my understanding is the the reverser/Johnson bar controls cutoff, the percentage of the piston stroke that steam is allowed to enter the piston. The longer the cutoff, the longer high pressure steam is forced against the piston. After cutoff, steam pressure will drop as the volume expands.
but cutoff needs to be reduced at higher speeds to reduce the amount of steam in the piston that needs to be force out when the piston changes direction.
presumably the cutoff can reduced significantly once at the desire speed to maintain that speed.
i've read that there is a throttle that "regulates" the steam pressure to the piston -- limits flow to reduce it to less than full boiler pressure.
while both the throttle needs to be open to allow the highest pressure and cuttoff delayed at long as possible to maximize horsepower when starting, it seems that both cutoff and thottle can be reduced to reduce horsepower.
can someone explain how cutoff and the throttle are properly used?
greg - Philadelphia & Reading / Reading
You can indeed 'regulate' the volume of steam, and therefore the pressure admitted to the cylinders, by 'cracking' open the throttle/regulator. It's like a choking point. But for any one setting of the reverser, the valves will still eventually cut off the admission of steam, and what steam has been admitted must now do all of the work to get the piston to the turn-around.
For short moves meant to be gentle, say coupling to the front of a passenger car, the hogger will crack open the throttle and set the cut-off long, up to 85%. This prevents the drivers from spinning wildly suddenly due to inertia, and the fire bed from being blown apart and hot cinders being sucked down the flues. He'll start out the same way once coupled, but if you watch videos, he'll go through a quick succession of jerks on the throttle as the train gains momentum. Once he's out on the main and working to get past 30 mph or so, he'll have the throttle wide open and let the valve timing (cut-off) do the real work of controlling how much steam has to be admitted to continue accelerating the train. While it may take quite a bit of horsepower to accelerate the train, especially quickly as in commuter service, it doesn't need all that steam once it has the momentum of 'track speed'. He can back the cut-off down to 30%, even less, and let much less steam shove the cylinders forward and backward in rapid succession.
This had been done for the better part of a century by using the reverse lever to shorten the valve travel. The crack "Throttle Artists" always thought they could tune it close enough to get the maximum compination of speed, power, and efficiency from an engine by carefully "notching back the Johnson Bar." Of course, they could feel the onset of a gradient and would have to gradually notch the lever back enough to maintain the speed and power. I never got to watch one operate, having only spent a half-hour or so in the cab of a Soo Line F7 while the crew picked up a baggage car of Railway Express from the House Track. (As Mork from Ork used to say, "Heavy sigh...")
Deano
One of the many booklets I'm fortunate to have in my little library is a 1901 issue from the New York Central & Hudson River Railroad called Fuel Economy.
I just scanned several pages that deal with cutoff. You can see the vintage of the locomotives as they use 145 PSI as the boiler pressure. 1901 was a year before the Century was inaugurated!
The pages I've scanned can be read here:
https://www.flickr.com/photos/gmpullman/albums/72157706562119484
NYC_Firing-Economy-cover by Edmund, on Flickr
NYC_Firing-Economy_58-59 by Edmund, on Flickr
Rather than take up too much space here, you can go to the album at Flickr and read the pages there.
Hope that helps, Ed
thanks for the responses. I found a book describing how much of an art the fireman's job is, balancing the coal bed and anticipating the need for steam. Seems that the engineer's job is also an art
Ed,
thanks for posting scans of the book. it looks promising
Deano,
what do you mean by "gradient"? (i have a math definition in mind)
Selector
i believe the cutoff should be maximized when starting. But your mention that the throttle limits the steam pressure controling the acceleration gives me a better understanding of how the throttle can be used
it's not clear to me if cutoff is simply determined by speed or with the notion of the steam volume that must be expelled from the piston. I found steam described in terms of pounds in the book posted by Ed. I think they're actually describing the weight of the steam, not its pressure.
gregcI think they're actually describing the weight of the steam, not its pressure.
Correct. Because they are describing steam, which varies in density by temperature and pressure, the consistent "quantity" is weight. Plus since the thrust of the whole thing is economy, a lb of steam = a lb of water and it takes a certain amount of fuel to raise a lb of water to the temperature and pressure in the boiler. Buy using "pounds" it is easy to illustrate fuel and water waste.
One of the first things that we learned to use in college thermodynamics were the "steam tables".
Dave H. Painted side goes up. My website : wnbranch.com
gregc Seems that the engineer's job is also an art
And the engineer could make the fireman's job miserable if he wanted to. By not "hooking up" soon enough he would be using more steam, thus more water. The more water the fireman had to put into the boiler the more coal he had to shovel to make it into steam. Some engineers didn't let the fireman use the stoker. "We didn't have no stokers in MY day — YOU don't need one, either.
If the engineer slipped his drivers the whole fire, if a light one, could be pulled right off the grates.
There were SO many other variables, too. Bad coal (clinkers and poor firing) being one and bad water (high mineral) being another. In later years some of the steam engines had so many leaks, especially staybolts leaking water into the firebox, plugged flues, plugged screens in the "front end" (smokebox), plugged superheater tubes, piston packing leaking so bad you couldn't even see the road ahead, broken grates (hole in the fire) and scale and mud built up in the low parts of the boiler. Boilers were supposed to be washed every 30 days but toward the end, that was only a half-hearted effort.
There was definitely more of a relationship between man-and-machine back then and some knew how to make peace with the beast and others didn't. Or maybe didn't want to bother.
There are good animations of the function of the locomotive valve gear here:
http://www.steamlocomotive.com/appliances/valvegear.php
There's scores of books out there in the study of steam locomotive design and operation. Here is a good one to start with:
https://tinyurl.com/ybcqybkg
IF you're in the mood for reading, the International Correspondence Schools Archive has many of their Railroad "Blue-Book" series available as .pdfs.
Scroll down to the 500 series and click to your heart's content. Locomotive Management is a good place to start but you can easily get addicted to these:
http://www.icsarchive.org/icsarchive-org/bb/
Cheers, Ed
seems to me that there once the loco is in motion, either cutoff or throttle can be reduced. In other words, there may not be one best setting.
gregcseems to me that there once the loco is in motion, either cutoff or throttle can be reduced.
Not unlike the throttle and gear shift in your car.
BUT if you are fighting a grade, like headed up the Allegheney summit out of Altoona, or perhaps Sherman Hill on the UP you might have had two, sometimes three locomotives all pounding away with wide-open throttle and the reverser "in the corner" at least for much of the trip.
In constant flux to respond to the contitions at hand. Some railroads adopter the Loco-Valve Pilot* as a watchdog and assistant in the setting of the reverser and throttle. It had some advantages but was difficult to maintain and it really didn't make good engineers out of bad ones. Getting the train over the road was a balancing act. Sometimes you had a good engine, good fuel and water, a good crew and good track conditions.
Put a clinker in any one of these and you're going to have to work harder for your day's pay.
Valve-Pilot by Edmund, on Flickr
Valve-Pilot_0001 by Edmund, on Flickr
Regards, Ed
gmpullmanNot unlike the throttle and gear shift in your car.
that's rather course
i think a better comparison would be between the throttle and continuously variable transmissions if the CVT didn't actually have predefined positions.
part of the problem is what are the optimal throttle and cutoff setting for any condition. Of course they're both pretty high going up a steep grade at slow speed.