4-8-4 vs 4-8-2

[ebtnut]

One thing that was mentioned peripherally, but is important is the difference in the grate area.  Those extra 16 square feet of grate area in the Governor meant greater steaming capacity, especially at high speeds.  Steam generating capacity is the real bottom line in over-the-road work.  The Pennsy duplexes were intended to reduce the wear and tear of high-speed running by dividing the drive train into lighter, and therefore better counterbalanced mechanims.  Would likely have worked out, but diesels were already arriving on the property. 

[pdlethbridge]

Then that begs the question of why is that engine more powerful, smaller grate lower steam pressure smaller wheels but slightly larger cylinders. It doesn't add up. TE is also a way to say horsepower. But even the loco weights are such that you'ld think the heavier engine would be more powerful, but thats not the case.  The driver size is not a significant difference to give the smaller engine that much more TE? Or is it?

[NarrowMinded]

Using a fixed pressure to run calculations shows that the smaller Engine would have more foot pounds of torque at the wheels.

I think the larger boiler pressure and grate size could have been more for volume of steam, then for pressure sent to cylinders, this would give the engine a larger margin in which to operate.


NM

[pdlethbridge]

Your saying that the higher boiler pressure and smaller cylinders would be more efficient than the lower boiler pressure and bigger cylinders? Then why doesn't it calculate at the axle?
The top speeds were 87mph for the RF&P and 83 mph for the B&M Not really much of a difference
But because the drivers are 4" bigger they lost in the tractive effort ( could have been 70000 )
Try this:http://www.steamlocomotive.com/misc/TractiveEffort.shtml

[pdlethbridge]

   If you take the engine weight divided by the wheels it will show that the B&M engine puts more weight on the drivers
466040/16=29127.5  4-8-4
416100/14=29721.4  4-8-2

[Atlantic Central]

pdlethbridge,

Locomotive weight is not evenly divided amoung all wheels. It is evenly divided amoung drivers, but leading truck and trailing truck axles can support more or less weight than the drive axles, and in most cases this was true.

Example - DT&I 800 class 2-8-2, adheasive weight 113T, EWWO 168T

lead truck axle - 40,000 lb

4 drive axles - 56,500 lb each

trailing axle - 70,000 lb

And remember, when compairing old specs, each was calculated by a different engineer who would have had his own opinion about fixed factors and such.

It is not just simple math, it hinges on the experiances, beliefs and preferences of the designer and builder. So compairing specs down the lb, psi, sq ft, etc, can be very misleading.

Same is true with automobiles. I built hot rods years ago, I beat lots of guys who's spec's said their car was faster.

[Atlantic Central]

The B&M loco breaks down like this most likely:

Lead axles  - 40,900 lbs each

Drive axles - 67,325 lbs each

Trailing axle - 65,000 lbs

The pivot points and lever lengths of the locomotives suspension made it possible to load one axle more or less than the others. Trailing and lead trucks are linked to the driver suspension to keep all wheels on the rail all the time and because of the multi wheel nature of such a system, these lead and trailing trucks could suppoer more or less weight than the drive axles.

Some 2-8-0's had as little as 10,000 or 20,000 lbs on their lead truck, with drive axle loads as high or higher than any Mike, Mountain, Berk or Northern.

While other locos had large amounts of weight of lead or trailing trucks.

Yes, it is a very complex science.

Sheldon

[NarrowMinded]

Your saying that the higher boiler pressure and smaller cylinders would be more efficient than the lower boiler pressure and bigger cylinders? Then why doesn't it calculate at the axle?
The top speeds were 87mph for the RF&P and 83 mph for the B&M Not really much of a difference
But because the drivers are 4" bigger they lost in the tractive effort ( could have been 70000 )
Try this:http://www.steamlocomotive.com/misc/TractiveEffort.shtml

No, I said it gives it a larger margin to operate in, nothing to do with efficiency.

Efficiency on a locomotive would be how hard the crew has to work to maintain steam and how far the loco will travel on a lump of coal and a gallon of water.

To understand why the small engine has a higher TE you have to look at the WHOLE package.

The numbers do add up, the smaller loco has more power per pound when sending the same psi to the cylinders.

200psi.X 28dia X 3.14 X 15.5 = 272552 / 12 = 22712 / 788,800 =

0.028793948613928329952670723461799

200psi.X 27dia X 3.14 X 13  = 220428 / 12 = 18369 / 842,940 =

0.021791586589792867819773649370062

[pdlethbridge]

. I think that, except for the discussion of different locos other than the 2 mentioned, this has been a great discussion. The fact that these 2 engines are so similar and yet so dissimilar, is amazing. The science behind the design done at the time, was all done in the head and on paper. No computers or calculators to make these work.
Over the years, locomotive design did have its failures but there were a lot of successes like these two engines

[Atlantic Central]

Sorry if I disrupted "your" discussion, but I sighted the information I posted to deminstrate other examples of how some of your basic assumptions were wrong - sorry to have bothered you.

Sheldon

[pdlethbridge]

Sheldon, you didn't disrupt it , you shed light on the subject.  My thinking was involved around these two particular statistics and what made them so different and what made them so important to their particular railroads. When you cook, if you add a pinch of this and a touch of that, everything changes. How these two were changed with a pinch of this and a touch of that is totally amazing. None of the changes were that great but what a difference.

[ebtnut]

To some extent, this discussion points up the series of issues that had to be addressed when designing a locomotive.  Generally, the head of the engineering department would develop a set of specs for what a new loco was to do - He had to take into account things like axle loading, clearances, type of fuel, minimum curves, grades, etc.  Then, unless he worked for someone like the PRR, N&W, SP, he would send the spec out to the builder for a quote and a more detailed design.  Once approved, the locos would be built.  There were always trade-offs.  High passenger speeds could be had with big drivers, but at the cost of tractive effort and factor of adhesion.  Lugging power could be had with a lot of small drivers (think
2-10-2), but then your speed might be limited to 25 mph.  One big advantage diesels had was that by making them in units that could be mated together and controlled from the head end was that the lash-up could match the need of each train. 

[pdlethbridge]

  The end of steam brought in an era of standardization and engineering productivity.. Steam engines were basically a custom build, with each being different in some way. Diesels, on the other hand, were assembly line built, all the same except for paint and numbering. A gp-7 was the same for hundreds of units, but not a 4-8-2 or a 4-8-4.  A quick look at steamlocomotive. com showed at least 4 makers of 4-8-4's, many different designs, 4-8-4's that started as something else. ( Reading t-1s that were originally I-10 2-8-0's) And maybe 20 different names. ( Niagara, northerns, governors, big apples, pocono's, J's etc