Valentas/maybachs/MTU's/Napier Deltics all run at 1500rpm
, the Napier did have a turbocharger being fitted to the T9's in the 'baby deltics', being a 2 stroke engine it did need force feeding and had a supercharger to force the air into the cylinders which both the 9 and 18cylinder versions had
Yes, I was about to reply in a similar way to the point that a Napier Deltic engine didn't have a turbocharger.
The potential for a turbo in a 2-stroke engine is very restricted. Its a very different process. But they DID have a "scrubber" which forced the evacuation of exhaust, and also, the 2 opposing pistons in each cylinder were staggered in time to provide additional force to remove exhaust and vacuum to draw in fuel.
Dont turbos mean less fuel consumption though?
A tubocharger improves torque during higher engine speeds. If the design of the whole engine+transmission can use that torque to reduce fuel consumption, then yes. But it doesn't follow automatically.
A turbo takes advantage of engergy (and heat) that might otherwise be lost and uses it to assist in fuel intake. Outside certain ranges of rev.s then it is simply a hinderance.
A supercharger (as kestrel hinted) performs a similar improvement to fuel injection at lower revs.
The best efficiency, and therefore the best fuel economy, is achieved at a single, constant, designed operating speed and load.
One of the design engineer's tasks is to maximise the potential to operate within that optimal range of speeds and loads. This is hard in stop-start rail engines and even harder in domestic motor cars, but a lot simpler in power generaton.