In thinking about the “might have beens” if steam had been kept longer and thinking about the “might yet happen” if we run out of oil and the ability to run locomotives on coal or pelletized switch grass becomes a necessity, I have run across an appliance called a steam turbo charger.
The concept is posed in the context of compound expansion, but what you do is use the spent steam of the LP cylinder to run a turbine, and that turbine in turn drives a compressor to boost the pressure of the HP cylinder exhaust before sending it to the LP cylinder.
When I first read of it, it sounded all confusing and complicated, but if you understand what a turbocharger does on a Diesel, it becomes straight forward. A turbocharger is a little jet engine, if you will, with a compressor connected to the same shaft as a turbine and without a power takeoff from that compressor-turbine spool. You take the compressor output as the input to a piston engine, and you feed the piston exhaust to the turbine, and the net effect is to run the piston engine as if it is running in a much denser atmosphere, giving much more power for the displacement of the piston engine.
Here, you are doing the same thing with steam instead of with air. In principle you don’t need compound expansion – you could apply a steam turbocharger to a simple expansion steam engine.
Why would you even want such a gadget? One of the issues with steam locomotives is that when you need a lot of tractive effort for climbing a hill or accelerating a heavy train, you operate at very low cutoff settings, sending exhaust steam with a lot of pressure left in it up the stack making for those picturesque photo runby’s of clouds of steam and drafted cinders flying up the stack in loud puffs. With the steam turbo charger, you could recover that wasted energy from the cylinder exhaust and use it to boost the intake pressure to the piston to get even more power from a given size steam cy
IIRC, the exhaust from a diesel engine has no job left to do on its way out the stack. Therefore, an exhaust turbochager taps residual energy that is otherwise wasted. The only downside is potential backpressure in the exhaust system.
Things are different on a steam loco. The exhaust does have something to do - provide draft for the firebox. Anything that compromises this job will adversely affect the steaming ability of the boiler. If to much exhaust steam is used for other purposes, the number of lbs of steam available to produce draft will be reduced.
One of the many problems of the often-dicussed triplexes (Erie, Virginian) was caused by 1/2 of the exhaust steam being wasted through the tender. It went straight to the atmosphere without providing draft.
Certainly not the whole answer, but something to think about.
The one possible refinement to the concept would be to put the superheater on the high pressure side of the turbocharger compressor – generally in heat engine cycles, you want to add heat on the high pressure side of things.
There is no perpetual motion involved any more than turbocharging a Diesel – it is using the pistons at what they are good for, compressing and expanding gas at high density and pressure, and using turbines and turbo compressors at what they are good for, compressing and expanding large volumes of gas (steam in this case) at lower pressure ratios.
Also, all of the turbo exhaust goes up the stack for the boiler draft, and the idea is to couple this with the more efficient ejectors.
And the steam mechcanics thought maintaining the internal third-cylinder was a nightmare. Attempting to add a turbo and keep it opperating efficiently… good luck with that one. I’ll just stick with my N&W moderern steam locos.
A turbo-charger on an infernal combustion engine is used to extract more of the possible energy from a given charge of fuel by providing more oxygen to the fuel than what can be obtained due to not supplying enough oxygen to the fuel without the tubby-charger. The higher pressure also puts more oxygen in closer proximity to the fuel to increase the rate of energy extraction so more of it gets used before the exhaust valve opens to release the pressure created.
The blower on a steam locomotive IS the “turbo-charger” of sorts as it puts more oxygen into the fire to extract the additional energy that is potentially available there.
Trying to add additional power to the steam already produced by the fire by using just the energy in the steam is an attempt to make perpetual motion and is doomed to failure.
If you use the energy in the steam to move the steam then there is less energy in the remaining steam to perform any other work.
A smaller cylinder is operated at lower cutoff, producing a higher pressure exhaust, part of the energy of which is used in a blowdown turbine with the remainder of the spent steam going into one of the considerable-higher efficiency ejector nozzles that were never adopted in US mainline steam practice in order to maintain the boiler draft – no perpetual motion is involved. The steam turbocharger is proposed as a way of getting the thermodynamic and mechanical benefits of increased steam pressure without going to a high pressure boiler, which always had been a problem in railroad applications – there is no end run around the Thermodynamic Laws, only an end run on the limits of boiler pressure. The turbocharger can sit in front of the smokebox in plain view instead of tucked inside the frame to drive a cranked axle of the third cylinder, widely used in European steam but disdained in American practice for maintenance considerations.
Were one to build a new generation of steam locomotives on account of peak oil (and while it is argued that the coal would be used to make synthetic oil, the net energy return from making liquid fuel from coal and using a Diesel may be comparable to burning the coal in a steam locomotive and cutting out the middle man of a very complicated refinery process), there is a continuum of ideas ranging from “bring back Norfolk and Western’s J, A, and Y class locomotives” to exotic power plants on wheels like that Union Pacific steam-turbine electric to NW’s Jawn Henry.
In the context of the ACE 3000, I am sympathetic to L. D. Porta’s idea of building an advanced generation Stephenson-style cylinder and rod drive with ejector boiler draft steam locomotive before going on to something like the ACE 3000 involving condensing tenders and mechanical
A smaller cylinder is operated at lower cutoff, producing a higher pressure exhaust, part of the energy of which is used in a blowdown turbine with the remainder of the spent steam going into one of the considerable-higher efficiency ejector nozzles that were never adopted in US mainline steam practice in order to maintain the boiler draft – no perpetual motion is involved. The steam turbocharger is proposed as a way of getting the thermodynamic and mechanical benefits of increased steam pressure without going to a high pressure boiler, which always had been a problem in railroad applications – there is no end run around the Thermodynamic Laws, only an end run on the limits of boiler pressure. The turbocharger can sit in front of the smokebox in plain view instead of tucked inside the frame to drive a cranked axle of the third cylinder, widely used in European steam but disdained in American practice for maintenance considerations.
