[Vwdiesel] Turbo in series or parallel
Shirley, Mark R
MarkRShirley at eaton.com
Thu Jan 29 09:07:59 EST 2004
See comments interspersed.
> Actually, the Roots blower on GM 2 strokes does both charge
> and scavenge. Charge, because, as noted, there is no intake
> stroke, and scavenge because there is little "loss" in doing
> so (just the energy to run the blower), and something to gain.
Charge by definition, is NOT the same exactly as BOOST.
> For those who aren't familiar: To follow a cycle, start at
> injection where the cylinder is closed up and fuel is
> introduced (by a unit injector driven from the cam). Like
> any other diesel, it does so as a "constant pressure" engine,
> or as close as it can. Part way down the power stroke, the
> exhaust valves (all 4 of them) open. This is why there is a
> particularly loud "bark" from these engines. Most of the
> exhaust escapes due to the cylinder pressure. Near the very
> bottom, the intake ports are exposed by the piston (they are
> in the wall of the cylinder). Air has been piling up in the
> intake tract due to the blower. Here you can see that,
> without the blower, there would not only be no air flowing
> INTO the engine, there would be exhaust going OUT the inlet.
> (This is also the reason why these engines won't run
> backwards). This is the charge part of the blower function.
> The exhaust valves are still open, so the continued air
> influx is allowed to flow out of the exhaust
> ports for better scavenging of end gasses (the other,
> secondary use of the blower). Then, the exhaust valves are
> closed, and, as the piston is on the way up, the blower
> finishes the job of charging the cylinder (and can, indeed
> produce "boost" or, more correctly, supercharge the cylinder)
It could, but doesn't on a GM 2 stroke diesel. The blower is used
to push the exhaust gas out. Any intake boosting is purely incidental.
> depending on sizing, efficiency, etc. From here on up, it is
> the compression part of the cycle, and we are back to
> (approaching) the top for injection/ignition.
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