I’ve read that steam locomotives are less efficient when running at low speeds, why would that be? And how does steam locomotive efficiency correlate with speed in general?
One contributing factor is a practice called “cutoff”. When a locomotive is just beginning to move, the “Johnson bar” would be put into the full forward position, letting the slide valve by the piston make a long stroak. This allows more steam to enter the cylinder in the beginning of the run, enabling more power to gain momentum. When the locomotive reaches a higher speed, however, the “Johnson bar” lever is pulled back slightly, decreasing the stroak of the slide valve. By doing this, the locomotive is made more efficient. This is because less steam is being sent into the cylinder due to the increased cutoff, and the expanding power of the steam is now doing a good deal of the work. This practice saves steam when the locomotive is working fast, but you could not start a locomotive with “cutoff” because of the weight behind the locomotive. So basically, a locomotive is less efficient starting off because it is using all its steam to gain momentum for its train. A locomotive is more efficient when it’s up to speed because the “cutoff” is saving steam in the boiler, and letting the expansive power of steam push the piston. Hope that helps! -W. Dancey
I was just thinking about the same question.
Generally speaking, a steam locomotive is most efficient when 1) steam is supplied to the cylinders at a very low cutoff to provide maximum expansion of the steam, and 2) the boiler is operating at its rated capacity for steam generation. These conditions are met at the upper end of the speed range of the locomotive, where the boiler is about able to keep up with the steam demand, that is, when the driver RPMs are high and the cutoff is low (say, at 25% in contrast with 80% at starting).
The locomotive is least efficient when starting and operated at high cutoffs (say, steam is admitted for fully 80% of the piston stroke). This gives the maximum tractive effort, but it sure uses a lot of steam and hence energy, and since this is at low speed, it doesn’t represent much horsepower.
The thing puzzling me relates to Fig. 82 on p 267 of David Wardale’s The Red Devil and Other Tales of the Age of Steam. Yes, an incredibly hard-to-find book, but there was a recent reprinting and there were posts on the Trains forum regarding where to order a copy.
That chart shows tractive effort on the vertical or y-axis as a function of locomotive speed on the horizontal or x-axis. The “islands” of constant thermal efficiency are plotted on that x-y scale. This chart is much like the torque-RPM “map” of an automobile engine also showing islands of constant thermal efficiency.
Having worked a long time ago for a major auto company, I can tell you that the auto engine map is measured in an engine dynamometer test cell. Race car teams use such a “dyno” to tune their race engines. Such a test appliance is called an “engine test plant” when it is applied to a whole locomotive, and the Pennsylvania Railroad, back in the day, famously had such a facility in Altoona. Wardale didn’t have such a “steam locomotive dyno”, and he shows results in
The responders are correct but the question remains. Why was the Johnson bar shoved to the corner? It was up to the engineer to limit the cutoff. The PRR built 598 I1s 2-10-0 locomotives with 50% cutoff as full throttle and later increased to 65%. They lasted until the end of steam when newer and more modern locos were being cut up for scrap. With 90,000 lbs of tractive effort at 20 revolutions there were very few 2 cylinder locos more powerful. Tractive effort fell off as speed increased so in essence they were more efficient at slower speeds.
Some steam locomotives were built to be efficient at slow speeds over faster speeds such as the I1s. The question should be. Why were not all locomotives built with limited cutoff?
Pete
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The 2-10-4 J’s lasted as long as the I1’s. In fact, PRR leased AT&SF Ripley 2-10-4’s to work along side its J’s on coal traffic while scrapping any I1’s that needed work. And all this while total dieselization had already been set as a goal.
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The I1 was designed as a low-speed locomotive with high efficiency at low speeds. This mean very large cylinders and small driving wheels as compared to boiler size. Wihtout limited cutoff, it would have been very easy to use full throttle, Johnson Bar in the corner, and simply spin the wheels by tryihg to obtain tractive effort beyond the capabilities provided by the factor of adhesion, and a the same time rapidly deplete boiler steam.
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All locomotives have limited cutoff to some extent. But a locomotive designed for greater efficiency at higher speeds, with boiler size, cylinder size, and driving wheel size, needs to have steam in the cylinders for a longer period of time on each stroke to have the needed tractive effort to start the train. This means reduced efficiency at low speeds and starting, but all possible tractive effort and power at those low speeds . (But still less than rated power, which is obtained only at higher speeds, but with reduced tractivfe effort.)
And why were many engines (like most I1s) that were built with short cutoff converted to longer cutoff? In the 1920s limited cutoff looked good on paper, but apparently it didn’t work out.
