How do smoke deflectors work?

[mainemandean]

What forces the wind upwards?  Seems like it would just pass behind the deflector, mostly unchanged in direction.

[tomikawaTT]

What forces the wind upward is the deflector itself - preventing the air displaced by the smokebox front from spreading sideways.  Then, too, the walkway steps narrow the available flow area and deflect the flow upward instead of straight back.

And then there is the rather small, but effective, smoke lifter found on Espee locos with skyline casings.  The casing around the stack is shaped something like a dust pan, and does have a ramp at the back.

Chuck

[selector]

Yes, that's about it.  The deflectors are long enough that their leading edges lead the face of the smoke box by a couple of feet or more in most cases.  As the train gains speed, the shape of the bow shock changes, and this strong force causes the air through which the train passes to move in vortices, but especially to "step aside".  Part of the stepping aside is also stepping up over the smoke box and stack since that air is also displaced.  That top air is compressed somewhat in being displaced, which makes it denser and more capable of forcing the rising smoke and steam to lie flatter longer along the boiler.  A lot of it gets blown right into the cab which is not exactly an ideal situation.  The lifters (elephant ears in common use) act as gates on a table saw, guiding much of the displaced air and making it conform to a path parallel to the axis of advance down the rails.   This air is also somewhat denser, or compressed, and acts like a keeper to the steam and smoke that would slowly drift downward over the flanks of the boiler.  It keeps the smoke aloft, in other words.

Part of the actual lift component comes from what is below and in front of the lifters.  There are usually steps at an angle between the boiler step plates and the pilot, and closer to the ground and rails is the pilot (cow catcher) normally shaped like a wedge.  Between these two items, air low to the ground is wedged upward with some velocity.  Even in the case of steamers with twin mounted air pumps behind vertical fairings (Berkshires, J1 Hudsons)  the tops of those fairings are usually angled.  The lifters don't do more than ensure these upward winds don't also drift sideways nearest the boiler...the same parallelizing effect is given to those updrafts.  When combined through the lifters lengths, the various air components have an effect of wafting upward, taking much (not all) of the smoke and steam with them.

Crandell

[timz]

mainemandean

What forces the wind upwards?  Seems like it would just pass behind the deflector, mostly unchanged in direction.

As I understand it you are 100% correct. The "deflectors" don't lift anything-- they just force the air to flow along the side of the boiler. To do that effectively, they need to extend ahead of the smokebox-- not just start alongside it.

There's no need to lift exhaust smoke-- it's already up there. What the plates are supposed to do is keep the smoke from being sucked down into the low-pressure areas that form alongside the boiler. So, get the air flowing along the side of the boiler (so no eddying is possible) and the problem is solved.