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Efficiency of Steam locos in reverse

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zwk500

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Why so? Are tender locos' reversing gear designed differently from tank engines?
I don't know the ins and outs of it, but it was one of the reasons given to me for avoiding tender-first running when I was involved in Steam Tours with NR. I think it's to do with the Chimney updraft not drawing air as efficiently through the firebox, restricting air for combustion, because of the airflow over the top of the loco. The reversing gear may also had something to do with it.
 

DelW

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I don't know the ins and outs of it, but it was one of the reasons given to me for avoiding tender-first running when I was involved in Steam Tours with NR. I think it's to do with the Chimney updraft not drawing air as efficiently through the firebox, restricting air for combustion, because of the airflow over the top of the loco. The reversing gear may also had something to do with it.
In addition to the effects on draught through the chimney, I'd imagine aerodynamics have an effect as well. Smokeboxes are not very streamlined, but they must be better than the flat rectangle at the back of most tenders, and their area is smaller too. Similarly cab fronts were often angled in later designs, while when running tender first the side sheets of an open cab will tend to scoop passing air in, disrupting the flow.
 

zwk500

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In addition to the effects on draught through the chimney, I'd imagine aerodynamics have an effect as well. Smokeboxes are not very streamlined, but they must be better than the flat rectangle at the back of most tenders, and their area is smaller too. Similarly cab fronts were often angled in later designs, while when running tender first the side sheets of an open cab will tend to scoop passing air in, disrupting the flow.
Given how un-aerodynamic the wheels, valve gear, cab, and all the other stuff on the loco is, I'd be willing to bet aerodynamics have a negligible impact on loco performance, other than by their impact on the draught through the firebox. Especially given the Merchant Navys and West Country/Battle of Britains were rebuilt from Air-Smoothed casing to convetional boiler barrels, and many LMS streamlined locos later had the casing removed.
 

HSP 2

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I don't know the ins and outs of it, but it was one of the reasons given to me for avoiding tender-first running when I was involved in Steam Tours with NR. I think it's to do with the Chimney updraft not drawing air as efficiently through the firebox, restricting air for combustion, because of the airflow over the top of the loco. The reversing gear may also had something to do with it.
In theory a steam loco should be as fuel effective in forward as reverse (think of a Garrett type loco, where one engine is in forward and the other on in reverses). the updraft at the chimney is caused by the blast pipe (the exhaust from the cylinders) not by any air passing over the chimney. So as long as the dampers at the front and rear of the ash pan are of a similar size you will get the same air flow through the fire bed.
I can't remember if B.R. restricted tender locos to 25M.P.H. or 45M.P.H.when running tender first. A tank engine with leading and trailing wheels was allowed to do the same speeds in both directions (there were restrictions to this).
 

norbitonflyer

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I doubt that it makes much difference at low speed, but the design of the top of the boiler certainly had an effect at high speed on the flow of air from the funnel. There is a possibly apocryphal story about the design of the A4 Pacifics that, during windtunnel testing, after several unsuccessful efforts to get the smoke to lift clear, a thumbprint was inadvertently left on the clay model, just behind the chimney. This succeeded in clearing the smoke and was incorporated into the final design.
If running in reverse the shape might be trying to force air down the funnel!
 

MarkyT

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I doubt that it makes much difference at low speed, but the design of the top of the boiler certainly had an effect at high speed on the flow of air from the funnel. There is a possibly apocryphal story about the design of the A4 Pacifics that, during windtunnel testing, after several unsuccessful efforts to get the smoke to lift clear, a thumbprint was inadvertently left on the clay model, just behind the chimney. This succeeded in clearing the smoke and was incorporated into the final design.
If running in reverse the shape might be trying to force air down the funnel!
The primary purpose of smoke lifting and deflection measures in a design is to ensure forward visibility is maintained at all times for the crew. When running tender first, the chimney would be behind the cab so there should be no visibility problems from the exhaust, although the massive tender of an A4 does tend to get in the way instead! Perhaps a rear-facing camera might be concealed in the passageway of a Gresley corridor tender, looking out of the round porthole window and linked to a cab monitor display for working tender first.
 

HSP 2

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In America in the 1930-40s the Union Pacific or Southern Pacific built some steam locos that ran in reverse for most of there mail line work these were called cab forwards.
cab forward steam locomotive - Bing images

These locos were built like this due to a (some) tunnels on the line filling the cabs of normal locos with smoke (oil fumes). these locos did not have a problem with air been forced down the funnel (chimney).

