From: craig@kralizec.zeta.org.au (Craig Dewick)
Newsgroups: rec.railroad
Subject: Re: Alcos, GG1's, etc.
Date: 13 Dec 1992 12:30:42 +1100
Organization: Kralizec Dialup Unix Sydney: +61-2-837-1183 V.42bis

kwyatt@ccscola.Columbia.NCR.COM (Kershner Wyatt) writes:

>Since this thread has been going, ALCOs have had exhaust driven turbos
>(which I thought always was the definition of a turbocharger, else it
>was supercharged), belt driven turbos, geared turbos.  Surely, we didn't
>have all of these, did we?

>What is it? Exhaust, belt, gear, or chain driven (figured I'd throw that in,
>no one else had)?

Ok, here's the undeniable truth about positive air pressure devices for
diesel engines...

Turbochargers are exhaust gas driven. They are NOT, never ever, mechanically
coupled to the crankshaft of the engine, as this, as has been stated
before, only loads the engine more, reducing efficiency.

Superchargers are gear driven off the crankshaft.

4 stroke diesels, not being scavenge-blown engines, don't need to have
positive air pressure present since it's the exhaust cycle of the piston
that exhausts the gases and not air. Turbos are used simply to increase
efficiency. A 4 stroke diesel will still run without a turbo, but at
anything above idle it will be extremely inefficient.

2 stroke diesel MUST ALWAYS have a pressurized air box, since they rely
on this to blow the exhaust gases out of the cylinder. 2 stroke diesels
don't have an exhaust stroke in their cycle. Now, while 2 strokers will
say that because of this their engines are more efficient, 4 strokers
say that this is garbage because, being gear driven, the supercharger
(usually a roots blower on older GM's) is ALWAYS consuming some of the
power output to provide the air pressure in the air box of the engine
surrounding the intake ports near the bottom of the cylinder walls.

Newer GM's (I think this started with the SD40 in the US, but here in
NSW, Australia we first got them on our 81 class diesels, which are
derived from the SD40 design), have what's called a turbo-supercharger.
This device is gear driven by the engine until the exhaust gas pressure
is sufficient to push it, after which it's purely exhaust gas driven.
There is some sort of one-way clutch arrangement that allows this.
 
When the engine slows down, and the exhaust pressure reduces again, it
will become gear driven once more.

To get back to the smoke issue, one thing that everyone has forgotten to
mention is that EMD build their engines with oil seperators in the 
exhaust line. This device is designed to seperate unburnt oil particles
from the exhaust gas stream, and to redirect some of this exhaust gas
stream back into the air box to be re-burnt. Alco's don't have this, so
any oil that gets past the exhaust ports ends up blanketting the
neighbouring countryside!

Thus, on a GM you usually only get blue or grey smoke, whereas with an
Alco you can get genuinely BLACK smoke, and whoa be tied anyone who
stands next to an oil-spluttering Alco when it's notched out and the
wind is blowing the exhaust your way (ie. don't where a white shirt!).
 
I suspect that the EMD oil seperators are also gear driven off the
crankshaft.
-- 
 Craig Dewick               |        - Railway Preservation Industries -
 Send email to:             |            Supporting the Preservation
 craig@kralizec.zeta.org.au |          of Australia's Railway Heritage