I’m looking for a little help with air compressors on steam engines. What were the deciding factors that led some railroads to use two compressors on an engine instead of one? Did mountain grades figure into the decision? And on a related subject - What figured into the decision to use multiple and/or larger air reservoirs on a particular engine?
I’m working on a kitbashed Yellowstone and would like it to be as believe-able as possible.
Thanks,
Gunnison142
Hello “Gunnison,”
There doesn’t seem to have been as much reason behind this as you might think. It’s just as easy to find examples of mountain engines with one compressor – Santa Fe 3800-class 2-10-2s – as it is to find flatland engines with two – Nickel Plate Berkshires. Two compressors should have been able to charge or recharge a train line faster, but according to an air brake professional this didn’t make much difference in practice. The two compressors were piped in parallel, both feeding into the first main air reservoir.
My advice on your Yellowstone is to use two compressors, since that seems to have been the norm on engines of that wheel arrangement.
So long,
Andy
A big factor in the sizing of locomotive air supply systems (number of compressors, number and size of tanks) was the expected quantity of air needed to pump up the brakes on the train to be pulled and the length of time the company was willing to take doing it.
If the locomotive (whether 2-8-4 on the flatlands or Yellowstone where the grades were steeper) would be expected to back down on a 120-car train with empty reservoirs, it made sense to provide two compressors. OTOH, if the usual assignment was to replace another locomotive that had just pulled into the division point an hour earlier, taking on a train with a fully-charged system, a single compressor would probably have been deemed adequate.
Some yards had fixed air systems which could be used to ‘pre-charge’ an assembled train, reducing the time needed when a locomotive finally coupled up.
Railroads operating in colder climates found that extreme cold = air leakage, so would be more likely to add extra capacity. (Yes, it can get below freezing in the south - but not a solid month of sub-zero temperatures.)
At least one Santa Fe 2-8-0 was fitted with two cross-compound compressors on the front of the smokebox (like Chessie!) in addition to the normal train air compressor and tanks. The loco was in ballast service, and the huge air capacity was needed to operate pneumatic side-dump ballast gons.
If I was building a Yellowstone, I would give it two cross-compound compressors. I can’t think of any American simple semi-articulated that had fewer.
Chuck (Modeling Central Japan in September, 1964 - one articulated with two compressors)
Available space on a particular locomotive design became an issue as well. As locomotive horsepower increased so did the need for larger volumes of superheated steam, thus boiler length and diameters were increased accordingly.
Hanging a Westinghouse cross compound on the lower side of the boiler was no longer an option due to clearance so the compressor had to be moved - usually to the pilot area or sometimes even bolted to the smokebox front.
Other considerations were how much room was needed for other appurtenances such as feedwater pumps and heaters, automatic train stop electrical gear, stoker and booster engines, turbogenerators (didn’t an SP GS-4 have three?) so available space meant some things had to be compromised.
Many times it was the cheif mechanical officer and the railroad’s motive power department that would have the final say when specifying which demands of the operating department would be met with a particular locomotive design.
ED
Thanks for the info. I was planning on two cross compound compressors on the smokebox front and your comments have clinched it. Sounds like the reservoir question will run the same course. I’m considering 3 if the other appliances will make room. I have to admit I love the Pennsy’s penchant for large reservoirs on the front of the pilot.
I know there was another post on a related topic and I understand how the brakes are applied, but am I correct in believing that the system loses air when the brakes are released - so the compressors need to replace the lost air in the main reservoirs? If that’s true I would expect that mountain roads need some additional compressor capacity? Do I have this right - or not?
Thanks,
Gunnison142
Air compressors on steam engines were commonly called “air pumps”. Typing air pump into the search box of the Trains.com forums will come up with a lot of items.
It’s been many years, but IIRC there was an ICC requirement for something like 25 or 30 feet of pipe between the air pump(s) and the first reservoir and another 25 or 30 feet of pipe between the first and second reservoirs. This was to allow the air to cool down before reaching the brake valves. This is the reason you often see those pipes snaking back and forth under the running boards on some engines.
Hello “Gunnison,”
Air is vented to the atmosphere both when the brakes are applied, by the engineer’s brake valve that reduces the train pipe air pressure, and when the brakes are released, by the control valves on the cars through the retaining valves. See “Air brakes for model railroaders,” by Fred Calrson, in the Novemeber 1994 Model Railroader, pages 100-105 for a full explanation of air brake functions. Also, you can find a 1945 Westinghouse instructional pamphlet, The “AB” Freight Brake Equipment, on the Web at www.psrm.org/faqs/brakes/5062/ab_brakes.html.
By the way, Fred Carlson is the air brake professional I referred to in my earlier post. He wrote in his MR article,
“The charging chokes in the service portion of the control valve regulate how quickly a train’s air brake system can be charged. More compressors on the locomotive or “notching up” a diesel to “run four” (half speed) may help, but not much. The reason some mountain steam engines had two compressors was that they used their brakes more often than engines in flat territory.”
He also mentions that on some very steep grades railroads use higher brake pipe pressures than normal, to produce higher brake cylinder pressures in service and emergency applications. This higher pressure requires greater compressor capacity, and Fred says this is another reason some mountain steam engines had two compressors.
However, those Santa Fe 2-10-2s I mentioned operated on the 3.5 percent grade of Raton Pass, the steepest mainline railroad west of the Mississippi.
So long,
Andy
Thanks again for the information. I’ll check out the article and the web site you mentioned.