The exhaust steam on a loco working at about 25 mph will be at about 25 PSI above atmosphere so the chance of air going down the chimney is zero.
 

Irascible

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I think the only difference you'll get to steaming if you're running in reverse is a slight ram-air effect into the firebox door - which iis probably not enough to make any difference whatsoever, and if it was could easily be controlled by closing the doors a little. The driver's controls being set up for looking forward would be more of an annoyance I'd think.

There's occasional back-blast issues that can get dangerous to the crew but the whole boiler works because there's gas forced out of the chimney, doesn't matter which way the relative wind is blowing.
 

HSP 2

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There's occasional back-blast issues that can get dangerous to the crew but the whole boiler works because there's gas forced out of the chimney, doesn't matter which way the relative wind is blowing.
In the old day it was called back blast normally caused by small bore tunnels and the front dampers been left open and the blower not been on. You also did not want to much exhaust steam as that caused its own problems.
 

Pigeon

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Nearly always the valve gear actually gives you slightly better timed valve events in reverse, which is kind of daft but is a result of various complicated compromises in the name of practicality which are very difficult to avoid, and were even more difficult to handle before we had computers to give us rapid and precise solutions to the mass of geometrical approximations comprising a valve gear instead of slow tedious approximate ones. So all else being equal a steam engine should actually perform slightly better in reverse. This is a minor justification for the cab-forward idea, and it is a thoroughgoing pain when designing Garratts.

It's hard to say what all else isn't equal, but in general there are lots of compromises in steam engine design and designers would of course always choose the one that gave a better result forward at the expense of backward whenever that was a factor.

Aerodynamics basically doesn't come into it, with the possible exception of different sizes for front and rear dampers. All steam engines are basically rotten aerodynamically whatever you do (the various forms of streamlining were all basically a joke, and a lot of the reason for doing it was fashion), and speeds are rarely high enough for long enough for it to be significant in any case.

But the biggest day-to-day variable in the performance of any given steam engine is the crew (simplest way to get a 10% improvement in anything is to paint it yellow and tell the driver it's an experiment). Crews almost universally hate running in reverse, and it's extremely possible that they simply don't perform as well in reverse because they're having a bad time in the first place.
 

Elecman

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In the old day it was called back blast normally caused by small bore tunnels and the front dampers been left open and the blower not been on. You also did not want to much exhaust steam as that caused its own problems.
Wasn’t it called Blowback as that’s how RAIB refer to it in thier reports ?
 

Loppylugs

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Crews almost universally hate running in reverse, and it's extremely possible that they simply don't perform as well in reverse because they're having a bad time in the first place.
With a tender loco, it was not nice to have coal dust blowing in your face even after attempting to dampen it down regularly. On the western we had 22xx and Manor class locos with small tenders. I often had bits frozen that I never knew existed after running tender first in the wee small hours on a cold winter's night!
 

ac6000cw

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In America in the 1930-40s the Union Pacific or Southern Pacific built some steam locos that ran in reverse for most of there mail line work these were called cab forwards.
cab forward steam locomotive - Bing images

These locos were built like this due to a (some) tunnels on the line filling the cabs of normal locos with smoke (oil fumes). these locos did not have a problem with air been forced down the funnel (chimney).
It was Southern Pacific - they eventually had a fleet of around 250, with the first ones built in 1910 and the last during WW2, big oil-burning Mallet-style articulated locos built by Baldwin. The problem they were designed to alleviate was operating trains through the 30+ miles of snow sheds and tunnels over Donner Pass (between Roseville and Truckee, California) - a steeply graded 80 mile climb for eastbound trains.
 

Irascible

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Nearly always the valve gear actually gives you slightly better timed valve events in reverse, which is kind of daft but is a result of various complicated compromises in the name of practicality which are very difficult to avoid, and were even more difficult to handle before we had computers to give us rapid and precise solutions to the mass of geometrical approximations comprising a valve gear instead of slow tedious approximate ones. So all else being equal a steam engine should actually perform slightly better in reverse. This is a minor justification for the cab-forward idea, and it is a thoroughgoing pain when designing Garratts.

Can you give any more detail on this? I'm intrigued, it's something I've missed somehow. Is it true of all types of gear?
 
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