Giant petrol cap thing on vans

[jumpinjackflash]

Something I always wanted to know and only remembered about it when I saw one the other day. What are those rotating circular black things that look like large petrol caps that you sometimes have on vans?

They rotate even when the van is parked and no engine running.

[Brendan]

I think what you're talking about is a ventilator. They keep airflow going in the van and run off the movement of air not the vans power.

[jumpinjackflash]

Oh, something as simple and boring as that! Thanks Brendan.