I recall reading an article about backpressure but I cant find this article in our magazine archive. Any information on backpressure or articles out there?

> I recall reading an article about
> backpressure in regards to operating a steam locomotive but I cant find this article in
> our magazine archive. Any information on
> backpressure or articles out there?

if answered correctly this could take a few pages. I will only answer in a brief explanation.

Back Pressure is basically the steam left in the cylinders of a steam locomotive during the exhaust cycle. This can be registered on a back pressure gauge or heard by a good steam engineer. This back pressure can actually rob the locomotive of performance. Cures to excessive back pressure are hooking up the valve gear properly or if mechanical then the exhaust nozzle size needs correcting. There is much more to this such as correct cylinder size in relationship to the valve ports, Lap and lead, exhaust nozzle size etc. etc.I am sure there are readers on this board who are capable of a much more concise and in depth answer.

It can get pretty esoteric. If you read the old steam journals, there is some talk of the beneficial effects of retaining some back pressure to "cushion" the end of stroke and make the change of direction less stressful.

More modern engineers talk about the compression of remaining steam in thermodynamic terms, and the efficiency of having the clearance (space between the piston at end of stroke and the head) and the passages between the valve seat and the cylinder full of steam from the previous stroke before admission for this one as opposed to more lead and the cost in power of compressing this bit of fog. Hugh Odom can better explain this than me.

I seem to remember a TRAINS article about 4449 and 844 leaving Sacramento simultaneously and 844 losing ground until she was dropped down closer to the corner, and ran faster with the increase in gage back pressure. Maybe I am confused? Bob?

Locomotives power outputs are so varied that fine tuning them for efficiency at a particular output isn't practical. Many stationary and marine steam plants exhausted into a vacuum to create negative back pressure to squeeze the last bit of expansive power from the steam. Of course, these systems didn't rely on the use of expanding steam in the blast pipe to create draft either.

Dave

irondave@bellsouth.net

> As I recall, this subject was addressed in an issue of Railfan Magazine from the mid-80's. I believe the same issue compared a couple of mainline steam locomotives operating at the time, included an article about a New Jersey shortline operating Baldwin diesels, and the impotance of the timely completion of smokebox patches.I recall reading an article about
> backpressure but I cant find this article in
> our magazine archive. Any information on
> backpressure or articles out there?


tom@buis-cs.com

Article has been located! It is a large footnote in an article by Rich Melvin about the comparison of 2 steam locomotives operating characteristics. The article is titled Will The Best Steam Locomotive Please Step Forward, and was in the March 1995 Railfan & Railroad magazine. Thanks to all replies. Hi Bill!

> It can get pretty esoteric. If you read the
> old steam journals, there is some talk of
> the beneficial effects of retaining some
> back pressure to "cushion" the end
> of stroke and make the change of direction
> less stressful.

Absolutely; it also has an effect on the draft.
An old rule of thumb was "not more than 12 pounds nor less than 2 pounds." At 0 back pressure, or negative back presure, the action of the pistons can pull cinders or sand from the exhuast into the cylinders, which can cuase a lot of problems.

> I seem to remember a TRAINS article about
> 4449 and 844 leaving Sacramento
> simultaneously and 844 losing ground until
> she was dropped down closer to the corner,
> and ran faster with the increase in gage
> back pressure. Maybe I am confused? Bob?

Yeah, you are confused. It was 1989 when 8444 and 4449 left San Bernardino at the same time and ran up Cajon Pass. The 4449 crew had to flog their engine a bit and run at highher back poressure than they were used to in order to keep up with 8444.