To invent a new type of solar cell car, I only have an idea, but I can also draw a structural diagram. Can I apply for a patent?

Yes, many patents are only an Idea when they are filed, as long as you can clearly describe the technical scheme in the application documents so that technicians in this field can implement it;

a patent only requires that the technical scheme can be implemented, and does not require that you have already implemented it and made a physical object.

From your description, your technical scheme should have been conceived completely, and it can be implemented for the technicians in this field. Therefore, you can apply for a patent for the technical scheme you have conceived now.