A and AE

Thu Feb 12 18:39:58 EST 2015

Nope, disagree, with the idea that the H8 generated 40% more horsepower than the Class A.   Eric, I believe you may have used the 7,498 DBHP figure for the H8 and 5,300 DBHP for the A.  These are indeed indicative of the 40% superiority of the H8.  However, comparisons of steam locomotive horsepower are almost impossible to bring off in any meaningful way.  For example, N&W rated the Class A's very (very very) conservatively,.   A figure of 5,300 for an A was what I would call "dispatchable horsepower", i.e. what could be expected of the locomotive any day, under any circumstances.  The 7,l498 DBHP figure for the H8 was achieved in regular service, but needs some perspective.  Take a look at page 204 of Eugene Huddleston's book on the Allegheny and you will notice that the scatterplot where this figure is displayed clearly shows it to be a very high figure, almost an outlier.  Further, the reading was taken at a known location, where track charts show that the train was running through a sag.  This would tend to make the figure slightly overstated.  Also, the curve drawn through the scatterplot diagram looks like the H8 was developing about 6,600-6,700 DBHP at the time, still higher than the 5,300 rated for a Class A.  Nonetheless, I don't think we should be content comparing a highly conservative rating of 5,300 DBHP with an outlier high figure of 7,498.  If you want to use something like 6,700 for the H8, I may agree that this is more reasonable, if not entirely equitable.  More later (I hope, if ambition strikes).
Dave Stephenson

      From: NW Modeling List <nw-modeling-list at nwhs.org>
 To: "List, NW" <nw-modeling-list at nwhs.org> 
 Sent: Wednesday, February 11, 2015 9:50 PM
 Subject: Re: A and AE

Hi Eric;
Sorry for the very short response: I would like to refer you to Col. Jeffries book: "N&W Giant of Steam"
Nigel
From: "NW Modeling List" <nw-modeling-list at nwhs.org>
To: "NW Modeling List" <nw-modeling-list at nwhs.org>
Sent: Wednesday, February 11, 2015 3:55:16 PM
Subject: A and AE

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I have been ruminating on a comment made in this forum several weeks ago to the effect that the N&W A was a superior engine to the VGN AE or C&O H-8, because the A produced more tractive effort with far less weight (and implied stress) on the track, despite the fact that it was an 8 year older design.   The claim of higher tractive effort for the A surprised me, but is theoretically accurate based on boiler pressure (300 psi for the A vs 260 for the AE and H-8), cylinder bore (24” for the A vs 22.5 for the AE and H-8), stroke (30” for the A vs 33” for the AE and H-8), and driver diameters (70” for the A vs 67” for the AE and H-8).  The net result is a theoretical tractive effort of 114,000 pounds for the A vs 110,000 for the AE and H-8.  Despite the larger drivers and shorter stroke, the A achieves its higher theoretical tractive effort through its larger cylinder bore and higher boiler pressure. But an (equally theoretical) analysis indicates that the A’s factor of adhesion was 3.44 (indicating a propensity to slip pretty easily), whereas the AE was 4.49 (indicating a VERY “planted” locomotive), and if the weight rumors of the H-8 were true, its (theoretical) factor of adhesion would have been a bit higher than even the AE. Both classes were designed to run at 70mph, but the design philosophies to do so were different.  The longer AE stroke and smaller diameters would have resulted in ~15% higher piston speed at any given speed, but the same dimensional differences would have resulted in ~15% lower static load on the main and side rods for a given tractive effort.  This and the ~18% smaller piston mass would have allowed ligher main and side rod mass and potentially easier dynamic balancing—thereby allowing the AE and H-8 to achieve the 70 mph design speed, even with 4.5% higher driver rpm than the A. I think it’s undisputed that the AE / H-8 design produced nearly 40% more horsepower than the A. So here are the questions I’m driving to:   Did the A slip on starting?  Were the AE and H-8 notoriously sure-footed on starting?  Could either type start markedly heavier trains than the other under similar consitions? Could the A run any train at 50mph that it could start? Could the AG start any train it could run at 50mph? Did N&W and VGN operate the A and the AE over similar profiles at any given time (e.g., Narrows to Roanoke, or Roanoke to Norfolk)?  Did they use coal of approximately the same grade on these runs?  Did one deliver more ton-miles per hour, over similar profiles, with approximately equivalent quality coal? -Eric Bott
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