Pressure at 8500' msl is about 71% of standard pressure, the half standard pressure elevation is about 17000' msl. A quick (and possibly inaccurate) look at a steam table suggests at most a 2% increase in efficiency (20% to 22%) at 8500'.
Blue Streak 1- thanks for an interesting question, and thanks Erik Mag for the additional info. Sorry I’ve not been paying attention.
I think the way it works is this. Although back pressures are usually quoted in units of psig, what really matters to the engine is units of psia. So, at 8500 feet, back pressure will be reduced by 4.2 psia. I don’t think there are any US mainlines at 8500’, but there are stretches at 7500’, where I will guess that backpressure will be reduced by 3 psia.
If we take our trusty K4 it will at sea level be working at a back pressure of about 22psia at 2500IHP and 75mph, consuming about 42600lb/hr steam (11.7% efficiency). At 19 psia back pressure it would produce about 2720IHP at 44300lb/hr steam (12.3% efficiency) at the same cut off, i.e. be about 5% more efficient, running e.g. from Riordan to Supai on the ATSF.
Unfortunately, it would struggle to get up there, so we need to consider a 2900 4-8-4. At what I think is a normal maximum service rate, back pressure would be about 22 psia also, working at 4500IHP and 75mph setting of from Needles down to Topock, 61900lb/hr steam, 13.9% efficiency. Once it got to the top at Supai it could cruise at 75mph (if the track allows), producing about 4770IHP at 64200lb/hr steam, or 14.3% efficiency, about 3% greater.
So, yes, there will be a small benefit compared to what you might measure on a sea level test plant at this high altitude. But there are many other factors which make test plant data not a reliable predictor of what happens in service, some greater than this. Test plant data are useful I think for establishing relative performance levels, all other things being equal, and, being collected under carefully controlled conditions extremely valuable in checking out theories on locomotive behaviour. They can be very misleading when locomotives are pushed to their limit, way beyond what is practicable in daily service.
An altogether different question is how the boiler, in particular the draughting, would cope with lower atmospheric pressure. Quite complex I think, but automotive engineers must know all about this, because I’ve driven standard cars up to 12000’ without problems!
So how much more efficient do you figure it would be at 40000 ft altitude? (Standard-day atmospheric pressure 2.7 psi)
Sorry, Tim z, can't do calculations at less than 14.7 psia back pressure. So the best i can do is about 17000', where the efficiency of the 4-8-4 would rise to 14.8%. Need to rustle up some funds to send 2926 to the Qingzang Railway to find out.
As a boy I read a story in a Sci Fi Mag where some aliens captured what looked like a nuclear powered version of the PRR S1, (an S1A maybe?) and dropped it on the moon.
I always thought this would be a rather splendid place to run it- no air resistance, low gravity and back pressure - only snag being the lack of water there.
I can't remember how the earth was saved, or who did it, but I'm sure the PRR got its S1A back.
Dreyfusshudson ...As a boy I read a story in a Sci-Fi Mag where some aliens captured what looked like a nuclear powered version of the PRR S1, (an S1A maybe?) and dropped it on the moon. I always thought this would be a rather splendid place to run it- no air resistance, low gravity and back pressure - only snag being the lack of water there. I can't remember how the earth was saved, or who did it, but I'm sure the PRR got its S1A back.
...As a boy I read a story in a Sci-Fi Mag where some aliens captured what looked like a nuclear powered version of the PRR S1, (an S1A maybe?) and dropped it on the moon.
That Sci-Fi Magazine really had a good taste in futuristic design! The S1 itself already looks like a nuclear rocket or nuclear-powered vessel for me. When I look at an HO scale brass model of it, those seams and lines on the streamlined shrouding make it looks like something from the year 2046.
Although the S1 was scraped 73 years ago, I believe S1 and PRR's duplexes stories will be continually remembered because of the coming back of T1.
Jones 3D Modeling Club https://www.youtube.com/Jones3DModelingClub
Ref “Caprotti poppet valves might be more steam tight than the Lentz arrangement because their weight is not carried on the valve spindle”... Can you explain this and where you get this information from as it is a serious functional defect – considering your Niagara’s were clocking around a 1000 miles day for almost a year on piston valves. Obviously the new P2 is using the Franklin B version of the Lentz gear (and I believe the T1 is similar) which is an allegedly beefed up version of the Lentz gear Greasley used. Beafed up would therefore suggest this weakness was overcome?
I can't easily see the statement you're referencing but it almost certainly refers to BRITISH Caprotti, which is a completely different arrangement from Len(t)z whether OC or RC. As with Cossart drop valves, the spindles are vertical so there is no eccentric wear of the glands over time, etc.
The Niagara piston valves were ring-borne, and this among other things limits the practical superheat that can safely be used in them. However by the time you get up in the speed range that some people (Arnold Haas for one) attribute to them, since there is no proportioning superheater damper the amount of superheat would have gone surprisingly high. I suspect a similar effect is the actual proximate cause of the valve failure that ended the PRR high-speed N&W J testing.
T1 5550 will receive a version of Franklin B-2, which is a RC valve drive acting (via bridges) on the 8-valve-per-cylinder type A valve arrangement. This is NOT the 3-valve type B as applied to ATSF 3752 -- that was brought up and rejected fairly early in the engineering committee discussions (which should still be in the repository). There is considerably more to the proposed 1948 revisions to the T1 arrangement than 'beefing up' the arrangement Gresley used -- Baldwin heavily touted that arrangement after they got the rights to it in the late '20s, even offering it on narrow-gauge power, and I know of no case where that gear, built as well as Baldwin could cost-effectively make it, survived for any particular time in actual service.
Thank you. I seen the lentz and caprotti valves on the lner historical site. I worked for BR as locomotive engineer (mechanical ie not a driver) and BR had very good experiance with caprotti valves particulerly on 71000. So whats your experiance on locomotives and can you send me the details on the new Frankline valves you mention ie 8 valves per cylinder please
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