They are a 4 stroke V-8 diesel that could be had both with and without a turbocharger. They are similar to their older two stroke brethren in that they use unit injectors rather than an injector pump to raise fuel pressures like you might find on other buses.
Mine has a Turbo Charger. It is cool when your going down the road and it whistles.
__________________
"Escapin' through the lily fields
I came across an empty space
It trembled and exploded
Left a bus stop in its place
The bus came by and I got on
That's when it all began
There was cowboy Neal
At the wheel
Of a bus to never-ever land"