Re: A's on the WMB

[keithm@happy.dca.gov.au (Keith Malcolm)]

In article <35076F6E.7C25@harnessnet.com.au> Craig Haber <albatross@harnessnet.com.au> writes:
>From: Craig Haber <albatross@harnessnet.com.au>
>Subject: A's on the WMB
>Date: Wed, 11 Mar 1998 21:15:26 -0800

>G'day,


>But why is the turbocharged A class not a significantly better
>performer? It enjoys a 700hp
>advantage over an S (+39%), and 1000hp over a B (+67%).  I discussed
>this with our
>Workshops manager, who was involved in the testing on N Vs S class in
>early days of WCR
>operation.  In short, the answer lies in the turbocharger, and the
>nature of the Warrnambool line.

>The Warrnambool line is relatively flat - i.e, there is no sustained
>climbing, most grades are short and sharp.  There are also a lot of
>speed curves.  So by it's nature, it's an "open throttle, close
>throttle" type line, that is, the drivers are opening and closing the
>throttle
>regularly.  This presents a problem to any turbocharged engine, due to
>the turbo lag.  When opening the throttle (to accelerate out of a speed
>curve for example), a percentage of the engine's output is dedicated to
>getting the turbo up to speed, rather than moving the train.
>This consumes fuel, and restricts horsepower until the turbo comes into
>it's optimum range, at which point it goes from having a negative, to a
>positive effect.  In short, you never get the most out of a turbocharged
>loco on such a line.  On something like the Bendigo, or Ballarat
>where there is sustained climbing, you put the loco in notch 8, get the
>turbo up to speed and maximum engine output, and leave it there - you
>don't waste any further time/fuel accelerating the turbo.  No turbo lag
>on a roots blown loco, you open the throttle, and it's power (and fuel
>consumption) all goes into moving the train.

>Hence despite having smaller and less powerful engines, an S or B on the
>Warrnambool can match the performance of a nominally more powerful
>loco.  And despite being an older design engine, by virtue of being
>smaller and by putting more output into moving the train (rather than
>winding up the turbo), the fuel consumption is much the same.

>There was some discussion about the role of the roots blowers in these
>engines in this newsgroup - again speaking with our fitters at Ballarat
>East, they tell me the roots blower *does* supercharge the engine,
>although only very mildly.  Yes it does also scavenge the
>exhaust gas from the cylinder.

>I have a time log I took from A73 on an N set back on 16/2/93, I'll dig
>out some runs with WCR S/B's on 3 cars and see how they compare some
>time.  

>Cheers,
>Craig.
>-- 
>Craig Haber
>albatross@harnessnet.com.au
>Manufacturing Systems Engineer (almost) 
>Web Page Designer, Harness Racing, Railways, and Essendon Football Club
>fanatic
>http://www.harnessnet.com.au/

Thanks for confirming my story that roots blowers are superchargers. However 
I fail to understand how power and fuel are wasted in getting a turbo up to 
speed as the turbo is driven by the exhaust gassesnot by a mechanical drive.

David Malcolm