My Father passed away this past Valentine's Day and being the only one in the family that shared his passion for railroading I naturally inheritted all his railroadiana. Browsing through a less than recent issue of Railfan/Railroad it showed the restoration of this engine (early 90's) and I noticed something odd about her that I couldn't find on quite a few (that I looked at) other 4-8-4's. Her steam chest. Most engines have high pressure and low pressure chambers...from what I understand...if this is wrong I apologize...its just MY understanding. There is an even smaller chamber ABOVE the small one on both sides of the steam chest. What does this smaller chamber do; what is it used for?
Does it have anything to do with the fact that she is an oil burner?
Just curious and again I am sorry for my lack of steam mechanics jargon. I am in a complete OPPOSITE industry...hospitality.
Thanks.

Grin,
You have pried me out of a four year lurking mode.

The third (uppermost) cylinder is a "drift" mechanism. As it was told to me, Santa Fe was notoriously tight with its fuel budget. The long grades over Raton and Glorieta passes required leaving the throttles open small amount, to allow for lubrication of the cylinders. The top cylinder allows for steam recirculation and lubrication with out having to "use steam" and therefore burn fuel while drifting down grade.

Our locomotive the 2926 also has the same setup.

http://www.nmslrhs.org/images/photos/1-18-09/slides/P1170158.html

Here is a photo from our site showing the top cylinder.

Regards,
Kevin

Steve,

Just to make a minor correction to your original statements- the two lower "chambers" are not high pressure and low pressure. The middle "chamber" houses the piston valve that controls the flow of steam into and out of the cylinders. The lower "chamber" houses the piston that's connected to the main rod that turns the driving wheels.

See this animation for an explanation:

The drift mechanism Kevin describes and seen in your photos is unique (more or less) to the Sante Fe's engines and is not shown in the animation.

Learn something new everyday. I think you still have to use some steam because of cooling


High pressure/LOw Pressure cylinders is more a feature on articulated engines IE like the N&W Y6B's



The front cylinders are low pressure and the rear cylinders are high pressure, the front cylinders are larger for a larger surface area for power transfer since the steam is already partly used when recieved from the rear cylinders. The Y6's could run either simple or compound, simple meaning both front/rear cylinders used live boiler steam, in compound is when steam from the high pressure cylinders is sent to the front low pressure cylinders. You no longer get the double stack talk of an articuated engine like this, the engine will sound like a non-articulated engine.

Articulateds with 2 engine units under one boiler were the most common type of compound engine (where steam is partly expanded in high pressure cylinders and then the rest of the way in low pressure cylinders) but not the only kind.

Cross compound - HP on one side, LP on the other.

4 cylinder balanced compound - had two HP cylinders cranking between the wheels while two LP cylinders were in the normal place.

3 cylinder compound - a single HP cylinder cranking between the wheels feeding 2 LP cylinders in the normal place. All cylinders would be of the same or similar size.

Tandem compound - HP and LP cylinders on both sides, one in front of the other with very long piston rods

Vauclain compound - HP and LP above and below each other, tied together at the crosshead and both driving the same main rods.

There are others but this last one, Vauclain, would look very much like what the original poster thought was the case with 3751.

But as others have said, with 3751 and indeed most steam locomotives, that round chamber is the valve not an HP cylinder.

Steve

Kevin is essentially correct, but allow me to add some details. They are specifically called Wagner bypass valves. At the ends of each chamber are piston like valves. A large spring (or springs in the case of 2926) sits in between the two valves pushing them away from each other. In this position, the two sides of the piston are interconnected with the intention that neither compression nor vacuum would develop in the cylinders when drifting down grade without steam. Once the throttle was opened, pipes visible on the outside of the timing valves would route pressure from the valve chamber to the far ends of the Wagners, pushing against the piston like valves which would travel inward compressing the springs and closing the interconnect passage between the main piston faces.

My understanding is that the throttle would periodically be opened on long down grades to refresh the lubrication (though the Santa Fe power that used these had lubrication points directly to the cylinder and timing valves as well as at the steam inlet). Test data showed that some pressure and vacuum would still develop in the cylinder, so it is likely that the periodic burp of steam may have helped relieve the situation...they may have retrofitted stronger springs after the tests I read about to improve their performance, but even so there was demonstrable savings of fuel and water. I have talked to at least one old timer who said they still tended to drift with a light throttle...I know at least one of our engineers has been studying up with the old timers on how best to use them, but am not sure if most of our guys drift with an open throttle or not...

Probably more than you wanted to know, but there you go...

Thanks Wolf,

I was there the day we disassembled the Wagner valves... Great care was taken so that the springs did not "Get Away" (Grin)

BTW have you seen the latest crop of photos, the show the inside of one of the firebox side "Legs" where the side sheet is being replaced. Looks nice and clean.

http://www.nmslrhs.org/images/photos/6-12-10/index.html

Regards,
Kevin