Dave Plowman (News) wrote:
In article ,
John Williamson wrote:
It's not possible to make a two stroke meet emissions regs in practice.
Their fuel consumption is also poor. This is why they're only used for
purposes where their low production costs outweigh this. In countries
where it doesn't matter.

Almost all large diesels as used in ships are two stroke. They also have
the highest fuel efficiencies of all internal combustion engines,
currently over 50%. Two stroke engines scale up better than four stroke
engines.

Large diesels used for this sort of application aren't comparable to what
is suitable for road use. For a start, they run at or near a constant
speed. 2 stoke supercharged diesels were once used on road trucks by
Commer - but not now. Just about all common trucks and cars are four
stroke.

Two stroke engines as used in motorcycles and cars are built as cheaply
as possible, so they need to use a fuel/ oil mix. They could be designed
not to, but that adds complexity, cost and weight.

As much as all the complicated valve gear used on a modern two stroke?

Assuming you mis-typed two for four there, yes, as not using the
crankcase to compress the charge leads to things like blowers to get the
charge into the cylinders to start with. That's why the Commer
two-strokes you mentioned had to use a supercharger. The penalty is less
if you're already using a turbocharger, though.

People don't generally use four stroke engines either for very small or
very large applications.

Efficiency and emissions don't much matter for very small engines. Weight
may be crucial, though. And of course cost. Most very large engines run at
a near constant speed.

Fuel efficiency isn't vital, no, nor is emmission control, though power
per gramme can be very important. As for constant speed, marine diesels
work over about a three to one speed range (60 - 200 rpm) at pretty
constant efficiency levels, which is wider than most automotive engines
nowadays. 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.

--
Tciao for Now!

John.