance of the BSCD is due to its cutting-edge structure and operational cutting mode. Compared with the TPSB, the BSCD has better resistance reduction and energy-saving performance.