“Tough Gear”and “High Quality” for New X-TRAIL - 5. NV Technology to Achieve EV-Level QuietnessFig. 13 Booming noise reduction effect of ANC on 1.5th order (rpm) under 0.1G accelerationFig. 14 Car body frame eigenvalue array table6.4 Low-sensitivity car body6.4.1 Reduction of frame eigenvalue density6.4.2 Improving eigenvalues (modes)Fig. 13 shows the booming noise reduction effect of the ANC.For the new X-TRAIL, a high-rigidity, low-sensitivity body was developed to cope with the increase in the vibration level caused by the high-output 3-cylinder engine and to further improve NVH performance. A body that does not transmit noise or vibration to the users was realized by (1) reducing the eigenvalues (modes) of the body frame in the low-frequency range and (2) making the mode less likely to generate noise.The eigenvalue of the body frame is the unique vibration mode of the body frame, such as twist and bending, and it is considered the main factor that constitutes the sound sensitivity level because it amplifies the volume change in the cabin caused by the input of the engine, suspension, etc. The car body is likely to be excited in various modes in response to various inputs (i.e., easily generating sound) if many frame eigenvalues exist in the low-frequency range (high density). Compared with the previous X-TRAIL, the new one adopts a high-output 3-cylinder engine that tends to increase the engine vibration level. Therefore, it is necessary to reduce the eigenvalue density to improve the sound sensitivity of the car body (Fig. 14).Specific measures to improve the eigenvalues (modes) and reduce the sensitivity are shown in Fig. 15. The conventional C-pillar structure uses a cross section on the outer side as a countermeasure against twist resonance, which is the first eigenvalue. The new X-TRAIL employs a circular structure (1) with a cross local rigidity 29section on the inner side (compression side), which contributes significantly to the rigidity and improves the eigenvalue of the frame from 30 to 40 Hz. For the lateral bending resonance of the engine compartment, which is the second eigenvalue, both the lateral rigidity and eigenvalue were improved by installing frame (2), which connects the tip of the hood ridge and the front side member. For the bending resonance in the longitudinal direction of the vehicle body, which is the third eigenvalue, the previously mentioned C-pillar inner circular structure suppresses the deformation mode in the vertical direction of the vehicle body at the rear part. Further, another circular structure (3) is provided in front of the floor tunnel to suppress the tunnel-opening mode and further increase the eigenvalue. The eigenvalue density in the low-frequency range is reduced and the sound sensitivity is significantly improved by applying these structures.Moreover, the noise radiating panels are identified and stiffened to suppress noise radiation at eigenmode (reduce the vibration gain). For longitudinal direction dash panel vibration modes, which is one of the sounding parts, the dashed upper cross section is added to form a closed cross section (4) in the right and left directions above the dash. Further, the 7th cross member (5) is installed to connect the mounting points of the rear suspension members, which suppresses the vertical mode of the rear side members and the rear floor panel vibration mode and thus reduces the noise from the respective parts.For other frameworks, the amount of offset from the center of the centroid is reduced, whereas the bending points of the ridgeline are minimized considering packaging restrictions. Thus, the is improved by minimizing the seating surface of each mounting part and by increasing the rigidity efficiency. Consequently, the thickness and weight of the main frame are reduced compared to those of the previous X-TRAIL, whereas the torsional rigidity and vertical bending rigidity are increased by 44% and 40%, respectively. In this manner, a high-rigidity, low-sensitivity body that supports the high-output 3-cylinder engine of the new X-TRAIL is realized.
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