Animated diesel piston

[punky]

Here's some diesel links I found while trying to find out about Rear Engined,Cat powered schoolbuses. I'm trying to find out which white filter can it is and what it's filtering, it's on the passenger side engine check bay(rear of bus) it's behind the power steering fluid cannister in plain view, mounted upside down on a mount of some type but I can't see any hoses or IP, vision is blocked there. There's one more similar filter under engine and forward of engine near chassis rails. Can barely see that one. My Instructor, a nice lady bus driver, not a mechanic,(I'm only a student) doesn't know exactly what they're for or which is which(i.e. fuel, oil or power steering fluid filter) even though I have to be tested on this knowledge. So if there are any Cat mechanics out there or bus drivers I'd appreciate the help.
diesel link to animated piston
More diesel links, maybe they've been posted before.