The reasons for short cutoff, at least as applied to the I1, were explained in my post, and so were the reasons for not applying short cutoff. Limited cutoff reduces the flexibility of the engineer’s judgement. If engnineers are well trained, they will have the Johnson Bar in the corner only when maximum tractive effort is needed (and conditions are such as remove slipping possibities, sanders working, or dry rail, etc.), and move it to shorten cutoff rapidly as speed bills up to conserve steam and improve efficiency. In the case of the I1, the PRR found it could start heavier trains and increase tractive effort, without always spinning wheels, by lengthening cutoff from 50% to 65% (still fairly short for maximum cutoff), and that its engineers would use this flexibility intelligently. Also.a few I1’s may have outlasted the last J in service, but only because so many I1’s were built.
The last revenue mile pulled by a steam locomotive on the PRR was pulled by an I1sa. Remember that these were built in the drag freight era. Long and slow was the rule of the day and trains pushed and pulled by multiple locomotives. The fast freight era was just starting out when diesels were coming on the scene. The J1 and the oil burning leased locos were better on the flat lands that ran shorter but faster freights. The taller drivers were better at faster speeds but needed much more power to get them started, thus used more steam to start.
We can debate limited cutoff, cylinder size, and driver size forever but I believe that the steam locomotives that were built were the best that can be done with steel wheel on steel rail. Without beefing up bridges and roadbed the maximum amount of weight on drivers will always be the limiting factor.
Pete
I “get” the part about a high-speed locomotive operated at steam-saving short cutoff at high speed, at a somewhat steam-wasting long cutoff at low speed to get enough tractive effort out of the comparatively small cylinders, short stroke, and large wheels so as to start the train.
I noticed, however, in Wardale’s charts that cutoff being equal, efficiency appears to drop off at lower speeds. That is, “cruising” at a moderate level of tractive effort at 20 MPH is less efficient than at a similar level of tractive effort and hence cutoff percentage at 40 MPH. What gives?
Is this a heat loss effect? That is, the steam sits in the cylinder long with each stroke and more heat is lost? Is this a boiler “turn-down ratio” effect? That at the same cutoff, at slower speeds you are drawing less steam from the boiler, and the boiler is less efficient at low rates of evaporation?
The argument has been made that the last steam locomotives were far from the best possible, that considerable improvements could have been made, even without going to “exotic” stuff like condensing cycles, steam turbines, and electric drives.
The people making that argument were Andre Chapelon in France, Livio Dante Porta in Argentina, and David Wardale, working in South Africa, the U.S. (on the ACE 3000 project), in China (on improving the QJ class) and most recently in England (on the 5AT project for a modern new-built passenger-excursion locomotive). There are others, but these are the main one’s who have written about their work – Chapelon through a book, Porta through numerous papers, Wardale also publishing a book.
If you are limited by weight on drivers to get the tractive effort, you can build a locomotive with more drivers, and the same bridge limits come into play with Diesel multiple units having a large number of axles.
It is not that the people building steam locomotives didn’t know what they were doing, but stationary power plants started out with equally low thermal efficiency to steam locomotives, and power plant and steam locomotive thermal efficiency improved over time, with advances in the science of thermodynamics as well as in materials. The thing is that steam locomotive development just quit whereas power plants went on to turbines, condensing, superheat and reheat, compounding, pulverized coal combustion, stack scrubbers.
Wardale’s view on why steam locomotive development “just quit” is that even when Diesel traction was just an experiment in the
Fascinating stuff, Paul. Power plants are switching rapidly to natural gas, not because of the boogeyman environmentalists (although much cleaner-burning than coal at the plants, some claim large emissions at the gas well heads) you seem to have an aversion to, but because it is cheaper and they want to maximize profits.
Seeming averse to bookeyman environmentalists, I’ll tell you all you need to know.
You think you can build even a natural-gas fired electric plant without opposition?
Some independent power company thought it could build such a plant – the Rockgen facility, gas-turbine “straight” cycle, peaking duty – in exurban Dane County (outside Madison, WI). Every self-styled environmentalist in the 5-county region was protesting that one, but they did get it built.
You think at the “U” we sit here in our ivory towers on Linden, smug in the belief that we are a “soft” “post-industrial” “knowledge industry” beyond criticism from the environmental lobby? Our then-chancellor, an Electrical Engineering professor, informed our faculty senate that laboratory fume hood “make up air” alone, accounted for 60 percent of total campus heat usage. These fume hoods make it safe for lab workers to do stuff like develop stem cells as cures for human disease, come up with treatments for childhood cancers, stuff like that. Prior to the natural gas conversion of the Charter Street central heating plant, those activities accounted for as many as 3 railroad cars of coal per day. You don’t think of such cutting edge lab work as a dirty, industrial activity, but it can be.
