When you talk about US "diesel fleet" are you talking about the surface fleet or submarines? Cause surface ships use gas turbines and jet fuel, not diesel. The Navy got rid of nuclear surface ships, except for aircraft carriers, in late 90s cause they were expensive.
If you are talking about submarines, the US Navy hasn't had diesel submarines since 1990.
The last US Navy diesel submarine, USS Dolphin (AGSS-555), was decommissioned in 2007.
And not all Navy surface ships are nuclear or turbine powered. Many classes of US Navy surface ships are diesel-powered, including some of the newest ones. Ships with gas turbine engines typically feature diesel powerplants also.
I'm sorry, I assumed the US Navy ship engines burned bunker fuel like container ships, in diesel engines like those Wärtsilä makes, rather than using turbines. Thank you for the correction. I was talking about primarily the surface fleet, but also didn't know the US had eliminated their non-nuclear submarines, which I have to admit undermines my point somewhat!
If you are talking about submarines, the US Navy hasn't had diesel submarines since 1990.