Dave Plowman (News) wrote:
In article ,
John Williamson wrote:
Most of the ones I use day to day have a useful power band
from 1250 to 1800 rpm, with maximum fuel efficiency being in a quite
narrow band centred on 1500rpm.

A decent car diesel will have a useful power band from about 1500 - 4500
rpm.

I'll admit that the ones I use are a tad larger than that. Look for the
Volvo DH12C engine specifications, although the Scania equivalent isn't
too different. The narrow power band is a by-product of the emmissions
control requirements. Valve and injection timing are both critical, and
the former can't easily be varied while the engine is running, despite
the attempts at VVT by certain makers in the past.

The cars we have will indeed rev up to 4000, but aren't happy doing it.
Again, the most economical cruising speed is at about 1700 rpm, which
has been optimised by the makers to be at 80mph in top, 56mph in fourth
and 30 in third, to get the best figures for the EU consumption tests.

I'd love to see the efficiency figures for a large ship diesel. Surely
most of the time it will run at a near constant speed once on the open sea?

Yes, and also at constant power, given the direct coupling to the
propellers. The makers all state 60 - 200 rpm as the operating speeds,
though, without mentioning efficiency at the lower speeds. Even in the
1950s, they were doing over 40% thermal efficiency at best speed/ power,
and are now up to 54% for the best of them. Train diesels aren't all
that far behind, which leads to the interesting thought that unless
you're generating your electricity by nuclear power, a diesel locomotive
may well generate less pollution overall than an electric one.

--
Tciao for Now!

John.