The MGE power company and the “U” partnered on displacing some of that coal usage as well as building reserve capacity by expanding the Walnut Street plant with a state-of-the-art gas-fired 150 MW electric co-ge
Everyone wants magic. There’s no magic. There are many infants born each day. 60 years ago the Children’s World Book of Knowledge claimed that 47,000 babies were born EACH DAY in India…alone.
I don’t know that any magic or technology is going to outrun our own hormones.
-Crandell
In answering your very specific question, Paul, I think you stated the answers yourself, your guesses are correct. Which is the major heat loss, I suspect Juniatha would have a more exact answer.
Incidentally, from my own observation of conversations with engineers who had steam experience, each locomotive was slightly different, even with the same class, and a good engineer would get the feel of the right combination of throttle position and Johnson Bar posiiton to make the schedule or make up time and still conserve fuel and water, for the different conditions of track speed, grade, and load.
Diesels in good condition are much more identacle, if of the same exact type.
On modern steam locomotives, the power reverse was used to adjust cutoff. In my viewing of in-cab videos of N&W 611 and 1218, I saw the engineer started with the reverse lever in “full forward” and would adjust it back according to how the engine was running. He had to train his ear to listen for when he could adjust the cutoff. I have heard that most of the operation of a steam locomotive was done by ear, as the crew used the sounds the locomotive was making as cues in their operations in the cab. In this way a good engineer was tuned in to the locomotive, and he and the locomotive could be said to be working together as a team. lois
This is true, the ear-tuning, Lois, although a recent edition of Classic Trains had an article about the Valve Pilot used on the NYC’s Hudsons, for one example, that taught everyone who ran and designed steam locomotives how the locomotive could be made even more efficient that an experienced hogger could make it using his ear.
-Crandell
Adjusting the cutoff is the most understandable response yet. On my last cab ride, I was told “stand there” and watch in silence. Once the throttle was opened, the engineer simply worked the train with the reverse handle (cutoff). That was all he needed for to adjust for speed and gradient, etc.
Last time I paced, it seemed to take 2 miles or more for 765 to get up 30 MPH. Suddenly in less than a mile the engineer pulled away from all the traffic like shifting into overdrive. The acceleration curve “seemed” to increase in the 45 MPH range. NKP got good performance on manifests with relatively short and consistent train lengths (maybe 50 cars, lots of produce) relatively mild grades and 60 MPH operation. Efficiency was achieved by engineers who knew the territory and the individuality of the locomotives. It has been written that they could tell you which group of the 700’s had the best reputation and which of that group were the favorites. Other less favorite 700s, were notoriously “consigned” to the Wheeling District in respnse to the ebb and flow of traffic.
[quote user=“STCALRR”]
Adjusting the cutoff is the most understandable response yet. On my last cab ride, I was told “stand there” and watch in silence. Once the throttle was opened, the engineer simply worked the train with the reverse handle (cutoff). That was all he needed for to adjust for speed and gradient, etc.
Last time I paced, it seemed to take 2 miles or more for 765 to get up 30 MPH. Suddenly in less than a mile the engineer pulled away from all the traffic like shifting into overdrive. The acceleration curve “seemed” to increase in the 45 MPH range. NKP got good performance on manifests with relatively short and consistent train lengths (maybe 50 cars, lots of produce) relatively mild grades and 60 MPH operation. Efficiency was achieved by engineers who knew the territory and the individuality of the locomotives. It has been written that they could tell you which group of the 700’s had the best reputation and which of that group were the favorites. Other less favorite 700s, were notoriously “consigned” to the Wheeling District in respnse to the ebb and flow of traffic.
Interesting. This confirms the graph I saw years ago between steam and diesel locomotive of 6000hp. The diesel had enormous starting tractive effort (which was electric traction to be exact) and immediate access to its horsepower. The steam locomotive starts with less tractive effort and lower horsepower but the steam locomotive generated an inverted-U horsepower curve that peaked at 60mph. It crossed the diesel curve, rising at 30mph and eventually coming down at 90mph. That explains why the steam is a slow starter and frankly poor on grades relative to electric traction whereas between 30mph and 90mph in runs away from electric traction. If you remember the Reading T-1s, they were slow starters but once above 30mph they ran
Oops , just saw this thread now - cats can be so incredibly slow sometimes …
Dunno if its still waranted to sew up something , a number of points has been mentioned already …?
As I generally say :
nothing is for free
in technology
Regards
Juniatha
View the power and torque curves of internal combustion engines - the same types of curves apply to steam engines.