It very much slows down. Just not quite as much as a gas engine. You still have friction losses (especially as you get higher rpm), losses from alternator, wate

It very much slows down. Just not quite as much as a gas engine. You still have friction losses (especially as you get higher rpm), losses from alternator, water pump, engine fans, oil pump, etc. One of my vehicles is a VW Jetta TDI (diesel, ALH engine).