Identifying Engine Parts

[Dhamm]

I tried searching on Google but maybe I didn't search the right part. My car starts to smoke when I come to a stop but i don't see it come from under the hood, more so from under the car. I think I may know from what but I don't know what the part is called. Can anyone identify this for me? I'm talking about the white knob thing, by the way. Thanks in advance.