The Middle Frame Design for an Electric Car


Large, thin wall and complex precision casting technology of investment casting that has been developed in the automotive field has won a new expansion application. For new energy vehicles, also known as electric or partially-electric cars, the advantages of lightweight component parts are obvious. In this application, the design and manufacturing process were integrated to create a first-class design.

In the original design, there is no big circular angle around the model, and the inner cavity is only 22mm. Because the inner frame of the middle frame is too narrow, it is easy to bleed from parting. It is not easy to make shells on account of the narrow inner cavity. It is an unacceptable structure for investment casting. The design resulted in many casting defects and serious deformation.

In order to compare the influence of a circular angle, analysis and comparison were made. On the basis of the original scheme, only the four corners were rounded, and the rest of the design was unchanged. The weight increased by 0.3 kg.

By comparing the free mode results, it was found that the first 6 order mode frequency of the big circular angle scheme was larger than the original scheme, with an increase of 3-12%. The torsional stiffness of the car body is one of the important indicators of body-in-white templates, so the torsional stiffness is paid special attention to, and the first order torsional frequency of the big circular angle scheme was raised by 1.3Hz, up by 3.4%.

The deformation of each condition is more than 60% less than the original scheme, indicating the increase of the circular angle structure can effectively improve the stiffness of the structure, and the stress of the two schemes is basically equal.

The simulation results show that compared with the structure without the circular angle, the increase of the circular angle can advance the structural stiffness of the middle frame significantly. The middle frame material is ZL114A, with an elongation rate of 3%. The deformation of the increase of the circular angle structure is reduced by more than 60%. Compared with the big circular angle scheme, the original scheme is a failure mode.

Because the cavity is too narrow and easy to bleed from parting, and the deformation of the casting is serious. By just changing the design of a small step, the effect is particularly obvious.

2. The Leading Design Ideas Have Significantly Improved the Performance of the Structure
The same design approach was taken for the battery case and oil pan of the car. This has made a huge impact in manufacture of structure mode, strength and deformation.

Under the constraint mode of the vehicle, the vibration frequency of the vehicle under the condition of the vehicle is investigated, and electricity quality is considered.

Compared with the scheme without a reinforcing bar, the first order mode frequency of strengthening bars increase 3Hz, rising up 21%. Similar to the central frame, only a stiffener is added, but the mode frequency enhancement is particularly obvious.

A series of methods have been used for improving the structure of the oil pan, such as adding ring bars and transverse bars in the outer side of the oil pan, changing the groove U shape to an H shape, and drawing right and left triangle bars to the middle.

The mode analysis of the oil pan shows that the first 10 order frequency of the improved model is higher than the original model. The frequency of the first order is increased by 135%, and the increase indicates the stiffness of the improved model has a significant improvement. The car’s engine is a six-cylinder four-stroke, with idling speed of 600r/min, rated rotation speed of 1900r/min, maximum torque speed of 1100~1450r/min, and the corresponding frequencies of 30Hz, 95Hz, 55~72Hz. The frequency of the improved model can avoid the vibration frequency of the engine effectively.

Static analysis of the oil pan shows that the deformation decreases 81%, the stress decreases 19%, and the stiffness and strength are significantly improved compared with the original one.

The case of the middle frame, battery case and oil pan of a new energy vehicle, making small changes to a design can have a powerful effect on manufacturability. Based on advanced precision casting technology, the material can be transferred according to the load in the process of structural design, which can not only reduce the waste of material and reduce the weight, but also optimize the layout of the structure, which is no longer limited by the original process. This design method can play an important role in the design of cars and the weight loss of new energy vehicles.