[Vwdiesel] Turbo in series or parallel

[Shirley, Mark R]

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.   
>