Were one to build a new generation of steam locomotives on account of peak oil (and while it is argued that the coal would be used to make synthetic oil, the net energy return from making liquid fuel from coal and using a Diesel may be comparable to burning the coal in a steam locomotive and cutting out the middle man of a very complicated refinery process), there is a continuum of ideas ranging from “bring back Norfolk and Western’s J, A, and Y class locomotives” to exotic power plants on wheels like that Union Pacific steam-turbine electric to NW’s Jawn Henry.
In the context of the ACE 3000, I am sympathetic to L. D. Porta’s idea of building an advanced generation Stephenson-style cylinder and rod drive with ejector boiler draft steam locomotive before going on to something like the ACE 3000 involving con
“Tubby”-charger? A supercharger powered by fat?[:P]
FTR, I understand that a turbocharger is simply a supercharger that operates from a turbine turned by the pressure of exhaust fuel rather than being linked to the crankshaft. Lower-pressure cylinders (usually between 8.0:1 and 8.5:1, but not as high as 9:1 on street vehicles) are used with turbocharged automobile engines. Nowadays, turbochargers have been largely superseded by multi-valve cylinder heads (i.e. more than two valves per cylinder, usually two intake and two exhaust).
BTW, what’s an “infernal” combustion engine…an engine built in Hell? [}:)]
Comparing a diesel engine with a coal fired boiler is a no-no. They do not function the same way. The diesel converts the thermal energy directly to mechanical motion (pistons), while the boiler heats water, to form steam (or just hot water in a PWR reactor), which then imparts mechanical energy to the pistons. Yes I am aware of turbines, steam and gas.
The turbo or super charger forces more oxygen (air) into the combustion chambers, so when that larger fuel/oxygen mixture ignites you can get more energy from the cylinder. An oil fired boiler doesn’t use a turbocharger either.
More importantly, the steam engine is NOT going to like back pressure - N&W was able to significantly improve the performance of the later Y engines by reducing back pressure. A diesel or gasolene engine will almost always have a higher exhaust pressure than intake pressure - this is due to the expansion ratio on most piston engines being equal to the compression ratio and the pressure in the cylinder immediately after ignition is higher than immediately before ignition - ergo the exhaust pressure will be higher than intake pressure.
It is this additional pressure that allows the exhaust turbine of the turbocharger to extract energy from the exhaust - effectively letting it expand back down to ambient. The penultimate application of this idea was with the Wright R-3350 turbo-compounds, where the exhaust turbines were geared to the crankshaft resulting in about 30% more power for the same fuel consumption.
Gas turbines are designed on the basis of pressure ratios, the compression ratio will almost always be smaller than the expansion ratio of the turbine. A similar scheme is the Miller Cycle used on the Prius engine, the intake valve closing is delayed to allow for a smaller compression ratio than expansion ratio.
The real trick with steam locomotives is getting the maximum possible draft with minimum possible back pressure - which is what’s behind the recent work on smokebox design.
I once discussed a possible variant on this theme with a knowledgeable NYC roundhouse foreman (HP steam powering conventional drivers, their exhaust powering two turbogenerators to run traction motors on the tender wheels.) His take on the idea was that any possible fuel savings due to greater thermal efficiency would be eaten up by the cost of maintaining some rather complex and delicate machinery. (Yes, Matilda, in railroad service, steam turbines are delicate!) That’s why three cylinder designs weren’t widely adopted, why those compound locos built in the first decade of the 20th century were all ‘simplified’ by the end of the second and why so few ‘really good’ ideas from overseas ever got more than a passing glance from American motive power departments.
When it came to locomotives that had to earn their keep, the KISS principle applied. Nobody understood that better than the Norfolk and Western - and look at their record.
I have found this discussion on ways of improving steam engine efficiency very interesting - but I suspect that the best ways to improve efficiency start at the other end of the steam cycle better boiler, firebox, and cylinder insulation to reduce radiated waste heat. The cab on the engine I fire is consistently about 40 degress F hotter than the outside temperature. And there is no end to the steam heated opportunities to burn one’s self - hence long sleeve shirts and guantlet gloves - even in the hottest weather.
N&W simplified their steam by reducing complexity, improving maintainability and providing fixed plant specifically designed to speed up (and ‘simplify’) servicing their locomotives. They also refined their designs for maximum efficiency - to minimize their operating expenses.
OTOH the next step in the ‘Y’ parade, the Y-7, was designed to be a simple articulated, not a compound. It was never built.
Some of what N&W did right with the Y-6 was to start with high pressure superheated steam and pay close attention to back pressure on the LP cylinders. Part of the bad rep of compounding was due to use with low pressure steam with little or no superheat. On the other hand, the D&H high pressure locomotives carried thing a bit too far.
But, that simple 2-8-8-2, be it the ‘Y7’ or whatever, never happened, even though the drawings pre-ceded the last Y6bs by several years. I like to think N&W looked, but did not like, as compared to the Y6s.
Before the last management change at VGN, they were looking at N&W Class As. Instead, they got H-8s, after the ex-C&O management came on board. But, that was about the last ex-C&O decision they had to live with.
