On Wed, 12 Mar 08, Capt John wrote:
The artical indicated the engine was turbo charged. If the engine were
2 stroke, it has to have some kind of air pump to scavange exhaust,
basically pushes whatever gasses don't escape on their own out of the
cylinders. All two stroke engines employ some kind of air pump in
their design. In the case of outboard 2 strokes and small displacement
2 strokes the air pump is the backward stroke of the piston, along
with ports in the cylinder walls and reed valves that keep the
pressurized air from escaping out the carb (remove the reed valves and
see what happens). This kind of design does require an open crankcase,
roller bearings, and oil in the fuel which lubricates the bearings and
cylinder walls. In the case of diesels, because of the pressures
involved during compression, open roller bearings won't work, they
need traditional bearings with a fair amount of oil pressure. As such,
it needs a seperate air pump. The blower you see on Detroit Diesels
are for scavaging purposes only, the turbo's are supplying the real
boost. At higher RPM's the turbo's are probably supplying enough boost
to scavange the cylinders, as well as increase the amount of air
forced into the cylinders. But at low RPM's, without the blower, you'd
probably be lucky if you could get it to idle well, if at all.

I'll have to let that sink in awhile. The only 2 cycle diesel I'm
pretty familiar with is a Detroit 4-53. It uses a blower and doesn't
have or need a turbo. I'm pretty sure a 6V-53 is designed the same
way. I can't imagine shrinking either of them down enough to clamp
onto a transom though so obviously I'm lacking something in design
knowledge.

Rick