Fire Out of the Tailpipe ?

A British sprinter driver had an unusual drive and asked for help, he writes:
I have a 2011 Mercedes Sprinter 313 cdi. Whilst driving I heard a knocking noise then the the van started to lose power. Pulled up on the side of the road and noticed there was fire coming out of the exhaust pipe. Does anyone know what could have caused this?

Mafia hit gone bad...

Poke a hole in a piston top and you'll be pumping engine oil out the back
Lose a glow plug (as in "pop", right out of the head) and there will be no compression in that cylinder... some of its fuel will go out the rear

--dick

My bet is a cracked piston